R.&C.

Norman
Dm Jm Waddington

Modern
Organic
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Modem Chemistry Series
Under the supervisory editorship of D. J. Waddington, BSc, ARCS, DIC,
PhD, Professor of Chemical Education, University of York, this series is
specially designed to meet the demands of the new syllabuses for sixth form,
introductory degree and technical college courses. It consists of self-
contained major texts in the three principal divisions of the subject,
supplemented by short readers and practical books.

Major Texts
Modem Inorganic Chemistry
G. F. Liptrot MA, PhD

Modern Organic Chemistry

R. O. C. Norman MA, DSc, C Chem, FRSC, FRS
D. J. Waddington BSc, ARCS, DIC, PhD

Modern Physical Chemistry

G. F. Fiptrot MA, PhD
J. J. Thompson MA, PhD
G. R. Walker BA, BSc

Readers
Organic Chemistry: a problem-solving approach
M. J. Tomlinson BSc, C Chem, MRSC
M. C. V. Cane BSc, PhD, C Chem, MRSC

Investigation of Molecular Structure

B. C. Gilbert MA, D Phil, C Chem, FRSC

Mechanisms in Organic Chemistry: Case Studies

R. O. C. Norman, MA, DSc, C Chem, FRSC, FRS
M. J. Tomlinson BSc, C Chem, MRSC
D. J. Waddington BSc, ARCS, DIC, PhD

Practical Books
Inorganic Chemistry Through Experiment
G. F. Fiptrot MA, PhD

Organic Chemistry Through Experiment

D. J. Waddington BSc, ARCS, DIC, PhD
H. S. Finlay BSc
Modern
Organic
Chemistry
R. O. C. Norman, ma, dsc, c Chem,
FRSC,FRS
Professor of Chemistry, University of York

D. J. Waddington, bsc, arcs, dig,

PhD
Formerly Head of the Science Department, Wellington College.
Professor of Chemical Education, University of York

Bell & Hyman

London
Published in 1983 by
BELL & HYMAN LIMITED
Denmark House
37-39 Queen Elizabeth Street
London SEl 2QB

First published in 1972 by

Mills & Boon Limited

Reprinted 1974
Second edition 1975
Reprinted 1977, 1978, 1980
Third edition 1981
Fourth edition 1983
Reprinted 1984

©ROC Norman and D J Waddington 1972, 1981, 1983

All rights reserved. No part of this publication

may be reproduced, stored in a retrieval
system, or transmitted in any form or by any
means, electronic, mechanical, photocopying,
recording or otherwise, without the prior
permission of Bell & Hyman Limited.

British Library Cataloguing in Publication Data

Norman. R. O. C.
Modern organic chemistry.—4th ed.
1. Chemistry, Organic
I. Title 11. Waddington, D. J.
547 QD251.2

ISBN 0 7135 1363 2

Printed by Blantyre Printing & Binding Ltd.,

London and Glasgow
Contents

Page
Preface vii
Acknowledgements ix

CHAPTER
1 Introduction to Organic Chemistry 1
2 Preparation and Purification of Organic Compounds 12
3 Determination of the Structure of an Organic Compound 33
4 Bonding in Organic Compounds 50
5 Alkanes 69
6 Alkenes 79
7 Alkynes 94
8 Aromatic Compounds 99
9 Halogen Compounds 116
10 Alcohols and Phenols 143
11 Ethers 166
12 Aldehydes and Ketones 171
13 Carboxylic Acids 189
14 Derivatives of Carboxylic Acids 211
15 Isomerism 235
16 Amines 251
17 Nitro Compounds 274
18 Naturally Occurring Compounds 281
19 Petroleum 301
20 The Petrochemical Industry 314
21 Polymers 327
22 Looking to the Future 345

APPENDIX
I Summary of Industrial Processes 348
II Questions 353
III Apparatus and Chemicals 373
IV Suppliers of Apparatus and Chemicals 376
V Teaching Aids and Materials 377
VI Physical Constants 379

381
INDEX

V
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Preface

This textbook is intended primarily to cover organic chemistry for

Advanced Level examinations and University scholarships, but we believe
that our readers will also find the book of great use in the beginning of their
University work.
As the subject has evolved over the last twenty or so years, so the rather
artificial divisions between organic chemistry and other branches of chem¬
istry, and between chemistry and other sciences, have been breaking down;
the subjects are now much more interdependent, and gain one from an¬
other. We have endeavoured to reflect these developments, so as to place
organic chemistry in its proper perspective.
First, whereas the subject was concerned at one time mainly with an¬
swering the question ‘What product is formed when A reacts with B?’, now
it is also concerned with the question ‘How does A react with B to give
CT—a field described as mechanistic organic chemistry. This at once
involves the methods and concepts of physical chemistry—for example,
reaction kinetics and bond energies.
Secondly, the organic chemist has learned to apply techniques pioneered
by the physicist in order to determine the structures of organic compounds,
and to do so with much greater rapidity than by the older, ‘classical’
methods. For example, infrared and nuclear magnetic resonance spectros¬
copy are now routinely used in organic research laboratories.
Thirdly, organic chemists and biologists have been increasingly working
together to elucidate the ways in which organic compounds and their re¬
actions play their crucial roles in living organisms; DNA, which underlies
the transmission of inherited characteristics, is after all, an organic com¬
pound, and when animals need energy they obtain it by the controlled
oxidation of another organic compound, glucose. Now we are on the thresh¬
old of another evolutionary wave of science in which organic chemists
are combining with biologists and others: biotechnology and genetic
engineering are likely to make important strides in improving agricultural
methods and medical treatment.
Finally, the importance of organic compounds in our economy has been
rising steadily for many years and will continue to do so. New plastics are
being invented, new medicines are being tailor-made for specific require¬
ments, there is a new awareness of the problems of pollution which has
meant that the organic chemist is concerned with the discovery of new fuels,
detergents and pesticides, and we are on the threshold of manufacturing
proteins for animal feedstocks which could revolutionise the economies of
both developed and developing countries.
We have tried to reflect all these factors in this book. We have set out the
preparations and properties of organic compounds in terms of functional
groups and against a background of the principles of bonding, energetics
and reaction mechanism. Although we believe that these principles can be
all the more easily understood in this way, rather than as isolated topics in
a physical chemistry course, the book has been written so that students can
leave out some sections if these are outside their immediate scope and can
go back later to read the book as a whole.

vii
PREFACE Chapter 2 describes the modern techniques for the preparation and
purification of organic compounds and Chapter 3 is concerned with the
methods now employed for studying their structures. In Chapter 18, we
have related the chemistry of some of the types of compound which occur
naturally to their functions in living systems, while Chapter 21 deals with
the man-made macromolecules we use as plastics and fibres. We have
shown throughout the book how petroleum, including natural gas, is still
vital for the chemical industry, and two Chapters, 19 and 20, are specifically
devoted to this. It is especially notable here how much the methods em¬
ployed in the chemical industry have changed since the first edition of this
book was written, some ten years ago. New processes have been developed
that are more efficient and use less energy, and now new feedstocks are
beginning to be employed as alternatives to oil. Finally, we have included
a short Chapter—‘Looking to the future’—in which we have suggested
some of the main directions in which organic chemistry is likely to move
over the next decade or so.
The introduction of new syllabuses at every level is encouraging for all of
us who teach and study Chemistry, all the more so since, hand-in-hand with
these positive changes in our ideas on how theory should be presented,
comes the desire to illustrate the work experimentally. We have therefore
suggested practicals, with very simple apparatus, at the end of the chapters.
Most of these practicals should take less than an hour.

York 1983 R.O.C.N.

D. J. W.
Acknowledgements

We are most grateful to Dr J. McIntyre and Mr D. J. M. Rowe of our

Department and to Dr M. B. Sparke of British Petroleum for their valu¬
able assistance in revising Chapters 19 and 20, and to Dr C. J. Garratt, also
of the University of York, for his help with revisions to Chapter 18.
Mr H. S. Pickering (Uppingham School), who made such a helpful con¬
tribution to the original drafts, has continued to give his valuable advice
and we are also very grateful to Mr T. J. Harrington (Bradford Grammar
School).
We have been fortunate, too, in the assistance we have been given by
many teachers who have written to us with helpful suggestions.
We thank Mr J. Olive for the photographs used in Chapters 1, 2 and 15,
and Mrs Ann Ferens and Mrs Morag Morgan who tried out new experi¬
ments that we wished to include and for the modifications they suggested.
We thank the Esso Petroleum Co. Ltd., Shell Petroleum Co. Ltd., British
Petroleum Co. PLC, Imperial Chemical Industries PLC, and British Gas
for permission to use photographs and for most helpful assistance with
technical data
We acknowledge permission to us to use examination questions by the
Colleges of the Universities of Cambridge and Oxford, the Local Exami¬
nation Syndicate of the University of Cambridge, the Delegates of Local
Examinations, University of Oxford, the Oxford and Cambridge Schools
Examination Board, the Joint Matriculation Board, the Southern Uni¬
versities’ Joint Board, the Associated Examining Board, the Welsh Joint
Education Committee, the Northern Ireland Schools Examination Coun¬
cil, and the University of London University Entrance and Schools
Examinations Council. Those questions marked London (Nuffield) are
those set for the Nuffield Science Teaching Project in Chemistry at A level
and those marked London (X) are from papers taken by schools who were
taking the London A examination following a Nuffield O course.

IX
 Nomenclature

We have followed, with two exceptions, those recommendations based

on the I.U.P.A.C. nomenclature made by the Association for Science
Education in their booklet ‘Chemical nomenclature, symbols and termi¬
nology’ (revised 1983) and accepted by the Examinations Boards.
The first exception is that we have retained the generic name ‘ether’; the
alternative—alkoxyalkanes, aryloxyalkanes and aryloxyarenes, as appro¬
priate—is cumbersome. Secondly, we have used the trivial names for
the important a-amino-acids, since these original names are still widely
used in Biology and Biochemistry. Both these exceptions are accepted
by I.U.P.A.C.

Safety

The experiments we have suggested at the end of chapters need safe

handling. We commend to all readers the series of articles by the Associa¬
tion for Science Education that has appeared in Education in Science over
the last few years and which has been brought together in Topics in Safety
published in 1982. Copies may be obtained from the Association for Science
Education, College Lane, Hatfield, Hertfordshire ALIO 9AA, U.K.

X
Chapter 1
Introduction to organic chemistry

1.1 Over two million compounds are known which contain the element carbon,
Introduction and about 80,000 new carbon compounds are made each year. It is therefore
convenient to study the compounds of carbon separately, and this branch
of chemistry is known as organic chemistry.
Originally the word organic applied to those substances that were pro¬
duced by living organisms. Berzelius wrote in 1815 that the essential differ¬
ence between inorganic and organic compounds was that the formation of
organic compounds could only be achieved by the influence of a ‘vital force’
which was present in nature. No organic material could be synthesised in
the laboratory. Sugar, dyes, starch, oils, alcohol, known since the earliest
times, could only be made by nature.
A conflicting point of view was put forward by Wohler in 1828. He found
that when an aqueous solution of ammonium cyanate is evaporated to
dryness, carbamide (urea) is obtained:

NH NCO -> H N-C-NH

4 2 2

Ammonium cyanate Carbamide

Ammonium cyanate is an inorganic compound whereas carbamide is

present in the urine of all animals and was therefore particularly well known
as an organic compound. It seemed, then, that ‘vital force’ was unnecessary.
However, other scientists argued that, since the ammonia and cyanic acid
from which Wohler had made ammonium cyanate were both of animal
origin, the true synthesis of an organic compound had not been achieved.
It was not until 1845 that a final refutation of the ‘vital force’ idea was
propounded, when Kolbe prepared the organic compound, ethanoic acid
(acetic acid), from its constituent elements, carbon, hydrogen and oxygen.
Today, the distinction between organic and inorganic chemistry is an
arbitrary one. Organic chemistry is regarded as the chemistry of compounds
of carbon other than its oxides, the metallic carbonates and related com¬
pounds.

The bonds which carbon forms are covalent: that is, each bond is formed
1.2 by the sharing of two electrons, one of which is provided by the carbon atom
Bonding and one by the other atom. Carbon has four electrons available for sharing,
so that it forms four bonds; a fuller description is given later (Chapter 4).
It is convenient to represent each pair of electrons which constitutes a bond
by a line, —; for example, a bond between carbon and hydrogen is shown
as C—H.
The bonds to a carbon atom have particular positions in space in relation
to one another. For example, in methane, CH , the bonds are directed
4

towards the corners of an (imaginary) regular tetrahedron of which the

carbon atom is the centre; the angle between each pair of C—H bonds is
109°28'.

1
INTRODUCTION TO ORGANIC For simplicity, a two-dimensional structure is usually drawn; for example,
CHEMISTRY
methane is written as

However, two-dimensional representations can be misleading and care must

be exercised in their use. For instance, it might appear that there would be
two compounds with the formula CH CI :2 2

H Cl
I I
Cl-C-Cl H-C-Cl
I I
H H

A three-dimensional representation shows that the two planar structures

actually represent the same compound, and indeed only one compound of
this formula exists.

Plate 1.1. The tetrahedral

arrangement of four atoms of
hydrogen around a carbon atom in
a molecule of methane: (a) ball
and spring, {b) space-filling

(a) (b)

It is useful to have a simple set of molecular models to consider problems

like this; they can provide a quick means of translating the planar repre¬
sentations on paper into the more realistic three-dimensional structures.
The simplest models are called ball and spring. The balls are coloured
differently to represent different atoms, and have holes drilled in them
corresponding to the number of bonds they can form. The balls are joined

2
INTRODUCTION TO ORGANIC
CHEMISTRY together by means of stiff springs fitted into the holes which represent the
bonds (Plate . ). Space-filling models are more useful when it is necessary
1 1

to obtain a more accurate idea of how near together different atoms will be
in the compound. In one sort, Stuart models, the atoms (generally con¬
structed in a plastic) are made to scale according to the relative atomic radii
of the elements they represent, and they are joined together by clips. The
construction of the model for CH CI (Plate 1.2) shows at once that only
2 2

one compound with this molecular formula exists.

Plate 1.2. Alolecular models of

dichloromethane, CHiCf:
(a) ball and spring, (b) space filling.
The atoms are coloured black
(carbon), white (hydrogen), grey
(chlorine).

(b)

A carbon atom can be attached to monovalent atoms other than hydro¬

gen. For example, one chlorine atom can replace one hydrogen atom to give
chloromethane (also called methyl chloride):

H
I
H-C-Cl
I
H

Again for simplicity, some or all of the bonds are usually omitted in repre¬
senting these compounds, so that chloromethane is written as CH —Cl or3

CH3CI. Replacement of more than one of the hydrogen atoms by chlorine

atoms gives the compounds dichloromethane (CH CI ), trichloromethane
2 2

(CHCI3) and tetrachloromethane (CCI4).

Carbon can be bonded to a divalent atom, as in methanol, CH3OH:

H
I
H-C-O-H
I
H

3
INTRODUCTION TO ORGANIC to a trivalent atom, as in methylamine, CH NH :3 2
CHEMISTRY

H
1
H-C-N.
1
H

or to another carbon atom, as in ethane, CH CH 3 3:

H H
1 1
H-C-C-H
1 1
H H

Notice that ethane can be written as above or as C2H6, but not as CH ; 3

although the formula CH describes the relative numbers of each kind of

3

atom correctly, it does not give adequate information about the total num¬
ber of atoms in the molecule. For this purpose, the molecular formula must
be used, that is, a description of the actual number of each kind of atom
present.

1.3 Why is it that carbon forms so many more compounds than all the other
elements? The answer can be given in terms of bond energies. It can be
The unique nature of shown that it requires about 1,652 kilojoules to break up one mole of
carbon methane into its carbon and hydrogen atoms. Since there are four
C—H bonds in methane, the bond energy of one C—H bond is one-quarter
of 1,652 = 413 kilojoules per mole (abbreviated to 413 kJ mol“^). It can
also be shown that it requires 2,823 kJ to break up one mole of ethane
fCHj—CH ) into its constituent atoms. Since this compound contains six
3

C—H bonds, each of which requires 413 kJ mol“^ for its rupture, the
energy of the C—C bond is calculated to be 345 kJ mol“^. This is a very
high value as compared with those for other elements joined by single bonds
(e.g. 163 kJ moP^ for N—N and 146 kJ moP^ for O—O). Thus, whereas
compounds containing O—0 and N—N bonds are not very stable, very
vigorous conditions—for example, the high temperatures produced in com¬
bustion—are necessary to destroy C—C bonds, and this underlies the oc¬
currence of large numbers of stable compounds containing many C—C
bonds. For example, in poly(ethene), a plastic (p. 328), many hundreds of
carbon atoms are linked together in one molecule. The occurrence of chains
of carbon atoms is known as catenation.
As well as forming long chains, carbon atoms can form branched chains,
e.g.
1
-C-
1 1 1
-c-c-c-
1 1 1

There are also compounds in which some of the bonds between carbon
atoms are double or triple bonds (1.5) or in which the atoms form rings (1.6).
All these possibilities increase still further the number of carbon compounds
which can be formed.

4
1.4 The large number of organic compounds fall into a comparatively small
number of series, known as homologous series. In a particular series, each
Homologous series and
member has similar methods of preparation and chemical properties to the
functional groups other members. In the series of alkane hydrocarbons (Chapter 5), the
simplest member is methane, CH . The next member is ethane, C H , then
4 2 6

propane, C H , and so on; the general formula of the series is C„H „+ - As

3 8 2 2

the series is ascended, a methylene group, CH2, is added to each successive

member. As each methylene group is added, the physical properties change
slightly. This is demonstrated in detail with the alkanes (Chapter 5).
We have seen already that there is a number of compounds which contain
the grouping CH3. This is called the methyl group (sometimes, the methyl
radical). Other collections of atoms which occur frequently are the amino
group, NH2, and the hydroxyl group, OH. Each of these is able to bond
to another group, as in methylamine, CH3NH2, and methanol, CH3OH.
They are examples of functional groups, which consist of an atom or group
of atoms which determine the properties of the homologous series. For
example, in the series of alcohols.

CH3—OH CH3—CH2—OH CH3—CH2—CH2—OH

Methanol Ethanol Propan-l-ol
CH —CH —CH —CH —OH
3 2 2 2

Butan-l-ol

there is a gradation of physical properties but the chemical properties are

very similar.
Several homologous series can be considered to be derived from the
alkanes, C„H „+ , a hydrogen atom being replaced by a functional group.
2 2

The group formed from the alkane, C„H „+i, is known as the alkyl group,
2

and is often represented by the letter R.

The names of alkyl groups are related to the names of the corresponding
alkanes:

Alkane Alkyl group

C„H2„+2(RH) C„H2„+i(R)

Methane, CH4 Methyl, CH3

Ethane, C2H6 Ethyl, C2HS
Propane, CsHs Propyl, C3H7
Butane, C4H10 Butyl, C4H9

The compounds we have described so far contain single covalent bonds, and
1.5 these compounds are described as saturated. There are also unsaturated
Unsaturated compounds compounds in which two atoms share either four or six electrons. For
example, the carbon atoms in ethene share four electrons (two originating
from each atom); the bond is described as a double bond:

H XXX
H
CiC
X X

H' H

5
INTRODUCTION TO ORGANIC As can be seen from Plate 1.3, the carbon and hydrogen atoms are in a
CHEMISTRY
plane, with bond angles of 120° (compare the tetrahedral structure of
methane). The molecule is described as planar and can be represented as

H H

H H

Plate 1.3. A space-filling

molecular model of ethene

In ethyne, the carbon atoms share six electrons (three originating from
each atom); the bond is described as a triple bond:

X
X

X
X

The carbon and hydrogen atoms lie in a straight line (Plate 1.4) and the
molecule is described as linear. A simple representation is
«

H-C=C-H

Other common unsaturated groups are C=N, as in ethanenitrile, and

C=0, as in ethanal and ethanoic acid.

CHj—C=N CHj—C—H CHj—C—OH

o o
Ethanenitrile Ethanal Ethanoic acid

6
 The groups C—N, C—O and CO H are called the nitrile, carbonyl and
2

carboxylic acid groups, respectively.

Plate 1.4. A space-filling

molecular model of ethyne

1.6 The compounds we have mentioned so far have had carbon atoms joined
in straight chains or branched chains, and these are described as aliphatic.
Aliphatic, alicyclic and There are also compounds in which some of the atoms formi a ring, e.g.
aromatic compounds
/H

C C
H- \ /
H-C-C- -H

H H
Cyclopentane

Rings with from three atoms to very large numbers are known, and these
compounds are described as alicyclic.
There is a special class of ring compound of which benzene is the parent:

H
I
C H

7
INTRODUCTION TO ORGANIC large number of compounds contain the same carbon ring but have atoms
CHEMISTRY
or groups other than hydrogen attached to it. For example, writing benzene
as CgHg, chlorobenzene is CgHj—Cl. Compounds of this nature were orig¬
inally termed aromatic because some of them have pleasant smells (Greek:
aroma, fragrant smell). The term has been retained because it provides a
useful classification; as we shall see, benzene has different properties from
the simple unsaturated compound, ethene, and the differences arise because
of the special nature of the bonding in benzene ( . ).
8 2

1.7 There are two compounds with the molecular formula C H O: 2 6

Isomerism H H H H
I I I
H-C-C-O-H H-C-O-C-H
I I I I
H H H H

Ethanol Dimethyl ether

However, although both compounds have the same number of each kind
of atom (two C, six H, one O), they have different physical and chemical
properties. They are described as isomers, and the phenomenon is described
as isomerism. Isomerism is said to occur when two or more compounds have
the same molecular formula.
The existence of a large number of isomers is illustrated by the alkanes.
There is only one compound of molecular formula CH (methane), C H
4 2 6

(ethane) or C Hg (propane). There are two of molecular formula C H :

3 4 10

CHg
I
CH3-CH2-CH2-CH3 CH3-C-H
CH3

and three of molecular formula C H :

5 12

CH3 CH 3

\ I
CH3-CH2-CH2-CH2-CH3 CH-CH2-CH3 CH3-C-CH3
CH3 CH 3

Because carbon atoms can be joined in straight and in branched chains, the
number of possible isomers increases very rapidly as the number of carbon
atoms increases; for example, there are 5 isomers with molecular formula
C H and 18 with molecular formula CgHig. When other atoms are intro¬
6 14

duced, the number of possible compounds with a particular molecular

formula increases still further; e.g., there is only one compound with mol¬
ecular formula CgHg but three with molecular formula CgHgO, two being
alcohols and one an ether:

CH3-CH2-CH2-OH CH3-CH-CH3 CH3-CH2-O-CH3

I '
OH
Propan-l-ol Propan-2-ol Ethyl methyl ether

8
1.8 As organic chemistry developed, each new compound to be discovered was
Nomenclature of carbon given its own name, so that a variety of unrelated names quickly grew up.
It eventually became necessary to introduce a systematic form of nomen¬
compounds clature in order that the structure of a compound could be readily deduced
from its name, and vice-versa. The nomenclature at present in use was laid
down by the International Union of Pure and Applied Chemistry
(I.U.P.A.C.), and the rules for naming some of the simpler compounds are
given here.
Methane, as we have seen, is the simplest alkane, and each member of
the homologous series of alkanes is given the suffix -ane. The first four retain
the names originally given to them: methane (CH4), ethane (C2H6), propane
(C3H8) and butane (C4H10). After that, the first part of the name is derived
from the Greek for the number of carbon atoms in the molecule: pentane
(C5H,2), hexane (C6H14), heptane (CyHig), octane (CgHig), and so on.
When the chain is branched, the name is taken from that of the longest
straight chain of carbon atoms in the molecule; the carbon atoms are num¬
bered from one end of the chain, and the position of the branch and the
nature of the group there are indicated by the number of the carbon atom
at which branching occurs and the name of the alkyl group which forms
the branch. For example,
1 2 3 4 5
CH3-CH2-CH-CH2-CH3
CH3
is termed 3-methylpentane. The compound

CH3-CH-CH2-CH2-CH3
CH3

might be called 2-methylpentane or 4-methylpentane, depending upon from

which end the chain was numbered. The rule is to number from that end
which gives the substituent the lower of the two possible numbers, so that
the correct name is 2-methylpentane.
The unsaturated compounds with a C=C double bond which are often
referred to as olefins are termed alkenes in the I.U.P.A.C. scheme. The
number of carbon atoms is described in the same way as for the alkanes.
Each member terminates in -ene, and the position of the double bond is
determined by inserting the lowest possible number before the suffix to
describe the carbon atom which forms one end of the double bond relative
to its position in the chain; e.g.

CH3—CH2—CH2—CH=CH2

is pent-1-ene, not pent-2-ene or 4- or 5-ene.

The unsaturated compounds with a C^C triple bond, which are often
described as acetylenes after the simplest member of the series, acetylene
itself (CH=CH), are termed alkynes in the I.U.P.A.C. scheme, and in¬
dividual members are described as for the alkenes but with the termination
-yne.
When an atom other than carbon or hydrogen is present, the name either
begins or ends with a description of the type of group which contains this
atom, preceded by the number of the carbon atom to which the group is
bonded. The remainder of the name is that of the hydrocarbon with the same
number of carbon atoms, the final e of this name being omitted when a suffix

9
INTRODUCTION TO ORGANIC is employed for the substituents whose first letter is a vowel. For example,
CHEMISTRY
if chlorine is present, chloro- is used as a prefix, so that
CH3-CH-CH3
Cl
is 2-chloropropane. If a hydroxyl group is present, -ol is used as a suffix,
so that
CH3-CH-CH2-CH3
OH
is butan-2-ol. Table 1.1 gives the names of some of the homologous series
and their functional groups which are met early in this book.

Table 1.1. Nomenclature of organic compounds

HOMOLOGOUS FUNCTIONAL PREFIX OR EXAMPLE OF I.U.P.A.C.

SERIES GROUP SUFFIX NOMENCLATURE

Alkanes (-H) -ane CH3CHCH2CH3

1
CH3
2-Methylbutane
Alkenes C=C -ene CH3CH2CH=CH2
But-1-ene
-yne
u
u
Alkynes CH3C^CCH3
III

But-2-yne
Alcohols —OH -ol CH3CH2CH2CH2OH
Butan-l-ol
Chloroalkanes —Cl chloro- CH3CH2CHCH3
(Alkyl 1
chlorides) Cl
2-Chlorobutane
Primary amines —NH2 amino- CH3CH2CHCH2CH3

NH2
3-Aminopentane
Aldehydes -C-H -al CH3CH2CHO
II Propanal
0

Ketones -c- -one CH3COCH2CH2CH3

II Pentan-2-one
0

Carboxylic acids -C-OH -oic acid CH3CH2CO2H

II Propanoic acid
0

Acid chlorides -C-Cl -oyl chloride CH3CH2CH2COCI

II . Butanoyl chloride
0

Acid nitriles —CN -nitrile CH3CN

Ethanenitrile

The nomenclature for each homologous series is described in detail in the

Chapter concerned with that series. Where the older, or ‘trivial’, names are
still often used, they are given, in brackets, after the I.U.P.A.C. name. Two
commonly used prefixes in the older nomenclature are n-, which indicates

10
INTRODUCTION TO ORGANIC
CHEMISTRY an unbranched chain of carbon atoms, and iso-, which indicates the presence
of the group (CH3)2C; for example:

CH3
CH3-CH2-CH2-OH CH3-CH-CH3 CH3-CH-CH3
I
OH
Propan-l-ol Propan-2-ol 2-Methylpropane
(n-Propanol) (Isopropanol) (Isobutane)

1.9 1. Make models for the following compounds: (a) ethane, (b) chloro-
ethane, (c) ethanol.
Practical work
2. Make models for compounds with the molecular formulae: (a) C3H8,
(b) C3H6, (c) C3H4. Name the compounds.
3. Make models for compounds with the molecular formulae: (a) C4H8,
(b) C4H6. Name the compounds.

1.10 1 How many isomers would you expect with the following molecular formulae:
CH2Br2; QHei C4H10O? How many isomers, each containing the group C=0,
Questions would you expect with the molecular formula C5H10O?
2 Write down the structures of the following compounds; 2-methylpentane; 2,2-
dimethylpropane; hex-2-ene; 1-bromobutane; 2-methylpropan-2-ol; pentan-3-
one.

3 Name the following compounds according to the I.U.P.A.C. rules:

CH3-CH2-CH-CH2-CH2-CH3 CH3-CH2-CH=0

CH,
I
CH3

CH3-CH2-CH-CH3 CH3-CHo-CH,-C-OH
NHa o

CH3-CH=CH-CH2-CH3 CH3-CH2-CH2-C=CH

11
 Preparation and purification of
organic compounds

2.1 There are three stages in the preparation of a pure organic compound. First,
conditions are found under which the required product is formed in a
Introduction relatively short time (that is, in a few minutes or at most a few hours, rather
than days). Secondly, the product is separated from other materials. Finally,
methods are employed to find out if the compound is pure.
The preparations of many organic compounds are described in detail in
later chapters. In order to simplify the operations, the number of pieces of
apparatus necessary has been kept as small as possible (a list is given in
Appendix III). We describe below the more commonly used techniques in
the preparation and purification of organic compounds.

2.2 Many organic reactions occur very slowly at room temperature. However,
the rates of all reactions are increased by raising the temperature, and a
Heating under reflux
reasonably rapid rate can often be achieved by carrying out the reaction at
the boiling point of the mixture of reactants, or of the solution of the
reactants if they are dissolved in a solvent. The apparatus for this purpose
(Fig. 2.1) consists of a water condenser attached vertically to the reaction
flask; when the mixture in the flask is boiled, its vapour condenses at the
cold surface of the condenser and the liquid runs back into the flask. This
is known as heating under reflux.
FIG. 2.1. An apparatus for heating
under reflux

12
2.3 When the product is a liquid or solid with a boiling point below about 250°C
and no other such volatile compounds are present, the simplest method of
Distillation
purification is by distillation. The apparatus is shown in Figs. 2.2 and 2.3;

FIG.2.2. A distillation apparatus,

with a water condenser

FIG.2.3. A distillation apparatus,

with an air condenser

13
PREPARATION AND PURIFICATION OF the water condenser is used for compounds boiling up to about 180°C and
ORGANIC COMPOUNDS
the air condenser for compounds of higher boiling point. The mixture is
heated to boiling and the vapour of the product liquefies in the condenser,
the liquid collecting in a receiving flask at the end of the condenser. It is
important to place the thermometer so that its bulb is fully immersed in the
stream of vapour which is about to enter the condenser; the recorded tem¬
perature can then be compared with the quoted boiling point for the re¬
quired compound. Since the boiling point of a liquid is dependent on the
external pressure, the atmospheric pressure should also be measured and
recorded when a boiling point is reported; e.g. ethanol, b.p. 78°C/760
mm Hg (the pressure in S.I. units is 101-33 kN m“^). Unless the pressure is
stated, you may assume that the reported boiling point was measured at a
pressure of 760 mm Hg.
Although some organic liquids decompose before they have a chance to
boil at atmospheric pressure, they can still be purified by distillation by
reducing the external pressure until their boiling points are below their
decomposition temperatures. This is known as vacuum distillation. The
receiving flask is attached to the condenser and to a thick-walled rubber
tube through which air can be withdrawn from the apparatus (Fig. 2.4). By

14
ORG^^mc^cfo^MPOUND^^^ attaching this tube to a water pump, the pressure can be reduced to a value
equal to the vapour pressure of water, which is about 12 mm Hg at room
temperature. Much lower pressures can be obtained with a rotary oil pump.
When the required product is accompanied by one or more other com¬
pounds of similar boiling point, fractional distillation must be carried out.
A glass column containing glass or stainless steel coils or beads (Fig. 2.5)
is attached to the boiling flask and, at its top outlet, to the condenser. In
the apparatus shown, liquids boiling at least 30°C apart (e.g. benzene, b.p.
80°C, and methylbenzene, b.p. 111°C) can be separated successfully but as
the difference in boiling points becomes smaller the efficiency of separation
decreases and longer columns are necessary. The theoretical basis for
fractional distillation is discussed in physical chemistry textbooks.

FIG. 2.5. An apparatus for

fractional distillation

Heai

15
 For example, nitrobenzene boils at 210°C. At 98°C, its vapour pressure
is about 50 mm, while that of water is 710 mm. The total vapour pressure
is equal to the atmospheric pressure, and the mixture boils. Thus, nitro¬
benzene is distilled at 112°C below its boiling point. It is separated from
water by using a separating funnel (Fig. 2.7) from which the denser liquid
(in this case nitrobenzene) is run off first.

2.4 If the material to be purified is soluble in one solvent, while the impurities
are not, the mixture can be partitioned between two immiscible solvents.
Extraction
For example, sometimes an organic compound is obtained as an aqueous

16
PREPARATION AND PURIFICATION OF
ORGANIC COMPOUNDS
solution, which is contaminated with inorganic materials. The aqueous
solution is shaken with diethyl ether (often referred to simply as ether) in a
separating funnel. Only the organic compound is soluble in ether, and the
residue remains in the aqueous layer. The two layers are separated, and the
ether solution is shaken in the presence of a solid drying agent such as
anhydrous magnesium sulphate to remove the small amount of water which
will have dissolved in it. The solution can then be filtered from the drying
agent and the organic compound can be separated from the ether by
distillation.
Extraction is more elficient if several small quantities of ether are used,
rather than one large quantity. This is considered theoretically in many
physical chemistry textbooks.

2.5 This is the commonest method for purifying a solid. It is based on the fact
that the solubility of organic compounds in a particular solvent increases
Recrystallisation
as the temperature is raised. The solvent is heated with that amount of the
solvent which gives a nearly saturated solution at the boiling point. The
solution must then be filtered very rapidly, and this is done using a fluted
filter paper, contained in a glass funnel from which the stem has been
removed, through which the filtrate runs into a conical flask; insoluble
impurities are thereby removed. The solution is then allowed to cool, and
crystals of the solid continue to be deposited until the solution has reached
room temperature. The crystals are filtered off in a Buchner funnel (a

FIG. 2.8. A Buchner funnel and

flask

To pump

porcelain funnel with a large number of holes in its base on which a filter
paper rests), which is attached to the filter flask (a Buchner flask) whose
side-tube is connected to a water-pump. When filtration is complete, the
crystals are washed with a small quantity of the pure solvent to remove any
impurities which might have deposited on the surface. The crystals are then
dried on a watch-glass to remove the solvent; drying may be at room tem¬
perature if the solvent is a particularly volatile one, but it is usually quicker
to place the watch-glass in an oven, taking care that the temperature of the
oven is below the melting point of the solid.

17
PREPARATION AND PURIFICATION OF If one recrystallisation does not yield a pure material, further recrystalli¬
ORGANIC COMPOUNDS
sations, preferably with different solvents, should be carried out.

2.6 The chromatographic methods of separation are particularly important for

complex mixtures of substances which are not otherwise readily separated.
Chromatography The principal methods are:
(a) Column chromatography
(b) High pressure liquid chromatography
(c) Thin-layer chromatography
(d) Paper chromatography
(e) Gas chromatography

(a) Column chromatography

This is a method for the separation of solids, and can be successful even
when the solids have very similar solubilities, when recrystallisation would
not be effective.
A glass column about 30 cm long and 2 cm in diameter, which narrows
at one end to an outlet tube, is used. A pure liquid such as benzene is run in
until the column is about half-full, and then a slurry of a solid adsorbent
(such as alumina) in the same solvent is added. The solid sinks to the bottom,
FIG. 2.9. Column chromatography forming an evenly packed column immersed in the solvent; a pad of glass
wool at the bottom prevents the solid running out, and a disc of filter paper
is added at the top of the column to avoid disturbance of the solid when
Solvent
a solution or solvents are added later. The solvent above the column of
damp solid is run off, care being taken that the column of solid remains
wetted, for otherwise cracks and channels appear and the solution to be
Disc of filter paper
added does not run evenly through the column.
A solution of the mixture in the same solvent is then added at the top
of the column and the liquid emerging at the bottom is collected; by attach¬
Solid and solvent
ing the receiver to a pump, the rate of passage through the column can be
increased (Fig. 2.9). Following addition of the solution, further quantities
of the solvent are added, and after a while solutions of the components of
Glass wool
the mixture will emerge successively. This process is known as elution. Each
solution is collected in a separate flask, and the solvent is then removed by
distillation to leave the pure compound.
If the solutes are coloured, they are seen as they pass down the column.
If they are colourless, a physical property, such as fluorescence under ultra¬
To pump violet light, can be employed to find out how each compound is moving
down the column so that a fresh flask can be inserted at the bottom just
when the solution of the compound is about to emerge.
The principle of the method is that each compound in the mixture has
a particular solubility in the solvent and a particular tendency to be adsorbed
by the solid in the column; no two compounds behave exactly alike in these
respects. Thus, compounds which are readily soluble and not strongly
adsorbed move rapidly down the column in the stream of solvent; those
which are not so soluble and are more strongly adsorbed are held on the
column for longer periods.
In some cases a particular solvent causes one component of a mixture to
be eluted rapidly while another component does not travel down the column
at a significant speed. It is then helpful to use a second solvent after the first
compound has been eluted. Plate 2.1 shows a column packed with alumina
in which a solution obtained from spinach is being separated.

18
Plate 2.1. Column chromatography.
(a)
Separation of pigments in a
solution of spinach in light
petroleum, (a), (b) The column is
being developed with a solvent.
The carotenes {coloured orange)
are being eluted, (c) The column ^Chlorophylls
is being developed with another (green)
solvent. The chlorophylls {coloured "Carotenes
green) are being eluted (orange).

|ag-Chlorophylls
“ (green)

19
PREPARATION AND PURIFICATION OF A recent development in column chromatography has been in the way
ORGANIC COMPOUNDS
the column is packed with the solid adsorbent. This is the dry pack method.
Nylon tubing (about 20 cm in length and 2 cm in diameter) is used instead
of glass. It is sealed at one end (by pressing the end with a hot iron). Sand is
poured in to a depth of 1 cm, followed by the adsorbent and finally by a
little more sand, to a depth of 0-5 cm. The packed column is then stood in
a measuring cylinder. A sample of the solution of the mixture is added,
followed by small portions of the eluting solvent. Elution is stopped when
the solvent reaches the end of the column.
The nylon column is then taken out of the cylinder, lain sideways and
sliced with a sharp knife. The sections containing the solutes are placed in
different beakers and each is extracted with a solvent. The extracts are
filtered and evaporated, leaving samples of the pure compounds.
While this method is often not as efficient in separating closely related
compounds, it is very quick and convenient and is becoming widely used.

(b) High pressure liquid chromatography

The separations achieved by column chromatography can be improved
by reducing the size of the particles of adsorbent and ensuring that they
are of uniform size. However, this results in the solvent moving very slowly
through the column, and for practical purposes it is necessary to use a high
pressure to speed up the process. This is the basis of high pressure (some¬
times called high performance) liquid chromatography (HPLC).
There were practical difficulties in developing HPLC, since the materials
used to make the columns must be able to withstand the high pressures,
the solvent should fiow at constant rate and a special system must be used
to inject the mixture into the column.
The difficulties have been overcome. The columns are made of thick-
walled glass (8 mm diameter with 2 mm bore) or stainless steel. Pressures
of 200-300 atmospheres are generally used and, with the use of valves, the
flow-rate of solvent can be kept constant. The sample is injected on to the
column via a special valve or syringe (Fig. 2.10).
Many column packings have been developed. One is a polymer (often
poly(phenylethene) is used) which is prepared so that it is porous. Small

FIG. 2.10. Outline of

a high-pressure liquid
chromatograph - Syringe

. Injection head

Saturator
• Analytical column

Solvent Thermostat
reservoir

Pump
Recorder

Fraction
collector

Pressure
gauge

20
PREPARATION AND PURIFICATION OF
ORGANIC COMPOUNDS
molecules tend to be trapped in the pores, while larger molecules are ex¬
cluded. Thus the larger molecules elute first, the smaller molecules later.
This is known as gel permeation chromatography. Solutions containing
a polymer whose molecules have diflferent chain lengths can be separated
into fractions with more or less similar lengths.
Ion-exchange resins are used as packing materials to separate mixtures
of amino-acids, a particularly useful technique in the elucidation of protein
structure (Chapter 18) and in the analysis of protein hydrolysates in
medical diagnosis.
Other column packings include silica and alumina, similar in chemical
nature to those used for column chromatography above but with particles
which are much smaller and of more even diameter. Another development
has been to produce column materials composed of a liquid coated on an
inert solid. The compounds in the mixture are partitioned between the
liquid on the inert solid (known as the stationary phase) and the eluting
solvent. The liquid tends to be washed off the column by the solvent, and
to avoid this the solvent is first saturated in the liquid before it is pumped
through the column. Another interesting development is to ‘sew’ the liquid
onto the column by chemical reaction so that it cannot wash off.
When the solvent emerges from the column it passes through a detector
which usually measures either refractive index or ultraviolet absorption
and can thereby show whether the solvent is pure or contains a solute; as
each of the latter fractions emerges, it is collected separately and the solu¬
tion is then evaporated to obtain the pure compound.
The detector is used in conjunction with a pen-recorder. The result of a
typical experiment is shown in Fig. 2.11. The sample was injected at time A.
The time taken for each solute to pass through the column [for phenyl-
ethanone, time (B — A)]—its retention time—is characteristic of the com¬
pound and the conditions of the experiment (the rate of flow of solvent,
the nature and concentration of the stationary phase and the temperature).
Provided that the conditions are constant, an unknown compound can
be identified by comparison of its retention time with those for samples of
known compounds.

FIG. 2.11. Separation of a

mixture of compounds
by high pressure liquid
chromatography:
1, benzene; 2, phenylethanone;
3, methyl benzoate. The retention
time for phenylethanone is
(B — A) min.
The column used was a silicone
‘sewn ’ on to an inert solid support
and the solvent was a mixture of
propan~2-ol, dichloromethane
and hexane.

Time/min

21
PREPARATION AND PURIFICATION OF The area under the peak of the chromatogram depends on the amount
ORGANIC COMPOUNDS
of the material present, so that the amount in a mixture can be measured
by calibrating the chromatogram with injections of measured quantities
of the compound and comparison of the resulting areas with that from the
mixture.

(c) Thin-layer chromatography

This method is similar to column chromatography but on a much smaller
scale. It is generally used for the identification of components in a mixture
rather than as a method of purification in preparative work.
A solid adsorbent such as silica gel or alumina is spread as a thin, even
Plate 2.2 (colour) layer on a glass plate [Plate 2.2(a)]. A solution of the sample to be analysed
faces p. 22 is placed as a spot on the surface of the plate and near one end. When the
solvent has evaporated from the spot, the plate is put in a tall beaker or jar
containing the same solvent, so that the spot is a little way above the level
of the liquid. The beaker is covered to reduce evaporation. The solvent rises
slowly up the plate by capillary action, and after it passes the spot, the
components in the mixture begin to move with the solvent at different rates,
just as in column chromatography [Plate 2.2(b)].
When the solvent has almost reached the top of the plate, the plate is
removed from the beaker and the solvent is allowed to evaporate. If the
components of the mixture are coloured, each will be seen as a coloured
area [Plate 2.2(c) and (d)]. If they are colourless, the plate is developed by
being stood in a beaker containing some crystals of iodine. After a few
minutes, a dark spot will appear where each compound is held on the plate.
The distances from the original spot to the solvent front and the position
of each compound are measured. It is convenient to calibrate the distance
travelled by the solvent from 0-1, and the fraction of this distance travelled
by each compound is described as its Rp value. For example, in Fig. 2.12,

FIG. 2.12. /?p values

Solvent front

100

075

(b

CD
>

025

22
Plate 2.2. Thin-layer and 2M ammonia solution {3:1:1
chromatography, {a) A thin-layer by volume) as solvent.
plate, {b) The development of a {d) Separation of pigments in a
thin-layer plate, (c) Separation of solution of spinach in light
black ink using butan-l-ol, ethanol petroleum

seep. 20
 ' .h-

’?»■ ■r
*'■.

r S .
'V
Y

rt •>

I
I
4■' -V
IT t *

m
. \
I
■<
k “*' •>. ■ -.-^r
1
'I*
H 1
L
* 'j‘ «*
,■*'
^' ».■

I ’ »
<v It-
V ‘ y.
,

•'ll-
r

w
■J

Th I

1
«

i «•

9 a •j
■ 'k 'IM^
FIG.2.13. Apparatus for paper the mixture was placed on the plate at A, and thus solute X has an value
chromatography of 0-25 and solute Y has an value of 0-75.
Different substances have different R^, values for the particular conditions
of the experiment, so that comparison of the values from the mixture with
Glass rod those from authentic samples of pure compounds enables the components
of the mixture to be identified. It is essential that comparison is made for
the same solvent and temperature, for the R^ value depends on these as well
as on the nature of the compound.

(d) Paper chromatography

This method is like thin-layer chromatography in being suitable for the
identification of the components of a very small sample of a mixture. The
procedure is similar except that a large rectangular piece of filter paper
mounted in a glass tank (Fig. 2.13) is used instead of solid adsorbent on a
glass plate.
A pencil line is drawn across the paper near the bottom and a drop of
Paper the solution (which contains, for example, a mixture of amino-acids) is
placed on the paper at this line by means of a fine glass pipette. Samples
of solutions of the compounds believed to be in the mixture are placed
alongside so that a direct comparison of R^ values can be made. Solvent
is then poured into the tank so that the pencilled line is just above the surface.
When the solvent reaches almost the top of the paper, the paper is re¬
Spots of solution moved from the tank and the solvent is allowed to evaporate. If the com¬
ponents are coloured, they are easily seen, but if not, a physical or chemical
Solvent
property must be employed to reveal their positions. In Plate 2.3, a chemical
reagent, ninhydrin, was used to form a coloured compound with each of
the amino-acids.

Plate 2.3. Paper chromatography.

Separation of some amino-acids
with a mixture of butan-l-ol,
ethanoic acid and water
as solvent. The paper has
been developed with ninhydrin
solution. On the extreme right-
hand side, a mixture of
amino-acids has been separated,
and the Rp values can be
compared with those for pure
amino-acids developed at the
same time

t BUTAN-l-OL/ETHANOIC ACID'WATER 12:3:5

23
PREPARATION AND PURIFICATION OF The underlying principle in paper chromatography is the partition of a
ORGANIC COMPOUNDS
solute between two solvents. One solvent is the one which travels up the
paper (the eluting solvent), and the other is water, the molecules of which
are adsorbed on the cellulose which constitutes the filter paper. Compounds
which are relatively more soluble in the eluting solvent travel faster (i.e.
have larger values) than the less soluble ones.
In some cases, two or three components in a complex mixture are not
separated completely in this way. It is an advantage to elute with one
solvent, remove the paper, dry it, turn the paper through 90° and elute with
a second solvent.
Thin-layer chromatography is a more rapid method than paper chromato¬
graphy. However, for compounds which are only slightly soluble in organic
solvents but moderately soluble in water, thin-layer chromatography is
unsuitable whereas paper chromatography is successful.

(e) Gas chromatography

There are two types of gas chromatography: gas-liquid chromatography
and gas-solid chromatography. Each is suitable for separating mixtures of
gases, liquids and volatile solids.
In gas-liquid chromatography, as in paper chromatography, the com¬
pounds in the mixture are partitioned between two solvents. One solvent
is fixed in position and is known as the stationary phase (cf. water in paper
chromatography); it consists of a non-volatile liquid (such as a long-chain
alkane) which is coated on an inert solid. The other solvent is a gas which
travels through the column containing the stationary phase (cf. the eluting
solvent in paper chromatography). It is known as the carrier gas, and must
be one which does not react with the components of the mixture. Nitrogen
is often used.
Gas-solid chromatography differs in employing a solid as the stationary
phase. The principle of separation is like that of column chromatography
save that the eluting solvent is a gas and not a liquid. The solids generally
used are silica gel or alumina.
The gas chromatograph consists of: (i) a supply of carrier gas at constant
pressure; (ii) a flow-meter to measure the rate at which the carrier gas passes
through the column; (iii) the column, in a thermostat; and (iv) a detector,
to determine when each component of the mixture is eluted. At the top of
the column is a rubber septum cap through which the solution for analysis
is introduced by means of a hypodermic syringe; the cap is self-sealing so
that when the syringe is withdrawn the sample and carrier gas cannot escape.
Figure 2.14 gives an outline of a gas chromatograph with a column 2 m long
and 4 mm in diameter.
The detector makes use of a physical or chemical property of the com¬
pounds in the mixture. Two detectors commonly employed are the katharo-
meter and the flame-ionisation detector. The katharometer (Fig. 2.14)
measures the change in the thermal conductivity of the emerging gas which
occurs when an organic compound is eluted in a stream of carrier gas. The
flame-ionisation detector measures the concentration of ions in a flame.
Hydrogen is mixed with the carrier gas as it emerges from the end of the
column and is burned at a jet, and the ions formed in the flame are collected
on a charged plate above it. While only hydrogen is being burnt, the con¬
centration of these ions remains steady, but when an organic compound is
eluted there is a change in the concentration.
These detectors are used in conjunction with a device which enables the
changes to be observed. A galvanometer is suitable, but it is helpful to have

24
FIG. 2.14. A gas chromatograph Pressure controller Flow-meter

a permanent record, and a pen-recorder is generally used. The result of a

typical gas chromatography experiment is shown in Fig. 2.15. There were
six compounds in the mixture.
In Figure 2.16 a gas chromatogram of a sample of one of the fractions
from the primary distillation of a crude oil is shown. It illustrates the re¬
markable power of the technique. Although in this example the separation
took three hours, it is more usual to allow the temperature of the oven
containing the chromatographic column to rise at a uniform rate. This
speeds up the separation and it is possible to complete an analysis such as
this in 10-20 minutes.
A recent development has been to connect the gas chromatograph to a
mass spectrometer. As the vapour of each solute is eluted at the end of the
column, part of it is channelled into the spectrometer, and by analysis of
the mass spectrum the structure of the materials can be deduced (3.7). Such
a system would be used to identify, for example, all the hydrocarbons in
the sample of the crude oil (Fig. 2.16).

25
FIG. 2.15. A gas chromatogram.
The separation of a mixture of
six substances: 1, diethyl ether;
2, propanone; 3, methanol; 4,
ethanol; 5, pentan-3~one; 6,
propan-l-ol. The retention time
for propanone is {B — A) min,
and for propan-I-ol, (C — A)
min. The area under each curve is
a measure of the amount of the
compound in the mixture.

The gas chromatograph described above is suitable only for analytical

purposes, since the quantity of a mixture injected into the column is only
about one-hundredth of a gramme. However, it is also possible to use
columns of larger diameter which enable several grammes of a sample to
be introduced, so that each component can be isolated as well as identified.
For this purpose, the emerging gas is divided into two streams, the smaller
of which passes into the detector and the larger of which flows through a
U-tube surrounded by a freezing mixture. After each peak is registered, a
fresh U-tube is inserted to collect the next compound.

FIG. 2.16. A gas chromatogram

of a sample of a low boiling
fraction {up to I50°C) of crude
oil, showing the relative
abundances of the lighter (D
(/>
hydrocarbons. There are 33 C
o
separate peaks, all of which have Q.
CO
been identified. Of these are 1, 0)
methane; 2, propane; 4, butane; (D
"D
7, pentane; 13, hexane; 16, O
benzene; 19, cyclohexane; 29, o
05

heptane; 33, 2,2,3,3- QC

tetramethylbutane. The others
are isomers of these compounds.
The column was packed with a I234S6 7 8 soil 12
L
13 MIS 1617
lU
1819 2021 23 25 2728 29 30 3132 33
22 24 26
CjQ alkane, known as squalane,
■3 hr 27 min-
coated on a solid support.
Nitrogen was the carrier gas. Time

26
2.7 Pure solids melt over a very small temperature range (approximately 1-2°C),
Criteria of purity whereas the presence of even 1 per cent of an impurity can increase the
range to several degrees. The melting point therefore provides a very good
criterion of purity.
A small sample of the solid is placed in a glass capillary tube which is
sealed at its lower end. The tube is attached to a thermometer so that the
sample is level with the bulb and the thermometer is suspended in a small
beaker of paraffin oil (Fig. 2.17). The oil is heated slowly (not more than
2° per minute near the melting point of the solid) and is stirred at the same
time. The temperature at which melting is first observed and that at which
it is complete are both noted, and the melting point is then recorded by
quoting both temperatures, e.g. ethanamide, m.p. 81-82°C.
FIG. 2.17. A simple melting point
apparatus

A melting-point method can also be used to confirm the identity of a

compound. Suppose that a compound has been isolated which is thought
to be ethanamide and which has the expected melting point for ethanamide
of 81-82°C. A small amount is mixed with about the same quantity of a
sample from an authentic source of ethanamide. The mixture is ground up
and its melting point determined. If the first compound is in fact ethanamide,
the melting point of the mixture will still be 81-82°C, but if it is not, then
melting will begin at a lower temperature and extend over a wider range of
temperature than that for a pure compound. This is known as the mixed
melting point method.
A second criterion of purity for a solid is that it should give only one spot
on a thin-layer or paper chromatogram. Small quantities of impurities can
be revealed in this way, but unfortunately it sometimes happens that an
impurity has the same Rp value as the required compound and is therefore

27
 not revealed. Thus, observation of a single chromatographic spot is a
necessary but not a sufficient criterion for purity.
Pure liquids boil within a range of about 1°C. If the temperature recorded
during a distillation covers a wider range, the resulting liquid cannot be
pure. However, the converse is not necessarily true, because sometimes a
distillate which boiled at a constant temperature is found to contain small
quantities of other materials. In this respect, the boiling point of a liquid
is a less satisfactory criterion of purity than the melting point of a solid,
while even fractional distillation of a liquid is not so successful at removing
impurities as recrystallisation of a solid.
Two other criteria for the purity of liquids are available. One is the
refractive index, which can be measured to five significant figures with a
modern refractometer and which is very sensitive to impurities. The second
is the combination of the gas chromatogram and the mass spectrum. The
chromatogram of a pure compound shows only a single peak, and gas
chromatography is so sensitive a method that it is possible to detect less than
1 per cent of an impurity even when only about 1 mg of the compound is
available. However, an impurity with the same retention time as that of the
compound is not revealed on the chromatogram, and to test for this possi¬
bility the eluting compound is fed into the mass spectrometer; an impurity
is revealed by the resulting spectrum.

2.8 Between them, the various types of chromatography provide methods for
(i) the separation of the components of a mixture on a preparative scale,
Summary of the use of
chromatographic
methods Chromatographic
method Separation Identification Purity

Column +
High pressure -t- + +
Thin-layer + +
Paper + +
Gas (small scale) -t- +
Gas (large scale) +

Mobile Stationary Principle of

Type
phase phase separation
Column Liquid Solid Adsorption
High pressure (a) Liquid Solid Adsorption
liquid (b) Liquid Involatile Partition
liquid on
solid
Thin-layer Liquid Solid or Adsorption
water adsorbed and/or partition
on solid depending on
conditions
Paper Liquid Water adsorbed Partition
on paper
Gas (a) Gas Involatile Partition
liquid on
solid
(b) Gas Solid Adsorption

28
PREPARATION AND PURIFICATION OF
ORGANIC COMPOUNDS (ii) the identification of unknown compounds in a mixture, even when only
minute quantities (e.g. 1 mg) are available, and (iii) testing the purity of a
material.

2.9 Column chromatography

Practical work Preparation of a column for the separation of dyes
Place a plug of glass wool in the narrow neck of the bottom of a glass tube
(about 12 cm long and 1 cm in diameter; Fig. 2.9). Fill the column half-full
of water and pour a slurry of alumina and water into it, tapping the tube.
The solid will sink to the bottom, giving a uniform column; there must be
no cracks or channels in the solid. Liquid is run out of the column until
the solid is just covered. Place a small piece of filter paper on top of the solid
to prevent it from being disturbed when the solution is added.
Pipette (carefully, so that the solid is not disturbed) 3 cm^ of an aqueous
solution of two dyes (OT per cent w/v of malachite green and methylene
blue). Elute with water and, when one dye has passed through, elute with
ethanol until the second dye has run olf. Make sure that the solid is always
covered with solvent, and adjust the water pump so that solvent passes
through the column at 5-10 cm^ per minute.

Preparation of a column for the separation of pigments in spinach

(a) Preparation of a solution of pigments from spinach
Crush about 20 g of frozen spinach with about 20 cm^ of methanol. Filter
off the methanol and crush the spinach with 50 cm^ of a mixture (2:1 by
volume) of light petroleum (b.p. 60-80°C) and methanol. Filter the mixture,
collect the solvents in a flask and transfer them to a separating funnel. Run
off the methanol layer, and then pour the petroleum layer into a conical
flask. Add about 2 g of anhydrous sodium sulphate and shake for a few
minutes to dry the solution.
Decant the solution into a flask and evaporate off the solvent using a hot
water bath (cf. Fig. 2.2) until about 5 cm^ of solution remain. Allow the
solution to cool, stoppering the flask to prevent further evaporation of the
solvent.

(b) Preparation of a column of alumina

Prepare a column of alumina as described above, using dry methylbenzene
instead of water. The empty column should be half-filled with methyl-
benzene and the slurry made with alumina and methylbenzene.

(c) Separation of the solution from spinach

Place 2 cm^ of the solution from spinach on the column of alumina, and
develop the column with methylbenzene until the first band, which are
carotenes and coloured orange, passes into the flask. Change the receiver
and develop the column with a second solvent containing butan-l-ol,
ethanol and water in the ratio of 3:1:1 by volume. This separates the
chlorophylls, coloured green, from the xanthophylls, coloured pink-brown.

Preparation of a dry-pack chromatography column

Cut a piece of nylon tubing, of about 2 cm diameter, to a length of 20 cm.
Seal the end, either with a hot iron or in the edge of a bunsen flame.
Using a small filter funnel, pour fine sand in the bottom to a depth of
about 1 cm, then alumina to a depth of about 12 cm, tapping the sides of

29
PREPARATION AND PURIFICATION OF the tube to ensure even distribution of the solid, and then more fine sand
ORGANIC COMPOUNDS
to a depth of about 0.5 cm.
Prick the bottom of the column with a pin (this will prevent rising air
bubbles from breaking up the column when solvent is added).
Stand the packed column in a 50 cm^ measuring cylinder or similar con¬
tainer (Fig. 2.18).

FIG. 2.18. Dry-pack chromatography

Separation of the isomeric nitrophenylamines

Dissolve ca. 0.1 g of 2-nitrophenylamine and 4-nitrophenylamine in 2-
3 cm^ of propanone and, with a dropping pipette, run about 1 cm^ of the
solution into the sand. Add propanone, dropwise, on to the sand until the
solvent almost reaches the bottom of the column. You will need about
15 cm^ of solvent.
The nitrophenylamines are toxic. You should wear protective gloves when
handling them.

If there is time, slice up the column with a sharp scalpel and put the sliced
sections containing the solutes in separate beakers. Add a few cm^ of
propanone to each, filter, collect the solutions and evaporate the solvent
to obtain pure samples of solutes. Determine their melting points. The
melting points of the 2- and 4-isomers are 72° and 147°C, respectively.
You may wish to compare the efficiency of separation using this method
with the older method described above. You will need to make the slurry of
alumina in propanone when preparing the column.

Thin-layer chromatography
Preparation of thin-layer plates
Make a slurry of silica gel (20 g) in 1,1,1-trichloroethane (50 cm^) in a
beaker. Dip two microscope glass slides (15x2 cm) back-to-back in the
slurry and withdraw them slowly but at a constant rate. Separate the slides
and, holding them horizontally, wave them gently while the solvent eva¬
porates. It is important to wipe the back and edges of the slide free of
solid.
Scratch a line across the slide about 2 cm from the bottom, leaving spaces
where the solution to be analysed is to be placed. Apply the solutions using
a fine glass capillary tube; the spot should be no wider than 3-4 mm. Place

30
PREPARATION AND PURIFICATION OF
ORGANIC COMPOUNDS the slide in a beaker containing solvent; the level of the solvent should be
just below the level of the line drawn across the slide. When the solvent has
risen over three-quarters of the way up the slide, remove the slide and allow
the solvent to evaporate.

/Is trichloromethane is used, teachers may prefer to do the next two experi¬
ments as class demonstrations.

Separation of pigments from spinach

Prepare a solution of pigments as described on p. 29. Develop the slide
with trichloromethane. Note the number of spots developed, measure their
values and describe their colours. Place the slide in a beaker in which
there are two or three crystals of iodine. Cover the beaker with a watch-glass
and note whether any additional spots are developed.

Separation of products from the nitration of phenol

place 3 spots on a thin-layer plate: (a) a solution of 2-nitrophenol in ethanol,
made up by dissolving a few crystals of the solid in 5 drops of ethanol; (b)
a solution of 4-nitrophenol in ethanol; (c) a solution of the reaction products
from the nitration of phenol (p. 278) (or an artificial mixture of 2- and
4-nitrophenol) in ethanol.
Develop the plate using trichloromethane.

Separation of dyes in black ink

Apply a small spot of black ink (for example, Quink) to a thin-layer plate
and develop it using a mixed solvent, butan-l-ol, ethanol and 2M ammonia
solution in the ratio of 3:1:1 by volume.

Paper chromatography
Apparatus for paper chromatography (ascending solvent front)
A gas jar can be used as a chromatograph tank (Fig. 2.13). The paper is
held in position by a piece of glass rod and clips.
Cut strips of Whatman No. 1 filter paper about 4 cm wide and 30 cm long.
Draw a pencil line across the paper near the bottom and apply the solutions
with a fine capillary tube (a melting-point tube is useful). To prevent the
spot spreading, it should be dried quickly (with a hair dryer, for example).
Pour about 40 cm^ of solvent in the cylinder and place the end of the
paper just below the level of the solvent.

Separation of amino-acids
Make up 100 cm^ of solvent by shaking together 40 cm^ of butan-l-ol and
50 cm^ of water in a separating funnel for about 10 minutes and then adding
10 cm^ of ethanoic acid. Shake the mixture again. On standing, the mixture
will separate into two layers. The top layer should be used as the solvent.
Make up solutions of (a) OT g of glycine in 10 cm^ of water, (b) OT g
of proline in 10 cm^ of water, (c) a mixture of OT g of glycine and OT g
of proline in 10 cm^ of water.
Place 3 spots, (a), (b), and (c) on the paper, set up the chromatography
apparatus and allow the solvent front to move at least three-quarters of the
way up the paper. Dry the paper (preferably in an oven at 100°C) and spray
it with a solution of ninhydrin (0-2 g of ninhydrin in 99 cm^ of butan-l-ol
and 1 cm^ of ethanoic acid). If no spray is availaole (a cheap scent spray is

31
PREPARATION AND PURIFICATION useful), draw the paper through a shallow bath of ninhydrin solution. Dry
ORGANIC COMPOUNDS
the paper for about 2 minutes at 100°C.
Glycine and proline react with ninhydrin to form a blue and a yellow
compound, respectively. The Rp values of the spots formed in the control
experiments (a) and (b) may be compared with those formed by the mixture.

Gas chromatography
Several types of apparatus which can be constructed in a school laboratory
are described in Organic Chemistry Through Experiment (p. 173-184).

Organic Chemistry Through Experiment. D. J. Waddington and H. S. Finlay (4th

2.10
ed. 1977). Bell and Hyman Ltd.
Further reading Thin-layer chromatography. D. A. Stephens, School Science Review, Vol. 48,
p. 376 (1967).
Simple chromatographic demonstrations. M. Taylor, School Science Review, Vol.
45, p. 75 (1963).

2.11 Chromatography (V) I.C.I.

Videotape

2.12 1 Summarise the various methods available for the purification of an organic
compound and discuss the physico-chemical principles underlying any two of
Questions these methods. (W(S))
2 When an organic compound is nitrated under certain conditions, the solid
mononitro and a small proportion of solid dinitro derivatives are formed. De¬
scribe how the mononitro compound is obtained in the pure state by recrystallisa¬
tion from ethanol explaining the reasons for all the techniques employed.
A melting-point determination is carried out after four successive recrystallisa¬
tions (a), (b), (c) and (d) with the following results:
m.p.
(a) no”)
Melting-point not sharp
(b) 125°j
(c) 127°'!
Melting point sharp
(d) 127°)
Describe, with the aid of a diagram, how the determination is carried out and
explain the significance of the results quoted. (S)

32
Chapter 3
Determination of the structure
of an organic compound

3.1 The determination of the structure of an organic compound necessitates the

following steps;
Introduction
1. The purification of the compound. Methods of purifying organic com¬
pounds, and of testing their purity, are described in Chapter 2.
2. Determination of which elements are present: qualitative analysis
(3.9).
3. Determination of the molecular formula. This can be achieved by
combination of quantitative analysis to find the proportions of each element
in the compound (3.3) and measurement of the formula weight (3.5). In
recent years it has become possible to find the molecular formulae of some
compounds directly by mass spectrometry (3.6).
4. Determination of the way in which the atoms are arranged in the
molecule. Both chemical methods and physical methods (spectroscopy) are
used.

3.2 Practical details for the detection of carbon and hydrogen are given in
Section 3.9, together with the Lassaigne test for nitrogen, sulphur and the
Qualitative analysis halogens.

3.3 Carbon, hydrogen and nitrogen

Quantitative analysis A known weight of the compound is heated to a high temperature in an
excess of dry oxygen. The compound burns to form carbon dioxide and
water. If nitrogen is present in the organic compound, a mixture of nitrogen
oxides (and sometimes nitrogen gas) is also produced; the oxides of nitrogen
are subsequently reduced by copper to nitrogen. The weights of carbon
dioxide, water and nitrogen are then found and the percentage composition
of carbon, hydrogen and nitrogen can be calculated.

Detectors
FIG. 3.1. A CHN analyser
/FT C

N2,He He
Combustion
He tube
O2 O2 j— --

Magnesium chlorate(vii)
DETERMINATION OF THE STRUCTURE Recently, analysers (known as CHN analysers) have been introduced
OF AN ORGANIC COMPOUND
which enable the compound to be analysed automatically. In one of these
(Fig. 3.1), the sample (about 1-3 mg) is weighed in a platinum boat placed
in a stainless steel combustion tube (Fig. 3.2). The tube is heated in a furnace
FIG.3.2. The combustion and
at about 900°C in a stream of dry oxygen and a carrier gas (helium). The
reduction tubes used for analysis of
carbon, hydrogen and nitrogen. compound burns, and the products pass along the tube. They first pass over
The combustion and reduction three materials which are present to remove elements which would other¬
tubes are in dijferent furnaces wise interfere with the analysis for C, H and N: silver removes halogens as

involatile silver halides, and silver tungstate(VI) and silver vanadate(V)

FIG. 3.3. Analysis of a compound remove elements such as phosphorus and sulphur with which they combine
containing C, H, N. The
to form involatile inorganic salts. The gases then pass through a second
percentage composition of the
tube, the reduction tube, which is filled with copper and heated to 650°C.
elements is found from the heights
This causes the oxides of nitrogen to be reduced to nitrogen and removes
of the peaks, subtracting values for
the blanks {values obtained when oxygen by formation of copper(II) oxide. The gas, which now contains only
the boat is empty). The height for carbon dioxide, water, nitrogen and helium, then passes through three
carbon dioxide is (// —CD)

Water

Carbon
dioxide

G Nitrogen
H

Blank for water

E F

Blank for carbon dioxide

c D

Blank for nitrogen

A B

0 5 15 25 35 45 55 65 75 85 95
Recorder response

34
DETERMINATION OF THE STRUCTURE
OF AN ORGANIC COMPOUND katharometers (p. 24); each of these consists of two thermal conductivity
cells which contain heated platinum wires, the two wires forming two
arms of a balanced Wheatstone bridge circuit. The gases pass through
the first cell of the first katharometer, then through a tube of magnesium
chlorate(VII) to absorb the water vapour, and then through the second
cell of the same katharometer. The conductivity of the gases in the two cells
is therefore different, so that the Wheatstone bridge circuit becomes out
of balance and a current flows; the current, which is proportional to the
weight of water absorbed, is recorded automatically (Fig. 3.3) so that the
weight of water, and hence the percentage composition of hydrogen in
the compound, can be calculated. The cells of the second katharometer are
separated by a tube of soda-lime which absorbs the carbon dioxide; thus,
the percentage of carbon can be calculated from the Wheatstone bridge
current in the same way as that of hydrogen. The remaining mixture of
nitrogen and helium passes into one cell of the third katharometer while
a second stream of helium, at the same flow-rate as in the mixture of gases,
passes through the second cell. The current in the Wheatstone bridge circuit
is proportional to the concentration of nitrogen, so that the percentage
composition of nitrogen can be calculated.

Nitrogen by Kjeldhal’s method

Another method for determining the percentage composition of nitrogen
in an organic compound is to boil a weighed sample with concentrated
sulphuric acid and anhydrous potassium sulphate (to raise the boiling point
of the acid). A catalyst (for example, mercury to which a little selenium is
added) is sometimes used. The resulting solution of ammonium sulphate is
diluted, excess of sodium hydroxide is added and the liberated ammonia is
distilled into excess of standard acid. The acid is then titrated against
standard sodium hydroxide to determine the amount of acid originally
neutralised by ammonia.

Halogens by Carius’s method

A weighed quantity of the compound is heated strongly with concentrated
nitric acid and solid silver nitrate in a sealed tube for some hours. The tube
is cooled and then broken under water. The silver halide is filtered, washed,
dried and weighed.

Once the percentage composition of each element is known, the ratio of the
3.4 numbers of atoms of each element present in the compound can be cal¬
Calculation of the culated. This is the empirical formula.
empirical formula The method is to divide the percentage composition of each element by
its relative atomic mass and to factorise the resulting numbers so as to
obtain simple whole numbers. For example, a compound X, a white solid,
was found by analysis to contain 23-30 per cent carbon, 4-85 per cent
hydrogen and 40-78 per cent nitrogen. It was known to contain no other
elements except oxygen, so that the composition of oxygen was

100 — (23-30 + 4-85 + 40-78) = 31-07 per cent.

35
DETERMINATION OF THE STRUCTURE Then:
OF AN ORGANIC COMPOUND

/o Relative Atomic Simplest

Element atomic mass ratio atomic ratio
Composition

Carbon 23-30 12 1-94 2

Hydrogen 4-85 1 4-85 5
Nitrogen 40-78 14 2-91 3
Oxygen 31-07 16 1-94 2

The empirical formula of X is C H N O . To determine the molecular

2 5 3 2

formula, the relative molecular mass must be found.

The determination of relative molecular masses of compounds is described

3.5 in detail in texts on Physical Chemistry. Those of gases are generally deter¬
Determination of a mined by the limiting density method, using a gas density balance, while
relative molecular those for volatile liquids and solids are found by an adaptation of the tradi¬
mass tional Victor Meyer’s method in which the volume of vapour from a known
weight of compound is determined. The relative molecular mass of an
involatile liquid or solid is often found from the depression of the freezing
point of a solvent.
In many cases, the relative molecular mass can be found quickly and with
a very high degree of precision by mass spectrometry. The vapour of the
substance, at a low pressure (10“^-10“^ mm Hg), is bombarded by a stream
of electrons. This causes the loss of one electron from a molecule of the
substance, giving the molecular ion (sometimes called the parent ion):

M + e^M'^' + 2e

These positive ions are accelerated in an electrostatic field and pass through
a slit in the negatively charged plate into an electrostatic analyser. This
causes them to be deflected by amounts depending on their kinetic energies,
and only one component of the beam of ions, with a well defined kinetic
energy, can emerge from the slit at the other end of the analyser. This com¬
ponent then enters a magnetic field in which it is again deflected, and the
strength of the field is altered until the ions impinge on a detector; this
generates an electrical signal which is shown on an oscilloscope and/or
recorded on photographic paper.

FIG. 3.4. A mass spectrometer

The radius of the deflection, r, depends upon the strength of the magnetic field, B,
and the kinetic energy of the ions; and the latter depends on the accelerating voltage,
V, and the relative molecular mass of M* . Therefore, by finding the value of B for
selected values of r and V, the relative molecular mass can be calculated.

36
DETERMINATION OF THE STRUCTURE
OF AN ORGANIC COMPOUND With modern spectrometers, the relative molecular mass can be meas¬
ured to the fourth decimal place.
For some compounds, the molecular ion decomposes so rapidly into
smaller particles (p. 38) that it cannot be detected. The relative molecular
mass cannot be determined by mass spectrometry for these compounds,
although important information about their structures can still be obtained
from identification of the fragments (3.7).

3.6 The molecular formula of a compound—the actual number of each kind

Determination of the of atom in the molecule—can be determined from the empirical formula
and the relative molecular mass.
molecular formula For example, the empirical formula of the compound X in Section 3.4
was found to be C2H5N3O2. The formula weight (determined, for example,
by freezing-point depression) is 103. In this case, therefore, the empirical
formula is also the molecular formula. On the other hand, if the relative
molecular mass had been found to be 206, the molecular formula would
have been C4HioNg04..
If a molecular ion is formed in the mass spectrometer, the molecular
formula can be found without determination of the empirical formula or
any other study of the compound. Thus, the molecular ion of the com¬
pound X was measured as 103-0382. Its molecular formula must therefore
be C2H5N3O2 since no other possible combination of elements corres¬
ponds to this mass. (Table 3.1 lists compounds of formula weights between
103-0170 and 103-0494, based on = 12-000000; see Appendix VI.)

Table 3.1. Organic compounds of relative molecular mass ca. 103

MASS NUMBER OF ATOMS IN THE MOLECULE

c H N 0

103-0170 5 1 3 _
103-0269 3 5 1 3
103-0382 2 5 3 2
103-0427 7 5 1 -

103-0494 1 5 5 1

3.7 The structural formula shows us which atoms are joined to which in the
molecule. Strictly speaking, a structural formula requires a three-dimen¬
Determination of the sional representation, but planar, two-dimensional representations are
structural formula usually more convenient (1.2).
In elucidating the structural formula, it is usual to consider the possible
structures based on the molecular formula. For example, the compound X
has the molecular formula C2H5N3O2; a possible structural formula is:

OHO
H. II I II H
/N-C-N-C-N
H H

37
DETERMINATION OF THE STRUCTURE In order to confirm this structure, we examine both the physical and the
OF AN ORGANIC COMPOUND
chemical properties of the compound. By studying the chemical reactions,
the presence or absence of the functional groups can be determined. For
example, for X, we would test for the presence of the groups —NH2,
—NH—, and C=0. There is also a special chemical test for the —NH—
group which is adjacent to C=0 (p. 229).
However, in more complex molecules, physical methods for the deter¬
mination of structure are always used as well, in particular three spectro¬
scopic techniques: mass spectroscopy, infrared spectroscopy and nuclear
magnetic resonance (NMR) spectroscopy.

Mass spectroscopy
When molecules, M, are bombarded by electrons in the mass spectrometer,
molecular ions, M"^ ', are usually formed (p. 36). A proportion of these
fragment into smaller species, one of which carries a positive charge and the
other of which is a neutral radical:

M+- + Y

These fragments can in turn break down into smaller ones. Each of the
positively charged fragments is recorded by the spectrometer, so that its
mass can be determined. The resulting array of detected ions is described
as the fragmentation pattern.
The abundances of the different positively charged fragments vary widely.
For convenience in interpreting the spectrum, the relative abundances of
the fragments are plotted against their masses, the most abundant ion being
given an arbitrary value of 100 units (the base peak); the plot is known as
a stick diagram. The molecular ion is rarely the most abundant one; indeed,
with some compounds, the molecular ion is not detected and the formula
weight cannot then be determined (3.5).
The fragmentation pattern of a compound depends on its structure. The
presence of particular groupings is associated with specific fragmentation
patterns, and so the determination of the fragmentation pattern for a com¬
pound of unknown structure can often enable the structure to be deduced.
For example, the stick diagram for hexadecane is shown in Fig. 3.5. The
molecular ion is at 226, and the principal fragments are at 29, 43, 57, 71, 85,
99,113, 127. This difference of 14 units between fragments is characteristic
of the behaviour of linear alkanes. It occurs because the molecular ion can
break in two ways: by losing a methyl radical, -CHg, or an ethyl radical,
FIG.3.5. The mass spectrum of •CH2CH3. Each new ion can then lose molecules of ethene successively.
hexadecane {formula weight 226)

38
DETERMINATION OF THE STRUCTURE
OF AN ORGANIC COMPOUND
For example, fragmentation of the molecular ion from octane occurs as
follows;
CH3CH2CH2CH2CH2CH2CH2CH3 +

•CH, •CH2CH3

CH3CH2CH2CH2CH2CH2CH2 (99) CH3CH2CH2CH2CH2CH2 (85)

-C,H^

CH3CH2CH2CH2CH2 (71) CH3CH2CH2CH2 (57)

-C2H4 -C2H4

CH3CH2CH2 (43) CH3CH2 (29)

If there had been branching in the carbon chain, fragmentation would have
occurred preferentially at the branch [as a more stable, secondary carbonium
ion is formed (p. 67)]. For example, 2-methylheptane has a molecular ion
at 114 and a significant fragment is at 99 (Fig. 3.7).

FIG. 3.6. The mass spectrum of

2-methylheptane {formula weight
114)

(114) (99)

In contrast, for the isomer, 2-ethylhexane, there is a major fragment at 85:

+•
CH2CH3
I +
CHg-CCH^jg-CH-CHa CHg-fCHajg-CH-CHg + -CaHg

(114) (85)

Infrared spectroscopy
The bonds in organic compounds undergo various types of vibration. For
example, a C—H bond can stretch:

- > -e -
> H
-C H <■

39
DETERMINATION OF THE STRUCTURE or bend:
OF AN ORGANIC COMPOUND

>
<r

For a particular bond, only a particular set of vibrational frequencies is

possible. Suppose a bond is vibrating at frequency v^, and its next available
higher frequency is V2; then, if electromagnetic radiation with frequency
(V2 — Vi) is incident on the compound containing this bond, some of the
radiation is absorbed and some of the bonds then vibrate at the higher
frequency. The appropriate radiation to excite the bonds in organic com¬
pounds in this way corresponds to the infrared region of the spectrum, that
is, to frequencies in the region of 10^^ Hz (1 Hz = 1 cycle per second). (The
frequency is usually quoted in reciprocal cm (cm”^); a frequency of v Hz
corresponds to 1/A cm where A is the wavelength of the radiation and
is related to v by the expression Av = c, the velocity of light (3 x
10^° cm s~^). For example, a frequency of 6 x 10^^ Hz corresponds to
2000 cm“^ or a wavelength of 5 x 10“"^ cm.)
The excitation frequency for a particular bond is, to a first approxima¬
tion, independent of the other bonds in the compound. Therefore, deter¬
mination of the frequencies in the infrared region which are absorbed by a
compound gives information about the types of bond which are present.
The apparatus for this purpose—an infrared spectrophotometer—con¬
sists of a cell (usually made of rock salt as glass absorbs infrared radiation)
which contains the organic compound, a source of infrared radiation such
as a Nernst glower (a rod made of metal oxides which is heated at 1500°C),
a means of separating the infrared beam into its constituent frequencies (a
prism or diffraetion grating) and a deteetor for infrared light (a sensitive
thermocouple). The infrared light is passed through the separator and a
particular emergent frequency then passes through the cell; behind the ceil
is the detector. By rotating the prism or grating, all the infrared frequencies
are passed in turn through the cell and, by coupling the detector to a pen
recorder, a record of the behaviour of the compound towards each fre¬
quency—the infrared spectrum—is obtained.
The characteristic frequencies for various types of bonds in organic com¬
pounds are in Table 3.2. Thus, a compound which is found to absorb in the
region 1700-1750 cm can be deduced to contain the earbonyl group,
C=0 (for example, propanone, the infrared spectrum of which is in
Fig. 3.7).

FIG.3.7. The infrared spectrum

of propanone

40
DETERMINATION OF THE STRUCTURE
OF AN ORGANIC COMPOUND Table 3.2. Characteristic infrared frequencies for organic bonds

BOND COMPOUND frequency/cm

C—H Alkane 2850-2960

C—H Alkene 3075-3095
C—H Alkyne 3300
C—O Alcohol, ether 1050-1200
c=o Aldehyde, ketone, carboxylic acid 1700-1750
O—H Alcohol, phenol 3590-3650
N—H Amine 3300-3500

Nuclear magnetic resonance spectroscopy

The atoms of some of the elements have a nuclear structure which causes
them to behave like small magnets. In organic compounds, the commonest
of these elements is hydrogen; the hydrogen nucleus—the proton—when
placed in a magnetic field, aligns itself so that its magnetic moment (some¬
times referred to as its ‘spin’) is either in the same direction as the applied
field or in the opposite direction.
The energies of these two possible states differ. The protons whose spins
are aligned with the field have a lower energy than those aligned against
the field; the energy difference, AE, is proportional to the strength of the
field, B:

yhB
AE
2k

where y is a constant for a particular nucleus and h is Planck’s constant.

If electromagnetic radiation is incident on the protons, then, providing
that its frequency v is such that its energy, hv, is exactly equal to the energy
difference, AE, for the proton’s two orientations, some of the radiation will
be absorbed and some of the protons in the orientation of lower energy will
‘flip’ over to the orientation of higher energy; the total energy of the system
remains constant. Thus, the condition for absorption of radiation (the
‘resonance’ condition) is

yB

This phenomenon can be studied with a nuclear magnetic resonance

spectrometer, shown schematically in Fig. 3.8. A sample of the compound
containing hydrogen atoms is placed in a cell between the pole pieces of a
magnet and is irradiated; a detector records whether a particular frequency
of the radiation is absorbed or not. For the proton, the size of y is such that,
with a magnetic field of about 1-5 T, the frequency absorbed is 60 MHz,
which is within the range of radio waves; it is usual to keep this frequency
constant and to vary the magnetic field (e.g. by varying the current in an
electromagnet) while searching for absorption of the radiation.
So far, we have considered only the resonance condition for a proton
which experiences the applied field. What makes NMR spectroscopy so

41
FIG. 3.8. An NMR spectrometer
Magnet

S
Magnet

useful in organic chemistry is the fact that, in any compound containing

hydrogen, the magnetic field to which a particular proton is subjected is
not the same as the field which is applied. This is because the applied field
induces a circulatory motion of the electrons in, the bond to the hydrogen
atom and this motion in turn produces a small magnetic field which op¬
poses the applied field. Thus, the magnetic field experienced by the proton is
slightly less than the applied field. Consequently, for the resonance con¬
dition to be fulfilled, the applied field must be increased slightly.
The magnetic field induced at the proton by the bonding electrons de¬
creases as the electrons are moved away from the proton. Now, the average
position of the pair of electrons in a bond relative to each nucleus depends
on the nature of the atoms concerned (4.8); for example, the electrons are
relatively further from the proton in the O—H bond than in the C—H bond.
Therefore, the applied magnetic field at which the resonance condition is
met is smaller for a proton in an O—H bond than for one in a C—H bond.
This is shown in the NMR spectrum of methanol (Fig. 3.9); the area under

FIG. 3.9. The NMR spectrum of

methanol. Methanol has been
dissolved in tetrachloromethane
and TMS added as an internal
reference standard

42
DETERMINATION OF THE STRUCTURE
OF AN ORGANIC COMPOUND
the peak corresponding to resonance for the C—H protons is three-times
that for the O—H proton since there are three times as many protons in
the former environment.
Since the NMR characteristics of protons in a wide variety of environ¬
ments are known, the measurement of the NMR spectrum of a compound
of unknown structure reveals the bonds present between hydrogen and other
groups and also, from their relative peak areas, the relative numbers of these
bonds. It is convenient to measure the NMR spectrum relative to a standard;
tetramethylsilane (TMS), (CH3)4Si, is usually chosen since, having only one
type of proton, it gives only one absorption peak and so causes minimum
interference with the peaks from the unknown compound. Suppose that, for
a given frequency, the protons in tetramethylsilane come into resonance at
an applied field and the proton in another bond comes into resonance
at an applied field B2. Then the chemical shift, S, for the latter proton is
defined as

Bi B2

To obtain simple numbers for 3, the value is usually multiplied by 10® and
then expressed as parts per million (p.p.m.). Since chemical shifts are ratios,
they are independent of the frequency used for the NMR measurements
and therefore provide a common scale. Values for protons in environments
which occur commonly in organic compounds are in Table 3.3.

Table 3.3, Chemical shifts for protons

TYPE OF PROTON CHEMICAL SHIFT/P.P.M.

R-CH 3 0-9

R-CH -R 2 1-3

R
1

R-CH-R 1-5

R-C-CH 3 2-1

11

R-O-CH 3 3-3

R C=CH
2 2 4-7

R-O-H 1-5

R-C-H 9-7
II
0

Still further information can be obtained from the NMR spectrum by

increasing the resolution of the spectrometer. Figures 3.10 and 3.11 show

43
DETERMINATION OF THE STRUCTURE the spectrum of bromoethane. The former spectrum has two peaks, cor¬
OF AN ORGANIC COMPOUND
responding to the CH2 and CH3 protons (relative areas 2:3). In the latter
spectrum, measured at higher resolution, the CHj and CH3 peaks are split
into four lines and three lines, respectively.

FIG. 3.10. The NMR spectrum of

bromoethane at low resolution.
Bromoethane has been dissolved
in tetrachloromethane and TMS
added as an internal reference
standard

FIG. 3.11. The NMR spectrum of

bromoethane at high resolution
showing spin-spin coupling.
Bromoethane was dissolved in
tetrachloromethane and TMS
added as an internal reference
standard

The splitting is the result of spin-spin coupling and has the following basis.
Consider the conditions for resonance of the protons in the methyl group
in a compound of the type

I
CH3-CH
I

The magnetic moment of the single proton on the adjacent carbon atom can
be aligned either with or against the applied magnetic field. For the former
orientation, the protons in the methyl group experience a magnetic field
slightly greater than the applied field, and for the latter orientation they
experience a magnetic field correspondingly slightly less. Consequently,
there are two values of the applied field at which the resonance condition
for the methyl protons is met; one is slightly less than would be the case

44
DETERMINATION OF THE STRUCTURE
OF AN ORGANIC COMPOUND in the absence of the single adjacent proton, and the other is correspond¬
ingly slightly greater. Thus, the methyl group appears as a doublet.
Consider now the condition for resonance of the single proton. The
magnetic moments of the protons in the methyl group can all be aligned
with the applied field; two can be aligned with, and one against, the applied
field; one can be aligned with and two against, the applied field; and all three
can be aligned against the applied field. There are therefore four possible
magnetic arrangements for the methyl protons, and so there are four values
of the applied field at which resonance occurs; the CH group appears as
a quartet. However, the resulting four peaks in the spectrum do not have
equal areas. If we designate as ^ or <- a proton whose magnetic moment
is aligned respectively with or against the applied field, then we see
that there are three times as many ways in which two are aligned with
the field and one against it (—>—>•<—, -»•<—>•, —>->) or the converse
as there are ways in which all three are aligned
either with or against (<- <- <-) the field. Therefore the four peaks
have relative areas 1:3:3:1. Likewise, it can be shown that two protons
interact to give a 1:2:1 triplet pattern. Thus, in the spectrum of
bromoethane, the quartet corresponding to the CH2 group results from
coupling with the three methyl protons, and the triplet corresponding to the
CH3 group results from coupling with the two CH2 protons (Fig. 3.12).

— C H2 - protons — CH3 protons

FIG. 3.12. Possible orientations
of nuclear spins of the protons in
an ethyl, CH^-CH2, group and
the expected spin-spin coupling

In summary, while the chemical shift of a proton gives information about

the nature of the group containing that proton, the spin-spin coupling
pattern gives information about other protons in adjacent groups.

Recently, NMR spectroscopy has become important. Although the com¬

monest isotope of carbon, does not possess a magnetic moment, does so,
just like the proton. The y-value for ^ is about one-quarter that for ^H, so that, for
the same magnetic field, the resonance frequency for is about a quarter that for
‘H (e.g. 15 MHz and 60 MHz, respectively, in a field of about 1-5 T). This also
means that the sensitivity is much smaller (about l/64th) for compared with ^H.
Moreover, since the abundance of in naturally occurring carbon compounds is
only 11 per cent, resonances in a spectrum are only about one six-thousandth
as intense as those in a 'H spectrum. Consequently, special measures must be
adopted to record a NMR spectrum. One way is to scan the spectrum
repeatedly, accumulating the resonances in a computer, but this means that days or
even weeks are necessary to obtain a reasonable spectrum. A much better method is
to use an electronic pulse which excites all the nuclei in a molecule
simultaneously; a computer then accomplishes a Fourier transform analysis of the

45
 electronic output, and a complete, though still very weak, spectrum is obtained in
about a second. Repeated pulses, with computer accumulation, then give a
reasonable spectrum in minutes or, at the most, hours. spectra display chemical
shifts and spin-spin coupling (normally with protons) just as do spectra.
3.8
Summary ORGANIC COMPOUND
Qualitative
Chemical
analysis
analysis
DETECTION and/or
OF ELEMENTS spectroscopic
methods
Quantitative
(mass spectroscopy
analysis
infrared spectroscopy
NMR spectroscopy)
EMPIRICAL FORMULA
FORMULA / WEIGHT
Y

FUNCTIONAL
GROUPS
MOLECULAR
FORMULA

MOLECULAR
STRUCTURE

3.9 Carbon and hydrogen

Practical work: Heat a mixture of the compound and an excess of dried copper(II) oxide
in a pyrex test-tube. Pass the gases evolved into a solution of lime-water
Qualitative analysis of (Fig. 3.13). If the solution turns milky, carbon is present in the compound.
an organic compound If a liquid forms on the side of the tube, test it with anhydrous copper(II)
sulphate. If it turns blue, the liquid is probably water and the compound
contains hydrogen.

FIG.3.13. Testing for carbon and

hydrogen in an unknown compound

Lassaigne sodium test for nitrogen, halogens and

suiphur
Take great care and wear safety glasses.
Place about 0-2 g of the compound in an ignition tube together with a
small pellet of sodium. Hold the tube in an almost horizontal position and

46
DETERMINATION OF THE STRUCTURE
OF AN ORGANIC COMPOUND warm it gently so that the metal becomes molten. Then hold the tube
vertically and heat it strongly. Plunge the tube into a small beaker containing
about 3 cm^ of water. Boil the mixture for a few minutes, filter it and divide
the filtrate into three parts.
Some of the elements in the organic compounds have now been converted
into inorganic salts of sodium. If nitrogen is present in the compound, it
will have formed cyanide ions, the halogens will have formed halide ions
and sulphur will have formed sulphide ions.

Sulphur
To one part of the filtrate, add a drop of an aqueous solution of disodium
pentacyanonitrosylferrate(III) (‘sodium nitroprusside’). A purple colour
indicates the presence of sulphur.

Nitrogen
To the second part of the filtrate, add an equal volume of a fresh solution
of iron(II) sulphate. The mixture now contains a green precipitate of iron(II)
hydroxide. Boil the mixture for a few minutes, then add 2 or 3 drops of
iron(III) chloride solution and acidify the mixture with dilute hydrochloric
acid. Centrifuge (or filter) the mixture. A residue of Prussian blue indicates
the presence of nitrogen in the compound:

Fe2+ Fe(CN)6‘'

4Fe3+ + 3Fe(CN)6^- -> Fe4[Fe(CN)6]3

Prussian Blue
Halogens
(a) If nitrogen is present. Cyanide ions interfere with the test for halide ions
and must therefore be removed. If it has already been shown that nitrogen
is present, acidify the third part of the filtrate with dilute nitric acid and
evaporate it to about a quarter of its original volume. Add a few drops of
silver nitrate solution.
A white precipitate which is soluble in an excess of dilute ammonia
solution indicates the presence of chlorine in the compound:
^ cr , 2NH3
Ag+ -> AgC4 -^ Ag(NH3)4 + Cl-

A pale yellow precipitate which is soluble in an excess of concentrated

ammonia solution indicates the presence of bromine.
A yellow precipitate which is insoluble in an excess of concentrated
ammonia solution indicates the presence of iodine.
(b) If nitrogen is not present. Acidify the third portion of the filtrate from
the sodium fusion with dilute nitric acid, add a few drops of silver nitrate
solution and proceed as in (a).

3.10 The Physics of Chemical Structure. R. J. Taylor (1968). Unilever Educational

Booklet. Advanced Series.
Further reading Investigation of Molecular Structure. B. C. Gilbert (1976). Bell and Hyman Ltd.
Organic Chemistry: a problem solving approach. M. J. Tomlinson and M.C.V. Cane
(1977). Bell and Hyman Ltd.
Molecular Identification and Bonding (1980). Open University. S246 4-6

3.11 Analysis by Mass (F) A.E.I.-G.E.C. Film. Guild Organisation Ltd.

Spectroscopy (V) Open University. 246/05V
Film and Videotape

47
 1 A compound P contains 85*7 per cent carbon and 14-3 per cent hydrogen. After
3.12 reaction with trioxygen and then with water, two of the compounds formed, Q
Questions and R, were distilled and purified. Both compounds absorbed infrared radiation
at about 1700 cm“^ Mass spectra were obtained for Q and R;

„fMass 29 44 43 42
I Abundance (%) 100 89 50 15
IMass 43 58 15
t Abundance (%) 100 33 30

What are P, Q and R? Describe carefully how you elucidated the structures
of Q and R from the evidence given.

2 Two hydrocarbons, X and Y, contain 82-8 per cent carbon and 17-2 per cent
hydrogen. They have the following mass spectra:

Mass 43 41 42 27 15 29 57
Abundance(%) 100 39 32 17 7 6 3
Mass 43 29 27 28 41 39 42
Abundance (%) 100 43 39 32 29 14 12

Write down the structural formulae for X and Y, giving reasons for your choice.

3 Compound A contains 22-2 per cent carbon, 4-6 per cent hydrogen and 73-2 per
cent bromine. The mass spectrum for compound A was:

(Mass 108 no 29 79 81
1 Abundance (%) 100 97 51 4 4

The following NMR spectrum (with TMS as standard) was obtained for A:

How do you account for the physical evidence given ?

4 The chemical shift, 8, for the CH2 protons for the following compounds are:
CH3—CH2—Cl 3-57 p.p.m.
CH3—CH2—Br 3-43 p.p.m.
CH3—CH2—I 3-20 p.p.m.
How do you account for these results ?

48
OF AN ORGANIC COMPOUND^'"^^^^ ^ Compound, F, contains 38-2 per cent carbon, 4-9 per cent hydrogen and 56-9
per cent chlorine. On reduction with hydrogen, it forms G which has the following
mass spectrum:
(Mass 64 28 29 27 66 26 49 51
(Abundance (%) 100 90 84 75 32 29 25 8
The NMR spectrum of G (with TMS as standard) is shown below. Give the
structural formulae for F and G and account for the physical data given.

6 Two compounds, P and Q, contain carbon, hydrogen and oxygen and have a
precise relative molecular mass, as determined by mass spectrometry, of 58-0419.
Their mass spectra are:
pfMass 43 15 58 27 42 26
(Abundance (%) 100 31 28 7 7 4
^(Mass 29 58 28 27 57 18
^ (Abundance (%) 100 83 82 57 26 8
(i) Using the precise data in Appendix VI, determine the molecular formulae of
P and Q.
(ii) What are the possible structural formulae for P and Q?
(iii) Using the data from the mass spectra, determine the structural formulae of
the two compounds.

49
 Chapter 4 Bonding in organic compounds

4.1 Chemistry is concerned with the making and breaking of bonds between
atoms, and the bonds are associated with the electrons that surround the
Introduction nucleus. Present ideas of the nature of chemical bonding are based on both
experiments and mathematical theory. We are only able to describe the
conclusions in a non-mathematical way, but this is still a valuable exercise
because many experimental observations can be rationalised if we under¬
stand the principles of the theory.

4.2 Our understanding of the behaviour of the electrons in atoms and molecules
has evolved from quantum theory. Four especially important principles are
Atomic orbitals involved.
First, an electron can only possess particular energies; for example, it
might have energy a, b, c, etc., but could not have an energy intermediate
between a and b or between b and c. That is, it has various quanta of energy.
Secondly, the behaviour of electrons can be described by the same equa¬
tions as describe a wave motion. This is not to say that electrons are waves,
but only that they behave in the same way as waves. An electron of mass m
and velocity v has a wave-length. A, defined by de Broglie’s equation:
A = hjmv, where his a constant (Planck’s constant).
Thirdly, it is not possible to describe simultaneously both the precise
position and the momentum of an electron (Heisenberg’s Uncertainty
Principle)-, if the momentum is determined with a high degree of precision,
then the position is known only approximately, and vice-versa.
Consider an electron in an atom. It can possess one of various specific
energies, each of which, because of de Broglie’s relationship, is associated
with a particular wave-length. Thus, the energy of an electron can be defined
by a series of equations which describe wave motion—wave functions—and
it is customary to refer to these wave functions as orbitals. Now, because the
energy of the electron is defined precisely, it follows from the Uncertainty
Principle that its position cannot be known with certainty. It is possible
only to say that there is a particular probability of finding the electron at a
given point, or to describe a volume of space in which there is, say, 99 per
cent probability that the electron will be found.
FIG. 4.1. The Is atomic orbital
The fourth principle is that a particular orbital can be associated with a
maximum of two electrons (PaulVs Principle); they are described as having
opposite spins, and are sometimes represented as t and |. When two
electrons are present, they are described as paired; when only one is present
it is described as unpaired. Now, since systems tend to adopt states in which
their potential energy is minimised, it follows from Pauli’s principle that the
electrons in an atom are associated with the orbitals of lowest energy, each
orbital being associated with not more than two electrons.
The hydrogen atom has one electron. It is associated with the orbital of
lowest energy, which is spherically symmetrical about the nucleus. A con¬
tour diagram is shown in Fig. 4.1; if lines are drawn from the nucleus, the
electron is as likely to be found at a particular distance along one line as at
Nucleus the same distance along any other. However, the probability that the

50
BONDING IN ORGANIC COMPOUNDS
electron will be found at a particular distance from the nucleus varies with
the distance, rising to a maximum at a distance of 50 picometres (pm)
(1 pm = 10^ ^^m) and then decreasing again.
It must be noted that the diagram in Fig. 4.1 represents an orbital (wave
function) and not the volume in which the electron is most likely to be found.
However, the probability of finding the electron at a given point is related
to the wave function (it is actually proportional to the square of the wave
function), so that the diagram gives an indication of the likely ‘distribution’
of the electron. It must also be emphasised that, strictly, it is incorrect to
describe an electron as ‘occupying an orbital’; it is simply a useful short-hand
notation to describe it in this way, remembering that an orbital, as a wave
function, is not like the orbit which describes the motion of a planet.
The orbital shown in Fig. 4.1 is described as the H orbital; the electron
in the hydrogen atom which occupies it is unpaired. In the next element,
helium, there are two electrons in the Is orbital, with their spins paired. The
next element, lithium, has three electrons. Two are in the H orbital and
the third is in the next lowest orbital in the energy scale. This is the 2s
orbital, and it is also spherically symmetrical about the nucleus (as are all
orbitals of s type). However, it is larger than the Is orbital, and the distance
from the nucleus at which the electron has the maximum probability of being
found is greater for the 2s than for the H orbital. The next element, beryl¬
lium, has two electrons in the \s and two electrons in the 2s orbital, each
of which is therefore complete.
Of the five electrons in the next element, boron, four are in the H and
2s orbitals and the fifth is in an orbital of different symmetry, namely, a 2p
orbital. There are three 2p orbitals which are mutually perpendicular and
are described as 2p^, 2py and 2p^ orbitals (Fig. 4.2); their shapes are identical
and are illustrated in Fig. 4.3. The electron is as likely to be found on one
side of the nucleus as on the opposite side, but there is zero probability of
its being found at the nucleus (known as the node).
The three 2p orbitals have the same energy, so that the choice between
the 2p^, 2py and 2p^ orbitals for the fifth electron in boron is an arbitrary
one. However, with the next element, carbon, a different choice is available:
the sixth electron could either go into the same 2p orbital as the fifth or
into one of the other two 2p orbitals. The latter is found, and moreover the
spins of the two 2p electrons are the same {Hund's rule) (Fig. 4.4). With the
next element, nitrogen, the third 2p orbital is occupied. In successive

51
 z
FIG. 4.3. The 2p atomic orbitals

elements, the three 2p orbitals are completed by the introduction of one

more electron into each, then the 3s orbital is occupied, after which
three 3p orbitals and the M orbitals (of different symmetry from the s and p
orbitals) are available.
The eleetronic eonfiguration of an atom is eonveniently described as in
Table 4.1; the superscript ^ indicates that there are two electrons in the
orbital concerned, and where there is no superscript the number is one.

Table 4.1. Electronic configurations

ATOMIC
ELEMENT ELECTRONIC CONFIGURATION
NUMBER Z

Hydrogen 1 Is
Helium 2 W
Lithium 3 Is^, 2s
Beryllium 4 ls\ 2s^
Boron 5 \s\ 2s\ 2p^
Carbon 6 2s^, 2p^, 2py
Nitrogen 7 ls^ 2s^ 2p^, 2py, 2p^
Oxygen 8 ls^ 2s\ 2p^^, 2py, 2p^
Fluorine 9 ls^ 2s^ 2p^^, 2py^, 2p^
Neon 10 ls^ 2s^ 2p,\ 2py\ 2/7,^

52
 Atoms tend to adopt an electronic configuration in which each orbital has
Chemical bonding its full complement of two electrons. Two atoms form a bond by the transfer
of an electron from one to the other or by sharing two electrons, one from
each atom, so that singly occupied orbitals become filled.
An electrovalent (ionic) bond is formed when one atom donates one or
more electrons to an atom of a different element, forming charged particles
known as ions. This happens, for example, with lithium and fluorine: the
FIG. 4.4. The electronic
configuration of carbon

FIG. 4.5. The lithium fluoride lithium atom donates its one Is electron to the fluorine atom, each ion then
lattice having an electronic configuration of filled orbitals. The ions, being op¬
positely charged, are held together by electrostatic forces. These forces act
equally in all directions, so that the electrovalent bond is non-directional;
for example, in the crystal of lithium fluoride, each lithium ion (Li^) is
equidistant from six fluoride ions (F“), and vice-versa (Fig. 4.5).
A covalent bond is formed by the sharing of two electrons, one being
contributed by each atom.
Suppose that two hydrogen atoms approach each other. The atomic
orbitals of each atom can overlap, with the result that two molecular orbitals
are formed; these are similar to atomic orbitals except that they are associ¬
ated with two nuclei instead of one. One of the molecular orbitals is of lower
energy than the atomic orbitals and is described as a bonding molecular
orbital ; the other, of higher energy, is an antibonding molecular orbital.
The two l5 electrons from the hydrogen atoms occupy the lower energy,
FIG. 4.6. The formation of the s-s
bonding orbital, with their spins paired, and the antibonding orbital
molecular orbital from two s remains empty. Consequently, the energy of the system is lower than that of
atomic orbitals the separate atoms, and the molecule is more stable than the two atoms.

s atomic orbital s atomic orbital molecular orbital

53
BONDING IN ORGANIC COMPOUNDS Notc that the two hydrogcn nuclei remain separated in the molecule. This
is because, as the nuclei approach each other, on the one hand, the degree
of overlap between the atomic orbitals increases and so therefore does the
effectiveness of the bonding, but on the other hand the repulsive force
between the nuclei themselves increases. Consequently, there is an optimum
distance of separation at which the total energy reaches a minimum; this is
the most stable situation. Therefore, each covalent bond is characterised by
a particular bond length and has a particular bond energy. Bond lengths can
be measured by Z-ray crystallography and by microwave spectroscopy;
they are mostly 100-200 pm and some typical values are in Table 4.2. Bond
energies can be measured by calorimetry and spectroscopic methods;
Table 4.2. Some bond lengths typical values are in Table 4.3.

pm Table 4.3. Some bond energies

kJ mol ^ kJ mol '

C—H 109
C—C 154
134 C—C 345 C—F 450
C=C
119 c=c 610 C—Cl 339
C=C
143 c=c 835 C—Br 284
c—0
122 C—H 413 C—I 213
c=o
147 C—N 304 N—H 391
C—N
130 C=N 889 0—H 490
C=N
116 c—0 358 N—N 163
C=N
96 c=o 749 0—0 146
0—H

Consider next the approach of two helium atoms to each other. As with
hydrogen atoms, the 1^' orbitals of each overlap to form two molecular
orbitals, one bonding and one antibonding. In this case, however, there are
four l.y electrons—two from each atom—to occupy the molecular orbitals;
consequently, each of these contains a pair of electrons, and the effectiveness
of the pair in the bonding orbital in holding the atoms together is nullified
by the effect of the pair in the antibonding orbital, so that no bond is formed
between two helium atoms.
From this example, it can be appreciated that, for the overlap of two
atomic orbitals to result in the formation of a bond, each should contain
only one electron; then the bonding molecular orbital will be filled and the
antibonding one empty. Therefore we can expect the number of bonds
which an element forms to be equal to the number of unpaired electrons in
its atomic structure. For example, the hydrogen atom has one unpaired
electron and forms one bond, as in Hj; the nitrogen atom has three un¬
paired electrons and forms three bonds, as in NH3. However, a problem is
posed by the carbon atom, which has two unpaired electrons and yet forms
four bonds. The explanation is that one of the 2^ electrons in carbon is
transferred to the 2p orbital, ‘thereby yielding four unpaired electrons so
that four bonds can be formed:

2s <-2p-> 2s <-2p-^

t t > t t t t
Carbon in its Carbon in its
ground state bonding state

Although the transfer of the electron from the Is to the 2p orbital (known
as promotion) requires energy, it is more than compensated by the release

54
BONDING IN ORGANIC COMPOUNDS
of energy which accompanies the formation of four bonds compared with
two. This can be represented as in Fig. 4.7. Although the energy change in
forming 2 C—H bonds from a carbon atom in the ground state is not
known, it is not likely to be more than half the value for the formation of
4 C—H bonds from a carbon atom in its bonding state.

Carbon atom
FIG. 4.7. An energy diagram 2s. 2p„, 2py.2p^
showing that carbon prefers to
form four bonds
Increasing
energy
kj moC^

-2057 kJ mol '

total energy change

ca -1028 kJ mol ^

4 (C-H)bonds
total energy change
-1652kJ moT'

4.4 In a simple molecule like methane it would appear at first sight that there
Saturated carbon would be two types of bonds: one type would be formed by overlap of the
singly occupied Is orbital of carbon with the singly occupied Ij' orbital of
compounds a hydrogen atom, and the other would be formed by overlap of each of the
three singly occupied 2p orbitals of carbon with the 1^ orbital of each of
three hydrogen atoms. However, methane is known to be a symmetrical
molecule, containing four C—H bonds of equal length and at equal angles
FIG. 4.8. The sp^ hybridised atomic to each other; if the carbon atom were placed at the centre of a regular
orbital, {a) Cross-section, {b) tetrahedron, the four hydrogen atoms would be at the four corners. This
Shape can be understood by considering the four unfilled carbon orbitals to be

(a) (b)

55
BONDING IN ORGANIC COMPOUNDS ‘mixed’ so that each has l,/4 s character and 3/4p character; the process of
mixing is described as hybridisation and the resulting orbitals as sp^ orbitals.
These orbitals have a different shape compared with both s and p orbitals.
FIG. 4.9. The formation of a
C—H bond from an s atomic
Like p orbitals, they are directional, but unlike p orbitals, one lobe is larger
orbital (hydrogen) and an sp^ than the other (Fig. 4.8). It is the larger lobe which overlaps with another
hybridised atomic orbital (carbon). atomic orbital, such as the b orbital of a hydrogen atom, to form molecular
Cross-section of the atomic and orbitals. For example, the formation of one C—H bond in methane can be
molecular orbitals represented as in Fig. 4.9. The process of hybridisation occurs because

2 sp3 1s C—H bond

the four directional orbitals which result allow greater overlap with the
atomic orbitals of other atoms than if carbon formed its bonds with one
Is and three 2p atomic orbitals; in this way, the total bonding is increased
and therefore the potential energy of the system is decreased.

4.5 There are two other ways in which the atomic orbitals of carbon can be
Unsaturated carbon hybridised. First, the one singly occupied 2s orbital and two of the three
compounds singly occupied 2p orbitals can be hybridised to give three sp^ orbitals,
leaving the remaining 2p orbital intact. The three sp^ orbitals are arranged
symmetrically in a plane, making angles of 120° with each other; the un¬
altered 2p orbital is perpendicular to this plane (Fig. 4.10).

FIG. 4.10. A carbon atom with

three sp^ hybridised atomic orbitals
and one p^ orbital. The three sp^
orbitals, represented by lines from
the carbon nucleus, are in a plane
at angles of 120° to one another

56
BONDING IN ORGANIC COMPOUNDS is the State of hybridisation adopted by the carbon atoms in ethene,
C2H4,. Each carbon atom forms one bond with the other carbon atom and
two to hydrogen atoms by means of its three sp^ orbitals. This leaves a
singly occupied p orbital on each carbon atom (Fig. 4.11), and these two
p orbitals can overlap laterally with each other to form a bonding molecular
orbital between the carbon atoms (Fig. 4.12).

FIG. 4.11. Two carbon atoms

joined by the overlap of sp^ atomic
orbitals. The three sp^ atomic
orbitals from each carbon atom
are shown by lines. The p orbitals
for each carbon atom are
perpendicular to the plane of the
sp^ orbitals and parallel to each
other

FIG. 4.12. The carbon-carbon

double bond. The two carbon
atoms are joined by the overlap of
two sp^ orbitals, represented by a
line joining the carbon nuclei,
and two p orbitals

Two properties of the carbon-carbon double bond in ethene and its

derivatives can be understood immediately in the light of this description.
First, the bond is shorter than a carbon-carbon single bond (Table 4.2).
This is because relatively more bonding is gained by the overlap of two
sets of orbitals as the nuclei approach each other in forming C=C than in
the overlap of one set of orbitals in formation of C—C, while the repulsive

57
BONDING IN ORGANIC COMPOUNDS forces between the nuclei are the same in each case. Secondly, the carbon-
carbon double bond is resistant to rotation, because this reduces the extent
of overlap of the p orbitals; if the bond was twisted by 90°, the p-orbital
overlap would be reduced to zero and the bond energy would be that of a
single bond, so that about 265 kJ mol“^ would be required. It is because
of this that geometrical isomerism (15.4) occurs; for example, the compounds

CH. CH 3 CH, H
/ \ /
c=c
\ / \
H H H CH

do not interconvert because the process of interconversion would neces¬

sitate destruction of the bond formed by p-orbital overlap.
The final type of hybridisation for carbon involves the mixing of one Is
orbital with one 2p orbital; two />-hybridised orbitals are formed, the larger
5

lobes of which point in diametrically opposite directions from the nucleus.

The remaining unpaired electrons are in the original 2p orbitals, which are
perpendicular to each other and to the direction of the sp orbitals.
Ethyne, C H , is the simplest compound formed from p-hybridised
2 2 5

orbitals. Each carbon atom bonds with the other carbon atom and with one
hydrogen atom by use of its sp orbitals, the two bonds being at an angle of
180°, and forms two further bonds with the other carbon atom by /i-orbital
overlap (Fig. 4.13). Just as the C=C bond is shorter than C—C, so C=C is
shorter than C=C (Table 4.2).

FIG. 4.13. The carbon-carbon

triple bond. The two carbon atoms
are joined by the overlap of two
sp orbitals, represented by a line
joining the carbon nuclei, and four
p orbitals, two from each carbon
atom

Bonds between two atoms which are symmetrical about the axis joining
the nuclei of the atoms are described as sigma bonds (o-bonds); examples
are the bonds formed by atomic orbitals (e.g. in H ) and by sp^, sp^ and
5 2

sp hybridised orbitals (e.g. in methane, ethene and ethyne), summarised in

Fig. 4.14. Bonds formed by lateral p orbital overlap are not symmetrical
about the axis joining the nuclei; they are described as pi-bonds (ir-bonds).

58
FIG. 4.14. The three types of
hybridised orbitals for the carbon
atom, (a) Tetrahedral sp^ hybrid
orbitals; {b) Coplanar sp^ hybrid
orbitals; (c) Collinear sp hybrid
orbitals

(a) Tetrahedral sp^ hybrid orbitals

(b) Coplanar sp^ hybrid orbitals

4.6 So far, all the molecular orbitals we have described have been constituted
from the overlap of two atomic orbitals and are centred around two nuclei.
Delocalised bonds
The bonds to which these molecular orbitals correspond are known as
localised bonds.
There are also delocalised bonds, in which pairs of electrons are associated
with bonding molecular orbitals which extend over three or more atoms.
They occur less commonly than localised bonds, but they confer special
properties on the compounds containing them.
Benzene, CgH^, contains delocalised bonds. The six carbon atoms are
arranged in the form of a regular hexagon and each forms three localised
bonds with /?^-hybridised orbitals, two to other carbon atoms and one to a
5

hydrogen atom. This leaves a singly occupiedp orbital on each carbon atom,
and each p orbital overlaps with the p orbital on either side of it (Fig. 4.15).
The overlap of these six atomic p orbitals gives rise to six molecular n
orbitals, of which three are of bonding and three are of antibonding type.
The six electrons from the six atomic p orbitals then occupy the three

59
bonding in organic compounds bonding molecular n orbitals as three pairs; thus, there are three delocalised
n bonds. The shape of one of these three molecular orbitals is shown in
Fig. 4.15; the overall effect of filling each with a pair of electrons is to give the
same distribution of electron density between each pair of carbon atoms.

FIG. 4.15. Structure of benzene.

Each carbon atom forms three
a bonds, two to other carbon atoms
and one to a hydrogen atom
{represented by lines). In addition
the six atomic p orbitals shown
in the upper diagram overlap
laterally to give delocalised tt
molecular orbitals, one of which
is shown in the lower diagram

Delocalised n orbitals are much larger than the localised n orbitals in,
say, ethene. The electrons in them can be found in a greater volume than
for localised orbitals; in a sense, they have a greater freedom of movement,
and it is a general principle that, the greater the freedom of movement of
an electron, the lower is its energy. Hence, a special property of compounds
containing delocalised bonds is that they are more stable than similar com¬
pounds which only contain localised bonds. The experimental evidence that
benzene is stabilised in this way, and fuller details of its structure, are dis¬
cussed later (8.2).
There is an alternative method of representing delocalised bonds. Thus,
if we were to represent benzene as

H H
C
II or I
C
H "C H
I
H

60
BONDING IN ORGANIC COMPOUNDS
it would imply that three localised Ti-bonds are present. Instead, a widely
used convention is to draw two structures:

The double-headed arrow <->• is taken to mean that the actual structure lies
between the two representations; in other words, each C—C bond is
neither a simple single nor a simple double bond but is of intermediate type.
This method is sometimes described as mesomerism (‘in-betweenness’), and
benzene is described as a resonance hybrid of the two structures. It is
important to realise that benzene does not oscillate between these
structures; it exists in only one form, in which the six C—C bonds are of
identical type.
In this text, delocalisation is sometimes described in terms of molecular
orbitals, but in other cases it is more convenient to represent the benzene
ring as

where the circle corresponds to the delocalised bonds.

4.7 The two electrons in a bond between two atoms are attracted by the two
nuclei. In a diatomic molecule in which the two atoms are the same, such as
The inductive effect H2 or Cl2, attraction by one nucleus is as strong as attraction by the other
and so it is as likely that the electrons will be found a particular distance
from one nucleus as from the other; the bond is symmetrical in the sense
that the centres of gravity of the negative and positive charges coincide.
However, in a bond between unlike atoms, the nucleus of one atom exerts a
stronger attractive force than that of the other; the electrons are more likely
to be found nearer the former nucleus and the centres of gravity of the
negative and positive charges do not coincide. For example, in hydrogen
chloride the centre of gravity of the negative charges is nearer to chlorine
than the centre of gravity of the positive charges, an effect which is
represented as H ^C1 where represents the tendency of the electrons to
lie nearer chlorine.

The magnitude of the opposite charges multiplied by the separation of their

centres of gravity is defined as the dipole moment of the compound. This can be
measured by finding how the compound affects the capacitance of a condenser; the
greater the dipole moment, the more strongly the molecules tend to align themselves
between the plates of the condenser (the positive end of the molecule being nearer
to the negative plate, and vice-versa) and the lower is the capacitance.
The bonds in organic molecules are also polarised in this way by
substituents such as the halogens. For example, chloromethane has a dipole
moment of which the negative end is the chlorine atom:

H-C- ■Cl
I
H

61
BONDING IN ORGANIC COMPOUNDS It is useful to have a scale for describing the effects of different atoms to
attract bonding electrons. For this purpose, hydrogen is chosen as a refer¬
ence point; an atom or group which attracts the bonding electrons more
strongly than hydrogen is described as having an electron-withdrawing
inductive effect, symbolised as —I. For example, since the electrons in the
C—Cl bond of CH3—Cl lie relatively further from carbon than those in the
C—H bond in CH3—H, chlorine is a - / substituent. On the other hand,
alkyl groups are electron-releasing { + !) compared with hydrogen.

Some types of organic grouping are acidic and others are basic. The com¬
4.8 monest acid group is the carboxylic acid group:
Organic acids and bases
—C—O—H

Carboxylic acids, R—C02F1, are weak acids compared with the mineral
acids such as H2SO4; that is, the equilibrium

R—C—O—H + H2O ^ R—C—O" + H30^

o o
lies on the left-hand side.
The equilibrium constant, K, for this reaction is given by

[RCQ2-][H3Qn
[RC02H][H20]

However, for dilute solutions of acids, [H2O] is essentially constant, and

it is customary to define the acid dissociation constant, K^, as:

[RCQ2-][Hn
“ [RCO2H]

For example, for ethanoic acid, = F7 x 10“^ mol dm“^.

There is some confusion about the use of units for equilibrium constants. As the
equilibrium constant is described above, it has units of mol dm“^. However, in
more advanced textbooks, you will usually find the constant described as
dimensionless. This is because relative activities of the ions and molecules, rather
than concentrations, are used and these are themselves dimensionless.
Despite the small values of for ethanoic and other carboxylic acids,
they are still acidic enough to form salts with alkalis and turn blue litmus
paper red. In contrast, alcohols, which also dissociate,

R—O—H + H2O R—O" + H3O +

have such small dissociation constants (K^ is ca. 10”^^) that they exhibit
few of the properties usually associated with acids; for example, although
they form salts and liberate hydrogen when treated with sodium,

ROH + Na ^ RO“Na+ + ^H2

they do not turn blue litmus paper red.

62
BONDING IN ORGANIC COMPOUNDS
The reason for the greater acid strength of carboxylic acids than alcohols
can be understood by considering the bonding in the corresponding anions,
RCO 2 and RO . In the former, the atomic p orbitals on the carbon and
two oxygen atoms of the carboxylate group interact to give three delocal¬
ised 71 molecular orbitals; the four p electrons occupy the two lowest energy
orbitals of these three, of which one is bonding and one is non-bonding.
The shape of the bonding t: molecular orbital is shown in Fig. 4.16.

FIG.4.16. p-Orbital overlap in a

carboxylate ion; the lower
diagram shows the delocalised

The anion is thus symmetrical, with the negative charge shared equally by
the two oxygen atoms. This can perhaps be seen more easily by representing
the anion in terms of the theory of mesomerism; two structures can be
drawn:

.O' O
R-C R-C
o O

63
BONDING IN ORGANIC COMPOUNDS and the actual structure is intermediate between the two, with the equivalent
of half a negative charge on each oxygen atom. For the ion RO , on the
other hand, the charge is localised on one oxygen atom. Now, it is a general
principle that the potential of a charged system decreases as the volume
associated with the charge becomes larger (in electrostatics, the potential
of a charged sphere is inversely proportional to the volume of the sphere).
Therefore, the carboxylate ion, RC “, in which the charge is delocalised,
02

is relatively more stable than the ion RO“ and is formed the more readily.
Another acid grouping is

O
II
-S-O-H
II
o
in organic sulphonic acids, R—SO H. These acids are much stronger than
3

the carboxylic acids and are as strong as mineral acids like H SO . This 2 4

again can be understood in terms of charge delocalisation; in the sulphonate

anion, three oxygen atoms share the negative charge, as represented by the
three structures

O O" O
II I II
R-S-0~ R-S=0 R -s=o
II I
o O 0 “

In addition to the effect of charge-sharing described above, the strengths

of acids are dependent upon the presence of substituents in the rest of the
molecule.
For example, chloroethanoic acid (ClCHj—CO H) is a stronger acid 2

than ethanoic acid (Table 4.4). This is because chlorine has a —/effect; that
is, the chlorine nucleus, by its strong attraction for the electrons in the
C—Cl bond, enables the negative charge in the ion Cl—CH —CO ~ to be 2 2

spread through the molecule more effectively than in the ethanoate ion.
Two chlorine substituents are more effective than one, and three are more
effective than two.
The fluorine atom, having a stronger attraction for electrons, causes
fluoroethanoic acid to be a stronger acid than chloroethanoic acid. Bromo-
ethanoic and iodoethanoic acids are, as expected, weaker acids (Table 4.4).

Table 4.4. Dissociation constants of some

halogeno-substituted carboxylic acids

ACID K, .AT 25°C

CH —CO H
3 2 1-7 X 10-=
CICH —CO H 2 2 1-4 X 10-=
CI CH—CO H
2 2 5-1 X 10-=
CI C—CO H
3 2 2-2 X 10-=
FCH —CO H2 2 2-2 X 10-=
CICH —CO H 2 2 1-4 X 10-=
BrCH —CO H 2 2 1-3 X 10-=
ICH —CO H2 2 6-9 X lO-'*

64
BONDING IN ORGANIC COMPOUNDS
From such evidence, it can be shown that the following groups exert a
—I effect with respect to hydrogen (in descending order of power):

NO2, CN, F, Cl, Br, I, OH, NH2

Groups which are electron-releasing with respect to hydrogen (+/

groups) reduce acid dissociation constants (Table 4.5). For example.

Table 4.5. Dissociation constants of some

carboxylic acids

ACID Ka AT 25°C

H—CO2H 1-7 X 10-“

CH3—COjH 1-7 X 10-"
CH3CH2—CO2H 1-3 X 10-*
QH5—CO2H 6-3 X 10-5
CsHsCHj—CO2H 4-9 X 10-5

ethanoic acid is weaker than methanoic acid (H—CO2H), and propanoic

acid (CH3—CH2—CO2H) is weaker still. Groups that exert a -t-/effect are,
in descending order of power :

(CH3)3C, (CH3)2CH, C2H3, CH3

The amino group, —NH2, is the most commonly found basic group in
organic chemistry. It is basic because it can form a bond with a proton by
means of the unshared pair of electrons on the nitrogen atom:

R—NH2 + H2O ^ R—NH3 + OH-

The equilibrium constant for this reaction is given by:

[R-NH3][0H-]
[R-NH2][H20]

However, usually K is determined for a solution in water, which is in con¬

siderable excess over the other components, so that [H2O] is approximately
constant. It is therefore customary to describe the base strength of an amine
hy K^, given by:

K,= [R—NH3] [OH-]

[R—NH2]

(the units are again omitted, p. 62).

Amines are weak bases; that is, is small. For example, methylamine
(CH3—NH2) has Kh (at 25°C) = 4-4 x 10"''. Thus, they are far weaker bases
than inorganic alkalis such as sodium hydroxide, although they are strong
enough to form salts with acids.

65
4.9 As we have seen, the structures of organic compounds are characterised by
the formation of four bonds by each carbon atom. However, there are also
Unstable intermediates
species in which the carbon atoms form fewer bonds, and although they are
in organic chemistry too unstable to exist as compounds which can be isolated, they are never¬
theless important in occurring as short-lived intermediates in organic re¬
actions. Two of the more important types with which this book will be
concerned are free radicals, in which one of the carbon atoms has three
bonds and one unpaired electron, and carbonium ions, in which one of the
carbon atoms has three bonds and possesses a positive charge.
The simplest free radical is methyl, 'GHj (the dot signifies an unpaired
electron). The radical has a planar structure in which the carbon atom forms
three bonds to hydrogen atoms by /»^-hybridised orbitals and possesses a
5

p orbital with one electron in it (Fig. 4.17).

FIG. 4.17. The shape of the methyl

radical. The three sp^ orbitals of
carbon, represented by lines from
the carbon nucleus, are in a plane
at angles of 120° to one another,
and overlap with Is orbitals of the
hydrogen atoms. There is one
electron in the p orbital which is at
right angles to the plane

Free radicals are particularly important as intermediates in the reactions of

alkanes. For example, in the chlorination of methane (5.4), the methyl
radical is formed from methane and a chlorine atom:

CH + Cl-
4 -CHj + HCl

and then reacts with a moleculfe of chlorine:

•CH + CI ^ CH CI + Cl-
3 2 3

A carbonium ion has the same structure as a radical except that the p
orbital is empty; thus, the carbon atom is associated with a positive charge.
Carbonium ions are particularly important as intermediates in the reactions
of alkenes, alkyl halides and alcohols. For example, in the addition of
hydrogen chloride to ethene, a carbonium ion is formed first;

CH2=CH2 + HCl ^ CH3—CH2 + Cl-

66
BONDING IN ORGANIC COMPOUNDS
and then rapidly reacts with a chloride ion:

CHj—CH + 2 cr ^ CHj—CH CI 2

The substituents in a compound are of importance in determining the

ease of formation of a carbonium ion. For example, of the two carbonium
ions

CHj—CH—CHj CH —CH —CH

3 2 2

the former is attached to two alkyl substituents which are electron-releasing

relative to hydrogen and the latter to only one. Consequently, the concentra¬
tion of the positive charge is reduced to a greater extent in the former case
and the former ion is therefore the more stable. This is important in govern¬
ing the direction of addition of a proton to propene, CH —CH=CH 3 2

(6.4).

4.10 Determination of dissociation constants of some

Practical work carboxylic acids
Some of the evidence for the inductive effect of atoms and groups in organic
molecules comes from a study of the dissociation constants of carboxylic
acids. The dissociation constants can be readily obtained.

Experimental procedure
Prepare 50 cm^ samples of several carboxylic acids of concentration
OT M (chosen from, for example, ethanoic, monochloroethanoic, dichloro-
ethanoic, phenylethanoic acids), in 100 cm^ beakers. Set up a pH meter and
adjust the electrodes in the beaker so that they are well immersed in the
solution but are not at the bottom of the beaker. Measure and record the
pH of the solution.
Add 0-5 cm^ of M sodium hydroxide solution from a burette, swirl the
contents to ensure mixing, measure and record the pH of the solution again.
Continue to do this until the end-point is nearly reached and then add the
alkali drop by drop between each addition until after the end-point has
been reached.

Calculation of the dissociation constant

For a weak acid, HX,

[HX]

Thus

1
^ 1 X [X-]
[H^ X/[HX]
u 1^ 1
or

When the solution was half-neutralised, [X ] = [HX] so that, at this point,

the pH of the solution has the same value as the pK„( —Ig KJ of the acid.

67
BONDING IN ORGANIC COMPOUNDS The pH of the half-neutralised solutions of carboxylic acids can be
found from the titration curves obtained experimentally above.

4.11 Chemical Bonding (F) (CHEM-study series). Guild Organisation Ltd.

Film
4.12 1 Describe the bonding in methane and ethene in terms of orbitals.
Comment on the following boiling points:
Questions CH OC HS, 1 rC;
3 2 CjHsSH, 36°C; (CHjIjCO, 56°C ;
CH CH CH OH, 97°C.
3 2 2

(C(T))

2 Discuss or explain the following.

(i) Ethanol (ethyl alcohol) boils a't 78-3°C; dimethyl ether boils at —24-9°C.
(ii) The pH of a solution containing ethanoic acid and sodium ethanoate is
changed only slightly by the addition of a small amount of hydrochloric
acid.
(iii) The effect of a magnetic field on the emissions from radio-active nuclei.
(iv) The H—C—H angle in methane is 109°28', whilst the H—C—H angle in
ethene is 120°. (C(N and T))
3 Explain the following.
(i) The carbon-carbon bond lengths in benzene are all identical.
(ii) The carbon-oxygen bond length in dimethyl ether (1-43 A) is longer than
in propanone (1-24 A).
(iii) Fluoroethanoic acid is a stronger acid than ethanoic acid.
4 Explain what you understand by the following terms; bond energy, hybrid¬
isation, mesomerism, delocalised bonds.
5 What do you understand by the term heat of hydrogenation ?
The heats of hydrogenation of ethene and benzene are —120 and —210 kJ
per mol, respectively. Comment.

68
Chapter 5
Alkanes

General formula
C„H 2n+2

5.1 The first four members of the series retain their original names. Alkanes
with a straight chain containing five or more carbon atoms are named by
Nomenclature combining a prefix derived from the Greek for the length of the chain with
the suffix -ane.

Table 5.1. Physical properties of some alkanes

—
density/
NAME FORMULA B.P./°C M.P./°C gcm“^
(20°C)

Methane CH4 -162 -183 gas

Ethane CzHs -89 -172 gas
Propane CjHs -42 -188 gas
Butane C4H10 -0-5 -135 gas
Pentane CsHn 36 -130 0-626
Hexane CeiHi4 69 -95 0-659
Heptane C7HX6 98 -91 0-684
Octane CgHis 126 -57 0-703
Nonane C9H20 151 -54 0-718
Decane C10H22 174 -30 0-730
Undecane C11H24 196 -26 0-740
Dodecane C12H26 216 -10 0-749
Triacontane C30H62 343 37 solid

One of the characteristics of a homologous series is that successive members

5.2 show a gradation of physical properties. This is illustrated for the alkanes
Physical properties of in Table 5.1. As the series is ascended, the boiling point and density increase.
alkanes The boiling point depends on the attractive forces between the molecules
of the liquid; the stronger these are, the more energy is needed to separate
the molecules in order to convert the liquid into the vapour, and so the
higher is the boiling point. The forces increase with the formula weight of
the compound, so that when a methylene group (CH ) is introduced into the
2

alkane chain, the boiling point rises. However, as the molecular weight
becomes larger, its percentage increase on introduction of a methylene
group becomes smaller, so that the difference in boiling points between
consecutive members of the homologous series decreases as the series is
ascended, giving the smooth curve in Fig. 5.1 when the boiling point is
plotted against the number of carbon atoms in the molecule.
The melting points of the alkanes do not fall on a smooth curve. As Fig.
5.2 shows, two curves can be drawn, one for the alkanes with an even number
of carbon atoms and a lower one for those with an odd number. This is
because, in the crystalline state, the molecules adopt a highly ordered ar¬
rangement in which the carbon chains form a zig-zag pattern. For the even

69
 <

FIG. 5.1. Plot of

boiling point against
the number of carbon
atoms in straight-
chain alkanes

FIG. 5.2. Plot of

melting point against
the number of carbon
atoms in straight-
chain alkanes

70
ALKANES
members, different chains pack closer together than for the odd-numbered
members, so that the attractive forces are larger for the members of the
former group than for members of the latter group of similar size. It follows,
in turn, that relatively more energy must be applied to separate the mol¬
ecules with even numbers of carbon atoms and enable them to adopt the
more random arrangement of the liquid state than to separate the molecules
with odd numbers of carbon atoms.
Branched-chain alkanes have lower boiling points than their straight-
chain isomers, and as branching increases the boiling point decreases still
further. The examples in Table 5.2 illustrate the trend.

Table 5.2. Boiling points of the isomeric pentanes

NAME FORMULA B.P./°C

Pentane CH3CH2CH2CH2CH3 36
CH,
1 ®
2-Methylbutane CH3CHCH2CH3 28
CH3

2,2-Dimethylpropane CH3-C-CH3 10

CH3

The explanation is that increased branching gives the molecule a more

nearly spherical shape and reduces the extent of contact between neighbour¬
ing molecules; consequently, the attractive forces are reduced and the boil¬
ing point decreases.
There is no regularity in the change of melting point with increasing
branching in an alkane. This is because, whereas the linear alkanes all adopt
a simple, close-packed structure in a crystal, the type of packing of branched
members (and hence the attractive forces between the molecules) is not a
simple function of structure.

The principal sources of alkanes are natural gas and petroleum. Natural gas
5.3 contains mainly methane, with smaller amounts of the other gaseous alkanes
Occurrence of alkanes such as ethane, propane and butane. Petroleum contains a wide range of
alkanes, from the low molecular weight gases to the high molecular weight
solids.
The uses of petroleum, both as a fuel and as a source of chemicals, are
of outstanding importance. They are mentioned throughout this book, and
are brought together in Chapter 20. The formation of deposits of natural
gas and petroleum is discussed in Chapter 19.

5.4 Structural formula

Methane H
I
H-C-H
I
H

71
 Plate 5.1. An aerial view of the
production platform ‘Cormorant' in
the North Sea. The platform is also
used for pumping and storage of oil
from other oil fields in the North
Sea. It is linked by pipeline to the
oil terminal at Sullom Voe in the
Shetlands. The dish aerials and
high mast on the right are part of
the telephone communication
system to other platforms in the
North Sea and to Scotland. An
idea of scale can be gained by
comparing the size of the
helicopter to that of the platform.
(Esso Petroleum Co. Ltd.)

Occurrence
Natural gas is by far the most important source of methane (19.2). The gas
is brought to Britain both by direct pipe-lines from the deposits below the
North Sea and by specially constructed tankers which contain gas from the
Libyan deposits which has been liquefied.

Chemical properties
1. Like the other alkanes of low formula weight, methane does not
react with acids, alkalis or oxidising agents in solution. The lack of reactivity
of alkanes towards inorganic reagents led to their being termed originally
paraffins (Latin; parum affinis, little affinity).
2. Methane burns in air, with a hot, non-luminous flame, to carbon
dioxide and water:

CH4 + 2O2 ^ CO2 f 2H2O

72
ALKANES
Under carefully controlled conditions, methane is oxidised to other
organic compounds. When a mixture of oxygen is compressed to a high
pressure and passed through copper tubes at 200°C, methanol is formed :

CH4 + i02 -> CH3OH

Methane is now the most important source of ethyne. With a specially

designed burner, it can be partially oxidised:

CH4 + i02 ^ CO + 2H2

The heat liberated in this reaction is used to raise the temperature of a

further amount of methane, which is heated for a very short time at 1500°C:

2CH4 CH=CH + 3H2

Methane is also oxidised when it is mixed with steam and passed over
nickel, and this reaction is used as a method for the manufacture of carbon
monoxide and hydrogen (‘synthesis gas’, p. 321):

CH4 + H2O CO + 3H2

3. When methane and chlorine are mixed together in the dark, no reaction
occurs. However, if the mixture is either heated or exposed to ultraviolet
light (from a mercury lamp), a mixture of products is formed ;

CH4 + CI2 ^ CH3CI + HCl

Chloromethane

CH3CI + CI2 ^ CH2CI2 + HCl

Dichloromethane

CH2CI2 + CI2 ^ CHCI3 + HCl

T richloromethane

CHCI3 + CI2 ^ CCI4 + HCl

Tetrachloromethane

With an excess of methane, chloromethane predominates, whereas with an

excess of chlorine, tetrachloromethane is the main product.
These are examples of substitution reactions: substitution is defined as the
replacement of one atom (or group of atoms) in a molecule by another atom
(or group). When the reaction is brought about by ultraviolet light, it is
described as a photochemical reaction.
A detailed examination of the relatively simple reaction between methane
and chlorine to form chloromethane and hydrogen chloride:

CH + CI ^ CH CI + HCl
4 2 3 zl//= -lOOklmopi

illustrates many interesting principles. First, although the reaction is exo¬

thermic, energy (in the form of either heat or light) must be supplied to the
mixture for reaction to begin. Methane is transparent to ultraviolet light of

73
 Plate 5.2. An undersea pipeline
for gas being laid by a semi-
submersible barge (above). The
pipeline (1 m in diameter) coming
ashore from the North Sea on to a
beach near Aberdeen, Scotland.
(Shell Photo Service).

74
ALKANES
the wavelength used, about 300 nm, but chlorine is not. Chlorine absorbs
the light, the energy of which is equivalent to about 400 kJ moP this is
considerably greater than the bond strength of the chlorine molecule
(242 kJ mol which consequently splits into chlorine atoms:

Cl—Cl Cl. +-Cl (1)

Each atom retains one electron of the pair which formed the covalent bond.
An atom or group of atoms which possesses an unpaired electron is called
a free radical (p. 66). The energy supplied to the chlorine molecule is not
enough to produce ions, CP and CP (1130 kJ moP^).
Each chlorine atom then reacts with a molecule of methane by abstract¬
ing a hydrogen atom to form hydrogen chloride and a methyl radical:

Cl- + CH4 -> HCl + -CHj (2)

The methyl radical reacts with a molecule of chlorine to form chloromethane

and a chlorine atom:

•CHj + CI2 ^ CH3CI + Cl- (3)

The chlorine atom produced by reaction (3) can then react with another
molecule of methane according to reaction (2); thus reactions (2) and (3)
can occur successively once an initial supply of chlorine atoms has been
provided.
However, there are other reactions which can intervene to stop the
successive occurrence of reactions (2) and (3); these are processes in which
two free radicals combine with each other:

Cl. + Cl. ^ CI2 (4)

Cl. + .CH3 ^ CH3CI (5)

.CH3 + .CH3 ^ C2H6 (6)

Now, at any instant, the concentrations of chlorine atoms and methyl

radicals are very small compared with the concentration of methane and
chlorine. Therefore, the chances of a collision between two atoms, an atom
and a radical, or two radicals, are small compared with the chances of a
collision between a chlorine atom and a molecule of methane or between
a methyl radical and a molecule of chlorine. As a result, many thousands
of molecules of chloromethane are formed by reactions (2) and (3) for each
molecule of chlorine which is decomposed by reaction (1).
The characteristics described above are typical of a chain reaction. There
is an initiating step—reaction (1); propagating steps, which keep the chain
reaction in operation—reactions (2) and (3); and terminating steps, which
bring the chain to an end—reactions (4), (5) and (6).
It is often difficult to test for whether a reaction occurs via free radicals.
Two methods in this case involve adding another substance. For example,
if tetraethyllead vapour is added, the reaction rate is increased considerably.
This is because the lead compound decomposes to supply more free radicals:

(C2H5)4Pb ^ 4.C2H5 + Pb

some of which react with chlorine to form chlorine atoms:

.C2H5 + CI2 ^ C2H5CI + Cl.

75
ALKANES On the other hand, if oxygen is added, the rate is reduced, probably because
oxygen reacts with methyl radicals and so prevents them taking any further
part in the chain reaction.
In reactions (1), (2) and (3) covalent bonds are broken so that one electron
of the pair in each bond becomes associated with each of the atoms or
groups. These are examples of hemolysis or homolytic fission (Greek: lysis,
splitting). A second way in which a bond can undergo fission is for both
electrons of the bond to become associated with one of the two atoms or
groups:.
A—B ^ A+ + B”

Examples of this process, heterolysis or heterolytic fission, are described in

the next chapter.

Uses
The uses of methane are discussed above and in Section 20.3.

5.5 Ethane, propane and butane are obtained in two ways: from ‘wet’ natural
gas (available in large quantities, for example, in the United States) (19.2)
Other alkanes and from the gas above, and dissolved under pressure in, oil deposits (for
example, in the North Sea, off the coast of the United Kingdom) (19.3).
The alkanes are heated to high temperature in absence of air, a process
known as cracking, to yield ethene and propene (20.4). In turn, these two
alkenes are the major starting materials for a wide variety of polymers
(21.2) and other important chemicals (20.4).
The other properties of ethane and propane are similar to those of
methane. They do not react with acids, alkalis or oxidising agents in
solution; they burn in air; and they are readily chlorinated. A large number
of chloroalkanes can be made; for example, ethane gives chloroethane
(C2H5CI), two dichloroethanes (CICH2—CH2CI and CH3—CHCI2),
two trichloroethanes (CICH2—CHCI2 and CH3—CCI3), two tetrachloro-
ethanes (CICH2—CCI3 and CI2CH—CHCI2), pentachloroethane
(CI2CH—CCI3) and hexachloroethane (CI3C—CCI3).
The uses of ethane and higher alkanes are described in Section 20.3.

5.6 Cycloalkanes are classified as alicyclic compounds. The simplest is cyclo¬

propane:
Cycloalkanes

H H
\ /
Hx
c—c U
1 1
H H

The angles between the atoms in the ring are 60°, whereas sp^ orbitals are
at angles of 109° 28'. Consequently, the overlap between the pairs of orbitals
is not as complete as in the non-cyelic alkanes, and so the C—C bond
strengths are less; the ring is said to be strained. This in turn makes cyclo¬
propane more reactive than, for example, propane towards reagents which

76
ALKANES
break C—C bonds. Thus, cyclopropane reacts readily with bromine in the
absence of light:

CH2
/ \
H C-CH
2 2 -^ Br-CH2-CH2-CH2-Br
1,3-Dibromopropane

and with sulphuric acid:

CH2
/ \ HjSO.
H2C-CH2 -^ CH3-CH2-CH2-O-SO2-OH
Propyl hydrogensulphate

In these respects it resembles an alkene such as propene (6.4).

In cyclobutane, the angles between the carbon atoms are 90°. Thus, the
ring is strained, although not so much as in cyclopropane. As would be
expected, cyclobutane undergoes the same reactions as cyclopropane but
less readily.

/CH2.^
H2C-CH2
I I
H2C ^^2
H2C-CH2 H2C-CH2
Cyclobutane Cyclopentane

Cyclopentane has bond angles of 108°, so close to the tetrahedral value

that the ring can be regarded as strainless. As expected, it resembles the
non-cyclic alkanes and does not undergo the ring-opening reactions of
cyclopropane and cyclobutane.
The structure of cyclohexane is of particular note. Were the carbon atoms
in a plane, the bond angles would be 120° and would therefore be strained,
though in the opposite sense compared with those in cyclopropane. How¬
ever, the preferred angle of 109° 28' can be obtained by ‘buckling’ of the ring,
and this is indeed the structure of cyclohexane; a two-dimensional repre¬
sentation is

and is sometimes referred to as the chair structure. All the cycloalkanes with
seven or more members in the ring have strainless or nearly strainless
structures, although the exact shapes of the larger ones are not known.
They all resemble the non-cyclic alkanes in their properties.

Carry out the following reactions with a liquid alkane (e.g. pentane or
5.7 hexane) and a liquid cycloalkane (e.g. cyclohexane).
Practical work 1. Place a few drops of the liquid on a watch-glass or an evaporating basin
and apply a lighted splint. Note the colour of the flame.

77
ALKANES 2. To a few drops of the liquids in separate test-tubes, add;
(a) 5 drops of an alkaline potassium manganate(VII) solution (made by
dissolving about O'l g of sodium carbonate in 1 cm^ of a 1 per cent
solution of potassium manganate(VII)), shake the mixture and
see whether the liquid is oxidised;
(b) 5 drops of a solution of bromine in tetrachloromethane, shake the
mixture arid see whether the bromine is decolorised.
3. Cracking of paraffin oil. See p. 311.

Lists of films, videotapes and further reading on prospecting for petroleum

5.8 and natural gas, and the subsequent manufacture of chemicals, are given on
Resource materials pages 313 and 324-5.

5.9 1 Outline two laboratory methods for the preparation of methane. Give two
instances of the natural occurrence of methane.
Questions From the properties of methane deduce the characteristic chemical behaviour
of the carbon-hydrogen linkage in organic chemistry. How is the behaviour of
this linkage modified when it occurs
H
(a) in the group (as in ethanal),

(b) in benzene?
To 30 cm^ of a mixture of methane and carbon monoxide are added 50 cm^ of
oxygen, and the mixture is exploded. After shaking with potassium hydroxide
solution, 20 cm^ of gas are left. Calculate the composition by volume of the
original mixture. (All volumes are measured at room temperature and pressure.)
(JMB)
2 Give equations for four methods of preparing ethane, naming the reagents and
stating the conditions required.
Indicate briefly how and under what conditions methane reacts with chlorine.
Give the molecular formula for the hydrocarbon of molecular weight 56, and
write down structural formulae for the isomers.

3 Give two methods for preparing ethane in the laboratory.

A mixture of 10 cm^ of a gaseous hydrocarbon and 100 cm^ of oxygen (excess)
was exploded. The volume after explosion was 75 cm^, and this was reduced to
35 cm^ on treatment with potassium hydroxide solution. Deduce the molecular
formula of the hydrocarbon and give its possible structural formulae. (All
measurements were made at the same temperature and atmospheric pressure.)
4 Give equations, and conditions, for the reactions (if any) between methane and
(a) chlorine, (b) bromine in the absence of light, (c) sulphuric acid.
How would you expect cyclobutane to react with these reagents ?
5 An equimolar mixture of methane‘and chlorine is made up.
(i) What, if anything, happens if the mixture is stored in the dark?
(ii) What would you expect to happen if the mixture were exposed to sunlight?
(iii) What is the importance of the sunlight in the above reaction?
(iv) What mechanism would you propose for the reaction in (ii)?
(v) Name the reactive species in this mechanism.

78
Chapter 6
Alkenes

General formula
C„H2„

6.1 The compounds are named as for the alkanes, but with the suffix -ene instead
of -ane and the inclusion before the suffix of a number to describe the posi¬
Nomenclature tion of the double bond in the chain where more than one is possible. For
this purpose, the chain is numbered from the end nearer to the double bond
and the lower number of the two which describe the positions of the carbon
atoms in the double bond is employed. For example:

CHj—CH2—CH=CH2 CH3—CH=CH—CH2—CHj
But-1-ene Pent-2-ene

CH3—CH—CH=CH—CH2—CH3

CH3
2-Methylhex-3-ene

The two lowest members of the series are sometimes described by their
original names: ethene (CH2=CH2) is known as ethylene, and propene
(CH3—CH=CH2) as propylene. 2-Methylpropene, (CH3)2C=:CH2, is
sometimes referred to as isobutylene or isobutene.

The melting points and boiling points of the alkenes are very close to those
6.2 of the alkanes with the same number of carbon atoms. Ethene, propene and
Physical properties of the butenes are gases at room temperature, and the higher members are
alkenes liquids (Table 6.1).

Table 6.1. Some alkenes

NAME FORMULA B.P./°C

Ethene CH2=CH2 -102

Propene CH3—CH=CH2 -48
But-l-ene CiHs—CH=CH2 -6-5
tra«5-But-2-ene* CH3—CH=CH—CH
cw-But-2-ene* CH3—CH=CH—CH
2-Methylpropene (CH3)2C=CH2 -6-5
Pent-1-ene C3H7—CH=CH2 30
Hex-1-ene C4H9—CH=CH2 63

Cyclohexene 83

Phenylethene CfiHs—CH=CH2 145

* These are geometrical isomers, which are discussed in Section 15.4.

79
6.3 Structural formula
Ethene
H
:c=c
H H

Manufacture

The naphtha fraction from the distillation of petroleum, which contains

alkanes with 4-10 carbon atoms, is passed with steam through pipes heated
at 700-900°C. The ethene formed is purified by fractional distillation (20.4).
Smaller alkanes, such as ethane and propane, which are available
(depending on geographic location) either in ‘wet’ natural gas or in the gas
above and dissolved in oil deposits (5.5), are cracked with steam in a similar
way.
Both processes yield a variety of products, of which ethene and propene
are the most important. They are separated and purified by distillation.

Chemical properties
(a) Addition reactions
Ethene, and all other alkenes, are characterised by their addition reactions
in which the double bond is converted into a single bond and atoms or
groups are added to each of the two carbon atoms. The general reaction is;

C=C + X-Y

X Y

The following are examples:

1. When ethene is mixed with hydrogen and passed over nickel at 150°C,
ethane is formed:
Ni as cat.
CH2=CH2 + H2 -> CH —CH 3 3

The reaction takes place on the surface of the metal, which acts as a catalyst.
Finely divided platinum or palladium are more active catalysts, and
reaction takes place at room temperature.

2. Ethene reacts with chlorine to form 1,2-dichloroethane:

CH2=CH2 + CI2 CICH2—CH2CI

1.2- Dichloroethane

Similarly, with bromine, it forms 1,2-dibromoethane:

CH2=CH2 + Br2 -> BrCH2—CH2Br

1.2- Dibromoethane
There is considerable evidence that these are ionic reactions (compare the
free-radical reaction by which methane and chlorine give chloromethane,
p. 75). Important information about the mechanism is provided by the
observation that, if bromine is added to a solution of ethene in which
sodium chloride is also present, l-bromo-2-chloroethane is formed in
addition to 1,2-dibromoethane. This points to the mediation of an organic
«
cation which can react with either bromide ion or chloride ion:

80
 Plate 6.1. Steam cracking of Hydrogen Propene Butenes
naphtha to form alkenes methane Hydrogen Ethene propane butane
(principally ethene and propene).
This photograph shows one of the
largest plants of its kind in Europe.
A—Naphtha storage, B—Furnaces
to crack naphtha, C—Primary
fractionating column. Other
fractionating columns are shown
which remove D—methane, E—
ethene and ethane, F—ethene from
ethane, G—propene and propane,
H—butenes and butane. Other
parts of the site include I—steam
boilers, J—compressors, K—
flame stacks. {Imperial Chemical
Industries PLC). See also Fig. 20.1.

D E F G H

8
ALKENES CH2 = CH2 + Br 2 —> BrCHg—CH + Br
2

BrCH2-CH2Br

BrCH2-CH2

BrCH2-CH2Cl

The cation BrCH2—CH2, and other cations in which a carbon atom

bears the positive charge, are given the general name carbonium ions. In the
first step the two electrons which form the new bond are both provided by
the alkene; thus, the reagent (bromine or chlorine) is described as electro¬
philic (i.e. ‘electron-seeking’).
-t-
There is evidence that the catiorr BrCHj—CH2 should be more properly
represented as

Br;...
\ +
CH2—CH2
in which there is an (electrostatic) interaction between an unshared pair of electrons
on bromine and the positively charged carbon atom. However, this idea does not
affect the main conclusions here.

When the reaction with bromine is carried out in water, the main product
is 2-bromoethanol. The first step of the reaction is the same as above, but
the carbonium ion is a very reactive species which reacts with a molecule of
water almost as readily as with a bromide ion; since there is much more
water than bromide ion in the solution, the reaction with water predomi¬
nates. The reaction occurs by the donation of an unshared pair of electrons
on the oxygen atom of water to the electron-deficient carbon atom in the
carbonium ion, followed by the loss of a proton:

BrCH2—CH + :0H2 ^BrCH2—CH —O:"

2 2

^BrCH2—CH2OH + H +
2-Bromoethanol

A similar reaction occurs between ethene and chlorine water, 2-chloro-

ethanol being formed.

3. Ethene reacts with hydrogen chloride to give chloroethane:

CH2=CH2 + HCl ^ CH3—CH2CI

This is also an ionic reaction. The first step is the formation of two ions,
the ethyl cation and the chloride anion:

ch2=ch2 + HCl ^ CH3—CH2 + cr

The two ions then rapidly combine to form the product. In this reaction,
hydrogen chloride is the electrophilic reagent.
Hydrogen bromide reacts more readily than hydrogen chloride, giving
bromoethane, and hydrogen iodide reacts more readily still to give iodo-
ethane.

82
ALKENES
4. When ethene is passed through concentrated sulphuric acid, ethyl
hydrogensulphate is formed:

CH2=CH2 + H2S04^CH3—CH2—O—SO2—OH
Ethyl hydrogensulphate

Once again a carbonium ion is formed first, by reaction with the electrophilic
sulphuric acid:

CH2=CH2 + H2SO4 -> CH3—CH2 + O—SO2—OH

This ion then combines with a hydrogensulphate ion:

CH3—CH2 + “O—SO2—OH -» CH3—CH2—O—SO2—OH

When the mixture is diluted and warmed, ethanol is produced:

CH3CH2—O—SO2—OH + H2O -> CH3CH2—OH + H2SO4

This is the basis of a method for manufacturing ethanol from ethene, but
it is now being superseded by direct hydration with a solid catalyst (phos¬
phoric acid on silica) at 300°C:
H3P04/Si02 as cat.
CH2=CH2 + H2O -^ CH3CH2OH

5. When ethene is passed through a dilute, alkaline solution of potas¬

sium manganate(VII), ethane-1,2-diol (ethylene glycol) is formed:

CH2=CH2 + H2O + ‘O’ ^ HO—CH2—CH2—OH

Ethane-1,2-diol

The manganate(VII) is the source of the necessary oxygen atom denoted ‘O’. The
oxygen atom is never free but is thought to be transferred from the manganatefVII)
ion to the alkene via a cyclic intermediate which is too unstable towards water to be
isolated:

n O
CH2 // H,0 CH2OH /P
II -I- I Mn I -I- 0=Mn
CH, H2C.0/ \^. CHoOH \
' O'

(Two MnOa" ions disproportionate to one Mn04^ ion and Mn02.)

6. When ethene is treated with a peroxoacid (a compound containing the

group —CO—O—OH), epoxyethane is formed; for example, peroxobenzoic
acid can be used:

CH2=CH2 + CgHs-CO-O-OH ^ H2C-CH2 + CeHs-CO-OH

Peroxobenzoic acid o Benzoic acid
Epoxyethane

Industrially, epoxyethane is made by passing a mixture of ethene and

oxygen over a silver catalyst at 250°C:

ch2=ch2 + i02 ^ H2C-CH2

o

83
ALKENES (b) Other reactions
1. When trioxygen is passed through a solution of ethene in trichloro-
methane, ethene ozonide is formed:

/Ox
+ O3 -> H2C CH,

0-0
Ethene ozonide

It is an unstable, explosive product, which is readily hydrolysed with water

to a mixture of methanal and hydrogen peroxide; half the methanal is then
oxidised to methanoic acid by the hydrogen peroxide.

H2C CH2 + H2O -> 2HCHO + H2O2

0 _Q Methanal

HCHO + H2O2 -> HCO2H + H2O

Methanoic
acid

Plate 6.2. Hortonspheres used to

2. When subjected to very high pressures at 120°C in the presence of
store liquefied gases { for example, oxygen as a catalyst, ethene undergoes polymerisation to give the polymer,
ethene, propane and butanes, poly(ethene) (also known as polythene) (p. 328). Polymerisation is also
chloroethene) {Esso Petroleum catalysed by a mixture of triethylaluminium and titanium(IV) chloride, and
Co. Ltd.) this gives a type of poly(ethene) with more useful properties (p. 328).

84
ALKENES
Uses
Ethene is one of the most important raw materials for the chemical in¬
dustry, particularly in making plastics (p. 328). One of these, poly(ethene),
is made directly from ethene (21.2). A second, poly(chloroethene), is made
from ethene via chloroethene. Ethene is chlorinated to form 1,2-dichloro-
ethane, which is then vapourised and heated at 500°C to form chloroethene:

CH =CH
2 2 + CI 2 CICH —CH CI
2 2

CICH —CH CI
2 2 CH2=CHC1 + HCl
Chloroethene

Hydrogen chloride is separated and then passed with more ethene and
oxygen over copper(II) chloride at 250°C. This is called an oxychlorination
process:

CH2=CH2 + HCl + i02 ^ CH2=CHC1 + H2O

In this way, no chlorine is wasted. Each mole of chlorine yields two moles
of chloroethene.
Ethene is also made into ethanol (used as a solvent and as a starting
material for other products), epoxyethane (used in the manufacture of
detergents and of ethane-1,2-diol), ethanal and higher straight-chain
alkenes, used to make detergents.
The uses of ethene are further discussed in Section 20.4.

6.4 Structural formula

Propene
H H
H
H /C-C^
H H

Manufacture
Propene is obtained with ethene from the cracking of alkanes. The pro¬
cesses are outlined on p. 80.

Chemical properties
The chemistry of propene is similar to that of ethene. It burns in air,
can be reduced with hydrogen over metal catalysts, is polymerised to
poly(propene) (p. 328) and undergoes addition reactions with the halogen
acids, the halogens, sulphuric acid, potassium manganate(VII) and peroxo-
acids. However, there is a feature in its reactions with acidic reagents which
does not apply to ethene; thus, reaction with an acid of general formula HX
could give either of two products:

CH3—CH=CH2 + HX ^ CH3—CH2—CH2X or CH3—CHX—CH3

85
ALKENES In practice, the second of these products is the major one. With more
highly substituted alkenes, the major products are as follows:

^'^C=CH, + HX R-C-CH3
R I
X

R
R uY I
/C=CHR + HX -> R-C-CH2R
R^ I

These observations were first made by Markownikoff and can be sum¬

marised as an empirical rule known as Markownikoflf’s rule: when an acid
HX adds to the double bond of an alkene, the hydrogen atom becomes
attached to the carbon atom which has the larger number of hydrogen
atoms.
The principle which underlies this empirical rule is as follows. The pro¬
portion of each of the two products from an alkene is determined by the
relative rates of the addition of the proton to each carbon atom, for once
this step has occurred the uptake of an anion X" follows immediately. Now,
of the two possible carbonium ions which can be formed during the first
step of the reaction of propene,

CHj—CH2—CHz CHj—CH—CHj

the first is a primary carbonium ion, with only one alkyl group attached to
the positively charged carbon atom, whereas the second is a secondary
carbonium ion, with two alkyl groups attached to the positively charged
carbon atom. Since an alkyl group is electron-releasing relative to a hydro¬
gen atom (4.8), it reduces the density of positive charge on the neighbouring
carbon atom, and this makes the ion more stable. Two alkyl groups are
more effective than one, so that the secondary carbonium ion is more stable
than the primary carbonium ion. Consequently, the secondary ion is formed
the more rapidly, and the major product from propene is of the type
CH3—CHX—CH3.
When three alkyl groups are attached to a positively charged carbon atom,
the stabilising effect is increased further; i.e. the order of stability of car¬
bonium ions is tertiary > secondary > primary. Thus,

R2C=CH2 forms R2C—CH3 faster than R2CH—CH2

tertiary primary

R2C=CHR forms R2C—CH2R faster than R2CH—CHR

tertiary secondary

Free-radical addition to propene

When propene reacts with hydrogen bromide in the presence of an organic
peroxide, the product is mainly 1-bromopropane:

CH3—CH=CH2 + HBr CH3—CH2—CHj—Br

whereas in the absence of a peroxide the main product is 2-bromopropane.

86
ALKENES
This is because, in the presence of a peroxide, a radical chain reaction occurs
which is more rapid than the electrophilic addition which gives 2-bromopropane.
The peroxide, containing the relatively weak O—O bond (p. 54), breaks down
to give two radicals:

R—O—O—R 2R—O-

These radicals react with hydrogen bromide to give a bromine atom:

R—0- + H—Br ^ R—O—H + Br.

The bromine atom adds to the alkene mainly at the unsubstituted carbon atom,
giving the radical CH3—CH—CH2—Br (which is more stable than the alternative
radical, CH3—CHBr—CHj):

CH3—CH=CH2 + Br- CH3—CH—CH2—Br

and this radical abstracts a hydrogen atom from another molecule of hydrogen
bromide:

CH3—tH—CH2—Br + H—Br ^ CH3—CH2—CH2—Br + Br-

The bromine atom formed can add to another molecule of propene, so that a
chain reaction is propagated; the chain ends when two radicals meet and com¬
bine. In summary:

RO—OR 2RO-
Initiation
RO- + HBr ROH + Br-
(Br- + CH3—CH=CH2 ^ CH3—CH—CH2Br
Propagation; .
iCH3—CH—CH2Br + HBr ^ CH3—CH2—CH2Br + Br-
2Br- Br,

Br- + CHg-CH-CHjBr -> CHo-CHBr-CHoBr

Termination ■

ICHj-CH-CHjBr CH-CH^
BrCHs CH2Br

Hydrogen chloride and hydrogen iodide do not react in this way. This is because,
although halogen atoms are generated in the initiation step in each case, one or
other of the propagating steps is so slow that the overall rate of the chain reaction
is less than that of the electrophilic addition. With hydrogen chloride, the slow
step is

CH3—CH—CH2CI + HCl CH3—CH2—CH2CI + el¬

and with hydrogen iodide it is

I- + CH3—CH=CH2 ^ CH3—tH—CH2I

Since alkenes form peroxides slowly when exposed to air, it is not always
necessary to add a peroxide in order to bring about the chain reaction with
hydrogen bromide. Indeed, if the product of the electrophilic addition is required,
it is necessary for the alkene to be a freshly prepared (or freshly distilled) sample.

87
ALKENES
Uses
As with ethene, the principal use of propene is as a raw material to make
plastics, the most important being poly(propene), poly(propenenitrile) {via
propenenitrile), perspex {via propanone) and the glyptal resins {via pro-
pane-1,2,3-triol). These are discussed in more detail in Chapter 21.
The single most important chemical made from propene is propanone
(12.3). The uses of propene are summarised in Section 20.4.

6.5 Small-scale preparation of cyclohexene

Practical work To 10 cm^ of cyclohexanol in a flask, add, with a dropping pipette, 4 cm^
of concentrated phosphoric acid, shaking the flask.
Assemble the apparatus (Fig. 6.1), and heat the flask gently, distilling over
the liquid.

FIG.6.1. Preparation of
cyclohexene

Pour the distillate into a separating funnel and add 2 cm^ of a saturated
solution of sodium chloride. Shake the mixture and allow the two layers to
separate. Run off the lower layer and then run the top layer, containing
cyclohexene, into a small flask. Add 2 or 3 pieces of anhydrous calcium
chloride, stopper the flask and shake until the liquid is clear.
* Decant the liquid into a clean distillation flask and distil it, collecting the
liquid boiling at 81-85°C.

88
ALKENES
Test-tube preparation of ethene
Place ethanol in a test-tube, to a depth of 2-5 cm. Add Rocksil until the
ethanol has been soaked up. Place about 1 g of aluminium oxide half¬
way along the tube (Fig. 6.2). Fit a cork and delivery tube to the test-tube

Rocksil and ethanol

FIG.6.2. Test-tube preparation of
ethene

and heat the aluminium oxide with a gentle flame. Collect 4 or 5 test-tubes
of ethene by displacement of water, placing corks in the test-tubes when
they have been filled.

Cracking of oii to form alkenes

For details, see Section 19.10.

Reactions of alkenes
Carry out the following reactions with a gaseous alkene (e.g. ethene) and a
liquid alkene (e.g. cyclohexene), comparing the results of experiments 1-3
with those obtained with an alkane (5.7).

1. (a) Ignite the ethene by applying a lighted splint to the mouth of the
test-tube.
(b) Place a few drops of cyclohexene on a watch-glass and ignite.
Note the colour of the flame.
2. (a) Add a few drops of a solution of bromine in tetrachloromethane to
a test-tube of ethene and shake the mixture.
(b) To a few drops of cyclohexene in a test-tube add a few drops of
bromine in tetrachloromethane and shake the mixture.
3. (a) Add 3-4 drops of an alkaline solution of potassium
manganate(VII) (made by dissolving about OT g of anhydrous sodium
carbonate in 1 cm^ of a 1 per cent solution of potassium manganate(VII))
to a test-tube of ethene and shake the mixture.
Note the colour of the solution and whether any precipitate is formed.
(b) Repeat this experiment using 2 or 3 drops of cyclohexene instead of
ethene.

89
ALKENES 4. Polymerisation of alkenes. The polymerisation of an aromatic alkene,
phenylethene, CgHs—CH=CH2, is described on p. 341.

5. Mechanism of the addition of bromine to an alkene.

Take especial care during this experiment. Bromine liquid and vapour are
harmful to the skin and to breathe. You are advised to wear protective gloves
as well as eye-protection and, if possible, to keep the experiment in a fume-
cupboard.
(a) Make up a solution of 5 cm^ of cyclohexene in 10 cm^ of ethanol in
a 100 cm^ conical flask. Add 2 cm^ of a saturated solution of sodium nitrate
in water. Some of this salt recrystallises out of solution. Add water dropwise,
with shaking, until the sodium nitrate redissolves.
Shake the solution vigorously, so that the two layers are thoroughly
mixed, and add bromine, from a 10 cm^ burette at a rate not greater than one
drop a second until a faint yellow colour persists. Note the burette reading
before and after the addition of bromine.
Transfer the mixture to a separating funnel and collect the lower layer in
a dry test-tube. Add some anhydrous magnesium (or sodium) sulphate,
cork the tube and shake for a few minutes. Filter oflT the drying agent and
collect the dry product in a dry test-tube.
Carry out the Lassaigne sodium fusion test for nitrogen on some of the
product (p. 46).
(b) Make up a solution of 5 cm^ of cyclohexene in 10 cm^ of ethanol in a
100 cm^ conical flask. Shake vigorously and add bromine with care from a
10 cm^ burette until the mixture is pale yellow. Note the burette reading
before and after the addition of bromine.
Shaking the mixture well, add 2 cm^ of a saturated solution of sodium
nitrate in water and add water until the salt just redissolves.
Transfer the mixture to a separating funnel and collect a dry sample of
the lower layer, as described in (a). Perform the Lassaigne sodium fusion
test for nitrogen on some of the product (p. 46).
(c) Answer the following questions:
(i) Did you find any significant difference in the volume of bromine
used in experiments (a) and (b)? Explain this result.
(ii) The densities of cyclohexene and bromine are 0-81 and 2-93 g cm“^
at 20°C. Calculate the number of moles of cyclohexene that react
with one mole of bromine.
From the results of the Lassaigne tests, do you find that nitrogen is
present in the products formed in experiments (a) or (b)? If so, what
deductions can you make about the mechanism of the reaction between
cyclohexene and bromine?

6. Mechanism of the addition of bromine to an alkene: an alternative

experiment.
A saturated aqueous solution of bromine containing chloride ions is pre¬
pared by adding bromine to a saturated solution of sodium chloride and
shaking or stirring the solution until no more bromine will dissolve.
Add the solution of bromine, a few drops at a time, to 5 cm^ of cyclo¬
hexene in a stoppered flask and shake until the bromine colour just persists.
Transfer the mixture to a separating funnel containing 50 cm^ of 10%
sodium thiosulphate solution, and rinse the reaction vessel with 20 cm^ of
diethyl ether, adding this to the separating funnel. After shaking well, allow
the layers to separate and run off and discard the aqueous layer. The

90
 ALKENES
ethereal solution is then run into another, stoppered, flask, and dried for a
few minutes over anhydrous magnesium sulphate.
The products in the solution can be separated by thin-layer chromato¬
graphy using either plates prepared as on p. 30 or specially-prepared com¬
mercial papers. 60-80 petroleum spirit is a good solvent. The solvent
should not be allowed to rise higher than three-quarters of the way up the
slide. After drying the slide, place it in a covered beaker containing a crystal
of iodine. Brown stains will develop indicating the positions of the
components.
The product from the bromine/chloride solution may yield 3 or more
spots, showing that several products are formed. These may be identified
by comparison with authentic samples of the possible products. If authen¬
tic samples are not available, most of them can be prepared readily by
‘test-tube’ techniques.

6.6 Lists of films, videotapes and further reading concerned with the manu¬
. Resource materials facture of chemicals from petroleum sources are given on pages 324-5.

6.7 1 Describe in detail two test-tube reactions to show whether a given compound
contains an unsaturated carbon-carbon link.
Questions To 30 cm^ of a gaseous mixture of butadiene CH2=CH—CH=CH2 and
but-l-ene C2H5CH=CH2, 100 cm^ of hydrogen were added and the mixture was
passed repeatedly over a hydrogenation catalyst in a closed system until no
further reduction in volume occurred. The total volume was then 90 cm^. What
was the composition by volume of the original mixture? (All volumes were
measured at the same temperature and pressure.)
Give two sets of reactions by which chloroethene may be prepared.
Indicate the structure of poly(chloroethene). Give one industrial application of
this material and mention the properties upon which this use depends. (JMB)

2 Describe how you would prepare a pure specimen of ethene from ethanol.
By what reactions can the following be obtained from ethene: (a) ethanol,
(b) ethyne, (c) ethane-1,2-diol.

3 Write an account of the laboratory preparation and of the properties of ethene,

indicating how this substance may be distinguished from methane and from
benzene.

4 (a) Name the organic products of the reaction of but-2-ene, CH3CH=CHCH3,

with each of the following reagents and write a balanced equation for each
reaction:
(i) hydrogen bromide,
(ii) bromine dissolved in trichloromethane,
(iii) a dilute alkaline solution of potassium permanganate.

(b) When a hydrocarbon C;cHy is completely oxidised in oxygen the reaction

which occurs can be represented by the following equation:

+ (x + >’/4)02 XCO2 + (y/2)H20

20 0 cm^ of a gaseous, unsaturated hydrocarbon A were exploded with 200 cm^
of oxygen and, after cooling to the original room temperature, 140 cm^ of gas
remained. This volume was reduced to 20-0 cm^ on shaking the gas with potassium
hydroxide solution. Deduce the molecular formula of A.

91
ALKENES When the gas undergoes oxidation at the double bond, one mole of A gives
one mole each of B and both of which have the molecular formula CsHeO.
Both B and C give an orange precipitate with 2,4-dinitrophenylhydrazine and
B reduces Fehling’s solution while C does not.
Write structural formulae for A, B and C and explain the reactions involved.
(L(X))

5 Draw a labelled diagram of an apparatus suitable for the preparation of a sample

of trioxygen. Describe a method for determining the percentage by volume of
trioxygen in the sample.
Write structural formulae (excluding cyclic structures) for the different isomers
corresponding to the molecular formula C4H8 and explain how trioxygen may
be used to distinguish between the isomers. (W)

6 Reaction of compound A (CgHig) with trioxygen and then water gave neutral
compounds B (CsHgO) and C (CgHiiO). B did not reduce Fehling’s solution,
but C did. Reduction of C with hydrogen and a catalyst gave D (C6H14O),
which, when heated with concentrated hydrobromic acid, gave E (CgHiaBr).
E was heated with a concentrated solution of potassium hydroxide in ethanol,
and gave F(C6Hi2). After being heated with alkaline potassium permanganate,
Fgave on acidification an acid G (CsHio02); treatment of G with (5?-morphine
gave two products, separated by fractional crystallisation.
Identify the compounds A-G, explaining your reasoning. (C(N, S))

7 Reaction of compound A (C11H14) with trioxygen and then water gave B

(CsHsO) and C (CsHeO) in equimolar amounts. C was unaffected by am-
moniacal silver nitrate, but B reacted with this reagent to give D (C8H8O2). When
D was treated successively with sulphur dichloride oxide (or phosphorus trichlor¬
ide) and ammonia, E (C8H9NO) was formed. E reacted with bromine and sodium
hydroxide to give F(C7H9N), a much weaker base than methylamine.
Identify the compounds A-Eso far as is possible and comment on the reactions.
Suggest further experiments (a) to confirm the identification of C, and (A) to be
carried out on compound B to complete the identification of compounds D-E.
(C(T, S))

8 Describe in outline the preparation of propene starting from propanone.

Write down the structural formulae of the compounds obtained when propene
is treated with the following reagents.
(a) Concentrated sulphuric acid.
(b) Bromine.
(c) Cold potassium permanganate solution.
(d) Trioxygen followed by water. (O and C)

9 When 25 cm^ of the gaseous hydrocarbon, A, were exploded with 200 cm^ of
oxygen, the residual gases occupied 150cm^. After shaking the residual gases with
excess aqueous sodium hydroxide, the final volume was 50 cm^.
(All volumes were measured at room temperature and pressure.)
(a) Why was there a decrease in volume when the residual gases were shaken
with aqueous sodium hydroxide? Give an equation.
(b) Calculate the molecular formula of A. Explain your working.
(c) Write the structural formulae of SIX possible compounds (cyclic and non-
cyclic) which A could be and give the systematic names for each of the six
formulae.
(d) Which of the six structural formulae show compounds which are
(i) structural isomers;
(ii) stereoisomers;
(iii) optical isomers? (SUJB)

10 Reaction of a hydrocarbon. A, of molecular formula, C10H20, with trioxygen

and then water gives two isomeric products, B and C, C5H10O.

92
ALKENES Oxidation of B leads to an optically active acid, D, C5H10O2, whereas oxida¬
tion of C leads to an acid E, C4H8O2, which cannot be resolved into optically
active forms.
Reduction of C gives F, C5H12O, dehydration of which, followed by reaction
with trioxygen and then water, gives propanone as one of the products.
Deduce the structure of A and elucidate the reaction sequence.

11 A compound A reacts with bromine giving B (C5HioBr2). Oxidation of A leads

to two compounds C (C2H4O2) and D (CsHgO). The silver salt of C contains
64-6 per cent Ag while D with 2,4-dinitrophenylhydrazine forms a derivative
having the composition C9H10N4O4. D also reacts with iodine and sodium
hydroxide forming tri-iodomethane. Deduce a structure for A.
12 (a) State two properties of carbonium ions.
(b) What is the formula of the carbonium ion formed when propene reacts with
hydrogen bromide to form 2-bromopropane?
(c) State whether this carbonium ion is a primary, secondary or tertiary species.
(d) How does carbonium ion theory explain the fact that the principal product of
the reaction between propene and hydrogen bromide is 2-bromopropane
rather than 1-bromopropane?
(e) What reagents and conditions would be used to prepare propan-2-ol from
2-bromopropane ?

(f) In what way would you alter the conditions in (e) in order to prepare propene
rather than propan-2-ol from 2-bromopropane?
(JMB)
13 (a) State the Markownikoff rule for predicting the direction of electrophilic
addition of hydrogen bromide to alkenes. Explain in detail the rule in terms of the
relative stabilities of primary, secondary and tertiary carbonium ions.
(b) Propene reacts with hydrogen bromide to give a substance A, CjH^Br.
Substance A, when heated with aqueous potassium hydroxide, gives an alcohol B.
(i) Derive structures for A and B.
(ii) Explain and illustrate the meanings of the terms base, nucleophile, and
inductive effect by referring to the reactions of substance A with potassium
hydroxide under various conditions.
(JMB)

14 Devise experiments to determine:

(a) the structure of the product(s) obtained by addition of hydrogen bromide to
propene, CH3 — CH = CH^;
(b) the structure of the product(s) obtained by elimination of hydrogen bromide
from 2-bromobutane, CH, —CHBr —CH2 —CH3.
(O Schol.)

93
Chapter 7
Alkynes

General formula
C„H 2n-2

7.1 The compounds are named as for the alkenes but with the suffix -yne. For
example:
Nomenclature 4 3 2 1
CH3—CH2—C=C—H

is but-1-yne. However, the first member, ethyne (CH=CH) is often des¬

cribed by its original name, acetylene, and its simple derivatives are some¬
times described as substituted acetylenes: e.g. propyne (CH3—C=CH) is
methylacetylene.

1.2 The melting points and boiling points of the alkynes are similar to those
of the alkanes with the same number of carbon atoms (cf. Table 5.1).
Physical properties of
alkynes Table 7.1. Some alkynes

NAME FORMULA B.P./°C

Ethyne CH^CH -83

0
X

X
Propyne III -23
But-1-yne CjHs—Ce^C—H 9
But-2-yne CH3—C=C—CH3 27
Pent-1-yne C3H7—CeeeC—H 40
Phenylethyne C^Hs—C^C—H 143

7.3 Structural formula

Ethyne H—C=C—H

Manufacture
1. Calcium dicarbide is obtained by heating coke with calcium oxide in

an electric furnace above 2000°C:

CaO + 3C CaC2 + CO

Ethyne is produced by the treatment of calcium dicarbide with water;

CaC2 + 2H2O ^ CH=CH + Ca(OH)2

2. Ethyne is now being increasingly made by the pyrolysis of methane at

1500°C (5.4).

94
ALKYNES
Chemical properties
(a) Electrophilic addition reactions

Ethyne resembles ethene in undergoing addition reactions with electro¬

philic reagents. Examples are:

1. It reacts readily with the halogens. Chlorine reacts explosively:

CH=CH + CI2 ^ 2C + 2HC1

To prevent explosions, ethyne and chlorine are generally mixed in retorts

filled with Kieselguhr and iron filings to absorb the heat of the reaction.
Addition compounds are then formed:

CH=CH C1CH=CHC1 CI2CH—CHCI2

1,2-Dichloro- 1,1,2,2-Tetra-
ethene chloroethane

Bromine reacts less violently, giving 1,2-dibromoethene and, with an excess

of bromine, 1,1,2,2-tetrabromoethane.

2. It reacts with the halogen acids, for example:

HCl HCl
CH=CH -> CH2=CHC1 -> CH3—CHCI2
Chloroethene 1,1-Dichloro-
ethane

(b) Other reactions

1. Ethyne resembles ethene in being reduced by hydrogen on certain
metal surfaces (e.g. nickel at 150°C or platinum or palladium at room
temperature). First ethene and then ethane are formed:

CH=CH CH2=CH2 CHj—CH3

2. Ethyne differs from ethene in reacting with water in the presence of

dilute sulphuric acid and mercury(II) sulphate (HgS04) at 60°C:

CHe^CH + H2O -> CH3CHO

Ethanal
This is the basis of a method for manufacturing ethanal, but it is now being
superseded by one which starts with ethene, a cheaper raw material than
ethyne (12.3).

3. Ethyne also differs from ethene in forming salts with several metals;
for example, if ethyne is passed through an ammoniacal solution of
copper(I) chloride, a red precipitate of copper(I) dicarbide is formed:

CH=CH + CU2CI2 + 2NH3 ^ CU2C2 + 2NH4CI

Copper(I)
dicarbide

Similarly, if ethyne is passed through an ammoniacal solution of silver

nitrate, a white precipitate of silver dicarbide, Ag2C2, is produced.

95
ALKYNES
Uses
Many of the industrial uses of ethyne are now losing importance as methods
for making the same compounds from ethene have been developed. The
increase in energy prices in the 1970s accelerated the trend as ethene is
cheaper to produce than ethyne; examples are the production of ethanal
mentioned above and the formation of chloroethene by addition of hydro¬
gen chloride to ethyne (p. 95), which is being superseded by the chlorination
of ethene (6.3.)
One small-scale use is in oxyacetylene welding, which is based on the very
high temperatures {ca. 3000°C) attained.when ethyne burns in oxygen:

2CH=CH + 5O2 ^ 4CO2 + 2H2O

7.4 The properties of other alkynes resemble those of ethyne except that only
those alkynes in which the triple bond is at the end of the chain form metal¬
Other alkynes lic compounds. For example, propyne forms a silver salt, CHgC^CAg,
but but-2-yne (CH3—C=C—CH3) does not. This provides a method of
distinguishing ethyne and alkynes of the type RC=CH, which are acidic
and form metal salts, from other alkynes and also from alkenes.

7.5 Small-scale preparation of ethyne

Practical work Place 2 or 3 small pieces of calcium dicarbide in a test-tube and arrange the
apparatus for collection of ethyne (Fig. 7.1). Add water dropwise and
collect 4 or 5 test-tubes of gas, putting a cork in the test-tube when it is full
of gas.

FIG. 7.1. Test-tube preparation of

ethyne

96
ALKYNES
Reactions of alkynes
Carry out the following experiments with a gaseous alkyne (e.g. ethyne) and
a liquid alkyne {e.g. phenylethyne). Compare the results of experiments 1-3
with those obtained with an alkane (5.7) and an alkene (6.5).

1. (a) Ignite the gas by applying a lighted splint to the mouth of the test-
tube.
(b) Place a small sample (a few drops) of the liquid alkyne on a watch-
glass and ignite it.
Note the colour of the flames.

2. (a) Add a few drops of a solution of bromine in tetrachloromethane

to a test-tube of gas and shake the mixture.
(b) To a few drops of the liquid alkyne in a test-tube, add a few drops
of bromine in tetrachloromethane and shake the mixture.

3. (a) Add 3-4 drops of an alkaline solution of potassium manga-

nate(VII) (made by dissolving about OT g of anhydrous sodium carbonate
in 1 cm^ of a 1 per cent solution of potassium manganate(VII)) to a test-
tube of gas and shake the mixture. Note the colour of the solution and
whether any precipitate is formed.
(b) Repeat the experiment with 2 or 3 drops of the liquid alkyne instead
of the gas, warming the mixture gently.

4. (a) Prepare an ammoniacal solution of copper(I) chloride by dissolving

about 0-1 g of copper(I) chloride in 1 cm^ of a dilute solution of ammonia.
Generate some ethyne, and pass the gas through the solution (Fig. 7.2).
(b) Will this reaction occur with (i) propyne (CHj—C=C—H), (ii) but-
2-yne (CHj—C=C—CH3)?

FIG. 7.2. Reaction between

ethyne and a copper (I) salt

Ammoniacal solution of
copper(l) chloride

97
ALKYNES (c) Try the reaction with phenylethyne by shaking 2 or 3 drops of the
liquid alkyne with 1 cm^ of an ammoniacal solution of copper(I) chloride.

N.B. If any solids are formed in 4(a) or 4(c), do not allow them to become
dry. Wash them down the sink with plenty of water.

1 With the aid of equations describe how and under what conditions ethyne
7.6 (acetylene) reacts with (a) hydrogen, (b) chlorine, (c) hydrogen cyanide and (d)
Questions a solution of copper(I) chloride (cuprous chloride) in ammonia.
Explain briefly how you would demonstrate the presence of carbon and
hydrogen in ethyne. Describe an experiment you would carry out to determine
its formula. (AEB)

2 Discuss the meaning of the term unsaturation, illustrating your answer with
reference to ethene and ethyne.
Outline the reactions by which ethyne may be converted to (a) ethanamide,
(b) chloroethene, CH2=CHC1, (c) ethane-1,2-diol, (d) benzene. (CfN))

3 Describe in outline one commercial preparation of ethyne.

State the reactions of ethyne with (a) chlorine and (b) aqueous mercury(II)
sulphate (mercuric sulphate).
One mole of a hydrocarbon S (CgHio) gives 2 moles of ethanal with trioxygen.
What are the possible structural formulae for 5? (O and C)

4 (a) When 20 cm^ of a gaseous hydrocarbon A was exploded with 150 cm^ of
oxygen, the residual gases occupied 110 cm^. After shaking these gases with
aqueous sodium hydroxide, the final volume was 30 cm^ (all volumes at the
same temperature and pressure). Calculate the molecular formula of A.
(b) Another hydrocarbon B, of molecular formula C4H6, formed a red pre¬
cipitate with ammoniacal copper(I) chloride (cuprous chloride) and reacted with
water in the presence of sulphuric acid and mercury(II) sulphate (mercuric
sulphate) to give a compound C. C was unaffected by potassium permanganate
but with iodine and warm aqueous sodium hydroxide gave a yellow crystalline
precipitate with a characteristic odour. Deduce the structural formulae of B and
C and explain the above reactions. (C(T))

98
Chapter 8
Aromatic compounds

8.1 The term ‘aromatic’ was first used to describe a group of compounds which
Introduction have a pleasant smell (aroma). These compounds include the cyclic com¬
pound, benzene, and its derivatives. The name aromatic has been retained
since it is useful to classify these compounds separately; this is because their
properties are so different from those of the aliphatic and aiicyclic com¬
pounds.

8.2 It has been known for over 100 years that benzene is a cyclic compound,
Structure of benzene with a six-membered ring of six carbon atoms and with one hydrogen atom
attached to each carbon atom. Bearing in mind that carbon and hydrogen
form four bonds and one bond respectively, it was natural to represent its
structure as

H
I
H H

II I
/C^C^C
H H
I
H

in which single and double bonds alternate round the ring. However, the
following evidence shows that this representation does not- adequately
describe the structure of benzene.
1. If the structure was correct, we should expect to be able to isolate two
isomers of any disubstituted benzene in which the substituents were ad¬
jacent, for example:

Cl Cl

However, in no case has more than one isomer of any such compound been
obtained.

2. As we shall see (8.3), benzene does not undergo the addition reactions
which are characteristic of ethene (6.3) and other compounds which con¬
tain C=C bonds.

3. Whereas the length of a carbon-carbon single bond is 154 pm and

that of a carbon-carbon double bond is 134 pm, the six carbon-carbon
bonds in benzene are of equal length (139 pm), somewhat longer than
double bonds but shorter than single bonds.

4. Benzene is a more stable compound than would be expected by com¬

parison of its structure with that of an alkene, as shown by thermochemical

99
AROMATIC COMPOUNDS measurements. When benzene is reduced to cyclohexane:

the heat evolved is 207 kJ mol ^ When cyclohexene is reduced to cyclo¬

hexane :

the heat evolved is 119kJmoP^ Now, the heat change in the latter
reaction is associated with the conversion of a double bond into a single
bond and the breaking of the H—H bond, each of which requires energy,
and the formation of two C—H bonds, which releases energy. If benzene
contained three alkene-like double bonds, we should expect that the heat
evolved during its reduction would be three times that for cyclohexene, i.e.
357 kJ mol"\ since the bond changes associated with the reduction of one
molecule of cyclohexene take place three times over on the reduction of one
molecule of benzene. The experimental value is (357 — 207) = 150 kJ mol” ^
less than this, from which we infer that benzene is more stable by this
amount of bonding energy than would be expected if it possessed alkene-like
double bonds.

The differences described above between benzene and the alkenes can all
be understood by consideration of the nature of the bonding in benzene.
Each carbon atom is j/^^-hybridised, so that it forms three coplanar bonds,
two with other carbon atoms and one with a hydrogen atom. These bonds
are at an angle of 120° to each other, just as in an alkene, and this means
that the six carbon atoms are in a plane and that there is no strain in the
ring (compare cyclohexane, p. 77, where a planar structure would be
strained because the normal bond angle for j'/7^-hybridised carbon is
109°28'). The remaining electron is in a/? orbital perpendicular to the plane
of this ring, just as in ethene each of the carbon atoms has a p orbital
perpendicular to the plane containing the atoms. Now, in benzene, each of
these p orbitals can overlap with both the neighbouring p orbitals. Fig. 4.15
(p. 60). Since bond energies are determined by the extent of orbital-overlap
(4.3), there is a greater degree of bonding in benzene than would be expected
by comparison with ethene. This underlies the thermochemical stability
of benzene and also its resistance to addition reactions, since a reaction
such as:

would result in the loss of the extensive /j-orbital interactions. It also enables
us to understand why the bond lengths in the ring are equal, because, as
Fig. 4.15 shows, there is no difference in the type of bonding between one
pair of adjacent carbon atoms and any other pair.

100
AROMATIC COMPOUNDS
The bonds associated with the p-orbital interactions in benzene are
termed delocalised n-bonds, in contrast to the localised 7i-bond in an alkene
which is confined to only two atoms. The extra thermochemical stability of
benzene as compared with what would be expected if it possessed alkene-
like double bonds is termed its delocalisation energy or stabilisation energy.
As described in Chapter 4, there is an alternative way of representing the
delocalised bonds in benzene, that is, by describing benzene as a resonance
hybrid of two structures:

Although delocalisation in this book is often described in terms of molecu¬

lar orbitals, it is sometimes easier to discuss the properties of aromatic com¬
pounds in terms of resonance hybrids. In equations, benzene is represented
as:

8.3 Manufacture
Benzene 1. Based on petroleum
(a) If the gasoline and naphtha fractions from the distillation of petroleum
(19.4) are passed over a catalyst (either platinum or molybdenum(VI) oxide,
suspended on alumina) in presence of an excess of hydrogen, the straight-
chain alkanes undergo cyclisation and dehydrogenation. For example,
hexane is converted into benzene.
CH,
HaC CH. -SH,
I -^
H^C /CH^ HaC^ /CH2
CH CH,

heptane into methylbenzene and octane into a mixture of ethylbenzene and

dimethylbenzenes. The process is known as reforming. The separation of
the mixture into pure aromatic hydrocarbons is described in Section 20.5.
(b) Benzene is one of the products when the light gas-oil fraction under¬
goes catalytic cracking (19.6).
(c) It is also formed from other aromatic hydrocarbons by hydrode¬
alkylation (20.5), for example:

CH.

Pt on AI2O3 as cat.
+ H, 600 °C
+ CH4

2. Based on coal
Benzene is obtained by the fractional distillation of coal tar, which is itself
obtained by the destructive distillation of coal. This only becomes an
important source of aromatic hydrocarbons when large quantities of coal
are converted into coke, for steel-making, or coal-gas (where the country
needs gas but does not have a ready source of natural gas).

101
Plate 8.1. A reforming unit. Physical properties
Naphtha is heated in a furnace
Benzene is a colourless liquid with a characteristic odour. It is insoluble in
(A), mixed with hydrogen,
compressed to a high pressure (B)
water but soluble in all organic solvents, and it is itself a very good solvent
and passed over a heated for organic compounds. It freezes at 5°C and boils at 80°C. Both the liquid
platinum catalyst. The reactors and the vapour are highly poisonous, so that benzene must be used with
are hidden in the photograph (C). care.
Low boiling hydrocarbons are
removed by fractional
Chemical properties
distillation, for example ethane,
propane, butane [D). The residue, (a) Substitution reactions
which is fractionated (E), contains Benzene takes part in a variety of substitution reactions with electrophilic
principally the aromatic reagents.
hydrocarbons, benzene,
methylbenzene and 1. When treated with a mixture of concentrated nitric acid and con¬
dimethylbenzenes {Esso centrated sulphuric acid at room temperature, nitrobenzene is formed:
Petroleum Co. Ltd.)

Nitrobenzene

102
AROMATIC COMPOUNDS
This reaction occurs in several stages. First, sulphuric acid is so strong an
acid that it transfers a proton to nitric acid:

H2SO4 + H-O-NO2 HSOi" + /O-NO2

H

The protonated nitric acid then undergoes a spontaneous heterolysis to

form the hitronium ion, N02^;

/O-NO2 -> H2O + 0=N=0

The nitronium ion is the electrophilic reagent with which benzene reacts.
The first step is an addition:

The positive charge in this adduct is actually delocalised over three of the
carbon atoms; the ion can be described as a resonance hybrid of three
structures:

H NO 2
+

In the final step of the reaction, a proton is removed from the adduct by
the hydrogensulphate anion:

+ Hsor —^

There is strong evidence for the existence of the nitronium ion. For
example, compounds such as nitronium perchlorate, NO2CIO4, have been
prepared and shown, by X-ray analysis, to contain the ion N02'^. In
addition, the depression of the freezing point of sulphuric acid by dissolved
nitric acid is four times greater than expected; evidently each molecule of
nitric acid provides four particles in the solution, consistent with the
ionisation:

HNO3 + 2H2SO4 ^ NO2+ + H3O+ + 2HS04‘

The mechanism described above is typical of the electrophilic substitution

reactions of benzene, and the others will not be described in such detail.

103
AROMATIC COMPOUNDS 2. If chlorine is passed through benzene at room temperature and in the
presence of a catalyst, substitution takes place:

CsHs + CI2 —^ CgHs Cl + HCl

Chlorobenzene

Suitable catalysts are iron filings and aluminium chloride; they are re¬
ferred to as halogen carriers.
The function of the catalyst is to withdraw the electrons from the bond
between the chlorine atoms, a process represented for aluminium chloride
as:

Cl-Cl AICI3
where the curved arrow represents the tendency for two electrons to move
into the vacant 3p orbital of the aluminium atom. As this happens, the
benzene ring provides two electrons to make good the deficiency of electrons
on one of the chlorine atoms, so that the whole process can be represented
as:

The carbonium ion then reacts with the AICI4 ion:

+ AICI4- + AICI3

so that the catalyst is regenerated.

When iron is used as the halogen carrier, it first reacts with chlorine to
give iron(III) chloride which then catalyses the reaction in the same way as
aluminium chloride.
In the presence of iron filings or aluminium bromide, benzene reacts
with bromine to give bromobenzene:

C^Hg + Br2 ^ C^Hs—Br + HBr

Bromobenzene

The mechanism of the reaction is the same as that in chlorination.

The iodination of benzene is usually brought about by refluxing benzene,

iodine and concentrated nitric acid:

QHe + l2-^QH5l + HI

It was once thought that the function of the nitric acid was to oxidise the
hydrogen iodide as it was formed and thereby prevent the reverse reaction.
However, this cannot be the correct explanation, for hydrogen iodide does not
react with iodobenzene to give benzene and iodine. It is now thought that the
acid serves to provide the active iodinating species, possibly

1—

OH
This is one example of the way in which our interpretations of the mechanisms
of organic reactions change in the light of experimental evidence.

104
AROMATIC COMPOUNDS
3. Benzene reacts with an alkyl halide in the presence of an aluminium
halide to give an alkylbenzene, for example:

CgHe + CH3CH2—Br CeHs—C2H5 + HBr

Ethylbenzene

The reaction, which is described as alkylation, occurs in a similar way to

chlorination.

The catalyst withdraws the pair of electrons from the C—Br bond, the benzene
ring provides a pair of electrons to form a bond to the alkyl group:

Methylbenzene is formed from benzene and iodomethane (the iodide

being used as it is the only methyl halide which is a liquid at room tem¬
perature) :
„ AlCUascat.
QHe + CH3I -> CeHj—CH3 + HI
Methylbenzene

A similar reaction takes place with acid halides to give ketones, for
example:

AICI3 as cat.
CeHg + CH3—CO—Cl -^-> CeHj—CO—CH3 + HCl
Phenylethanone
The process is known as acylation.
The reactions of aromatic compounds with alkyl halides and acid halides
are known as Friedel-Crafts reactions, after the names of their two dis¬
coverers.

4. Benzene reacts with an alkene in the presence of an acid to give an

alkylbenzene, for example:

CgHg + CH3—CH=CH2 ^ CgHj—CH(CH3)2

(1-Methylethyl)benzene

Reaction occurs by addition of a proton to the alkene to give a carbonium ion

which then reacts with the aromatic compound:

CH3—CH=CH2 + H+ ^ CH3—CH—CH3

CfiHg + CH3—CH—CH3 -> C6H5—CH(CH3)2 +

5. If a mixture of benzene and concentrated sulphuric acid is heated for

about 8 hours, benzenesulphonic acid is formed:

CgHe + H2SO4 ^ CeHs—SO2OH + H2O

Benzenesulphonic
acid

105
AROMATIC COMPOUNDS The reaction is another example of an electrophilic substitution. The reagent is
sulphur trioxide, which is present in a solution of concentrated sulphuric acid
and accepts a pair of electrons from benzene:

Note that in this organic sulphur compound, carbon is linked to

sulphur, whereas in ethyl hydrogensulphate (p. 83) it is linked to oxygen
(CH3—CH2—O—SO2—OH).

(b) Other reactions

1. If a mixture of hydrogen and benzene vapour is passed over nickel at
150°C, cyclohexane is formed:

2. In the presence of ultraviolet light, benzene reacts with chlorine by

addition:

The resulting product, hexachlorocyclohexane, is a mixture of geometrical

isomers, one of which is an important insecticide, sold as Gammexane.

Uses
The uses of benzene are discussed in Section 20.5.

8.4 Structural formula

Methylbenzene

Manufacture
Methylbenzene (toluene) is obtained both from coal and from petroleum in
the same way as benzene.

Physical properties
Like benzene, methylbenzene is a colourless liquid which is insoluble in
water but soluble in organic solvents. It melts at — 95°C and boils at 111°C.
Note that the melting point is lower than that of benzene although methyl-

106
AROMATIC COMPOUNDS
benzene has the higher formula weight. This is because the planar molecules
of benzene can pack closely together in the crystal and the cohesive forces are
strong, whereas the methyl group in methylbenzene prevents such close
packing.

Chemical properties
Methylbenzene undergoes three types of reactions: (a) electrophilic
substitution in the ring, (b) addition to the ring and (c) substitution in the
methyl group.

(a) Electrophilic substitution reactions

Methylbenzene reacts with all the electrophilic reagents which attack ben¬
zene, and in each reaction it is more reactive than benzene. There are three
possible isomeric products in each case; for example, for nitration ;
CH, CHg CH3

u Ql 0 NO2
Methyl-2-nitrobenzene Methyl-3-nitrobenzene Methyl-4-nitrobenzene

The prefixes ortho, meta and para, usually abbreviated to o, m and p, are
often used instead of the numbers 2, 3 and 4, respectively, for describing the
relative positions of the substituents in a disubstituted benzene; for example,
methyl-2-nitrobenzene can be called o-methylnitrobenzene.
Of these three products, the principal ones from methylbenzene are
always the 2- and 4-isomers; for example, in nitration, with a mixture of
concentrated nitric and sulphuric acids, the relative amounts of the three
products, expressed as percentages, are 2-, 59; 3-, 4; 4-, 37. The methyl
group in methylbenzene is described as 2-,4-directing {ortho, /jam-directing).
The reasons for both the greater reactivity of methylbenzene than benzene
and the predominance of 2- and 4-substitution in methylbenzene can be
understood by considering the first step in the reaction. Thus, in nitration:

In each case a carbonium ion is formed which is a resonance hybrid of three

structures; the positive charge is shared by three of the ring-carbon atoms.

107
AROMATIC COMPOUNDS
Since the methyl group is electron-releasing (-t- /) and serves to stabilise a
positive charge (4.9), each of the three adducts is more stable than that
from benzene and is formed faster; thus, methylbenzene is more reactive
than benzene. Of the three adducts, those formed by reaction at the 2- and
4-positions have positive charge adjacent to the methyl group, so that
they are more stable than the adduct formed by reaction at the 3-position
in which a carbon atom is interposed between the positive charge and the
methyl group; consequently, reaction occurs faster at the 2- and 4-positions
than at the 3-position, that is, methylbenzene is 2-,4-directing.
Other examples of electrophilic substitutions in methylbenzene are:

Chloro-2-methylbenzene
+ CI2

Cl
Chloro-4-methylbenzene

CH.
^SOaOH

Qj + H=0

2-Methylbenzenesulphonic acid
+ H,
CH3

Q + H,0

SO2OH
4-Methylbenzenesulphonic acid

(b) Addition reactions

Methylbenzene resembles benzene in reacting with hydrogen on nickel at
150°C:

Methylcyclohexane

However, when treated with chlorine in the presence of ultraviolet light,

it preferentially reacts in the methyl group (p. 109).

108
AROMATIC COMPOUNDS
(c) Substitution in the methyl group
1. When chlorine is passed through boiling methylbenzene which is
exposed to ultraviolet light, substitution of chlorine for hydrogen occurs:

(Chloroniethyl)benzene

CHoCI

Cl,

(Dichloromethyl)benzene

CHCl,

CI2 + HCl

(Trichloromethyl)benzene

These reactions occur via free radicals, as in the chlorination of methane

(p. 73). They are in contrast to the ionic reactions which occur when
methylbenzene is treated with chlorine at room temperature in the presence
of a halogen carrier, which give mainly 2- and 4-chloro-derivatives in the
same way as benzene gives chlorobenzene (p. 104).

2. The methyl group in methylbenzene can be oxidised to the aldehyde

group, —CH=0, or to the carboxylic acid group, —CO2H, depending on
the oxidising agent:
CrOjCh
CgHs—CH3 -^ QH5—CHO
Benzaldehyde
KMn04
C6H5-CH3 -> CfiHs—CO2H
alkaline
Benzoic acid

Uses
The uses of methylbenzene are discussed in Section 20.5.

8.5 As we have seen, the methyl group in methylbenzene is 2-,4-directing in

electrophilic substitutions because it is an electron-releasing group and
Substitution in other preferentially stabilises the adducts formed by the reaction at the 2- and
aromatic compounds 4-positions. Other alkyl groups have the same effect; for example, chlorina¬
tion of ethylbenzene in the presence of a halogen carrier gives mainly
chloro-2- and chloro-4-ethylbenzene:

109
AROMATIC COMPOUNDS

Chloro-4-ethylbenzene

When an electron-attracting group such as —NO2 or —CN (p. 65) is

attached to the benzene ring, it reduces the stability of the intermediate
cation in an electrophilic substitution reaction. Consequently, compounds
such as nitrobenzene (C6H5NO2) and benzonitrile (CeHjCN) react less
rapidly than benzene. This destabilising effect is greatest for reaction at the
2- or 4-position, for the electron-attracting substituent is then adjacent
to one of the carbon atoms which shares the positive charge, for example.

whereas for reaction at the 3-position an extra carbon atom is interposed

between the substituent and the charge, for example:

Hence these electron-attracting substituents are 3-directing.

An illustration of the inhibiting effect which the nitro group has on
electrophilic substitution, as well as its 3-directing capacity, is provided
by the nitration of nitrobenzene: when benzene is treated with a mixture of
concentrated nitric and concentrated sulphuric acids at room temperature,
nitrobenzene is formed, but the temperature has to be raised to about 50°C
for further reaction to occur to give 1,3-dinitrobenzene.
In contrast, those electron-attracting groups bonded to benzene which
contain an atom with an unshared pair of electrons directly attached to the
aromatic ring are 2-,4-directing in electrophilic substitution; examples are
—Cl, —OH and —NH2.
The reason is again apparent when the intermediate adducts in the
substitution are considered. For example, when chlorobenzene is nitrated,
the possible adducts are:

110
AROMATIC COMPOUNDS

In the cases of the 2- and 4-adducts, the positive charges are delocalised not
only on to three carbon atoms but also on to the chlorine atom; that is, a p
orbital on chlorine can overlap with the adjacent carbon p orbital, the result
being that the pair of electrons in the chlorine p orbital is partly donated to
the carbon p orbital, so reducing the deficiency of electrons in the latter.
This can be represented as follows:

This corresponds to the sharing of the positive charge by the chlorine atom
in addition to the carbon atoms, and could be represented alternatively by
the structures:

This extra delocalisation of the charge, which cannot occur when the
reagent adds to the 3-position, makes the adducts formed by reaction at the
2- and 4-positions more stable than that formed by reaction at the 3-position;
hence the chlorine substituent is 2-,4-directing.
The hydroxyl (—OH) and amino (—NH2) substituents, each of which
contains unshared pairs of electrons, act in the same way as the chlorine
substituent; their ability to stabilise the 2- and 4-adducts formed during
electrophilic substitution can be represented as follows:

Ill
AROMATIC COMPOUNDS

Oxygen and nitrogen release p electrons in this way more readily than does
chlorine, and as a result phenol (QHs—OH) (p. 159) and phenylamine
(C^Hj—NH ) (p. 256) are much more reactive than chlorobenzene or
2

benzene. For instance, they are so reactive towards chlorine and bromine
that it is impossible to stop the reaction before all the activated positions
have been substituted, even in the absence of a halogen carrier, for example:

OH OH

> 3HBr

Br
2,4,6-Tribromophenol

Summary of directing power of substituents

2-,4-directing 3-directing
(ortho,para-directing) (meta-directing)

Substituent is electron-
Substituent is
attracting (-/effect) but Substituent is electron-
electron-releasing possesses an unshared
(+/ effect) attracting (-/ effect)
pair of electrons

—Alkyl, as in —Halogen (—F, —Cl, —NO2, as in nitrobenzene

methylbenzene —Br, —I), as in CfiHs—NO2
QHs—CH3 chlorobenzene.
CfiHs—Cl —CHO, as in
benzaldehyde.
—OH, as in phenol, CfiHs—CHO
CsHs—OH
—COR, as in
—NH2, as in phenylamine, phenylethanone
CeHs—NH2 CeHs—COCH3

—CO2H, as in benzoic
acid, CsHs—CO2H

—SO2OH, as in
benzenesulphonic acid,
CfiHs—SO2OH

112
8.6 Reactions of methylbenzene
Practical work Compare your results with those obtained with an alkane {p. 77) and an
alkene {p. 89).

1. To 5 drops of methylbenzene in a test-tube, add 1 cm^ of a solution of

bromine in tetrachloromethane and shake.

2. Place 5 drops of methylbenzene in each of two test-tubes, and to one of

them add a few iron filings. Add 3 drops of bromine to both test-tubes and
note whether there is evolution of gas from either. It may take a few minutes
to see whether a gas is evolved. Test the gas with moist blue litmus paper.

3. To 10 drops of concentrated nitric acid in a test-tube, carefully add

10 drops of concentrated sulphuric acid, shaking the mixture and cooling
the test-tube under a stream of cold water. Add the mixture of acids to 5
drops of methylbenzene in another test-tube, and shake this mixture under a
stream of cold water; then pour into a beaker containing about 10 cm^ of
cold water. Observe whether a new liquid is formed and note its smell.

4. To 4 drops of methylbenzene, add 10 drops of concentrated sulphuric

acid in a test-tube. Warm until the methylbenzene has dissolved into the acid
layer. Pour the mixture into 10 cm^ of a cold saturated solution of sodium
chloride. White crystals of a mixture of sodium methylbenzenesulphonates
are formed.

5. Make up about 25 cm^ of an alkaline solution of potassium manga-

nate(VII) (add OT g of sodium carbonate to a 10 per cent solution of potas¬
sium manganate(VII)).
To 10 drops of methylbenzene, add the alkaline solution of potassium
manganate(VII) in a flask. Heat the mixture under reflux (Fig. 2.1) until
the purple colour disappears.
Cool the mixture and add dilute sulphuric acid until the mixture is acid
to litmus. Add solid sodium metabisulphite until the brown solid (man-
ganese(IV) oxide) has dissolved. Filter the white crystals of benzoic acid and
recrystallise them from hot water. Find the m.p. of benzoic acid.

1 Describe two reactions which show that ethene (an alkene) and benzene are
8.7 different and two reactions which show them to be similar.
Questions Show by diagrams how you consider benzene to be structurally related to
ethene and then briefly explain why it differs in its behaviour. (JMB)

2 How, and under what conditions, does (i) chlorine, (ii) nitric acid, react with
(a) benzene, (b) methylbenzene?
Outline how benzene may be converted into methylbenzene and vice versa.
(C(N))
3 Distinguish between an aliphatic and an aromatic compound.
Give one reaction to illustrate the saturated nature and one reaction to illustrate
the unsaturated nature of the benzene molecule.
Explain how you would convert benzene into (a) benzene carboxylic acid
(benzoic acid), (b) methylbenzene (toluene), (c) hydroxybenzene (phenol), and
also how each of these products may be reconverted into benzene. (AEB)

4 The average bond energies associated with the C—H bond, the C—C bond and
the C=C bond are respectively 98-7, 82-6 and 146-0 kcal mole"h Use these values

113
AROMATIC COMPOUNDS to calculate the theoretical enthalpy of formation of the molecule;

H H

/C,
H H

In practice, the enthalpy of formation of benzene from atoms is 1317 kcal

mole“*. How do you account for the difference in these quantities? Discuss the
action of bromine on benzene in the light of your answer.
The normal length of a C—C bond is 1-54 A, and for a C=C bond is T33 A.
Suggest values, which might be observed experimentally, for the lengths of the
various bonds in
(a) benzene,
(b) graphite,
(c) diamond, and
(d) buta-l,3-diene(CH2=CH—CH=CH ). 2

Indicate briefly the reasons for your answers. (L(X))

5 There are three typical ways in which methylbenzene might react with chlorine.
State what these are and indicate the conditions necessary for two of them to
take place.
How can methylbenzene or its chlorination products be converted into
benzaldehyde?

6 (a) Write equations to show how, given supplies of benzene and methylbenzene,
you would prepare the following compounds. (Full practical details are not
required, but reagents and conditions should be indicated.)

(i) QH5I, (ii) QHsNHCOCHj, (iii) QH5CHO.

(b) A compound is believed to have structure X.

OCHa - CO-OQH 5

NHa X

By what chemical reactions would you show the presence of:

(i) the benzene ring;
(ii) the amino group;
(iii) the ester group ? (C(N, S))

7 How is methylbenzene obtained industrially and how may it be prepared from

benzene? By what reactions may (a) benzaldehyde, (b) benzoic acid, (c) a
chloromethylbenzene, and (d) benzene be obtained from methylbenzene?

8 Both ethene and benzene are unsaturated compounds. They both react with chlo¬
rine under different conditions and by different mechanisms.
(a) State what is meant by unsaturated compounds.
(b) Give the structures of ethene and benzene and discuss the similarities and
differences between them.
(c) State how ethene and benzene react with chlorine, giving the conditions and
the mechanisms of the reactions in each case. (AEBi

114
AROMATIC COMPOUNDS 9 Give the name and formula of one example of an alkane, an alkyne and an aromatic
hydrocarbon. Compare and contrast the reactions of the named compounds
with (a) bromine, (b) potassium manganate(VII) (potassium permanganate), and
(c) sulphuric acid.
Give a mechanism for (i) the reaction of the alkane with bromine, (ii) the reaction
of the alkene with sulphuric acid, (iii) the reaction of the aromatic hydrocarbon
with a nitrating mixture. (L)

115
Chapter 9 Halogen compounds

The four halogens (fluorine, chlorine, bromine and iodine) are contained
9.1
in several types of organic compound:
Introduction Alkyl halides, in which the halogen atom is attached to a saturated carbon
atom (e.g. bromoethane, CH3—CH2—Br). These can be subdivided into
three classes, according to how many alkyl groups are attached to the carbon
atom which is bonded to the halogen:

H R R
j 1 1
R-C-X R-C-X R-C->
1
1 1
1
H H R
Primary Secondary Tertiary

These descriptions correspond to those of the related alcohols (10.2).

Unsaturated halides, in which the halogen atom is attached to a carbon
atom which forms a double or triple bond (e.g. chloroethene,
CH2=CH—Cl).
Aryl halides, in which the halogen atom is attached to an aromatic ring
(e.g. chlorobenzene, CgHj—Cl).
There are also polyhalides, which contain more than one halogen atom
(e.g. 1,2-dichloroethane, CH2CI—CH2CI; trichloromethane, CHCI3).

9.2 Alkyl halides are named as derivatives of the corresponding alkane, a

number being inserted to indicate the position of the halogen atom in the
Nomenclature carbon chain where there would otherwise be ambiguity. The simpler ones
can also be named by combining the names of the appropriate alkyl group
and the halide; for example, CH3CI is chloromethane or methyl chloride.
Unsaturated halides are generally named as derivatives of the correspond¬
ing alkene or alkyne. Examples are in Table 9.1. Aryl halides are described
in Section 9.6.

This section considers chlorides, bromides and iodides. Fluorides are dis¬
cussed separately (9.7).

9.3 Physical properties

Alkyl halides Chloromethane, chloroethane and bromomethane are colourless gases at
room temperature. The other lower members are colourless liquids with a
sweet smell. Iodides have a higher boiling point than bromides, which, in
turn, boil at higher temperatures than the chlorides (Table 9.1).
The alkyl halides are insoluble in water, but are soluble in organic
solvents.
Alkyl chlorides are less dense than water, but the bromides and iodides
are denser.

116
HALOGEN COMPOUNDS
Table 9.1. Some aliphatic halides

NAME FORMULA B.P./°C

Chloromethane CH3CI -24

Bromomethane CHsBr 4
lodomethane CH3I 42
Chloroethane CH3CH2CI 12
Bromoethane CH3CH2Br 38
lodoethane CH3CH2I 72
1-Chloropropane CH3CH2CH2CI 47
Cl

2-Chloropropane CH3CHCH3 36
1-Chlorobutane CH3CH2CH2CH2CI 78
Cl

2-Chlorobutane CH3CH2CHCH3 68
2-ChIoro-2-methylpropane (CH3)3CC1 51
l-Bromobutane CH3CH2CH2CH2Br 102
Dichloromethane CH2CI2 40
1,1 -Dichloroethane CH3CHCI2 57
1,2-Dichloroethane CICH2CH2CI 84
1,2-Dibromoethane BrCH2CH2Br 131
T richloromethane CHCI3 61
Tri-iodomethane CHI3 m.p. 119
Tetrachloromethane CCI4 77
Chloroethene CH2=CHC1 -14

Laboratory preparations
1. From alcohols
(a) Alkyl chlorides
(i) By treating the alcohol with sulphur dichloride oxide (a liquid, b.p.
77°C). Sulphur dioxide and hydrogen chloride are evolved:

R—OH + SOCI2 -> R—Cl + SO2 + HCl

An organic base (for example, pyridine) is often added to neutralise the

hydrogen chloride.
(ii) By treating the alcohol with hydrogen chloride:

R—OH + HCI R—Cl + H2O

To obtain good yields from primary and secondary alcohols, anhydrous

conditions are necessary and even then primary alcohols react slowly unless
anhydrous zinc chloride is used as a catalyst with the hydrogen chloride.
However, tertiary alcohols react readily even in the presence of water and
concentrated hydrochloric acid can be used.

(b) Alkyl bromides

(i) By treating the alcohol with a mixture of red phosphorus and bromine:

2P + 3Br2 2PBr3

3R—OH + PBrj 3R—Br + H3PO3

117
HALOGEN COMPOUNDS (ii) By treating the alcohol with hydrogen bromide. It is convenient to
generate the hydrogen bromide in situ by the reaction of potassium bro¬
mide with concentrated sulphuric acid:

KBr + H2SO4 KHSO4 + HBr

R—OH + HBr ^ R—Br + H2O

(c) Alkyl iodides

(i) By treating the alcohol with a mixture of red phosphorus and iodine:

2P + 3I2 2PI3
3R—OH + PI3 ^ 3R—I + H3PO3

(ii) By treating the alcohol with a concentrated solution of hydriodic

acid:

R—OH + HI ^ R—I + H2O

The hydrogen iodide can be prepared in situ from potassium iodide and
phosphoric acid:

3KI + H3PO4 ^ 3HI + K3PO4

(Sulphuric acid is not used because it oxidises hydrogen iodide.)

2. From alkenes
Hydrogen halides react with alkenes to form alkyl halides. The orientation
in the addition reaction is described by Markownikoff’s rule (6.4), for
example:

CH3—CH=CH2 + HCl ^ CH3—CHCl—CH3

2-chloropropane

Manufacture of alkyl chlorides

1. By addition of hydrogen chloride to an alkene (6.3). For example,
over 90% of chloroethane is made by this route, from ethene and hydrogen
chloride.
2. By direct chlorination of alkanes (5.5).

Chemical properties
(a) Substitution reactions
The most important reactions of alkyl halides are those in which the
halogen atom, X, is replaced by another group. They can be represented
by the general equation:

R—X + Y—Z ^ R—Z + Y—X

An example is the hydrolysis of an alkyl halide with sodium hydroxide:

R—X + NaOH -> R_OH + NaX

118
HALOGEN COMPOUNDS
The mechanisms of these reactions have been studied in great detail and
are well understood. There are two general mechanisms. The first applies
to primary halides, RCH2X. The reagent (e.g. hydroxide ion) approaches
the carbon atom of the C—X bond in the halide from the side opposite to
FIG. 9.1. The mechanism of an the halogen atom. As it does so, it begins to form a bond to the carbon
Sn2 reaction of an alkyl halide, atom, while the bond between the carbon atom and the halogen begins to
RCH2X. (a) The approach of a break (Fig. 9.1). The pair of electrons which forms the new bond is supplied
hydroxide ion; {b) The transition
state, showing that the alkyl group,
RCH2, is planar; (c) The reaction
product, RCH2OH, is formed and
the four bonds are tetrahedrally
arranged around the carbon atom

by the reagent, and the pair of electrons in the C—X bond is gradually
acquired completely by the halogen atom. A convenient representation of
these processes is:

I py I
HO: CHo-X -> HO-CH2 + X
• • “

where each curved arrow represents the movement of a pair of electrons.

Eventually, the new bond is fully formed and the C—X bond is completely
broken.
The energy change during the reaction is shown in Fig. 9.2. At first, more
energy is needed to break the C—X bond than is supplied by the formation
of the new C—O bond, and the energy of the system increases. A peak is
reached, corresponding approximately to the situation in which the C—X
bond is ‘half-broken’ and the C—O bond is ‘half-formed’; the system is
then described as being at its transition state. The energy then decreases,
corresponding to the completion of the formation of the C—O bond. The
difference in energy between the transition state and the reactants is the

119
FIG. 9.2. The variation in energy
as the reactants RCHjX + 0H~
are converted into the products Transition state
RCH2OH+ X-

Extent of reaction

activation energy of the reaction. There is in effect a barrier to the reaction;

a collision between the reactants can only lead to the formation of products
when the reactant molecules possess between them enough excess of kinetic
energy to surmount the barrier. In many cases, only a small proportion
possesses sufficient energy, and reaction is slow. However, kinetic energy
rises with temperature, and so the rate is increased by heating.
Other reagents which possess at least one unshared pair of electrons can
replace the hydroxide ion in this reaction. Such reagents are described as
nucleophiles (‘nucleus-seeking’) or nucleophilic reagents. Since two species
—the reagent and the alkyl halide—are brought together in formation of
the transition state, the reaction is bimolecular. The overall reaction is
referred to as an 5^2 reaction (Substitution, nucleophilic, bimolecular).
The second mechanism applies to tertiary halides. These ionise spon¬
taneously in solution, forming a carbonium ion and a halide ion, for
example:

CH, CH3
I I ^
CH3 -c-ci + cr
I CH3 CH3
CH3

The rate of ionisation is fairly small, and the equilibrium lies well to the left-
hand side. However, the carbonium ion is very reactive and is attacked by
other nucleophiles which may be present. For example, if water is the
solvent, water itself acts as the nucleophile and an alcohol is formed:

CH, CH, CH,

C ^ /O-C-CH3 ->H0-C-CH3 + H+
CH3'^ ^CHs H I
CH3 CH,

120
HALOGEN COMPOUNDS
If the solvent is an alcohol, an ether is formed :

CH, CH, CH3

R R I
I
/O-C-CH3 »R0-C-CH3 +H+
H H I
CH3 CH3
These reactions can be represented as in Fig. 9.3. The slow step is the
breaking of the C —Cl bond to form the carbonium ion. This can then
either revert to the reactants by recombining with chloride ion, or give
products by reacting with water. Both these steps are fast, and although the
recombination with chloride ion has the lower activation energy, most of
the carbonium ions react with water because it is present in far higher
concentration than chloride ion. The overall reaction is described as uni-
molecular because only one molecule is involved in the transition state of
the slow step, and it is also described as an ^yyl reaction.

FIG.9.3. The variation in energy

during the hydrolysis of a tertiary
halogenoalkane by the Sn /
reaction

The difference between tertiary and primary halides arises because a

tertiary carbonium ion is a relatively more stable species than a primary
carbonium ion (6.4) and is formed much faster. The activation energy for
formation of a primary carbonium ion is so great that it is not formed at
a significant rate under most conditions; instead, a nucleophilic reagent is
needed to assist the breaking of the C—X bond.
Secondary alkyl halides (e.g. 2-chloropropane, (CH3)2CH—Cl) are inter¬
mediate in behaviour between primary and tertiary halides; they react partly
by the bimolecular Sj^l reaction and partly by the unimolecular S^^l
reaction.
The following are examples of these nucleophilic substitution reactions
with primary alkyl halides.

121
HALOGEN COMPOUNDS (i) Preparation of an alcohol. By treatment of the halide with an aqueous
solution of sodium hydroxide, for example:

C2H5—Br + NaOH C2H5—OH + NaBr

(ii) Preparation of an ether. By treatment of the halide with a solution of

a sodium alkoxide in the corresponding alcohol, for example:

C2H5—Br + C2H5—ONa+ -^C2H5—O—C2H5 + NaBr

Diethyl ether

The nucleophile is the alkoxide ion, C2H50”. However, with higher

homologues, an alternative reaction—elimination of the hydrogen
halide—is also important and is often the major reaction (p. 123).

(iii) Preparation of amines. By heating the halide with concentrated

ammonia in a sealed tube. A mixture of amines is formed, for example:

C2H5—I + NH3 ^ C2H5—NH2 + HI

Ethylamine
C2H5—I + C2H5—NH2 ^ (C2H5)2NH + HI
Diethylamine
C2H5-I + (C2H5)2NH ^(C2H5)3N + HI
Triethylamine
C2H5-I + (C2H3)3N ^ (C2H5)4NM-
Tetraethylammonium
iodide

Ammonia is a nucleophile:

H3N:

Likewise, ethylamine, diethylamine and triethylamine are nucleophilic

reagents.

In each of the first three equations, the first-formed product is therefore a

substituted ammonium salt. However, this is in equilibrium with the amine, e.g.
+
C2H5—NH3 r ^ C2H5—NH2 + HI

and it is the amine which takes part in subsequent steps. Each amine is thereby
removed from the equilibrium so that eventually, with a sufficient excess of the
halide, the quaternary salt (tetraethylammonium iodide) is formed.

(iv) Preparation of an ester. By treatment of the halide with the silver salt
of a carboxylic acid, for example:

C2H5—Br + CH3—CO2 Ag+ ->C2H5—O—CO—CH3 + AgBr

Silver ethanoate Ethyl ethanoate

The nucleophile is the ethanoate anion:

CH3—C—Q- CH3—C—O—CHj + Br-

O CH3 O CH3

122
HALOGEN COMPOUNDS
(v) Preparation of a nitrile. By refluxing a solution of the halide and
potassium cyanide in ethanol, for example:

C2H5—Br + KCN C2H5—CN + KBr

Propanenitrile
The nucleophile is the cyanide anion, N=C“.

If silver cyanide is used, an isocyano-compoundis formed:

C2H5—Br + AgCN -> C2H5—-N^C + AgBr

Isocyanoethane

(vi) Preparation of a nitroalkane. By refluxing a solution of the halide in

ethanol with silver nitrite. A mixture of a nitroalkane and an alkyl nitrite
is formed and can be separated by fractional distillation, for example:

C2H5-NO2 + Agl
Nitroethane
C2H5-I + AgN02

C2H5-0-N = 0 + Agl
Ethyl nitrite

The nucleophile is the nitrite ion, NO2 . It can react at either an oxygen atom
or the nitrogen atom:

0=N-0~^^ CH2-I 0=N-0-CH2 + 1“

/ \
CH3 CH,

O
N: CH2-I ■> ,N- ■CH, +
/ "O
CH3 CH 3

(vii) Preparation of alkylaromatic compounds. By reaction of the halide

with the aromatic compound in the presence of an aluminium halide
(Friedel-Crafts reaction, p. 105), for example:
AlBr, as cat.
QHe + C2H5—Br -^^ CgHs—C2H5 + HBr
Ethylbenzene

These reactions can be regarded either as electrophilic substitutions in the

aromatic compound or nucleophilic substitutions by the aromatic com¬
pound on the alkyl halide (p. 105).

(b) Elimination reactions

When 2-bromopropane is refluxed with a solution of sodium hydroxide in
ethanol, both an ether and propene are formed:

CH3-CH-CH3-I- NaBr
I
CH3-CH-CH3 +C2H5-0-Na+ OC2H3
Br CH3-CH = CH2+C2H50H + NaBr

The ether is formed by the 5^2 reaction and propene is formed by an

elimination reaction: the base, ethoxide ion, abstracts a proton from the

123
HALOGEN COMPOUNDS halide at the same time as the halide ion breaks away. The movements of
electron-pairs are represented as follows:

CoH.-O--^ C2H5-OH
H
I > CH2=CH—CH3
CH2-CH-CH3
GBr Br-

At the transition state, the C—H and C—Br bonds are partially broken
and the C=C and O—H bonds are partially formed:

C2H5-9

H
CH2-^CH-CH3
Br

Since two molecules are involved in the formation of the transition state,
the reaction is bimolecular; it is described as an El reaction {E, elimination;
2, bimolecular).
Competition between the Sj^l and El reactions occurs with other primary
and secondary halides. The relative importance of each type of reaction
depends on the solvent, the temperature and the structure of the halide; the
ratio of elimination to substitution increases as:

(i) the solvent is changed from water (where the reagent is hydroxide
ion) to an alcohol, ROH (where the reagent is the corresponding
alkoxide ion, RO“);

(ii) the temperature is increased;

(iii) the number of alkyl groups adjacent to the double bond in the
resulting alkene is increased (for example, the reaction of bromo-
ethane with a solution of sodium hydroxide in ethanol gives only
1 per cent of ethene, whereas 2-bromopropane, under the same
conditions, gives 80 per cent of propene).

Tertiary alkyl halides, and to some extent secondary alkyl halides,

undergo elimination by a different mechanism in which the first step is
heterolysis of the carbon-halogen bond, for example;

CH, CH,
1
CHg-C-Br
1 '"CH,
CH3

CH,
1 CH,
c+ -> /C=CH2
CHg'^

This is described as an E\ reaction {E, elimination; 1, unimolecular). The

intermediate carbonium ion is the same as is involved in substitution
reactions, so that the E\ elimination competes with the S^l substitution.
Under most conditions, the E\ reaction is the major process.

124
HALOGEN COMPOUNDS
(c) Reactions with metals
Alkyl halides react with sodium to give alkanes, for example;

2C2H5—I + 2Na ^ CH3CH2CH2CH3 + 2NaI

Butane

This is the Wurtz reaction.

With magnesium, they form Grignard reagents (alkylmagnesium halides),

for example:

C2H5—Br + Mg C2H5—MgBr
Ethylmagnesium
bromide

These reagents are of especial value in synthesis (9.8).

Uses of alkyl halides

Alkyl halides are of great value in organic synthesis because of the variety
of compounds which can be made from them by nucleophilic substitution
and via Grignard reagents (9.8).
Tetraethyllead, which is the principal ‘anti-knock’ additive in petrol
(19.4), is made by heating chloroethane with a lead-sodium alloy:

4C2H5—Cl + Pb + 4Na ^ (C2H5)4Pb + 4NaCl

9.4 Compounds in which two halogen atoms are attached to adjacent carbon
atoms are known as z;/c-dihalides (vicinal, adjacent). Compounds in which
Polyhalides two halogen atoms are attached to one carbon atom are known as gem-
dihalides (gemini, twins). For example:

H H H H
I I I I
H-C-C-Cl H-C-C-H
I I I I
H Cl Cl Cl
1,1 -Dichloroethane 1,2-Dichloroethane
(a gew-dihalide) (a iJ/c-dihalide)

There are also compounds in which three or four halogen atoms are
attached to one carbon atom [for example, trichloromethane (CHCI3),
tri-iodomethane (CHI3), tetrachloromethane (CCI4)].

vic-Dihalides are prepared by the addition of the halogen to an alkene,

for example:

CH2=CH2 + Br2 ^ CH2Br—CH2Br

The properties of y/c-dihalides are very similar to those of alkyl halides.

Thus, they undergo both substitution and elimination reactions, for
example:
1. They undergo hydrolysis when heated with aqueous sodium hydrox¬
ide:

125
HALOGEN COMPOUNDS CH2CI—CH2CI + 2NaOH CH2OH—CH2OH + 2NaCl
Ethane-1,2-diol

2. They react with a solution of potassium cyanide in ethanol to give

dinitriles:
CH2CI—CH2CI + 2KCN ^ CH2—CH2 + 2KC1

CN CN

3. They undergo elimination with a hot solution of sodium hydroxide in

ethanol:

CH2CI—CH2CI + C2H5—O- Na+ CH2=CHC1 + C2H5OH + NaCl

CH2=CHC1 + C2H5—O- Na+ ^ CH=CH + C2H5OH + NaCl

gem-Dihalides are prepared from aldehydes or ketones with phosphorus

pentahalides, for example;

CH3—CH=0 + PCI5 ^ CH3—CHCI2 + POCI3

Hydrolysis regenerates the aldehyde or ketone, for example:

CH3—CHCI2 + 2NaOH ^ CH3—CHO + H2O + 2NaCl

Hence, they can readily be distinguished from i;/c-dihalides, which give diols
(dihydric alcohols) on hydrolysis.

Trichloromethane (chloroform) can be made in the laboratory by heating

ethanol with bleaching powder. The bleaching powder provides chlorine,
and reaction occurs in three stages, which can be represented by the follow¬
ing equations;

Oxidation:
CH3—CH2OH + CI2 -» CH3—CHO + 2HC1
Chlorination:
CH3—CHO + 3CI2 ^ CCI3—CHO + 3HC1
Hydrolysis:
2CCI3—CHO + Ca(OH)2 ^ 2CHCI3 + (H—C02)2Ca
Calcium
methanoate
(Ca(OH)2 ^ 2HC1 CaCl2 + 2H2O)

Trichloromethane is a colourless liquid with a characteristic sickly smell. It

is almost insoluble in water but is soluble in most organic solvents. Its
chemical properties are as follows:

1. It is oxidised in the presence of light and air to carbonyl chloride:

CHCI3 ^ i02 COCI2 + HCl

Carbonyl
chloride

126
HALOGEN COMPOUNDS
Trichloromethane is stored in dark bottles to prevent the formation of
carbonyl chloride, as it is intensely poisonous.

2. It is hydrolysed with alkali :

CHCI3 + 4NaOH -> H—C02“ Na+ + 3NaCl + 2H2O

Sodium
methanoate

This reaction occurs by a different mechanism from those in the hydrolysis

of other aliphatic halides. Trichloromethane is weakly acidic, and in basic solution
ionises to a small extent to form a carbanion, CCI3 “ :

CHCI3 + OH- ^ CCI3 + H2O

The carbanion eliminates a chloride ion to form a carbene:

CCI3 :CCl2 + Cl-

Dichloro-
carbene

Dichlorocarbene has two electrons available for bonding and is therefore very
reactive; thus, it is attacked by water:

:CCl2 + H2O ^ HO—CHCI2

The resulting unstable species reacts further:

H-O-CHCl - Cl —> 0=CH-C1 + HCI

0 = CH-C1 + H2O -> HO-CH-CI

I
OH

H-O-CH-Cl -> H-CO2H + HCI

I
OH

The high reactivity of dichlorocarbene is also shown by its reaction with alkenes
to give cyclopropane derivatives:

R2C = CR2 + iCCIj -^ R2C-CR2

c
/ \
Cl Cl

3. It reacts with a primary amine, in an ethanolic solution of potassium

hydroxide, to form an isocyano-compound (carbylamine reaction):

CHCI3 + 3KOH + R—NH2 R—N=C + 3H2O + 3KC1

Isocyano-compounds have characteristic powerful and unpleasant smells,

and their formation has sometimes been used as a test for primary amines.

127
 HALOGEN COMPOUNDS This reaction also occurs via dichlorocarbene, which is formed in the basic
solution:

CHCI3 + OH- ^ CCI3 + H2O

CCI3 -H- :CCl2 + Cl-

r„NH2 + :CCl2 R—NH2—CCI2 ^ R—NH CHCI2

r_NH—CHCI2 + 20H- -> R—N=C + 2C1- + 2H2O

Tetrachloromethane (carbon tetrachloride) is manufactured by the chlori¬

nation of carbon disulphide. Aluminium chloride is used as a catalyst.
AICI3 as cat.
CS2 + 3CI2 -> CCI4 + S2CI2
Disulphur
dichloride

It is a colourless liquid which is insoluble in water but soluble in all

organic solvents. It is inert to most reagents; for example, it is not hydro¬
lysed by alkali. It is used as a fire-extinguisher (Pyrene) because it is non-
inflammable and its dense vapour prevents oxygen getting to the flame.

9.5 Chloroethene (vinyl ehloride), CH2=CHC1, is the most important of the

unsaturated halides.
Unsaturated halides It is manufactured from ethyne (7.2) and, to an increasing extent, from
ethene (6.3). Its principal use is for the manufacture of PVC (poly(chloro-
ethene)) (p. 330).
The reactions of chloroethene are strikingly different from those of alkyl
chlorides in that it does not react with nucleophilic reagents; for example,
it is not hydrolysed by sodium hydroxide.

FIG. 9.4. p-Orbital overlap in

chloroethene; the lower diagram
shows one of the delocalised v
orbitals

128
HALOGEN COMPOUNDS
The reason for this difference is that the C—Cl bond in chloroethene is
stronger than one in an alkyl chloride such as chloroethane and is therefore
less readily broken. This in turn is because, in chloroethene, a p orbital on
chlorine interacts with the p orbital on the adjacent carbon atom (Fig. 9.4),
providing additional bonding as compared with an alkyl chloride.

Trichloroethene, CHCl=CCl2, is manufactured by the chlorination of

1,2-dichloroethane (p. 85) in presence of a catalyst:

CH2CI—CH2CI + 2Cl2 3^ CHCl=CCl2 + 3HC1

It resembles chloroethene in being unreactive towards nucleophilic

reagents, and because of its chemical stability and its property of dissolving
oils and greases, it is used as a cleaning agent. Surfaces are cleaned by
suspending the object (for example, an aeroplane wing or large engine) in
the vapour of trichloroethene, which condenses on the surface and ‘strips
off’ the grease and dirt.

9.6 There are two types of halides which contain an aromatic ring, examples of
which are given in Table 9.2.
Aromatic halides
(a) Aryl halides have the halogen atom attached to the aromatic ring.
They are named as the halogen derivatives of the aromatic compound, for
example:

Chlorobenzene 1,3-Dichloro- 3,5-Dibromo-

benzene nitrobenzene

Aryl halides (aryl, from aromatic) have different properties from alkyl
halides.
(b) Some aromatic halides have the halogen atom in a side-chain, for
example:

(Chloromethyl)- (Dichloromethyl)-
benzene benzene

These behave like aliphatic halides, and their reactions are compared with
those of alkyl and aryl halides in the practical section (p. 137).

Preparation of aryl halides

1. Chlorobenzene and bromobenzene are prepared by the reaction of
the halogen with benzene at room temperature in the presence of a ‘halo¬
gen carrier’ (8.3); iron, or aluminium chloride or bromide, are usually
employed. The mechanism of the reaction is discussed on p. 104, and
details of the laboratory preparation of bromobenzene are given on p. 136.

129
HALOGEN COMPOUNDS Table 9.2. Some aromatic halides

NAME FORMULA B.P./°C

C6Hs—F 85
Fluorobenzene
CfiHs—Cl 132
Chlorobenzene
CsHs-Br 156
Bromobenzene
CfiHs-I 189
lodobenzene

Chloro-2-methylbenzene 159

Chloro-3-methylbenzene 162

ChIoro-4-methylbenzene 162

Cl
(Chloromethyl)benzene CsHs—CH^CI 179
(Dichloromethyl)benzene CsHs—CHCI2 206
(Tr ichloromethyl)benzene C^Hs—CCI3 221

2. All aryl halides can be prepared from the corresponding aromatic

amine via the diazonium salt (16.8).

Manufacture of chlorobenzene
Chlorobenzene is now manufactured by the chlorination of benzene in the
liquid phase using iron(III) chloride as the ‘halogen carrier’ (8.3).

Chemical properties of aryl halides

1. They are unreactive towards nucleophilic reagents under ordinary
laboratory conditions. Thus, they resemble unsaturated halides and differ
from alkyl halides. However, some reactions with nucleophiles can be
effected under very vigorous conditions, and one which is of industrial
importance is the hydrolysis *of chlorobenzene with sodium hydroxide
solution at 200°C under a pressure of 200 atmospheres:

CgHs—Cl + 2NaOH CeHj—O “Na^ + H2O + NaCl

Phenol is liberated on the addition of acid:

CgHs—0“Na+ + H+ -^CgHs—OH + Na +

2. Aryl halides undergo substitution in the aromatic ring with electro¬

philic reagents (8.5). The 2- and 4-derivatives are the major products, for

130
HALOGEN COMPOUNDS
example:

Cl

+ H2O
Cl
Chloro-2-nitrobenzene
cone. HNO3
cone. H2SO4
Cl

NO2
Chloro-4-
nitrobenzene

3. Bromobenzene and iodobenzene form Grignard reagents with mag¬

nesium (9.8). Details of the preparation of phenylmagnesium bromide are
given on p. 203.

Uses of chlorobenzene
Chlorobenzene is used in the manufacture of phenol (10.8) and the im¬
portant insecticide D.D.T., by reaction with trichloroethanal in the presence
of concentrated sulphuric acid.

Cl

^ y For convenience, the alkyl fluorides, R—F, and other fluorine derivatives of the
* , hydrocarbons are discussed together. They are much less reactive than the other
r luorocarbons halides because of the much greater strength of the C—F bond (485 kJ mol“')
than C—Cl (339 kJmori), C—Br (284kJmori) or C—I (213kJmori).
In general, they behave like alkanes (hydrocarbons), so that they are usually
referred to as fluorocarbons. Many have become of industrial importance during
the last 20 years.

Physical properties
Fluorine derivatives have physical properties (for example, boiling points) which
are similar to those of the parent alkane:

n= 1 2 3 4

C„H2„+2 B.p. °C -162 -89 -42 -0-5

C„F2„+2 B.p. °C -128 -79 -38 -5

131
HALOGEN COMPOUNDS Preparations
1. By fluorination of alkanes. Fluorine reacts far more vigorously with the
alkanes than do the other halogens, and it is necessary to use nitrogen as a diluent
for the fluorine to help remove the heat evolved in the reaction. A complex mixture
of products is formed and the carbon skeleton of the alkane is often broken down.
2. By substitution of fluorine for other halogens, (a) A halogen atom is replaced
by fluorine when an alkyl halide is heated with anhydrous potassium fluoride in
ethane-1,2-diol, for example:

CH3—Cl + KF CH3—F + KCl

(b) Antimony trifluoride, in the presence of antimony pentachloride, can also

be used as a fluorinating agent, for example:

3CCI4 + 2SbF3 ^ 3CF2CI2 + 2SbCl3

Addition of anhydrous hydrogen fluoride regenerates antimony trifluoride from

the antimony trichloride.
3. By substitution of fluorine for oxygen. Sulphur tetrafluoride replaces oxygen
by fluorine in alcohols (R—OH —^ R—F), ketones (R2C=0 ^ R2CF2) and
acids (R—CO2H -> R—CF3).
4. By electrolysis. When an organic compound in pure hydrogen fluoride is
electrolysed with nickel electrodes at 0°C, it undergoes fluorination at the anode.
For example, diethyl ether gives C2F5—O—C2FS, and ethanoic acid gives
CF3—CO—F, which on hydrolysis gives trifluoroethanoic acid, CF3—CO2H.

Chemical properties
Unlike the other halogen derivatives of alkanes, the alkyl fluorides are chemically
stable. They do not react with oxidising or reducing agents, or with strong acids
and alkalis. They react slowly with sodium or potassium metal at elevated tem¬
peratures. However, Grignard reagents have been prepared, but the magnesium
compound is only stable below —20°C.
The fluorocarbons are regarded as parents of a new branch of chemistry,
similar to the organic compounds formed from hydrocarbons. Chains of —CF2—
are stable, similar to —CH2— chains. Thus there is a wide range of fluorocarbon
derivatives of the type RpZ, where Rp is the fluorocarbon group (CF3—, CHF2—,
CH2F—, etc.), and the functional group Z can be —CO2H, —CHO, —CH2OH,
—OH, etc., and a vast new series of compounds is now being developed.

Uses
1. Fluorocarbons are generally very stable. They are used as oils, sealing
liquids and coolants.
2. Tetrafluoroethene, C2F4, is the fluorine analogue of ethene, C2H4. It is pre¬
pared by the fluorination of trichloromethane, generally by antimony trifluoride;
reaction occurs in two stages:

SbFj 700°C
2CHCI3 -^ 2CHF2CI -> CF2=CF2 + 2HC1

In the presence of a catalyst, tetrafluoroethene undergoes a free-radical

polymerisation (21.2) to give a plastic, poly(tetrafluoroethene) (PTFE), marketed
as Teflon, which is highly resistant to all chemicals.
3. The fluorochloro derivatives of methane and ethane, for example CF2CI2
and CFCI3, are used as refrigerants (sold under the name of Freons) and as
aerosol propellants in dispensers for fly-killers, shaving cream, etc.
4. l-Bromo-l-chloro-2,2,2-trifluoroethane is used as an anaesthetic
(Fluothane).

132
9.8 The preparation of organic compounds containing magnesium was first
described by Grignard in 1900. The importance of the compounds lies in
Grignard reagents their usefulness in organic synthesis, and Grignard was awarded the Nobel
Prize for his work in 1912.

Preparation of Grignard reagents

A Grignard reagent is prepared by refluxing an alkyl or aryl bromide or
iodide, dissolved in dry ether, with small magnesium turnings. A small
crystal of iodine is sometimes added to initiate the reaction:

R—Br + Mg -> R—MgBr

Grignard reagents cannot be isolated. The ethereal solution of the reagent

is used for further reaction.
Alkyl chlorides also form Grignard reagents, though less readily than the
bromides or iodides, but aryl chlorides react too slowly. Iodides react more
readily than bromides, and are used in the following examples.

Reactions of Grignard reagents

The Grignard reagent, dissolved in ether, is usually kept in the apparatus
used for its preparation, and the other reagent, which is sometimes dissolved
in ether, is added from a tap-funnel.
1. With water, to form an alkane:

R—Mgl -f H2O ^ R—H + Mg(OH)I

2. With methanal, to form a primary alcohol. Methanal gas is passed

into the solution of the Grignard reagent, and the mixture is then hydrolysed
with dilute acid:

R—MgI + CH2=0 R—CH2—O—Mgl

R—CH2—O—Mgl + H2O -> R—CH2—OH + Mg(OH)I

3. With other aldehydes, to form a secondary alcohol:

R-Mgl + R'-CHO -> /CH-O-Mgl

R
/CH-OH + Mg(OH)I

4. With ketones, to form a tertiary alcohol:

R'

R-Mgl + R'aCO ^ R-C-O-Mgl

I ^
R'

R'
H,0
> R-C-OH + Mg(OH)I
I ^
R'

133
HALOGEN COMPOUNDS 5. When carbon dioxide is passed through the solution of the Grignard
reagent and the mixture is then hydrolysed, a carboxylic acid is formed.

R—Mgl + CO2 -» R C 0 Mgl

R C OH + Mg(OH)I

Details of this reaction are given on p. 203.

6. With epoxyethane, to form a primary alcohol:

R —Mgl + HjC CHj -> R—CH2—CHg-O—Mgl

0

> R-CH2-CH2-OH + Mg(OH)l

In this method for primary alcohols, two carbon atoms are introduced into the
Grignard reagent, whereas when methanal is used one carbon atom is added.
The reactions of Grignard reagents described above share a common feature
in their mechanisms. The C—Mg bond in the Grignard reagent is strongly
polarised by the electropositive metal:
1
-C^ Mo-I
1 ^
As a result, the carbon atom tends to break away with the bonding pair of
electrons; that is, it behaves as a nucleophilic reagent, for example:

0 X O'
I-Mg-CHj CH2=0 -I- + Mg"+ + CH3-CH2-O-

9.9 Small-scale preparation of bromoethane

Practical work Place 6 cm^ of ethanol in the flask, immerse the flask in cold water and add,
slowly, 7 cm^ of concentrated sulphuric acid. Shake the mixture gently
while adding the acid. Add 6 g of potassium bromide and set up the
apparatus (Fig. 9.5).
Heat the flask very gently until all the alkyl halide has distilled over into
the receiver, which should be surrounded by ice.
Transfer the distillate to a separating funnel, add about 3 cm^ of a dilute
solution of sodium hydrogencarbonate, fit the stopper and shake. Remove
the stopper several times to relieve the pressure.
Allow the mixture to settle and run off the lower organic layer. After
discarding the upper layer, wash the alkyl halide in the separating funnel
with water and then run it into a test-tube. Add some anhydrous calcium
chloride, stopper the test-tube and shake it.
Redistil the dry bromoethane, collecting the fraction boiling between
35 and 40°C (Fig. 2.2); heat the flask with a beaker of hot water.

Small-scale preparation of 1-bromobutane

Place 6 g of potassium bromide in about 6 cm^ of water in the flask and add
6 cm^ of butan-l-ol. Immerse the flask in cold water and add, slowly, 6 cm^
of concentrated sulphuric acid. Shake the mixture gently while adding the
acid. Reflux the mixture using a small bunsen flame for 30 minutes
(Fig. 2.1).
Rearrange the apparatus (Fig. 2.2). Distil the halogenoalkane until no

134
FIG. 9.5. Preparation of
bromoethane

more oily drops come over.

If a pure sample is required, transfer the impure compound to a test-tube and add
about 2 cm^ of concentrated hydrochloric acid. Shake the mixture to remove
unchanged butan-l-ol and then remove the acid layer (using a dropping pipette).
Add, slowly, about 5 cm^ of a saturated solution of sodium carbonate (to remove
any excess acid) and remove the aqueous layer (with a dropping pipette). Finally,
shake the mixture with 5 cm^ of water. Remove the water layer yet again, and add 2
or 3 lumps of anhydrous calcium chloride to the halogenoalkane (to remove the last
traces of water).
Redistil (Fig. 2.2) and collect the fraction boiling between 99 and 103°C.

Small-scale preparation of 2-chloro-2-methylbutane

Shake 5 cm^ of 2-methyIbutan-2-ol with 20 cm^ of concentrated hydro¬
chloric acid for 10 minutes in a separating funnel. Run olf the lower layer
(acid) and add slowly 10 cm^ of dilute sodium hydrogencarbonate solution.
Shake, making sure that you do not allow an excess of pressure of carbon
dioxide to build up.
Discard the lower aqueous layer, run the alkyl halide into a test-tube, add
a few pieces of anhydrous calcium chloride, stopper the tube and shake.

135
HALOGEN COMPOUNDS Decant the dry alkyl halide into a distillation flask and purify it by distilla¬
tion (Fig. 2.2). Collect the fraction boiling between 84 and 86°C.

Small-scale preparation of bromobenzene

The experiment must he carried out in a fume-cupboard, and take great care
not to breathe in the vapour of either benzene or bromine. Benzene is highly
toxic and must only he used under supervision.
Set up the apparatus shown in Fig. 9.6, with 6 cm^ of benzene and 0-2 g
of iron filings in the flask. Run 3 cm^ of bromine slowly into the flask, and
slowly raise the temperature of the water-bath to 70°C. Maintain this
temperature until no more hydrogen bromide is evolved.

FIG.9.6. Preparation of
bromobenzene

Remove the flask, cool it in cold water and pour the mixture into a
separating funnel. Purify the aryl halide in a similar way to that described
for bromoethane (p. 134), collecting the fraction boiling between 152 and
158°C.

136
HALOGEN COMPOUNDS
Rates of hydrolysis of some halogen compounds
(a) To compare the rates of hydrolysis of chloro-, bromo- and iodoalkanes
To three separate test-tubes, add 2 cm^ of ethanol and place them in a
beaker of water kept at about 60°C. When the ethanol has reached this
temperature, using separate dropping pipettes, add 5 drops of 1-chloro-
butane to one test-tube, 5 drops of 1-bromobutane to the second and 5 drops
of 1-iodobutane to the third. Then, as quickly as possible, add 1 cm^ of
OTM silver nitrate solution to each. Shake the test-tubes and observe (a)
the order in which the precipitates appear, (b) the colour and density of the
precipitates.

(i) What are the precipitates ?

(ii) Test whether they occur when the silver nitrate is added to any of
the alkyl halides by themselves.
(iii) Deduce why the precipitates are formed. Write equations for the
reactions.
(iv) What is the effect on the rate of these reactions arising from changing
the halogen in the alkyl halides?

(b) To compare the rates of hydrolysis of some bromobutanes

Repeat the experiment above, at room temperature, using 1-bromobutane,
2-bromobutane and 2-bromo-2-methylpropane. The test-tubes can be
placed in a test-tube rack. Add the reagents as quickly as possible with
shaking, and observe the test-tubes carefully for at least five minutes.

(i) What are the precipitates ?

(ii) Write equations for the reactions.
(iii) What effect does the alkyl group have on the rate of reaction?
Suggest reasons.

(c) To compare the reactivity of the halogen atoms in aliphatic and aromatic
halogen compounds {Carry out these experiments in a fume-cupboard.
{Bromomethyr)benzene is a powerful lachrymator)
Repeat the experiments given in (a), first at room temperature and then in
hot water (at 60°C), with 1-bromobutane, bromobenzene and (bromo-
methyl)benzene.
(i) What are the precipitates ?
(ii) Write equations for the reactions.
(iii) What effect does the phenyl group have on the reactivity of the
halogen atom ?
Take care that there is enough ethanol present to dissolve the aromatic
halogen compounds. A slight turbidity on mixing may be due to an emulsion
of the organic compound with water; to test for this, add a few drops of
ethanol and shake.

Reactions of 1,1-dichloroethane and

1,2-dichloroethane
1. To 2-3 drops of 1,1-dichloroethane in a test-tube, add 2 cm^ of dilute
sodium hydroxide solution. Shake and gently boil the mixture. Allow the
mixture to cool, acidify with dilute nitric acid, then add silver nitrate
solution.
(a) Note the smell of the vapour evolved.
(b) Note whether a precipitate is formed.

137
HALOGEN COMPOUNDS Write an equation to explain your observations.
Repeat the experiments using 1,2-dichloroethane. Do you observe the
same results? Write equations for the reactions.
2. Add a few pellets (about 0-5 g) of potassium hydroxide to 2 cm^ of
ethanol in a test-tube. Warm the test-tube gently until the pellets have
dissolved. Add 6 drops of 1,1-dichloroethane to the alcoholic solution of
potassium hydroxide. Shake gently, then introduce a plug of Rocksil to
absorb the solution. Fit the test-tube with a delivery tube dipping into 2 cm
of an ammoniacal solution of copper(I) chloride. Warm the Rocksil plug
gently and observe what occurs to the copper(I) chloride solution.
Repeat the experiment with 1,2-dichloroethane, writing equations for
both reactions.

Preparation of a Grignard reagent

The preparation of phenylmagnesium bromide from bromobenzene is
described on p. 203.

Reaction mechanisms: An introduction. 1980. Open University. S246 7-9

9.10
Further reading

9.11 S,,,! or Sf^2 (F,V) Open University. S246/08F

Films and Videotapes

9.12 1 Outline how iodoethane (ethyl iodide) may be prepared from (a) ethene (ethylene),
(b) ethanol. How from iodoethane would you prepare (i) ethane, (ii) ethene,
Questions (iii) butane, (iv) diethyl ether?
Give the structural formulae of the isomers of C2H4CI2 and describe the action
of aqueous alkali on these isomers. (AEB)
2 Describe a laboratory method for the preparation of a named alkyl halide, giving
a diagram and full practical details.
An alkyl bromide, A (0-615 g), was boiled under reflux with 100 cm^ ofO-125M
sodium hydroxide solution. The mixture was allowed to cool and then titrated,
using methyl orange as indicator, with 0-125M hydrochloric acid, of which 60 cm^
was required. Assuming that A contains only one bromine atom in its molecule,
calculate its molecular weight.
The organic product of the hydrolysis of A is found to be easily oxidised to a
ketone. Suggest the probable structural formula of A and indicate the reasoning
by which you arrive at this result. (H = 1, C = 12, Br = 80.) (L(X))
3 Describe how you would prepare a pure specimen of bromoethane from ethanol
(ethyl alcohol). Draw a sketch of the apparatus you would use to obtain the crude
product.
How and under what conditions does bromoethane react with: (a) silver oxide,
(b) potassium cyanide, (c) sodium hydroxide? Describe the experiments you
would carry out to identify the organic product in one of these reactions. (O)

4 How would you prepare a pure sample of bromoethane from ethanol ?

Outline the reactions by which bromoethane may be used in the preparation
of (i) propanonitrile, (ii) ethane, (iii) ethylbenzene.

138
HALOGEN COMPOUNDS
Chloroethane reacts with an alloy of sodium and lead to produce a liquid
compound of the composition C, 29-7 per cent; H, 6-2 per cent; Pb, 64-1 per cent.
Suggest a structural formula for this compound. (C(T))
(H = 10, C = 12, Pb = 207.)
5 Assume that the chemical properties of 1-bromopropane are the same as those
of bromoethane. Deduce what products are formed when 1-bromopropane
reacts with (a) hydrogen, (b) potassium hydroxide, (c) sodium, (d) sodium
ethoxide. In each case give the essential conditions of reaction and name the chief
product formed.
Give outline schemes of reactions for converting 1-bromopropane into (i) an
acid, (ii) a primary amine, each containing the original number of carbon atoms
per molecule. (AEB)
6 By means of equations with brief notes on reagents and experimental conditions
show how an alkyl halide may be converted into (a) the corresponding hydro¬
carbon, (b) an alcohol, (c) an alkene, (d) a nitrile, and (e) a primary amine.
What prevents a good yield of the primary amine in (e) ? (JMB)
7 Describe two methods of introducing a chlorine atom into (a) an aliphatic com¬
pound and (b) an aromatic compound. Give one example of each method.
By consideration of the reaction between methylbenzene and chlorine illustrate
the importance of experimental conditions in determining the reaction products.
Discuss the mechanism operating in each case.
Describe the reactivity of typical aliphatic monohalogen compounds towards
(i) potassium hydroxide, (ii) potassium cyanide, and (iii) ammonia. Discuss the
mechanisms of these reactions. (JMB (Syllabus B) Specimen question)
8 Give reasons for the items that are underlined in the following directions for the
laboratory preparation of approximately 30 g of bromoethane.
Fit a 500 cm^ round-bottomed flask with a bent tube connected to a double¬
surface condenser set for downward distillation. To the lower end of the con¬
denser attach an adapter. (These parts must be fitted together with tight-fitting
joints.) Arrange for the end of the adapter to dip below the surface of about
50 cm^ of water contained in a 250 cm^ flask which is surrounded by an ice/water
mixture.
Place 37 cm^ (30 g) of ethanol in the round-bottomed flask and add slowly
40 cm^ (74 g) of concentrated sulphuric acid. When the mixture has cooled add
50 g of powdered potassium bromide, reconnect the flask to the condenser and
heat gently, at the same time ensuring that a copious supply of cold water is
passing through the condenser. Continue heating until nc more droplets pass
from the end of the condenser.
Pour the contents of the receiving flask into a separating funnel and run off and
retain the lower layer. Discard the upper layer. Return the lower layer to the
separating funnel and wash it first with dilute aqueous sodium carbonate and then
with water, retaining the lower layer each time. Add a few pieces of anhydrous
calcium chloride to the lower layer and leave for 20 min in a stoppered flask.
Filter the solution through a fluted filter paper directly into a 50 cm^ distilling
flask containing a few chips of unglazed porcelain. Fit the flask with a 100°
thermometer and a double-surface condenser having as before a copious supply
of cold water running through it. Collect the fraction b.p. 36-40°C.
The bromoethane obtained in this way contains approximately 15 per cent
diethyl ether. How do you account for the presence of this impurity?
Given that the product contains 30 g of bromoethane, calculate the percentage
yield for the overall reaction. (JMB)
9 (a) Chloromethane reacts with an alloy of aluminium and sodium to form the
compound J.
J has the following composition by mass; C = 50%, H = 12-5%, A1 = 31-5%.
0-24g of J react with excess water to produce 0-224 dm^ of the gas, K, and a white
gelatinous precipitate, L.
L dissolves in hydrochloric acid and in sodium hydroxide solution.
25 cm^ of K require 50 cm^ of oxygen for complete combustion. (All gas volumes

139
HALOGEN COMPOUNDS were measured at s.t.p.)
[H = 1-0, C = 12-0, A1 = 27-0; molar volume of a gas at s.t.p. = 22-4 dm^ mol \]
(i)
Calculate the empirical formula of J.
(ii)
Suggest a structural formula for J.
(iii)
Identify the compounds K and L.
(iv)Write equations for:
the reaction of chloromethane with the Al/Na alloy;
the reaction of J with excess water;
the complete combustion of K;
the reaction of L with hydrochloric acid;
the reaction of L with sodium hydroxide solution.
(b) How, and under what conditions, does bromoethane react with
(i) potassium hydroxide;
(ii) silver ethanoate (acetate);
(iii) benzene?
Write equations for the reactions described. (SUJB)
10 Given a supply of ethanol and the usual laboratory reagents, describe how
you would prepare a sample of , -dibromoethane (ethylene dibromide).
1 2

Outline methods for the conversion of the dibromoethane into

(a) CH2OH (b) CH2COOH (c) CH2.O.CO.CH3

I I I
CH2OH CH2COOH CH2.O.CO.CH3 (L)
11 Give the structural formulae of the isomers represented by C H CI and outline
2 4 2

how each may be prepared.

Describe how and under what conditions each isomer reacts with potassium
hydroxide and name the chief products formed. (AEB)
12 Outline schemes of reactions for the preparations of (chloromethyl)benzene and
chlorobenzene from benzene, and for the preparation of chloroethane from
methane.
How do these products react with (i) aqueous sodium hydroxide, (ii) alcoholic
ammonia, (iii) lithium aluminium hydride ? (AEB)
13 The hydrolysis of chloroethane C H CI is a nucleophilic, bimolecular substitution
2 5

reaction, whereas the hydrolysis of -chloro- -methylpropane,

2 2

CH3

CH3—C—Cl

CH3

is a nucleophilic, unimolecular reaction.

(a) Explain the terms in italics.
(b) Discuss the mechanisms of the two reactions.
(c) Describe in outline one experiment to show that the tertiary halogen
compound hydrolyses at a faster rate than the primary.
(d) Give, and explain the results of, the reactions of each of the halogen
compounds with sodium ethoxide in ethanol solution. L(S)
14 Write the names and structural formulae of the products of mono-nitration of
bromobenzene. Indicate any necessary conditions for the nitration.
Compare the chemical reactivity of bromoethane and bromobenzene to
aqueous sodium hydroxide.
Outline how bromobenzene may (a) be obtained from benzene and 4-bromo-
benzaldehyde respectively, (b) be converted to methylbenzene. (W)
15 Suggest possible structural formulae for aromatic compounds of empirical
formula C7H7CI. What (if any) would be the reactions of each of these compounds
with sodium hydroxide?

140
HALOGEN COMPOUNDS
Describe, giving essential experimental details, how you would detect the
presence of chlorine in one of these isomers. How, and why, would you modify
your method if the compound also contained nitrogen ?
Which isomer would you use as a starting material for the preparation of
benzoic acid ? Describe briefly how you would perform the conversion in the
laboratory. (AEB)

16 Explain the meaning of the terms nucleophile, electrophile, and free radical, giving
one example of each, and emphasising those structural features which are
responsible for their behaviour.
Give two examples of electrophilic addition, two examples of electrophilic
substitution, and two examples of nucleophilic addition.
Why do many gas phase reactions involve free radicals rather than ions?
(O and C(S))

17 (a) Describe the meaning of the terms homolytic and heterolytic with reference to
the reactions of chlorine with (i) ethene, (ii) methane. Indicate the mechanisms
of the reactions you choose.
(b) Calculate the weight of bromine needed to convert 4.6 g of methylbenzene
into compound K below.
(c) Describe test-tube experiments which would enable you to distinguish
chemically between K and L.

18 The following results are those from an experiment in which equal volumes
of equimolar solutions of a bromoalkane of formula C4H9Br and of potassium
hydroxide were mixed and then 20 cm^ samples taken at intervals, the reaction
quenched in an excess of ice cold water, and titrated against a standard acid
solution.
Time (seconds X 10^) 0 0-45 0-9 1-8 2-7 3-6 4-5 5-4 6-3 7-2 8T
Titre (cm^ of acid) 20 11-5 8-0 5-0 3-55 2-8 2-35 2-0 1-75 1-55 1-4
(a) Use these results to find the overall order of the reaction.
(b) From your result in (a), deduce the most probable mechanism for the
hydrolysis, explaining your deduction.
(c) What is the most probable formula for the bromoalkane? Explain your
reasoning carefully. (L (Nuffield trial) (S))

19 Devise experiments to determine:

(a) the structure of the product(s) obtained by addition of hydrogen bromide
to propene CH3—CH=CH2;
(b) the structure of the product(s) obtained by elimination of hydrogen bromide
from 2-bromobutane CH3—CHBr—CH2—CH3. (O Schol.)

20 Discuss the differences in chemical properties between chloroethane and chloro¬

benzene. How would you expect the following compound to behave:

21 Write equations for some of the reactions of alkyl halides giving examples of as
many different types of reaction as possible. How could you distinguish chemically
between 1-chloropropane and 2-chloropropane ?
When 1-chloropropane is further chlorinated the ratio of 1,2-dichloropropane
to 1,3-dichloropropane in the product is 9:7. How does this result differ from
that which might have been expected ? (O Schol.)

141
 = ■

>£»». , t , >(1'
HALOGEN compounds " a 22 Writej
K'iJil'*— L’*"' /o~ >'
of the haloalkanes with alkali vary f
tf’' . ^ '..'tv
lit 'U..f)H/oh»^ X{pC‘/t pr. -. ■ " .
(JMB) I 1

SWr ■ .r ^ •.

7
!i=r
lil ;, ' /t,.if,J<hVr

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V. I.i-i?
S
^
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y - "■ ■" ,W-Kvo*.h*yi ttjj
^,7-4 fejO-'t .'i.'iT<V>-',^.4*^^^’j5
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142 fH ■
*’f'. SifH/; ., jV- -p I
Chapter 10
Alcohols and phenols

General formula
R—OH

10.1 Alcohols are compounds containing one or more hydroxyl groups attached
to saturated carbon atoms. Those with one hydroxyl group are known as
Introduction
monohydric alcohols; examples are ethanol, C2H5—OH (an aliphatic
monohydric alcohol) and phenylmethanol, CgHj—CH2—OH (an aromatic
monohydric alcohol). There are also polyhydric alcohols, which contain
more than one hydroxyl group; examples are ethane-1,2-diol,
HOCH2—CH2OH (an aliphatic dihydric alcohol) and propane-1,2,3-triol,
HOCH2—CH(OH)—CH2OH (an aliphatic trihydric alcohol).
Phenols are compounds containing one or more hydroxyl groups at¬
tached to aromatic carbon atoms; the parent member of the series is phenol
itself, CgHj—OH. Many of the properties of phenols are different from
those of alcohols, and they are described separately (10.8).

10.2 Monohydric alcohols are named by replacing the final -e in the correspond¬
ing alkane by -ol. The position of the hydroxyl group in the carbon chain
Nomenclature of is given by numbering the carbon atoms as for alkanes. For example:
monohydric alcohols
CH,
4 3 2 1 4 3 I2 1
CH, -CH, CH-CH, CH3-CH2-C-CH3
OH OH

Butan-2-ol 2-Methylbutan-2-ol

There are three classes of alcohol. They differ in the number of alkyl
groups attached to the hydroxyl-bearing carbon atom:

H R R
1 1 1

R-C-OH R-C-OH R-C-OH

1 1 1
1 1 1
H H R

Primary Secondary Tertiary

The three classes have many similar chemical properties, owing to the
presence of the same functional group, —OH. However, there are also
differences which are due to the different numbers of hydrogen atoms on
the hydroxyl-bearing carbon atom.
The names and structural formulae of some alcohols are in Table 10.1.
There are two isomers with formula C3H7OH, one a primary and the
other a secondary alcohol. There are four isomers of C4H9OH, two of
which are primary alcohols, for 2-methylpropan-l-ol is a primary alcohol
as it possesses a —CH2OH group, even though it contains a branched alkyl
chain.

143
 Table 10.1. The structural formulae, class and physical properties of some alcohols

NAME FORMULA STRUCTURAL FORMULA CLASS M.P./°C B.P./°C

Methanol CH3OH H—CH2—OH Primary -97 64

Ethanol C2H5OH CH3CH2—OH Primary -117 78
Propan-l-ol C3H7OH CH3CH2CH2—OH Primary -127 98
CH3^
Propan-2-ol C3H7OH CH-OH Secondary -89 82
CH3
Butan-l-ol C4H9OH CHgCHjCHaCHa—OH Primary -90 118
CH3^
2-Methylpropan-1 -ol C4H9OH .CHCH2-OH Primary -108 108
CH3
CHgCH^^
Butan-2-ol C4H9OH ^CH-OH Secondary 100

CH3^

CH3
2- Methylpropan-2-ol C4H9OH CH3-C-OH Tertiary 25 83
/
CH3
Pentan-l-ol C5H11OH CH3CH2CH2CH2CH2—OH Primary -79 138
Hexan-l-ol C6H13OH CH3CH2CH2CH2CH2CH2—OH Primary -51 157
Phenylmethanol CfiHsCHjOH C6HsCH2—OH Primary -15 205

10.3 As with alkanes, the boiling points of the alcohols increase fairly regularly
on the addition of each methylene (—CH2—) group; the increment is about
Physical properties of
20°C among the lower homologues (Table 10.1). Again, as with the alkanes,
monohydric alcohols and for the same reason (5.2), increase in branching of the carbon chain is
accompanied by decrease in boiling point; this is illustrated by the four
isomeric alcohols, C4H9OH (Table 10.1).
However, the boiling points of alcohols are considerably higher than
those of alkanes of approximately the same formula weight, as illustrated
in Table 10.2.

Table 10.2. The boiling points of alkanes and alcohols of similar formula
weight

ALKANE F. WT. B.P./°C ALCOHOL F. WT. B.P./°C

Ethane 30 -89 Methanol 32 64

Propane 44 -42 Ethanol 46 78
Butane 58 -0-5 Propan-l-ol 60 98

This is because of the occurrence of strong attractive forces, known as

hydrogen-bonds, between the molecules of the alcohol in the liquid phase.
The bonds are electrostatic in nature; the proton in the hydroxyl group of
one molecule is attracted by an unshared pair of electrons on the oxygen
atom of another:
H
R-of
H H
^O'
I
R

144
ALCOHOLS AND PHENOLS
The hydrogen-bonds, represented by dotted lines, are longer than covalent
bonds and are not as strong. A typical value for the strength of a hydrogen-
bond is 20 kJ mol“\ which is much larger than the usual attractive forces
between molecules, which are generally of the order of l-2kJmoP^
These differences in the attractive forces are reflected in the higher tempera¬
tures needed to separate the molecules of an alcohol compared with the
molecules of an alkane. Hydrogen-bonding in a liquid alcohol can be
detected by infrared spectroscopy (Fig. 10.1).

FIG. 10.1. Part of the infrared

absorption spectrum of ethanol. In
{a), ethanol is in the vapour phase,
and the —OH group is absorbing
radiation at 3700 cm~^. In {b),
ethanol is dissolved in an inert
solvent and the —OH group is
absorbing radiation at 3300 cm~^.
The reduction in frequency
indicates that the bond strength is
reduced due to hydrogen-bonding
between the molecules of ethanol

Frequency/cm" (b)
(a)

The smaller alcohols are miscible with water, but as the number of carbon
atoms increases, solubility decreases, for example:

Alcohol Methanol Ethanol Propan-l-ol

Solubility (g 100 cm Miscible Miscible Miscible

Alcohol Butan-l-ol Pentan-l-ol Hexan-l-ol

Solubility (g 100 cm“^) 8-3 2-6 1-0

Alcohols are more soluble in water than are alkanes of similar formula
weight because of the attractive forces (hydrogen-bonds) between molecules
of the alcohol and molecules of water:

R H
^o-H o:^
H..

10.4 General methods

Methods of preparation 1. By the hydration of an alkene (6.3), for example:

of monohydric alcohols CH2=CH2 + H2SO4 ^ CHj—CH2—O—SO2—OH

CH3—CH2—O—SO2—OH + H2O ^ CHj—CH2—OH + H2SO4

145
ALCOHOLS AND PHENOLS 2. By the hydrolysis of an alkyl halide with an aqueous solution of an
alkali (9.3), for example:'
(CH3)2CH—Cl + NaOH ^ (CH3)2CH—OH + NaCl

3. By the reaction of a Grignard reagent with an aldehyde or ketone

(9.8). Methanal yields a primary alcohol, other aldehydes yield secondary
alcohols and ketones yield tertiary alcohols.
4. By the reduction of an aldehyde (giving a primary alcohol) or a ketone
(giving a secondary alcohol) (12.5).
5. By the reduction of a carboxylic acid with lithium tetrahydrido-
aluminate, giving a primary alcohol (13.5).

Manufacture of methanol
From natural gas. Methane, obtained from natural gas (19.2), is passed
with steam over nickel at about 900°C and under pressure:

CH4 + H2O CO + 3H2

This mixture of gases, known as synthesis gas, can be converted into

methanol. The gases are passed over a heated copper catalyst under
pressure:
250°C; 70 atm. pressure /-,tt

Manufacture of ethanol
1. Ethene is passed under pressure through concentrated sulphuric acid
at 80°C to form ethyl hydrogensulphate:

CH2=CH2 + H2S04^CH3—CH2—O—SO2—OH

The mixture is then diluted with water and distilled to give an aqueous
solution of ethanol:

CH3—CH2—O—SO2—OH + H2O ^ CH3CH2OH + H2SO4

2. A more recent method is to hydrate ethene directly by passing a

mixture of the alkene and steam over a solid acid catalyst (phosphoric acid
on silica) at 300°C and a pressure of about 70 atmospheres:

CH2=CH2 + H2O ^ CH3CH2OH

Only about 4 per cent of the ethene reacts, but the remaining ethene and
steam are recirculated over the catalyst many times to obtain a good yield
of ethanol.
3. An older method is by the fermentation of starch (18.3).

Absolute ethanol
Regardless of the method of manufacture, all aqueous solutions of ethanol
yield, on fractional distillation, a ‘constant boiling mixture’ of 96 per cent

146
ALCOHOLS AND PHENOLS
ethanol and 4 per cent water, known as rectified spirit. This means that
further fractionation would remove no more water, since the distillate has
the same composition as the liquid.
In the laboratory, the rectified spirit is stored over quicklime (freshly
prepared by heating calcium carbonate). Subsequently the mixture is
refluxed over quicklime for about 6 hours, and then allowed to stand over¬
night. The pure product, known as absolute ethanol, is then distilled off",
precautions being taken to prevent absorption of water vapour by the
hygroscopic alcohol.
In industry, benzene is added to the rectified spirit. Distillation yields
three fractions:
At 65°C, a constant boiling mixture of ethanol, benzene and water (a
‘ternary azeotrope’).
At 68 °C, a constant boiling mixture of ethanol and benzene (a ‘binary
azeotrope’).
At 78°C, pure ethanol.

Manufacture of propan-2-ol
Propan-2-ol is made from propene (obtained from petroleum (20.4)) by
methods analogous to those for the manufacture of ethanol from ethene:

H2SO4 H2O
1. CH3—CH=CH2 ^ CH3—CH—CH3
^ ^ 30°C ^ I ^
OSO2OH
CH3—CH—CH3 + H2SO4

OH
Tungsten(VI)
oxide as cat.
2. CH3—CH=CH2 + H2O -^ CH3—CH—CH3
250°C, 200 atm. |
OH

10.5 The reactions of alcohols can be grouped in three classes:

(a) Reactions of the —OH group.
Chemical properties of (b) Oxidation reactions, which depend on whether the alcohol is primary,
monohydric alcohols secondary or tertiary.
(c) Elimination reactions, which depend on whether or not the alcohol
contains at least one hydrogen atom attached to the carbon atom next
to the C—OH group.

(a) Reactions of the —OH group

1. Alcohols react with sodium to form salts (sodium alkoxides) and
hydrogen, for example:

2C2H5OH + 2Na ^ 2C2H50-Na+ + H2

Sodium ethoxide

147
ALCOHOLS AND PHENOLS 2. Alcohols react with the hydrogen halides to form alkyl halides;

R—OH + HCl -> R—CI + H2O

R—OH + HBr ^ R—Br + H2O

R—OH + HI ^ R—I + H2O

The reaction with hydrogen chloride is catalysed by zinc chloride. A test

(Lucas test) to distinguish between simple primary, secondary and tertiary
alcohols is based on this reaction and the fact that tertiary alcohols react
faster than secondary alcohols which in turn react faster than primary
alcohols. The alcohol is shaken with a solution of zinc chloride in con¬
centrated hydrochloric acid. Immediate cloudiness (due to formation of the
alkyl chloride) indicates a tertiary alcohol; if the solution turns cloudy
within about five minutes, a secondary alcohol is indicated; while primary
alcohols show no cloudiness at room temperature.
The mechanism of the reaction with a hydrogen halide depends on whether the
the alcohol is primary, secondary or tertiary. In each case, the first step is the
same: the alcohol reacts reversibly with the hydrogen halide:

R-OH + HX R-O-H + X“
I
H
That is, in the presence of a strong acid, the oxygen atom of the alcohol is acting
as a base, accepting a proton from the hydrogen halide. Alcohols are very weak
bases and the equilibrium lies to the left.
With primary alcohols, the next step is displacement by the halide ion of a
molecule of water. This is an S^l reaction, analogous to the reaction of a primary
alkyl halide with hydroxide ion (9.3):

X--> R—CHj—X + H^O

R ^ H

With tertiary alcohols, the second step is heterolysis:

R R
I I.
R-C-O^ -> ,c+ + H,0
I H R R

and this is followed by reaction of the carbonium ion with the halide ion:
R R
I + + X
- -> R-C-X
I
R R I
R

Tertiary alcohols differ from primary alcohols because a tertiary carbonium ion,
R3C , is relatively more stable,' and is formed more rapidly, than a primary
carbonium ion, RCHz"^ (9.3).
Secondary alcohols are intermediate in behaviour between primary and tertiarv
alcohols.

3. Alcohols react with phosphorus tribromide and phosphorus tri-iodide

to form alkyl bromides and alkyl iodides;

3R—OH + PBrj ^ 3R—Br + H3PO3

3R—OH + PI3 ^ 3R—I + H3PO3

148
ALCOHOLS AND PHENOLS
These phosphorus trihalides are conveniently prepared in situ from red
phosphorus and the halogen.
Alkyl chlorides can be obtained from the alcohol with phosphorus
pentachloride or with sulphur dichloride oxide at room temperature;

R—OH + PCI5 ^ R—Cl + POCI3 + HCl

R—OH + SOCI2 ^ R—Cl + SO2 + HCl

The liberation of hydrogen chloride from an alcohol and phosphorus

pentachloride is typical of the behaviour of organic compounds containing
the hydroxyl group.

4. Alcohols react with concentrated sulphuric acid to form products

which depend on the nature of the alcohol and the reaction conditions.
At 0°C, an alkyl hydrogensulphate is formed;

R—OH + H2SO4 ^ R—O—SO2—OH + H2O

When the acid is added to an excess of a primary alcohol and the mixture
is heated to about 140°C, an ether is formed, for example:

H2SO4
2C2H5—OH C2H5—O—C2H5 + H2O
140°C^
Diethyl ether

The reaction occurs by displacement of the hydrogensulphate group by the

excess of the alcohol, as in the Sn2 substitution of an alkyl halide (9.3):

o-.' CHo-0-SO,-OH /O-CH,

H V
CH, CH,
+

■0-S0,-0H
+ H,SO,

However, when there is an excess of the acid and a still higher temperature
is used, elimination can occur and an alkene is formed (p. 150).
5. Alcohols react with organic acids to form esters. The reaction is slow
unless an acid catalyst is used; hydrogen chloride or concentrated sulphuric
acid are suitable catalysts. For example:

CH3-C-OH + CH3-OH CH3-C-O-CH3

3 II + HoO

O ^ o
Ethanoic acid Methyl ethanoate

This type of reaction (esterification) is discussed on p. 212.

Alcohols also form esters when treated with acid halides or acid an¬
hydrides (14.3, 14.4), for example:

CH3-C-CI + CH3-OH CH3-C-O-CH3 + HCl

II
O O
Ethanoyl chloride Methyl ethanoate

149
ALCOHOLS AND PHENOLS (b) Oxidation reactions
Primary and secondary alcohols are readily oxidised to aldehydes and
ketones, respectively. Tertiary alcohols are resistant to oxidation.
Oxidation in solution can be brought about with acidified sodium or
potassium dichromate:

Na2Cr207
RCH —OH 2 RCH=0
H2SO4
Na2t^r207
R CH—OH 2 R2C=0
H2SO4

In the case of the primary alcohol, the resulting aldehyde undergoes further
oxidation to the carboxylic acid unless precautions are taken to prevent it
(12.3).
Potassium manganate(VII) in acid solution also effects these oxidations.
Oxidation in the gas phase can be brought about either by passing the
vapour of the alcohol, together with oxygen, over silver at about 500°C, for
example,
Ag as cat.
CH OH + ^02 —^-> HCHO + H O
3 2
500°C

or by passing the vapour of the alcohol alone over heated copper, for
example:
„ Cu as cat.
CH3CH2OH -^ CH3CHO + H2
^ ^ 500°C ^

C'U 3.$ C^t

(CH3)2CH0H —^ •> (CH3)2C0 + H2

Secondary alcohols which contain the grouping CH —CH(OH)— are 3

oxidised by iodine in the presence of sodium hydroxide to tri-iodomethane:

CH3—CH(OH)—R + I2 + 2NaOH ^ CH3—CO—R + 2NaI + 2H2O

CH3—CO—R + 3I2 + 4NaOH ^ CHI3 + R—C02"Na+ + 3NaI + 3H2O

One primary alcohol, ethanol, also undergoes this reaction:

CH —CH —OH + I + 6NaOH ^

3 2 4 2

CHI + HC02-Na+ + 5NaI + H O 3 5 2

Other primary and secondary alcohols are also oxidised and iodinated but do not
then give tri-iodomethane, for example:

CH —CH —CH —OH + I + 2NaOH ^

3 2 2 2

CH —CH —CH=0 + 2NaI + H O 3 2 2 2

CH3—CH2—CH=0 + 2I2 + 2NaOH

CH —CI —CH=0 + 2NaI + H O
3 2 2 2

(c) Elimination reactions

Alcohols which possess at least one hydrogen atom on the carbon atom
next but one to the hydroxyl group undergo dehydration (elimination of

150
ALCOHOLS AND PHENOLS
water) when heated with concentrated sulphuric acid:

H-C-C-OH /C=CC + H2O

II ^ ^
The reaction occurs via the alkyl hydrogensulphate, for example:

C2H5OH + H2SO4 -^ CH3—CH2—O—SO2—OH + H2O

170°C
CH3—CH2—O—SO2—OH -> CH2=CH2 + H2SO4

This elimination reaction competes with the substitution reaction which

occurs (p. 149), for example:

CH3—CH2—O—SO2—OH + C2H5OH ^
CH3CH2—O—CH2CH3 + H2SO4

The substitution reaction is promoted by the presence of an excess of the

alcohol, and the elimination reaction is promoted by the use of higher
temperatures.
An alternative dehydrating agent is phosphoric acid. A practical example,
the dehydration of cyclohexanol, is given on p. 88.
Alcohols can also be dehydrated by passing their vapour over aluminium
oxide at about 300°C, for example:

„ Al203 ascat.
CH3—CH2—OH -> CH2=CH2 + H20

Summary of the differences between primary,

secondary and tertiary alcohols
1. Oxidation. Primary alcohols give aldehydes, secondary alcohols give
ketones and tertiary alcohols are resistant to oxidation by mild oxidising
agents in solution such as acidified potassium dichromate.

2. Elimination. All three types of alcohols containing the grouping

I 1
H—C—C—OH are dehydrated by sulphuric acid to alkenes, but tertiary

alcohols react far more readily than secondary alcohols which in turn react
more readily than primary alcohols.
3. Reaction with halogen acids. All three types of alcohol react, but
tertiary alcohols react the most readily (e.g. with concentrated hydrochloric
acid, p. 117) and primary alcohols the least readily (anhydrous conditions,
with zinc chloride as a catalyst, are necessary).

10.6 Methanol
Uses of monohydric 1. In the manufacture of methanal, which is used to make thermosetting
plastics such as Bakelite (p. 333).
alcohols 2. To make methyl 2-methylpropenoate, which is used in the manu¬
facture of Perspex (p. 331).
3. In the manufacture of methanoic and ethanoic acids, using carbon
monoxide as the other starting material (p. 191).

151
ALCOHOLS AND PHENOLS 4. In the manufacture of single-cell protein animal foodstuffs. Methanol
is used as the food for the .micro-organisms (p. 345).
5. As a solvent for varnishes and paints.

Ethanol
1. In the manufacture of ethanal (12.3).
2. In the manufacture of ethyl esters (p. 214).
3. As a solvent for many organic compounds.

Propan-2-ol
1. In the manufacture of propanone (20.4) and of hydrogen peroxide
(20.4).
2. As a solvent for spirit polishes and varnishes.

10.7 Dihydric alcohols

Polyhydric alcohols According to the I.U.P.A.C. nomenclature, these are known as diois, for
example:

B.P./°C

Ethane-1,2-diol HO—CHj—CH2—OH 197

Propane-1,2-diol HO—CH2—CH—CH3 189

OH
Propane-1,3-diol HO—CH2—CH2—CHi—OH 215

The older name is glycol; for example, ethane-1,2-diol is often referred to as

ethylene glycol or simply as glycol.

Physical properties of diois

The lower members are viscous, colourless liquids which are soluble in
water. Their boiling points are very much higher not only than those of
alkanes of similar formula weight but also than those of monohydric
alcohols of similar formula weight; this is because the presence of two
hydroxyl groups gives rise to very extensive hydrogen-bonding. This is also
the reason for their high viscosity, since neighbouring molecules in the
liquid, being bonded by hydrogen-bonds, cannot move freely relative to
each other. Finally, their solubility in water stems from their forming
hydrogen-bonds with water molecules.

Manufacture of ethane-1,2>diol
Ethane-1,2-diol is manufactured by hydration of epoxyethane (11.6):

H2C-CH2 H2C-CH2
\ / -f- H2O -I I
O HO OH

This is carried out in acid solution at about 60°C or with water at 200°C
under pressure.

152
ALCOHOLS AND PHENOLS
Chemical properties of diols
Ethane-1,2-diol (ethylene glycol) is taken as a typical example. Its two
primary alcohol groups behave in the same way as the one such group in
a monohydric primary alcohol, except that more vigorous conditions are
sometimes needed for reaction of the second of the two groups. For
example:
1. It reacts with sodium to form a monoalkoxide and, at higher tempera¬
tures, a dialkoxide;

CH2OH Na CH20-Na+
I I
CHjOH 50 CH2OH

CH2O- Na+ Na CH2O- Na+

C^H^OH Na^ "

2. It reacts with phosphorus halides:

CH2-OH CH2-Br
3 I + 2PBr3 -> 3 I + 2H3PO3
CH2-OH CH2-Br

3. It reacts with carboxylic acids to form esters:

excess of
CHj—OH CH3-CO2H CH2—O—CO —CH3CH3-CO2H CHj—O—CO—CH3
CH2-0H “ CH2-0H CH.2-0-C0-CH3

When esterified with a dibasic acid, it forms polymers, for example:

«(HOoC CO2H) + «(H0-CH2- CH2-OH)-^

Benzene-1,4-dicarboxylic acid

H-0-(C0 C0-0-CH2-CH2-0)n - H + (2«-1)H20

4. On oxidation, with nitric acid, both primary alcohol groups are

oxidised, first to aldehyde and then to carboxyl groups. Ethanedioic acid is
then oxidised to carbon dioxide and water:

CH=0
CH = 0

CH2-OH CH=0 CO2H CO2H

I -> I I -► I
CHj-OH CH2-OH CHO CO2H

T
2C02 + HjO

153
ALCOHOLS AND PHENOLS Uses of ethane-1,2-diol
1. In the manufacture of Terylene (p. 337).
2. As an anti-freeze for car radiators and as a de-icing fluid for aeroplane
wings. Other chemicals (anti-oxidants) are added to inhibit the formation
of acids, by oxidation of the diol, which would cause corrosion.

Propane-1,2,3-triol (glycerol)
Propane-1,2,3-triol is the simplest trihydric alcohol (triol):
CH2—CH—CHi
I I I
OH OH OH
It is a colourless, very viscous liquid which is soluble in water and ethanol.
Its chemical properties are similar to those of monohydric alcohols.

Manufacture of propane-1,2,3-triol
From propene (obtained from petroleum; 20.4) in two ways:
1. via 3-Chloropropene:

CHg-CH^CHg -CH2-CH=CH2 -1122^

3 2 400—600 “C I ^ ^
Cl

CH2-CH-CH2 > CH2-CH-CH2

I I I I \ /
Cl Cl OH Cl o

150 °C NaOH/H„0

CH2-CH-CH2
I I I
OH OH OH
2. wa Propenal:
02
CH3-CH = CH2 > 0=CH-CH = CH2
CuO as cat.
350 °C Propenal +0

H0-CH2-CH=CH2 —> CH2-CH-CH2

WO3 as cat. I I I ^
OH OH OH

3. It is a by-product in the manufacture of soap (13.10).

A

Uses of propane-1,2,3-triol
1. In the manufacture of nitroglycerin, a constituent of several
explosives:

CH2-OH CH2-O-NO2
I I
CH-OH + 3HNO3 CH-O- NO2 + 3H2O

CH2-OH CH2-O-NO2
Nitroglycerin

154
ALCOHOLS AND PHENOLS
It should be noted that nitroglycerin is not a nitro-compound as its name
may suggest. It is a nitrate ester (propane-1,2,3-triyI trinitrate).
Nitroglycerin is a colourless, oily liquid which is violently detonated on
slight shock. Oxygen is present in the molecule, and carbon dioxide, water
vapour and nitrogen are liberated to produce a very large pressure.
Dynamite, invented by the Swedish chemist, Nobel, is made by allowing
kieselguhr to absorb nitroglycerin. Although it retains its explosive pro¬
perties, the nitroglycerin is less sensitive to shock. Nobel also introduced
gun-cotton (cellulose trinitrate), and blasting gelatin, a mixture of 90 per
cent nitroglycerin and 10 per cent gun-cotton.
Cordite is a slower burning powder (30 per cent nitroglycerin and 65 per
cent gun-cotton) and is used as a propellant for shells, bullets, etc.
2. In the manufacture of glyptal plastics (p. 335).

10.8 Phenols are the hydroxy-derivatives of aromatic compounds. Examples of

some monohydric phenols:
Phenols

Phenol itself is chosen as a typical member of the group.

155
ALCOHOLS AND PHENOLS Physical properties of phenol
V

Phenol is a colourless, crystalline solid which becomes discoloured on

exposure to air and light. It is only slightly soluble in water but is very
soluble in organic solvents.

Preparation of phenol
1. From benzenesulphonic acid (obtained by the sulphonation of
benzene; 8.3). The sodium salt of the acid is fused with sodium hydroxide
at 300°C:

CeHj—SOaO-Na^ + NaOH ^ CgHs—OH + NajSOj

CeHj—OH + NaOH CgHj—0-Na+ + H2O

s Sodium phenoxide

Phenol is released from sodium phenoxide with dilute acid:

CeHs—0-Na+ + H2SO4 ^ QHj—OH + NaHS04

Phenol

2. By warming an aqueous solution of benzenediazonium chloride (16.8).

Manufacture of phenol
The process via (1-methylethyl)benzene (cumene) now accounts for about
80 per cent of the total phenol produced, the older processes via benzene¬
sulphonic acid and chlorobenzene having been largely superseded.
1. The Cumene process. Benzene is alkylated with propene, either in the
liquid phase with aluminium chloride as catalyst or in the gas phase with
phosphoric acid on an inert solid as catalyst:

CeHg + CH3—CH=CH2 ^ CgHs—CH(CH3)2

(1 -Methylethyl)benzene

Air is passed through (1-methylethyl)benzene (often called cumene)

to form its hydroperoxide which is then decomposed with warm, dilute
sulphuric acid:
CH,
I
CeHs-CHlCHg), + O2 -> CgHa-C-CHg
O-OH

QH5-OH + (CH3)2C=0

The final reaction involves a rearrangement: as the O—O bond in the protonated
hydroperoxide breaks, so the phenyl group migrates from carbon to oxygen:

CH, CH,
CeHs-C-CH, Jilx QH5-C-CH3 CH3^+^CH3
I O + H2O
O.
OH Co" QH5-O

156
ALCOHOLS AND PHENOLS
The carbonium ion reacts with water to form the products:

CH3
/CH3 I
Ce H5-O-C + H2O -> QHs-O-C-CHg + H +
^CH3
OH

CH3
QH3-O-C-CH3 -> CeHs-OH + (CH3)3C=0
OH

This process is particularly valuable because of the formation of pro-

panone as well as phenol.

2. Chlorobenzene process.

Cl, NaOH H
CeHg C.H,—Cl O'Na -CgHs-OH
Fed, cat. 330°C

3. Benzenesulphonic acid process (p. 156).

The continued superiority of the Cumene process over the others is de¬
pendent on there being a ready market for the propanone which is also
produced, for in terms of selling phenol alone, it is a relatively expensive
method.

Chemical properties of phenoi

Like alcohols, phenol reacts with acid chlorides to form esters, for example:

CgHs—OH + CH3—CO—Cl -^CgHs—O—CO—CH3 + HCl

Phenyl ethanoate

In the following respects, phenol behaves differently from both aliphatic

and aromatic alcohols. In Section 10.9 experiments are suggested which
compare and contrast the reactions of phenol with those of ethanol and
phenylmethanol.
1. Phenol does not react with hydrogen halides to form aryl halides.
2. Phenol does not react with phosphorus tribromide or tri-iodide to
form bromobenzene or iodobenzene.
3. Phenol is not oxidised in the same way as alcohols. Oxidation occurs
readily, but the products are generally complex, polymeric materials.
4. Phenol does not undergo elimination reactions.
5. Phenol is a considerably stronger acid than an alcohol; thus, K is
1-3 X 10“ for phenol and 10“^^ for methanol.

This can be understood by considering the bonding in the phenoxide ion. One
of the p orbitals on the oxygen atom, which contains two electrons, interacts
with the singly occupied p orbital on the adjacent carbon atom. The latter also
takes part in the p-orbital interactions which are characteristic of the benzene
ring (p. 60), so that a total of seven n molecular orbitals is formed. The eight p
electrons—six from carbon atoms and two from the oxygen atom—fill four of
these; one is shown in Fig. 10.2. Consequently, the charge on the phenoxide ion
is not confined to oxygen but is delocalised and therefore stabilised; in contrast,
the charge on an alkoxide ion is confined to the oxygen atom. Thus, phenol has a
greater tendency to dissociate than an alcohol.

157
ALCOHOLS AND PHENOLS Phcnol is not as strong an acid as carbonic acid or a carboxylic acid. This
affords a method for distinguishing phenol from a carboxylic acid, for
phenol does not react with an aqueous solution of sodium carbonate,
whereas carboxylic acids react to liberate carbon dioxide. The separation
of a mixture of phenol and a carboxylic acid is based on the same principle
(13.5).

FIG. 10.2. v-Bonding in the

phenoxide ion; the lower diagram
shows a delocalised w orbital
formed by overlap of the p
orbitals shown in the upper
diagram

The acidity of a phenol is increased when electron-attracting substituents are

introduced into the aromatic ring. For example, 2,4,6-trinitrophenol is quite a
strong acid which liberates carbon dioxide from sodium carbonate
solution; it is often described as an acid (picric acid) for this reason.

OH

2,4,6-Trinitrophenol
(Picric acid)

6. Phenol reacts with a neutral solution of iron(III) chloride to give a

%
violet colour. This is a characteristic reaction for compounds containing a

158
ALCOHOLS AND PHENOLS
hydroxyl group adjacent to an unsaturated carbon atom:

—C=C^
I ^
OH

This is known as an enol group. Another example of a compound containing

an enol group is ethyl 3-oxobutanoate (p. 215).

Reactions of the aromatic ring

1. Phenol is reduced to cyclohexanol when passed over nickel at about
200°C:

OH OH

Cyclohexanol

Cyclohexanol is used in the manufacture of nylon (p. 320).

2. Phenol is very reactive towards electrophilic reagents, undergoing

substitution at its 2- and 4-positions under mild conditions. The reason for
the greater reactivity of phenol than benzene in these reactions has been
described earlier (8.5). For example:
(a) With bromine, 2,4,6-tribromophenol is formed:

OH OH

3HBr

Br

Likewise, chlorine gives 2,4,6-trichlorophenol.

(b) With dilute nitric acid, a mixture of 2- and 4-nitrophenol is formed:

OH

H2O
OH

2-Nitrophenol

OH

NO2
4-Nitrophenol

159
ALCOHOLS AND PHENOLS 2-Nitrophenol possesses an internal hydrogen-bond:

In contrast, the 4-isomer forms hydrogen-bonds by attraction of the hydroxyl

hydrogen atom of one molecule to the nitro group of another:

Consequently, more energy is needed to separate the molecules of 4-nitrophenol

from each other, so that its boiling point (279°C) is higher than that of the
2-isomer (216°C). This makes it possible to separate the two compounds; a con¬
venient method is by distillation in steam (2.3), the 2-compound having the
higher vapour pressure and therefore being the more volatile in the steam.
With excess of nitric acid, 2,4,6-trinitrophenol (picric acid) is formed.
The dry compound is explosive, but solutions have been used as yellow dyes,
and it was probably the first artificial dye (1849).

Uses of phenol
1. In the manufacture of phenol-methanal plastics, e.g. Bakelite (p. 333).
2. As a starting material for the production of cyclohexanol, which is
used in the manufacture of nylon (p. 320).
3. To make substituted phenols, which are used to make epoxy resins
(p. 335).
4. To make 2,4-dichlorophenol which is used to make 2,4-dichloro-
phenoxyethanoic acid (known as 2,4-D), a selective weed killer:

^/CHa-COaH
O" Na+

(1) Cl-CHg-COrNa-^.
(2) dilute acid ^

Cl Cl

5. To make 2,4-dichloro-3,5-dimethylphenol, a powerful antiseptic

(‘Dettol’):
OH

Cl

10.9 Reactions of alcohols and phenols

Practical work For reactions with alcohols, unless stated, use either ethanol or phenyl-
methanol (which must be freshly distilled), and use phenol itself as an
example of a phenol.

160
ALCOHOLS AND PHENOLS
Reactions of the —OH group
1. Dissolve 5 drops of ethanol in 5 cm^ of water in one test-tube and
0-5 g of phenol in 5 cm^ of water in another tube. To each solution, add
1 drop of blue litmus solution.
2. (a) To 1 cm^ of an alcohol in a test-tube, add a small pellet of sodium.
Note the effervescence and test the gas evolved with a lighted splint.
(b) When all the sodium has reacted, evaporate the solution to dryness
to obtain a white residue. Add 3 drops of water to the residue and test the
solution with litmus solution.
Comment on whether these reactions would occur with phenol.
3. Warm a mixture of 5 drops of an alcohol and 5 drops of ethanoic acid
with 1 drop of concentrated sulphuric acid. Note the characteristic smell
of the product.
Comment on whether this reaction would occur with phenol.
4. To 5 drops of an alcohol in a test-tube, add 2 or 3 drops of ethanoyl
chloride. Repeat the experiment with a few crystals of phenol.
5. Schotten-Baumann reaction. To 0-5 g of phenol, add 5 cm^ of 10 per
cent sodium hydroxide. Add 5 drops of benzoyl chloride and shake. Filter
the precipitate of phenyl benzoate, wash with water and recrystallise from
hot ethanol; the product should melt at 69°C:
CgHs—OH + NaOH ^ CeH;—0-Na+ + H2O
CgHs—0-Na+ + CgHs—CO—Cl ^ CeHj—O—CO—CgHj + NaCl
Phenyl benzoate
Would you expect alcohols to undergo this reaction ?
6. To 1 cm^ of an alcohol in a test-tube, add about OT g of phosphorus
pentachloride. Test the fumes evolved by (a) moist blue litmus paper, (b)
breathing upon them.

Oxidation reactions
7. To 5 drops of ethanol, add 10 drops of dilute sulphuric acid and 2 drops
of potassium dichromate solution. Warm gently, noting (a) the colour of
the solution and (b) the smell of the product.
Repeat the experiment with (i) propan-2-ol, (ii) 2-methylpropan-2-ol,
(iii) phenylmethanol.
8. Introduce about 10 cm^ of methanol into a 100-cm^ beaker. Introduce
a red-hot spiral of platinum wire above the alcohol as shown in Fig. 10.3.

FIG. 10.3. Catalytic oxidation of

methanol

161
ALCOHOLS AND PHENOLS The spiral continues to glow and the pungent odour of methanal is noticed:
N

Pt 3.S Cd.t
2CH30H + 02-^ 2HCHO + 2H20

9. The iodoform test. To 5 drops of ethanol, add 5 drops of iodine solution

(Appendix III) and then add dilute sodium hydroxide solution dropwise
until the colour of the iodine is discharged. A yellow precipitate of tri-
iodomethane (iodoform) is obtained. Filter, dry and view the crystals under
a microscope and note their characteristic shape (hexagonal plates).
Repeat this experiment with (i) methanol and (ii) propan-2-ol.

Elimination reactions
10. The test-tube preparation of ethene in Section 6.5 is an example.
Would you expect (i) methanol, (ii) propan-2-ol, (iii) 2-methylpropan-2-
ol, (iv) phenylmethanol, to undergo this reaction ?

Substitution in the aromatic ring of phenol

11. To a few crystals of phenol in a test-tube, add 1 cm^ of water. Shake
and warm the solution. Allow to cool and add bromine water dropwise,
until a precipitate of 2,4,6-tribromophenol is obtained.
What products would you expect to obtain when bromine water is added
to (a) ethanol, (b) phenylmethanol ?
12. Phenol is nitrated with dilute nitric acid (p. 278).

Test for the enol group in phenol

13. Make up a neutral solution of iron(III) chloride by adding ammonia
solution to 2 cm^ of iron(III) chloride solution, until a precipitate just
appears. Add the original iron(III) chloride solution, dropwise, until the
precipitate just disappears.
Place 5 drops of this solution in two test-tubes. To one add 3 drops of
an alcohol and to the other add 1 crystal of phenol.

Reactions of polyhydric alcohols {choose either ethane-1,2-diol or propane-

1,2,3-triol)
14. To 2 cm^ of the alcohol in a test-tube, add a small clean pellet of
sodium. Note whether there is any effervescence, and then warm the
mixture gently. Test any gas evolved with a lighted splint.
15. To 2 cm^ of the alcohol in a test-tube, add ethanoyl chloride dropwise
(taking great CARE). Test the gas evolved.
16. To 1 cm^ of a dilute acidified solution of potassium manganate(VII),
add 5 drops of the alcohol. Warm the mixture gently.

Preparation of propane-1,2,3-triol from a fat

17. Details of the saponification of a fat are given on p. 227.

10.10 1 How and under what conditions does ethanol react with (a) sodium, (b) phos¬
phorus trichloride, (c) sulphuric acid, (d) chlorine and (e) ethanoic acid.
Questions Suggest a scheme for preparing from ethanol a compound containing four
atoms of carbon per molecule. (AEB)

2 Name and give the formula of one aliphatic monohydric alcohol and describe
« how it behaves with (a) phosphorus pentachhsride, (b) concentrated sulphuric
acid.

162
ALCOHOLS AND PHENOLS
Name and give the formula of one aliphatic dihydric alcohol, and write down
the formulae of all its possible oxidation products.
Distinguish between a primary, secondary and tertiary alcohol and explain how
each behaves on oxidation. (AEB)

3 Name the four alcohols represented by the molecular formula C4H9OH, and write
their structural formulae.
What is the effect of oxidation upon each of these compounds? Outline an
experiment by which, using an acidified dichromate solution as a relatively mild
oxidising agent, you could differentiate as far as possible between these four
alcohols by recognition of the character of their oxidation products.
Outline the procedure by which pure ethanol can be obtained industrially from
starch. (JMB)

4 Write structural formulae for the isomers corresponding to the molecular formula
C4H10O.
One of these isomers, W, reacts with sodium. A ketone, X, is formed when W
is oxidised. Dehydration of W gives a mixture of two hydrocarbons Y and Z,
each containing 85-7 per cent of carbon. Explain how these reactions enable W
to be identified and specify an appropriate reagent for its oxidation and dehydra¬
tion respectively. Predict how hydrogen bromide would react with Y and Z.
(W)
5 Outline how you would prepare a pure sample of ethanol in the laboratory.
What evidence would you cite for the presence of (a) a hydroxyl group (—OEf),
(b) a methyl group (CH3—) and (c) a methylene group (—CH2—) in ethanol ?
(L)
6 60-0 cm^ of a gaseous hydrocarbon. A, was exploded with 400 cm^ of oxygen. On
cooling to room temperature and pressure, the residual gas occupied a volume of
280 cm^. On shaking with aqueous potassium hydroxide, the volume left occupied
40 cm^. Treatment of compound A, with hydrogen chloride gas, yielded a com¬
pound B. When B was treated with aqueous sodium hydroxide, compound C was
obtained which when reacted with ethanoic acid, produced a compound D. Com¬
pound D had an empirical formula of C3H6O. Compound C was found to be
resistant to oxidation.
(a) Determine the molecular formula of A and write the structures of the pos¬
sible isomers of this compound.
(b) Give the names and structural formulae of B, C and D and explain, with the
aid of equations, the reasons for your deductions.
(c) Explain, using equations, how you would convert
(i) B into A
(ii) A into methanal (formaldehyde)
(iii) C into B. (AEB(I))

7 A compound A, containing C, 60 0; H, 13-3; O, 26-7 per cent, and having mol¬

ecular weight 60, gave on oxidation B, containing C, 62T; H, 10-3; O, 27-6 per
cent. B did not reduce Fehling’s solution.
When A was heated with concentrated sulphuric acid it gave C. Addition of
bromine to C gave D, which on boiling with aqueous sodium carbonate gave E.
Write down the structural formulae of the compounds A to E. (O and C)

8 Describe, giving essential reagents and conditions, how phenol can be prepared in
the laboratory, starting from nitrobenzene.
(a) How would you confirm that the product is a phenol?
(b) What are the products of the reaction of phenol with (i) ethanoic (acetic)
anhydride and (ii) benzenecarbonyl (benzoyl) chloride? Give equations for
these reactions. (JMB)

9 Describe the preparation of phenol from benzenesulphonic acid. Compare and

contrast the chemical reactivity of the hydroxyl group in ethanol with that in
phenol. (W)

163
ALCOHOLS AND PHENOLS 10 Outline (a) one process for the manufacture of phenol, (b) a laboratory prepara¬
tion of phenol from phenylamine.
How, and under what conditions, does phenol react with (i) sodium carbonate,
(ii) 50 per cent nitric acid, (iii) concentrated sulphuric acid, (iv) iodomethane
(methyl iodide)? (AEB)
11 (a) A polyhydroxylic compound, C4H8O4., was heated with excess ethanoic
anhydride. On refluxing 2-87 g of the product with 50 00 cm^ of molar sodium
hydroxide solution, the residual alkali required 15 00 cm^ of molar hydrochloric
acid for neutralisation. Calculate the number of hydroxyl groups per molecule
of the original compound.
(b) A compound X has the following percentage composition by weight:
C = 64-9 percent, H = 13-5 percent, O = 21-6 per cent. Oxidation yields a neutral
compound Y which does not react with sodium. Further oxidation of Y yields
an acid Z. When 8 cm^ of the vapour of Y is sparked with excess oxygen, 32 cm^
of carbon dioxide is formed. Z forms only one silver salt which contains 64-7
per cent by weight of silver.
Identify X, Y, Z and explain your reasoning. (SUJB(S))
12 Outline how phenol is manufactured from petroleum. Explain what happens
when phenol reacts with (i) iodomethane, (ii) benzoyl chloride, (iii) bromine
water, (iv) nitric acid.
13 Compare and contrast the reactions of the —OH group in phenol and in ethanol.
How, and under what conditions, does phenol react with (a) bromine, (b)
benzenediazonium chloride ?
14 Describe how phenol may be prepared from benzene.
Outline the simplest methods for effecting the following changes: (a) phenol
to phenylamine, (b) phenol to phenyl ethanoate. What action has bromine on
phenol ?
15 Four isomeric liquids stand side by side on a shelf, each with the label C4H10O,
and no other information. Two are known to be alcohols, but two have been
shown by their infrared spectra to lack an —OH group. Suggest formulae for
these compounds and outline a scheme by which they may each be identified.
(C Schol.)
16 By what reactions may ethane-1,2-diol be obtained from ethene? Give the
structural formula of ethane-1,2-diol and show how this formula may be justified.
What substances may be formed from ethane-1,2-diol by oxidation? Give their
structural formulae and show how any one of these may be confirmed by an
independent method of formation. What are the uses of ethane-1,2-diol?
17 Describe one method by which methanol is manufactured. How does it react
with (a) sodium, (b) phosphorus pentachloride, (c) ethanoic acid?
Describe one chemical test which would enable you to distinguish between
methanol and ethanol.

18 A compound. A, C3H8O2, was ethanoylated; 0-236 g of the ethanoyl derivative

was boiled with 50 cm^ of M/10 sodium hydroxide and the resulting solution
required 30 cm^ of M/20 sulphuric^acid for neutralisation. When A was oxidised
it gave B, CjHgOj, which could'not be ethanoylated, but of which 0-225 g
required 25 cm^ of M/10 sodium hydroxide for neutralisation. Assign possible
structures to A and B and account for the above results. (C Schol.)

19 An optically active compound Wundergoes the following reactions:

dilute __ NaOBr __ alkaline
C9H12O > C9H10O ^ CgHgOj ->■ C8Hg04
nitric acid potassium
W manganate(VII) Z
Z is unchanged on heating. Elucidate these reactions and give structures for W,
X, FandZ.

164
ALCOHOLS AND PHENOLS
20 Compare and contrast the properties of phenol and phenylmethanol in their
reactions, if any, with sodium carbonate, sodium hydroxide, potassium, bromine,
ethanoic acid and an acidified solution of potassium manganate(VII).

21 How do primary, secondary, and tertiary alcohols differ in their reactions with
oxidising agents ?
Investigation of the rates of the following reactions of primary, secondary, and
tertiary alcohols shows that in (a) the rates are in the order tertiary > secondary
> primary, whereas the reverse is true in (b) and (c):
(a) ROH + HBr = RBr + H2O
(b) 2ROH + 2Na = 2RONa +
(c) ROH + CH3COOH = RO. CO. CH3 + H2O
What can you deduce about the esterification reaction (c) from these observa¬
tions ? (O Schol.)

22 Contrast the properties of the hydroxyl groups of ethanol and phenol. Place the
following in order of diminishing acidity: CeHsOH, C2H5OH, CH3COOH,
H2CO3, and describe simple tests which would enable you to justify your order.
(O Schol.)

165
Chapter 11 Ethers

General formula
R—O—R'

The two R groups in the structural formula R—O—R' can be the same (the
11.1 simple ethers) or different (the mixed ethers), and can be either alkyl groups
Nomenclature or aromatic groups. According to the I.U.P.A.C. rules, the RO— group
is regarded as a substituent of., the hydrocarbon R'H; for example,
CHj—O—CH —CH is methoxyethane. However, it is common practice
2 3

to use the name compounded from the two groups R and R' followed by
ether, as in Table 11.1.

Table 11.1. Some ethers

NAME FORMULA B.P./°C

Dimethyl ether CH3—0—CH3 -24

Ethyl methyl ether CH3—0—CHj—CH3 11
Diethyl ether CH3—CH2—0—CH2—CH3 35
Methyl phenyl ether CH3—0—CsHs 154
Diphenyl ether CfiHs—0—CsHs 259

11.2 Dimethyl ether is a colourless gas, and the other lower homologues are
colourless liquids with the characteristic ‘ether’ smell. Their boiling points
Physical properties of
are much lower than those of the isomeric alcohols, but are about the same
ethers as those of the alkanes of similar formula weight (Table 11.2). Molecules of
ethers are not associated by hydrogen-bonding in the liquid phase, unlike
alcohols.

Table 11.2 The boiling points/°C of alkanes, alcohols and ethers

(The formula weights are given in brackets)

ALKANE ALCOHOL ETHER

Propane (44) Ethanol (46) Dimethyl ether (46)

-42 78 -24
Pentane (72) Butan-l-ol (74) Diethyl ether (74)
36 118 35
Heptane (100) Hexan-l-ol (102) Dipropyl ether (102)
98 157 91

166
11.3 Laboratory methods
Methods of preparation 1. Simple ethers can be prepared by the dehydration of an excess of an
alcohol with concentrated sulphuric acid at about 140°C (10.5), for example:
of ethers
C2H5—OH + H2SO4 ^ C2H5—O—SO2—OH + H2O
C2H5—OH + C2H5—O—SO2—OH C2H5—O—C2H5 + H2SO4
Diethyl ether

2. Simple and mixed ethers can be prepared by the reaction between an

alkyl halide and the sodium derivative of an alcohol (the alkoxide):

R—OH + Na ^ R—0-Na+ + iH2

R—0-Na+ + R'—X ^ R—O—R' + NaX

For example:

C2H5—0-Na+ + CH3—I -> C2H5—O—CH3 + Nal

Ethyl methyl
ether

Manufacture
Diethyl ether is obtained as a by-product during the manufacture of ethanol
from ethene and concentrated sulphuric acid.

11.4 Ethers, like alkanes, are inert towards most inorganic reagents. For example,
they are not attacked by sodium or, in the cold, by phosphorus penta-
Chemical properties of chloride, and can therefore be readily distinguished from alcohols. They
ethers have three general properties:
1. They are highly flammable, and mixtures with air are dangerously
explosive.
2. They react with a hot, concentrated solution of hydriodic acid to form
alkyl iodides, for example:

C2HJ—O—C2H5 + 2HI ^ 2C2H5—I + H2O

Reaction occurs by the reversible protonation of the ether,

C2H5—O—CiHs + HI ^ C2H5—6—C2H5 I-
1
H

followed by displacement of a molecule of the alcohol by the nucleophilic iodide

ion:

-> I-CH2CH3 + HO-QHs

CH3 H

The alcohol reacts with a further molecule of hydrogen iodide.

167
ETHERS 3. They react with phosphorus pentachloride when heated. No hydrogen
chloride is evolved, showing that ethers do not contain a hydroxyl group
(10.5):
R—O—R' + PCI5 ^ R—Cl + R'—Cl + POCI3

11.5 Diethyl ether is used as an anaesthetic. Inhalation of the vapour depresses

the activity of the central nervous system.
Uses of ethers
Ethers, particularly diethyl ether, are used as solvents for fats, oils and
resins.
The ability of ethers to dissolve a wide range of organic compounds,
coupled with their resistance to chemical reaction, makes them valuable
solvents for organic preparations (e.g. Grignard reagents; 9.8) and for the
separation and purification of compounds by extraction.

11.6 Cyclic ethers with five or more members in the ring are, like the correspond¬
ing cycloalkanes, relatively strainless and have the properties of their non-
Cyclic ethers
cyclic analogues. An example is tetrahydrofuran. However, those with
three- and four-membered rings are strained and, like cyclopropane and
cyclobutane (5.6), are very reactive towards reagents which are capable of
opening the ring. An example is epoxyethane (ethylene oxide).

HX-CH2 HX-CH2
\ / 7 \
O H.,C^ -
GH2
Epoxyethane
Tetrahydrofuran

Manufacture of epoxyethane
By passing ethene and oxygen at 250°C over a silver catalyst;

CH2=CH2 + IO2 -> H2C-CH2

Physical properties of epoxyethane

It is a volatile liquid, b.p. 13°C, which is soluble in water and in organic
solvents.

Chemical properties of epoxyethane

1. It is hydrolysed by steam at 200°C under pressure, or by dilute acids at
60°C and atmospheric pressure:

HoC-CHj CH2-CH2
' \ / ^ H2O —> I I
O OH OH

Ethane-1,2-diol

« This is the basis of the manufacture of ethane-1,2-diol.

168
ETHERS
2. It reacts with the halogen acids, for example:

H2C-CH2 CH,-CH2
\ / ' + HCl ^ I “ I
O Cl OH
2-Chloroethanol

3. It reacts with Grignard reagents to yield primary alcohols (9.8).

Uses of epoxyethane
1. In the manufacture of ethane-1,2-diol.
2. In the manufacture of diol ethers which are used as de-icing fluids,
brake fluids and solvents. The diol ethers have an ether and a primary
alcohol group:

H2C-CH2 CH2-CH2
\ / + R-OH I I
O OR OH

3. In the manufacture of non-ionic detergents (13.10).

11.7 1 Describe the laboratory preparation of diethyl ether.

Deduce the probable structural formula of an ether which contains by weight
Questions 60 per cent of carbon and 13-3 per cent of hydrogen, and suggest how it could be
prepared.
*
2 Starting with a sample of radioactive potassium cyanide KCN, suggest reaction
schemes enabling you to label each of the carbon atoms in ethyl methyl ether
CH3OCH2CH3.
How would you confirm the position of the labelled carbon atom in
CH3OCH2CH3?

3 Describe the preparation of a pure sample of diethyl ether.

Four unlabelled bottles contain samples of diethyl ether, pentane, propanone,
and ethanol. What chemical tests would you use to identify each compound?
(C(T))

4 Write an equation for the laboratory preparation of diethyl ether from ethanol.
Why is an excess of ethanol used in the preparation? What is the main organic
impurity in the distillate likely to be and how may it be removed?
Explain why ether is particularly suitable as a solvent for the extraction of an
organic compound from an aqueous solution. What is the main disadvantage in
using ether for this purpose?
Calculate the weight of phenylamine which would be extracted from 100 cm^
of an aqueous solution containing 5-0 g phenylamine by shaking with
(a) 50 cm^ of ether is one portion,
(b) two successive 25 cm^ portions of ether.
Comment on the results.
(Partition coefficient of phenylamine between ether and water = 5.) (L(X))

5 y4 is a liquid containing carbon, hydrogen and iodine only. 0’150g of T in a

Victor Meyer’s apparatus displaced 25-3 cm^ of air, collected over water at 15°C
and 763 mm pressure.

169
ETHERS Another compound, B, containing carbon, hydrogen and oxygen only, reacts
vigorously with metallic sodium when hydrogen is liberated and a white solid, C,
is formed.
When A was heated under reflux with C and the mixture subsequently distilled,
a compound D, containing carbon, hydrogen and oxygen only was obtained.
D contained C, 64-9 per cent and H, 13-5 per cent and it was not attacked by
sodium even on warming.
Identify A and then show that there are two possible compounds for each of
B, C and D.
The saturated vapour pressure of water at 15° is 13-0 mm. (L)
6 16 cm^ of a gaseous aliphatic compound A, C„H3„Om, was mixed with 60 cm^
of oxygen at room temperature and sparked. At the original temperature again,
the final gas mixture occupied 44 cm^. After treatment with potassium hydroxide
solution the volume of gas remaining was 12 cm^. Deduce the molecular and
structural formulae of A, and name it.
Give the name and structural formula of a compound B isomeric with A, and
state briefly how A and B react separately with (a) sodium, (b) hydrogen iodide,
(c) phosphorus trichloride.
(If there is no reaction in any one case, make this clear.)
Outline a reaction scheme, stating reagents, by which A might be prepared
from B. (SUJB)
7 A neutral compound, P, C9H12O2, fumes when treated with phosphorus penta-
chloride and, when heated with acidified sodium dichromate solution another
neutral compound, Q, is formed. Q produces a characteristic orange-red pre¬
cipitate with a solution of 2,4-dinitrophenylhydrazine but Q does not react with
an ammoniacal solution of silver oxide.
P gives a yellow precipitate with iodine and alkali and if the filtrate from this
reaction is acidified, an acid, R, CsHgOj, is produced. R, on boiling with hydrogen
iodide, gives a further acid, S. A familiar smell of oil of wintergreen is produced
if S is warmed with methanol containing a little concentrated sulphuric acid as
catalyst.
Deduce the nature of the compounds P, Q, R and 5 and explain fully all the
reactions.
What is the significance of the reaction between 5 and ethanoic anhydride ?
(S(S))

170
Chapter 12
Aldehydes and ketones

12.1 General formula

Introduction R-C-H R-C-R
il II
o o
Aldehyde Ketone

Both aldehydes and ketones contain the carbonyl group (^C=0). This
group has characteristic properties which are shown by both classes of
compound, so that it is convenient for the two homologous series to be
considered together. However, the attachment of a hydrogen atom to the
carbonyl group in an aldehyde gives aldehydes certain properties which
ketones do not possess and which enable the two classes of compound to be
distinguished from one another.

The I.U.P.A.C. nomenclature uses the suffixes -al for aldehydes and -one
for ketones; the main carbon chain is named as usual and, for ketones, the
position of the carbonyl group is specified by inserting the number of its
carbon atom from the nearer end of the chain. For example:

CHj—CH2—CH2—CH2—CHO CHj—CH2—CO—CH2—CH2—CH3
Pentanal Hexan-3-one

12.2 The simpler members of the series are often known by their original names.
Some examples, with the original names in parentheses, together with their
Nomenclature boiling points, are in Table 12.1.

Table 12.1. Some aldehydes and ketones

NAME STRUCTURAL FORMULA B.P./°C

Methanal (formaldehyde) H—CHO -21

Ethahal (acetaldehyde) CH3—CHO 21
Propanal (propionaldehyde) CH3—CH2—CHO 49
Benzaldehyde CgHs—CHO 179

Propanone (acetone) CH3—CO—CH3 56

Butanone (ethyl methyl ketone) CH3—CO—CH2—CH3 80
Pentan-3-one (diethyl ketone) CH3—CH2—CO—CH2—CH3 102
Phenylethanone (acetophenone) CgHs—CO—CH3 202
Diphenylmethanone
(benzophenone) CfiHs-CO—CeHs 306

Methanal is a gas, other aldehydes and ketones of relatively low formula

12.3 weight are liquids and the remainder are solids. Methanal dissolves readily
Physical properties of in water; a 40 per cent solution is known as formalin. The liquid aldehydes
aldehydes and ketones and ketones of low formula weight are also readily soluble in water (for

171
ALDEHYDES AND KETONES example, ethanal and propanone are miscible with water), but solubility
decreases as the formula weight increases.

12.4 General methods

Methods of preparation 1. Aldehydes are obtained by the oxidation of primary alcohols, and
ketones by the oxidation of secondary alcohols.
of aldehydes and ketones
A convenient oxidising agent is an acidified solution of sodium di¬
chromate. For example:

3CH3CH2OH + Na2Cr207 + 4H2SO4

3CH3CHO + Na2S04 + Cr2(S04)3 + 7H2O
Ethanal

3(CH3)2CH0H + Na2Cr207 + 4H2SO4 ^

3CH3COCH3 + Na2S04 + Cr2(S04)3 + 7H2O
Propanone

Sodium dichromate can be regarded as a solution of chromium(VI) oxide:

NaiCrjOv + H^O ^ 2Cr03 + 2NaOH

Reaction with the alcohol occurs by formation of an unstable chromium(VI)

ester which breaks down to give the carbonyl compound and a chromium(IV)
ion:
O
II
RjCHOH + CrOg “ R,CH-0-Cr-0H
II
O

H 0'S O-
K P I
R,C-0-=^Cr-0H RoC — O + Cr- OH +
II II
O O

Three chromium(IV) ions disproportionate to give two chromium(III) ions and

one chromium(VI) species:

SHCrOj- + 9H+ ^ CrOj + 2Cr^+ + 6H2O

A difficulty arises in the preparation of aldehydes in this way: the

aldehyde is itself readily oxidised (12.5). It is necessary to remove the
aldehyde as soon as it is formed in order to prevent this happening, and this
can readily be done because aldehydes have lower boiling points than the
corresponding alcohols (whose boiling points are relatively high because of
their hydrogen-bonded structure; 10.3). Thus, the alcohol is added slowly
to the hot oxidising agent so that the aldehyde boils off as fast as it is formed
whereas the higher boiling alcohol remains in solution until it is oxidised.
Details of the laboratory preparation of ethanal (p. 183) and propanone
(p. 183) are given.

2. Aldehydes and ketones are formed by the hydrolysis of gem-

dichlorides (RCHCI2 and R2CCI2), for example:

QH5—CHCI2 + H2O ^ CeHs—CHO + 2HC1

%
Benzaldehyde

172
ALDEHYDES AND KETONES
This method is of limited use for aliphatic compounds because of the difficulty
of obtaining the dichloro-compounds; in fact, these compounds are usually made
from the corresponding aldehyde or ketone with phosphorus pentachloride (12.5).
However, the method is particularly useful for aromatic aldehydes because the
dichloro-compounds can be obtained by the free-radical chlorination of the
corresponding methyl compound (8.4), so that to obtain benzaldehyde, methyl-
benzene would be the starting material:

CsHs—CH3 + 2CI2 '*^^1 CfiHs—CHCI2 + 2HC1

heat

Specific method for aldehydes

Acid chlorides are reduced to aldehydes by hydrogen on palladium which
is supported on barium sulphate;

R-C-Cl r-c-h
II + H2 -> II + HCl
O o
Sulphur and quinoline are added as a poison to prevent the reduction of
the aldehyde to the primary alcohol.

Specific method for aromatic ketones

Many aromatic compounds react with acid chlorides in the presence of
aluminium chloride to give aromatic ketones (Friedel-Crafts reaction; 8.3),
for example:
AlCU as cat.
CftHg + CH3—CO—Cl -=-> CeHj—CO—CH3 + HCl
Phenylethanone
AICI3 as cat.
CfiHg + CgHj—CO—Cl -^-> CeHj-CO—CgHj + HCl
Diphenylmethanone

Manufacture of ethanal
1. Wacker process. By oxidising ethene with palladium(II) chloride in
water:

CH2=CH2 + H2O + PdCl2 ^ CH3CHO + 2HC1 + Pd

By carrying out the reaction in the presence of a copper(II) salt, the pal¬
ladium which is formed is oxidised back to palladium(II) ion:

Pd + 2Cu2+ ^Pd^"- + 2Cu +

In the presence of air, the copper(I) ion is oxidised back to copper(II) ion;

4Cu+ + O2 + 4HC1 ^ 4Cu^+ + 2H2O + 4Cr

Therefore, the reaction requires only catalytic amounts of palladium and

copper salts, so that the principal cost is that of ethene. Since the Wacker
process is essentially one-stage from ethene, and ethene is cheaper than
ethyne, this process has superseded the two older routes, outlined below.

173
ALDEHYDES AND KETONES 2. By passing ethyne through dilute sulphuric acid, with mercury(II)
sulphate as catalyst, at 60°C:

CH=CH + H2O CH3—CHO

3. By oxidation of ethanol (which is manufactured from ethene) in the

gas phase over a silver or a copper catalyst:

CHjCHjOH + iO, ‘ CH3CHO + HjO

CHjCHjOH CHjCHO + H,

Manufacture of methanal
By the oxidation of methanol vapour over heated copper or silver:

CH3OH +
3
i2 02^ 500°C
HCHO + H2O

Manufacture of propanone
1. As a co-product from the Cumene process for the manufacture of
phenol (10.8).
2. By passing the vapour of propan-2-ol over copper at 500°C. The
alcohol is obtained from propene (10.4).

(CH,),CHOH (CH3),C0 + H,

3. By oxidation of propan-2-ol in the liquid phase (20.4).

4. As a by-product in the manufacture of ethanoic acid from naphtha
(13.4).

The reactions can be divided into three types:

12.5
(a) Reactions of the carbonyl group.
Chemical properties of (b) Reactions of the alkyl group(s) adjacent to the carbonyl group.
aldehydes and ketones (c) Oxidation reactions. Reactions of this type constitute the principal
difference between aldehydes and ketones.
Many of the reactions which follow are shown by all aldehydes and
ketones, but some members of each series show exceptional behaviour
which is mentioned below and also in a section on anomalous properties
(p. 182).

(a) Reactions of the carbonyl group

These can be subdivided into (i) addition reactions and (ii) condensation
reactions.

(i) Addition reactions

Both aldehydes and ketones undergo addition reactions. The following are
examples:
1. They react with hydrogen cyanide to form 2-hydroxynitriles (cyano¬
hydrins), for example:

174
ALDEHYDES AND KETONES
OH

CH3CHO + HCN ^ CH3—C—H

CN
2-Hydroxypropanenitrile

These reactions occur very slowly, but their rates are greatly increased by
the addition of some alkali. This is because the slow step is the addition
of the cyanide ion to the carbonyl group; the resulting anion then takes
up a proton, for example:
o o- OH
I I
CH 3 c- H CH3-C-H CH3-C-H
f I I
c C C
III III
N N
N
Thus, hydrogen cyanide itself adds slowly because, being a very weak acid
{Ka = 5 X 10“^°), its solution contains only a very small proportion of
cyanide ions, whereas in the presence of alkali a much larger concentration
of cyanide ions is present since the equilibrium.

HCN + OH- CN- + H2O

lies to the right.

In practice, in the laboratory, it is more convenient, and safer, to replace
hydrogen cyanide and alkali by an alkali-metal cyanide like KCN (whose
solution contains a high proportion of cyanide ions) and acid; but too
much acid must be avoided for otherwise the concentration of cyanide ion
is reduced to so low a level that the rate of reaction is again too slow for
practical purposes.

It was the observation that alkali catalyses the addition of hydrogen cyanide to
carbonyl compounds that led to the reaction mechanism described above. This was
the first organic mechanism to be elucidated, in 1902. The carbonyl compound
chosen for study was a coloured one which gave a colourless addition product so
that it was possible to assess the rate, roughly, by following the loss of colour by eye.
It was found that reaction between the ketone and hydrogen cyanide alone took
8-10 hours to go to completion, and when some mineral acid was added there was
no detectable reaction even after 14 days; but the addition of a drop of an aqueous
solution of potassium hydroxide caused the reaction to go to completion in a matter
of seconds.

The mechanism of formation of cyanohydrins is typical of all nucleo¬

philic additions to aldehydes and ketones: that is, reaction occurs by addi¬
tion of a nucleophile to form an anion (Fig. 12.1) followed by uptake of
a proton.
An understanding of the mechanism enables us to understand why the
double bond in an aldehyde or ketone is reactive towards nucleophilic
reagents whereas that in an alkene is not. In the anion formed by addition
to a carbonyl group, the negative charge resides on oxygen:

o-
CH3-C- H
I
CN

175
However, if a nucleophile adds to an alkene, the negative charge resides
on carbon; since carbon is much less strongly electron-attracting than
oxygen, this species is less stable and less readily formed.
Aromatic aldehydes, such as benzaldehyde, react with potassium
cyanide in a different manner from other aldehydes or ketones (p. 183).

2. Aldehydes and ketones react with sodium hydrogensulphite to form

addition compounds, for example:

/OH
(CH3)2C=0 + NaHSOa
S02-0~ Na^

In this reaction, both the hydrogensulphite and the sulphite ions act as
nucleophilic reagents:

O CH,
II I
o=s-c-o~
I I
HO CH3

O CH,
II I
o=s-c-o^
I I
O- CH3

The sulphite ion reacts the more rapidly, but hydrogensulphite ion is a weak acid
(K^= 11 X 10“^), so that the equilibrium
HSOj- -h H2O S03^“ -h H3O +
lies well to the left and under the usual reaction conditions [HS03“] is much
greater than [SOj^'], serving to offset to some extent the greater reactivity
of the sulphite ion. Which of the two ions makes the more important contribution
to the overall reaction depends on a variety of factors, including the structure
of the carbonyl compound and the pH.

176
ALDEHYDES AND KETONES
Ketones only undergo this reaction if at least one of the two groups attached
to the carbonyl group is methyl. The probable reason is that methyl is the smallest
group, and when two larger groups are attached, their size hinders the approach
of the nucleophilic reagent to the carbonyl group.

3. Aldehydes and ketones are reduced to alcohols by treatment with

sodium tetrahydridoborate in water. Aldehydes give primary alcohols and
ketones give secondary alcohols, for example;

4CH3—CH-=0 + NaBH4^(CH3—CH2—0)4B-Na +

(CH3—CH2—0)4B”Na+ + 3H2O ^
4CH3—CH2—OH + NaH2B03

The reducing agent is the tetrahydridoborate anion, BH4“. It acts as a nucleo¬

phile by transferring a hydride ion to the carbonyl group:

H3B-4r~^^CH=Q) -> BH3 + CH3-CH2-O'

CH3

The species BH3 immediately reacts with the alkoxide ion,

CH3—CH2—O" + BH3 ^ CH3—CH2—O—BH3

and the resulting anion then reduces another carbonyl group. Further reactions
of this type occur until all four hydrogens of the BH4“ anion have been replaced.

Lithium tetrahydridoaluminate, LiAlH4, can be used in place of sodium

tetrahydridoborate. It is a much more powerful reducing agent and
reacts violently with water, so that it is necessary to use an inert sol¬
vent such as ether. The product of the reaction is a salt of the alcohol,
(R2CHO)4Al“Li^; excess of LiAlH4 is destroyed by adding ethyl etha-
noate (which acts by being reduced to ethanol), and the alcohol is then
liberated from the salt by addition of dilute sulphuric acid:

4R2CO + LiAlH4 ^ (R2CH0)4A1-Li+

2(R2CHO)4ALLi+ + 4H2SO4 ^ 8R2CHOH + Al2(S04)3 + Li2S04

The aldehyde or ketone reacts with the tetrahydridoaluminate anion, A1H4“, in

a similar way to the tetrahydridoborate anion.

4. Aldehydes and ketones are also reduced with sodium amalgam and
water, with sodium and ethanol or with zinc and ethanoic acid, for example:

C2H5CH=0 + 2Na + 2C2H5OH C2H5CH2OH + 2C2H50-Na+

(CH3)2C=0 -f Zn + 2CH3CO2H -> (CH3)2CH0H + (CH3C02)2Zn

These reactions do not occur by nucleophilic attack but by the transfer of

electrons from the electropositive metal to the carbonyl group and the uptake
of two protons from the water, ethanol, or acid; a simple representation is:

V = 0 + 2e + 2H+-> Vh-OH
/ /

Sodium provides one electron, so that two atoms of sodium per molecule of
carbonyl compound are required, whereas one atom of zinc, which can provide
two electrons (giving Zn^'^), reduces one molecule of carbonyl compound.

177
ALDEHYDES AND KETONES 5. Aldehydes and ketones react with phosphorus pentachloride to give
gem-dichloro compounds,'for example:

CHj—CH=0 + PCI5 CH3—CHCI2 + POCI3

1,1 -Dichloroethane

CH3—CO—CH3 + PCI5 ^ CH3—CCI2—CH3 + POCI3

2,2-Dichloropropane

(ii) Condensation reactions

Aldehydes and ketones react with compounds which contain the —NH2
group with the elimination of water.
Hydroxylamine forms oximes, for example:

CH3CH=0 + NH2OH ^ CH3CH=N—OH + H2O

^ Ethanal oxime
(CH3)2C=0 + NH2OH -> (CH3)2C=N—OH + H2O
Propanone oxime

Hydrazine forms hydrazones which, since they still contain an —NH2 group,
can react with more of the carbonyl compound to give azines, for example:

CH3CH=0 + NH2NH2 CH3CH=N—NH2 + H2O

CH3CH=N—NH2 + CH3CH=0 ^ CH3CH=N—N=CHCH3 + H2O

Phenylhydrazine forms phenylhydrazones, for example:

C6H5CH=0 + H2N—NH—CftH, ^ C6H5CH=N—NH—CfiHs + H2O

Phenylhydrazine Benzaldehyde
phenylhydrazone

2,4-Dinitrophenylhydrazine forms 2,4-dinitrophenylhydrazones, for

example:

Phenylethanone
2,4-dinitrophenylhydrazone

Primary amines form imines (also called Schiff bases), for example:

C6H5CH=0 + H2N—CeHs CeHjCH^N—C^Hj + H2O

« However, many of the derivatives with amines are unstable.

178
ALDEHYDES AND KETONES
The reactions with hydroxylamine, phenylhydrazine and 2,4-dinitro-
phenylhydrazine are used for the characterisation of aldehydes apd ketones
because the products are mostly crystalline solids and the melting points of
the derivatives from closely similar aldehydes or ketones are usually suffi¬
ciently different to enable the carbonyl compound to be recognised.

The reactions all occur by nucleophilic addition to the carbonyl group followed
by the movement of a proton from one atom to another and then the elimination
of water, for example:

H R
1+ I
> HO-N- C-O
I
H R

R R
I
HO-N-C-OH HO-N=-C^ + H2O
I I
H R R

(b) Reactions of the akyl group(s)

1. Aldehydes and ketones which possess at least one hydrogen atom on
the carbon atom adjacent to the carbonyl group (i.e. ^CH—CO—, etc.)
undergo condensation reactions in the presence of a base. For example,
ethanal and dilute alkali give 3-hydroxybutanal;
NaOH
2CH3CHO ^ CH3—CH—CH2—CH=0

OH
3-Hydroxybutanal

If the solution is warmed, water is eliminated:

CH3—CH—CH2—CH=0 CH3—CH=CH—CH=0 + H2O

But-2-enal
OH

Propanone gives 4-hydroxy-4-methylpentan-2-one;

NaOH
2(CH3)2C0 (CH3)2C—CH2—C—CH3

OH O

With concentrated alkali, ethanal forms a resin which precipitates from

the solution. It is a polymeric compound of structure

CH3-fCH=CH^CH=0

where x is large and varies with the conditions of the reaction.

However, those aldehydes which do not contain at least one hydrogen
atom on the carbon next to the carbonyl group (e.g. methanal and benzalde-
hyde) do not undergo condensation reactions with alkali (see above) but

179
ALDEHYDES AND KETONES instead, with Concentrated alkali, undergo the Cannizzaro reaction in which
one half of the quantity of the aldehyde is oxidised and the other half is
reduced, for example:

ICeHj—CH=0 + NaOH ^ CgHj—C02“Na+ + C^Hj—CH2OH

Sodium benzoate Phenylmethanol

Reaction occurs by addition of hydroxide ion to the carbonyl group of one

molecule of the aldehyde, to give an intermediate which transfers a hydride ion
to a second molecule of the aldehyde. Reaction is completed by transference of
a proton;
0“
I
QH5-CH = 0 + OH --> CeHj-C-H
OH

Oj -i p
QH5-c4t~^CH=Q) -> + CeH^-CH^-O”

OH QHs OH

O
-> QH3-C^ + CeHs-CH^-OH
o~

2. Aldehydes and ketones which possess at least one hydrogen atom on

the carbon atom adjacent to the carbonyl group react with the halogens in
the presence of dilute acid or alkali; one or more of these hydrogen atoms
is replaced by halogen atoms. In the acid-catalysed reaction, it is possible
to isolate the monohalogenated product, since introduction of a second
halogen atom occurs more slowly than that of the first, for example:

CHj—CH2—CO—CH2—CHj + I2
CH3—CHI—CO—CH2—CHj + HI

However, in the base-catalysed reaction, the reaction rate increases as each

halogen atom is introduced, so that it is possible to isolate only the product
in which all the appropriate hydrogen atoms have been replaced, for
example:

CH3—CH2—CO—CH2—CHj + 4I2
CH3—CI2—CO—CI2—CH3 + 4HI

If the carbonyl compound contains the group —CO—CH3 (i.e. ethanal

or a methyl ketone), reaction in the presence of alkali leads first to the
replacement of all three hydrogen atoms in the methyl group by halogen
atoms, for example,

CH3—CH2—CO—CH3 4- 3CI2 + 3NaOH ->

CH3—CH2—CO—CCI3 + 3H2O -b 3NaCl

and then, with excess of alkali, to breakage of a C—C bond;

CH3—CH2—CO—CCI3 -b NaOH ^
CHj—CH2—CO2- Na+ + CHCI3
Trichloromethane

180
ALDEHYDES AND KETONES
Similarly, bromine gives tribromomethane, CHBrj, and iodine gives
tri-iodomethane, CHI3. Tri-iodomethane is a yellow crystalline solid which
is easily recognised, and its formation from a carbonyl compound indicates
that this must have been either ethanal or a methyl ketone, since no others
contain the group —CO—CH3. This reaction is known as the iodoform test,
after the original name for tri-iodomethane. Since alcohols which contain
the group —CH(OH)—CH3 (e.g. ethanol and propan-2-ol) are oxidised
by iodine to give the group —CO—CH3, these also give a positive iodoform
test (p. 150).

(c) Oxidation reactions

The principal difference between an aldehyde and a ketone is that the
—CH=0 group of an aldehyde is readily oxidised to the carboxylic acid
group, —CO2H, whereas ketones are difficult to oxidise in solution.

Oxidation of aldehydes
Aldehydes are oxidised to carboxylic acids by sodium (or potassium)
dichromate in acidic solution:

Na2Cr207/H2S04
R—C—H -^ R—C—OH

0 0

Likewise, they are oxidised by potassium manganate(VII).

The ready oxidation of aldehydes compared with ketones enables the two
types of carbonyl compound to be simply distinguished. Three reactions
for this purpose (details for two of which are on p. 185) are:

1. A solution of silver nitrate in an excess of a solution of ammonia,

which contains the complex ion, Ag(NH3)2’^, is reduced by an aldehyde to
silver, which deposits on the wall of a test-tube as an easily recognised
mirror.

2. Fehling’s solution, which is made by mixing a solution of copper(II)

sulphate with an alkaline solution of a salt of 2,3-dihydroxybutanoic
(tartaric) acid and which contains a deep-blue complexed copper(II) ion,
is reduced by an aldehyde to copper(I) oxide, which deposits as a red
precipitate.
However, some aldehydes, such as benzaldehyde, do not react in this
way, so that a negative result must be interpreted with care.

3. Fuchsin is a pink dye which forms a colourless complex when treated

with sulphur dioxide. The addition of an aldehyde to this colourless solution
restores the pink colour of the dye (Schiff’s test).

Oxidation of ketones
Ketones are oxidised by strong oxidising agents, such as alkaline potassium
manganate(VII) and hot nitric acid. The bond between the carbonyl
group and the adjacent carbon atom is broken, for example:

CH3—CH2—CO—CH2—CH3 CH3—CH2—CO2H -h CH3—CO2H

181
ALDEHYDES AND KETONES The acids formed contain fewer carbon atoms than the ketone. The acid
formed on oxidation of an aldehyde contains the same number of carbon
atoms.

Specific reactions of aldehydes and ketones

Methanal, HCHO, differs from other aldehydes in the following ways:

1. When an aqueous solution of methanal is gently evaporated, a linear

polymer, polymethanal, is formed. Polymethanal has the structure

HOfCHa—0>^CH20H

When it is distilled from an acidified solution, methanal forms the cyclic

trimer: ^

H2C CH2
3HCHO > I 1

2. It does not undergo a condensation reaction of the type ethanal

undergoes when treated with alkali, but with concentrated alkali it under¬
goes the Cannizzaro reaction;

2HCHO + NaOH ^ H—C02-Na+ + CH3OH

This behaviour of methanal is typical of that of aldehydes which do

not have a hydrogen atom attached to the carbon atom next to the carbonyl
group.
Ethanal, CH3CHO, forms cyclic polymers with acids:

CH,
\
CH-O CH,
\/
4CH,CHO O CH
0 °C

CH, O-CH
^ \
CH3
Ethanal tetramer

CH3 O /CH3
C^i CH
3CH,CHO
room temp.

I
CH3

Ethanal trimer

Benzaldehyde, CgHsCHO, does not reduce Fehling’s solution or undergo

condensations with alkali, but it undergoes the Cannizzaro reaction. In

182
ALDEHYDES AND KETONES
these respects it is typical of aldehydes which do not have a C—H bond
adjacent to carbonyl.
Benzaldehyde is peculiar in not giving a cyanohydrin with potassium
cyanide. Instead, it undergoes a condensation reaction to form 2-hydroxy-
1,2-diphenylethanone, in which cyanide ion acts as a catalyst;

CN-
IQHsCHO -> QHs-C-CH-QHg

O OH
2-Hydroxy-1,2-diphenylethanone

12.6 Methanal
Uses of aldehydes and 1. In the manufacture of thermosetting plastics, in particular Bakelite,
carbamide-methanal resins and polyoxymethylene (p. 332, 333).
ketones
2. In solution (formalin) it is used as a disinfectant and to preserve animal
specimens.

Ethanal
In the manufacture of ethanoic acid (p. 191).
However, ethene (from which ethanal is made) is becoming progressively
more expensive relative to the starting materials for other routes to
ethanoic acid (p. 191), and these are now displacing the route via ethanal.

Propanone
1. In the manufacture of Perspex (p. 331).
2. In the manufacture of ethenone, used to make ethanoic anhydride
(14.4).
3. As a solvent for plastics, varnishes and greases.

12.7 Small-scale preparation of ethanal

Practical work To 6 cm^ of water in a flask, add 2 cm^ of concentrated sulphuric acid, and
set up the apparatus as shown in Figure 12.2.
Make up a solution containing 5 g of sodium dichromate in 5 cm^ of
water, add 4 cm^ of ethanol and pour the solution into the dropping funnel.
Boil the acid in the flask and then turn out the bunsen flame. Add the
mixture containing ethanol slowly so that the dilute acid remains near its
boiling point.
Collect the distillate and redistil (Fig. 2.2), using a hot-water bath.
Collect the fraction boiling between 20 and 23°C.

Small-scale preparation of propanone

Repeat the experiment above, using 4 cm^ of propan-2-ol in place of 4 cm^
of ethanol. Collect the distillate and redistil it, collecting the fraction boiling
between 54 and 57°C.

183
FIG. 12.2. Preparation of
ethanal

Reactions of methanal, ethanal, benzaldehyde

propanone, pentan-3-one and phenylethanone
(a) Reactions of the carbonyl group
Addition reactions
1. Preparation of ethanal hydrogensulphite. Make up 10 cm^ of a saturated
solution of sodium metabisulphite, in a flask, and pass sulphur dioxide
through it for about 3 minutes. Add 1 cm^ of ethanal, shake, stopper the
flask and leave for 1-2 hours. Crystals of the addition product, ethanal
hydrogensulphite, are formed.
The experiment can be repeated with other aldehydes and ketones, for
example with propanone (1 cm^) or benzaldehyde (1 cm^), instead of
ethanal.

Condensation reactions
2. To 5 drops of one of the aldehydes or ketones in a test-tube, add
methanol until the compound just dissolves. Add 5 cm^ of the solution of
2,4-dinitrophenylhydrazine (see Appendix III, p. 374, for the preparation

184
ALDEHYDES AND KETONES
of this solution). Cork the test-tube and shake the mixture. Allow it to
stand. If a precipitate is not formed within 5 minutes, add dilute sulphuric
acid dropwise.
Filter the precipitate using a small Buchner funnel and flask and wash
it with a minimum amount of methanol. Transfer the precipitate to a filter
paper and squeeze it between two papers to dry it. Recrystallise the solid
from the minimum quantity of a (1:1) mixture of ethanol and water. Filter,
dry the crystals and obtain the melting point (p. 379).

(b) Reactions of the alkyl group

3. To 3 drops of ethanal in a test-tube, cautiously add concentrated
sodium hydroxide solution drop by drop. A brown resin is formed with a
characteristic smell.
4. To 5 drops of propanone, add 10 drops of iodine solution and 10 drops
of 2M hydrochloric acid. Warm the mixture gently.
Repeat the experiment with pentan-3-one.
5. The iodoform reaction. In four separate test-tubes, add 5 drops of
(i) ethanal, (ii) propanone, (iii) benzaldehyde and (iv) phenylethanone,
followed by 10 drops of iodine solution in each (Appendix II). Add dilute
sodium hydroxide solution dropwise until the brown colour just disappears.
Observe what happens (a) in the cold, (b) when the mixtures are gently
warmed.

(c) Oxidation reactions

6. Silver mirror test. Clean 3 test-tubes by washing them thoroughly with
distilled water and then with propanone and drying them.
To 3 cm^ of a solution of silver nitrate, add 1 drop of a dilute solution
of sodium hydroxide. Add dilute ammonia solution dropwise until the
brown precipitate of silver oxide just redissolves. Divide this solution into
three test-tubes.
To one of the test-tubes, add 3 drops of ethanal, to the second 3 drops of
propanone and to the third 3 drops of benzaldehyde.
Warm the test-tubes in a beaker of boiling water for about 5 minutes.
7. Fehling's test. Make up some Fehling’s solution by adding solution II
(an alkaline solution of sodium potassium 2,3-dihydroxybutanedioate)
to 5 cm^ of solution I (a solution of copper(II) sulphate) until a deep blue
solution is formed (Appendix III). Divide the solution equally between 3
test-tubes.
To one, add 3 drops of ethanal, to the second, 3 drops of propanone
and to the other, 3 drops of phenylethanone. Boil the mixtures for a few
minutes.
8. Reaction with potassium manganate{VII). To 10 cm^ of dilute sulphu¬
ric acid in a test-tube, add 2 cm^ of a 1 per cent solution of potassium
manganate(VII) and divide the solution between 5 test-tubes. Add 3 drops
of the following to the test-tubes: (i)methanal, (ii) ethanal, (iii) propanone,
(iv) benzaldehyde, (v) phenylethanone. At first shake the mixtures without
warming, then warm them very gently.
In particular, contrast the behaviour of (a) ethanal and propanone,
(b) propanone and phenylethanone.

(d) Specific reactions of some aldehydes and ketones

9. Pour 1-2 cm^ of methanal solution (formalin) on a watch-glass,
and place it on a beaker containing water that is boiling gently.

185
ALDEHYDES AND KETONES 10. Pour 1-2 cm^ of ethanal into a test-tube and place the tube in a
beaker containing an ice-salt freezing mixture. Add two drops of con¬
centrated sulphuric acid and stir the mixture gently. Observe whether there
is a rise in temperature (why ?) and whether a new compound is formed.
11. The Cannizzaro Reaction. To a cool solution of potassium hydroxide
(5 g in 5 cm^ of water) in a boiling tube, add about 5 cm^ of benzaldehyde.
Cork the boiling tube, shake and allow it to stand overnight. Add about
20 cm^ of water to dissolve the potassium benzoate, and extract the aqueous
layer with ether. To the aqueous portion, add dilute hydrochloric acid to
precipitate benzoic acid. Filter and recrystallise from hot water (m.p. 121 °C).
Dry the ethereal extract over solid potassium carbonate. Fractionate the
dry extract and collect phenylmethanol (b.p. 204-207°C). Benzaldehyde
does not form a resin (cf. reaction 3 with ethanal).

(e) To identify an aldehyde or ketone

12. You are given a compound which is either an aldehyde or ketone.
(i) Test the compound with Fehling’s solution (reaction 7) to see whether
it is either an aldehyde or ketone.
(ii) Prepare a solid derivative of the compound, the 2,4-dinitrophenyI-
hydrazone (reaction 2), and determine its melting point. A list of melting
points of 2,4-dinitrophenylhydrazones is given in Appendix VI (p. 379).

12.8 1 Give the structural formulae of the compounds obtained when propanone reacts
with
Questions
(a) hydroxylamine,
(b) sodium hydrogensulphite,
(c) concentrated sulphuric acid,
(d) lithium tetrahydridoaluminate.

2 To a solution of propanone (y g) in water was added an excess of bromine water

followed by sodium hydroxide solution. The tribromomethane, CHBrs, obtained
weighed 4-365 g. Calculate the value of y. (O and C)

3 (a) State the conditions, name the organic product and write its structural
formula, for the reactions of each of ethanal and propanone with:
(i) sodium tetrahydridoborate(III),
(ii) 2,4-dinitrophenylhydrazine.
(b) Describe one simple reaction to distinguish between the members of each
of the following pairs of compounds.
(i) Ethanal and benzaldehyde.
(ii) Propanone and phenylethanone (phenyl methyl ketone).
(iii) Aqueous solutions of ethanal and methanal.
State what is observed for both compounds in each pair. (AEB)

4 What do you understand by (a) homologous series, (b) isomerism'l

Give the full structural formulae for (i) all compounds having the molecular
formula C4H10O, (ii) two compounds having the molecular formula CjHgO.
Name four of the compounds in (i) and describe two experimental methods by
which you could distinguish between the two compounds in (ii). (C(N, T))

5 Compare the reactions of ethanal, benzaldehyde and propanone with (a)

hydrogen cyanide, HCN, (b) aqueous sodium hydroxide, (c) concentrated
hydrochloric acid, (d) ammoniacal silver nitrate, (e) phenylhydrazine
CeHsNHNHa. (O)

186
ALDEHYDES AND KETONES
6 Outline the preparation of propanone starting from ethanol.
Write the structural formulae of all compounds with the general formula
C .H O. Indicate which of these compounds give the iodoform reaction.
4 10

How is the iodoform test performed? (C(N))

7 (a) Illustrate the similarities and differences in the chemical properties of alde¬
hydes and ketones by considering the reactions of ethanal and propanone with:
(i) hydrogen cyanide in the presence of a trace of potassium cyanide;
(ii) potassium manganate(VII);
(iii) sodium tetrahydridoborate, NaBH ; 4

(iv) ammoni'acal silver oxide.

(b) Name one naturally-occurring compound containing both aldehyde and
hydroxyl groups.
(c) Describe what is meant by an addition-elimination reaction, using a reaction of
ethanal as your example.
(d) Explain why ethanal reacts with hydrogen cyanide but ethanoic acid does not.
(O)

8 Describe analytical and synthetic methods you would employ to establish that the
product of a reaction possessed the structural formula:

Cl

CO-CH3 (W(S))
9 Propanone reacts with iodine in aqueous solution according to the following
reaction, which is catalysed by H :
I + CH3-C-CH3->CHoI-C-CH3
2
3 II 3 ^ ,1 *
+ HI
o o
In three experiments the rate of the reaction was studied in aqueous acidic
solutions using concentrations of propanone much larger than that of the iodine.
The results are given below:
Initial concentrations [I ] in millimoles per dm^ at time t
2

in minutes
[Propanone] [H+] 0 5 9 12 14 15
Exp. 1 1-00 M 0-100 M 2-50 1-65 0-97 0-46 0-12 0
Exp. 2 1-00 M 0-150 M 3-20 1-93 0-90 0-14 0 0
Exp. 3 2 00 M 0 -100 M 6-00 4-30 2-94 1-92 1-24 0-90

Plot the concentrations of iodine against time.

How does the reaction rate depend on the concentrations of the various species ?
Can you express all the data with one rate expression containing only one rate
constant ?
Given that
CH2=C-CH3

OH

is an intermediate in the reaction, devise a mechanism which will explain the data.
(O Schol.)
10 (a) Name two reagents which will add to alkenes and two which will add to alde¬
hydes. Write an equation for each reaction.
(b) Compare the electronic structure of ethene (ethylene) with that of methanal
(formaldehyde).
(c) Make a mechanistic comparison between the addition reactions of alkenes
and the addition reactions of aldehydes, illustrating your answer with suit¬
able reactions. (O and C)

187
ALDEHYDES AND KETONES 11 Discuss the chemistry of phenylethanone, CgHjCOCHj, by considering the
following:
(a) a method of synthesis,
(b) addition and addition-elimination reactions of the carbonyl group,
(c) a method of preparing benzenecarboxylic (benzoic) acid from phenyl¬
ethanone. (JMB)

12 Three different compounds, each of molecular formula CgHgO, give yellow pre¬
cipitates with 2,4-dinitrophenylhydrazine and are reduced to compounds of
formula CgHioO by lithium tetrahydridoaluminate (LiAlH ). Given that these
4

compounds do not differ merely by substituent orientation around a ring, suggest

structures for each one.
How, using chemical tests, would you decide which compound is which?
Suggest how the following compounds could be synthesised, assuming all three
of the above compounds are available as the only organic starting materials.
(a) benzoic acid
(b) one of the isomeric methylphenylmethanols

CH2OH

CH3

(c) C6H5CH2CH(0H)CH(C6H5)CH0. (O and C)

188
Chapter 13
Carboxylic acids

13.1 General formula

Introduction R—CO2H

Carboxylic acids contain the carboxyl group

O
//
-C
\
OH

which is a combination of the carbonyl group, ^C= , and the hydroxyl

0

group, —OH. It will be seen that the properties of each group separately
are modified when they are combined.
There are compounds with one carboxylic acid group (monocarboxylic
acids), two (dicarboxylic acids) and more than two.

13.2 Monocarboxylic acids are named, according to the I.U.P.A.C. system,

by replacing the final e of the corresponding hydrocarbon by oic acid, for
Nomenclature of example;
monocarboxylic acids
CH,

CH —CH —CH —CH —CO H

3 2 2 2 2 CH —CH—CH —CO H
3 2 2

Pentanoic acid 3-Methylbutanoic acid

The two lowest members, methanoic and ethanoic acid, are often known
by their original names: formic acid and acetic acid.

Table 13.1. Some monocarboxylic acids

NAME FORMULA M.P./°C B.P./“C

Methanoic acid H—CO2H 8 101

Ethanoic acid CH3—CO2H 17 118

Propanoic acid CH3CH2—CO2H -22 141
Butanoic acid CH3CH2CH2—CO2H -5 163
2-Methyl- (CH3)2CH—CO2H -47 154
propanoic acid
Benzoic acid CfiHs—CO2H 121

189
13.3 The lowest members are liquids with pungent odours. Ethanoic acid smells
of vinegar, and the higher acids smell of rancid butter, which is partly
Physical properties of butanoic acid. Methanoic acid and ethanoic acid are miscible with water, but
monocarboxylic acids as the formula weight increases the solubility decreases.
The formula weight of lower members of the series as determined by, for
example, the depression of freezing point of a solvent such as benzene is
about twiee that expected for the moleeular formula R—CO2H. This is
beeause carboxylic acids exist as dimers: pairs of molecules are linked by
two hydrogen bonds, for example:

O H-q
// \
CR-C C-CH3
0-H -0

A further eonsequence of this is that the boiling points of carboxylic acids

are much higher than those of alkanes of similar formula weight, since more
energy is required in order to break the hydrogen bonds in the vaporisation
of carboxylic acids; for example, ethanoic acid (formula weight, 60) boils at
118°C whereas butane (formula weight, 58) boils at — 0-5°C.

13.4 General methods

Methods of preparation 1. By the oxidation of primary alcohols and aldehydes with acidified
sodium dichromate solution (10.5, 12.5):
of monocarboxylic acids
N3.2Cr207 Na2Cr207
R—CH2OH -> R—CHO -> R—CO2H

2. By the hydrolysis, with dilute mineral acid or alkali, of acid nitriles

(14.6) and acid amides (14.5):
H+ or OH-
R—CN + H2O -> R—CONH2
or OH'
R—CONH2 + H2O -> R—C02“ NH4+

R-CO., Na^ + HoO T NH-

R-CO.,

R-CO.H + NH4CI

3. By the reaction of carbon dioxide with a Grignard reagent followed

by hydrolysis (9.8):

R—MgX + CO2 ^ R—CO—O—MgX R—CO2H + Mg(OH)X

Details of the preparation of benzoic acid from phenylmagnesium bromide

are given on p. 203.

Methanoic acid
By heating a solution of ethanedioic acid in propane-l,2,3-triol at 150°C:
CARBOXYLIC ACIDS
Benzoic acid
By the oxidation of methylbenzene with hot, alkaline potassium manga-
nate(VII) solution, followed by acidification:
_ KMn04
C5H5-CH3 —QHs-COr Na+

CeHj—C02“ Na+ + HCl -> CeHj—CO2H + NaCl

Benzoic acid
Practical details are described on p. 113.

Manufacture of monocarboxyiic acids

Methanoic acid is mainly produced as a by-product of the manufacture of
ethanoic acid from naphtha (see below).
It is also manufactured by the carbonylation of methanol, in the liquid
phase,

CH3OH + CO--' > HCO2CH3

150 C pressure

followed by hydrolysis of the ester:

HCO2CH3 + H2O HCO2H + CH3OH

Ethanoic acid is manufactured by three routes:

1. From butane or naphtha. They are dissolved in a solvent (ethanoic

acid itself) and heated, under pressure, with air at 180-200°C. A catalyst,
cobalt(II) ethanoate, is sometimes used. A range of co-products is formed
which includes methanoic acid, propanoic acid and propanone. They are
separated by fractional distillation.
2. From methanol, by carbonylation at 180°C and 30 atmospheres pres¬
sure, using a rhodium catalyst:

CH3OH + CO —CH3CO2H

This is a new process. By the mid-1980s, it is expected to be used to pro¬

duce half the world’s needs for ethanoic acid as, unlike the butane and
naphtha routes, it produces a pure product.
3. A small amount of ethanoic acid is produced from ethanal:
(CHjCO^ljMn
as cat.
CHj—CHO + i02 -^ CHj—CO2H
60-70°C

Benzoic acid is made by passing air under pressure into methylbenzene at 150°C
in the presence of an organic cobalt salt as a catalyst:

C6H3-CH3 C6H5-CO2H

1. They are weak acids, dissociating to a small extent (1-2 per cent) in
13.5
water:
Chemical properties of
monocarboxyiic acids R—C02H;=±R—C02“ + H +

191
CARBOXYLIC ACIDS The dissociation constants, K^, of some typical members of the series are

Ka at 25°C
H—CO2H 1-7 X 10-^
CH3—CO2H 1-7 X 10-^
CH3CH2—CO2H 1-3 X 10“^
CsHs—CO2H 6-3 X 10-"
C6H5CH2—CO2H 4-9 X 10“"

Thus, the hydroxyl group is far more acidic than in an alcohol, its properties
being modified in this respect by the carbonyl group, for a reason described
in Section 4.9. They are also stronger acids than phenols, but weaker than
sulphonic acids.

2. They react with bases to form salts, for example:

R—CO2H + NaOH ^ R—CO2" Na+ + H2O

They react with carbonates and hydrogencarbonates to liberate carbon

dioxide:

R—CO2H + NaHCOj ^ R—CO2- Na+ + H2O + CO2

The last reaction enables them to be distinguished from simple phenols, for
although phenols are acidic enough to turn blue litmus red and to form
salts with sodium hydroxide, they are weaker acids than carbonic acid and
thus do not liberate carbon dioxide from sodium hydrogencarbonate (10.8).
The reaction with sodium hydrogencarbonate also enables carboxylic
acids to be separated from simple phenols. For example, if a mixture of
benzoic acid and phenol is partitioned between a solution of sodium
hydrogencarbonate and ether, the acid dissolves in the aqueous layer (with
liberation of carbon dioxide) and the phenol mainly dissolves in the ether.
The two solutions are separated; the ether is evaporated to leave phenol,
and hydrochloric acid is added to the aqueous solution to precipitate
benzoic acid:

CgHs—COj- Na+ + HCl QHj—CO2H + NaCl

R—CO2H + R'—OH -- R—CO—O—R'+ H2O

The reaction is discussed on p. 212.

4. They react with phosphorus halides to form acid halides:

R—CO2H + PCI5 > R—CO—Cl + POCI3 + HCl

Red P/Bf:
R—CO2H + PBr > R—CO—Br + POBrj + HBr

Sulphur dichloride oxide (thionyl chloride) can also be used:

R—CO2H + SOCI2 ^ R—CO—Cl + SO2 + HCl

192
CARBOXYLIC ACIDS
5. They are reduced to primary alcohols by lithium tetrahydridoalu-
minate:

.r LiAlH4
R—CO2H -^ R—CH2OH

However, unlike aldehydes and ketones (p. 177), they are not reduced by
sodium tetrahydridoborate.

6. A C—H bond adjacent to the carboxyl group is converted into a C—Cl

bond when chlorine gas is passed into the hot acid in the presence of
ultraviolet light, for example:

(CH3)2C—CO2H + CI2 (CH3)2C—CO2H + HCl

I heat I
H Cl

When there is more than one such C—H bond, further reaction can occur,
for example:

u.v. light
CH3—CO2H + CI2 - > CH2CI—CO2H + HCl
Chloroethanoic
acid
U.V, light
CH2CI—CO2H + CI2 —. > CHCI2—CO2H + HCl
Dichloroethanoic
acid
u.v. light
CHCI2—CO2H + CI2 - > CCI3—CO2H + HCl
Trichloroethanoic
acid

7. Although carboxylic acids contain the carbonyl group, C=0, they

do not undergo the addition reactions with, e.g., sodium hydrogensulphite
or the condensation reactions with, e.g., hydroxylamine which are charac¬
teristic of this group in aldehydes and ketones.
This is because an orbital on the oxygen atom of the hydroxyl group which
contains two unshared electrons interacts with thep orbital of the adjacent carbon
atom (Fig. 13.1). This provides extra delocalisation in the molecule, as compared
with that in an aldehyde or ketone, which would be lost by addition of a reagent
to the carbonyl group, so that a carboxylic acid is more resistant to addition than
an aldehyde or ketone.

Methanoic acid
Methanoic acid, H—CO2H differs in the following respects from the
other monocarboxylic acids:
1. It is dehydrated by concentrated sulphuric acid:

H2SO4
H—CO2H —U CO + H2O

This is the basis of a laboratory preparation of carbon monoxide.

2. It reduces Fehling’s solution and gives a silver mirror when warmed

with a solution of silver nitrate and ammonia. These are characteristics of

193
 aldehydes (12.5), and it is because methanoic acid contains the readily
oxidised aldehydic group (HO—CH==0) that it undergoes these reactions,
that is:
H—C—OH —^ (HO—C—OH) ^ CO2 + H2O

O O
3. It does not form acid halides.

13.6 Ethanoic acid is used in the manufacture of ethenyl ethanoate, required for
the production of poly(ethenyl ethanoate) (21.2), and ethanoic anhydride
Uses of monocarboxylic (14.4), required for making cellulose ethanoate (18.3) and other ethanoate
acids esters.
Long-chain monocarboxylic acids are used in the manufacture of soaps
and detergents (p. 201).

13.7 Some examples of dicarboxylic acids, together with their original names, are
in Table 13.2.
Dicarboxylic acids

194
CARBOXYLIC ACIDS
Table 13.2. Some dicarboxylic acids

NAME FORMULA M.P./°C

Ethanedioic acid HO2C—CO2H 189

(oxalic acid)
Propanedioic acid HO2C—CH2—CO2H 136 (decomposes)
(malonic acid)
Butanedioic acid HO2C—CH2—CHj—CO2H 185
(succinic acid)
Hexanedioic acid HO2C—(CH2)4—CO2H 149
(adipic acid)

Benzene-1,2-dicarboxylic
acid (phthalic acid) 200 (decomposes)

CO.,H

Benzene-1,4-dicarboxylic
300 (sublimes)
acid (terephthalic acid)
O' CO.>H

They are white crystalline solids. The lower members are soluble in water
and ethanol but are insoluble in ether.

Laboratory preparations of dicarboxylic acids

Etbanedioic acid is made by heating sodium methanoate at 400°C to give
disodium ethanedioate,

2H—CO2" Na+ ^ Na+ OaC—COz” Na+ + H2

dissolving this salt in water and adding calcium hydroxide to precipitate

calcium ethanedioate,

Na^“0.,C-C0.rNa^+ Ca(OH)2

“0.,C
-> Ca2+ I + 2NaOH
“O.2C

and then adding to the dried calcium salt the exact quantity of dilute
sulphuric acid needed to liberate the acid:

-o.,c
Ca2+ “ I + H2SO4 > H0,,C-C02H + CaS04
“0.,C

Calcium sulphate precipitates, and ethanedioic acid is crystallised from the

filtrate as the hydrate, (C02H)2.2H20.

Propanedioic acid is made by the reaction of sodium cyanide in water with

the sodium salt of chloroethanoic acid, followed by hydrolysis of the nitrile

195
CARBOXYLIC ACIDS with concentrated hydrochloric acid:

Cl—CH2—CO2" Na+ + NaCN ^ N=C—CH2—C02“ Na+ + NaCl

N=C—CH2—CO2' Na+ + 2H2O + 2HC1

HO2C—CH2—CO2H + NH4CI + NaCl

Butanedioic acid is prepared from 1,2-dibromoethane:

Br—CH2—CH2—Br + 2NaCN N=C—CH2—CH2—C=N + 2NaBr

N=C—CH2—CH2—C=N + 4H2O + 2HC1

HO2C—CH2—CH2—CO2H + 2NH4CI

Manufacture of dicarboxylic acids

Ethanedioic acid is manufactured by the oxidation of carbohydrates (e.g.
corn starch) with nitric acid in the liquid phase.
Hexanedioic acid, required for the production of nylon-6.6 (21.3), is
manufactured from cyclohexane (20.6), and benzene-1,4-dicarboxylic acid,
required for the production of Terylene, is made from 1,4-dimethylbenzene
(20.5).

Chemical properties of dicarboxylic acids

In most respects dicarboxylic acids resemble monocarboxylic acids. They
react with bases to form two series of salts, for example:

HO2C—CO2H + NaOH HO2C—C02“ Na+ + H2O

Sodium hydrogen-
ethanedioate
HO2C—C02~ Na+ + NaOH ^Na+ “O2C—C02“ Na^ + H2O
Disodium ethanedioate

and with phosphorus halides and sulphur dichloride oxide to form acid
halides, for example:

HO2C—CO2H + 2PCI5 ^ Cl—CO—CO—Cl + 2POCI3 -f 2HC1

Ethanedioyl chloride

The lower members of the series have certain special properties:

1. Ethanedioic acid and propanedioic acid are decarboxylated on being

heated strongly:

HO2C—C02H-^^H—CO2H + CO2

HO2C—CH2—C02H-‘^CH3—CO2H + CO2

2. Ethanedioic acid is a reducing agent, being readily oxidised to carbon

dioxide and water, for example with a warm solution of acidified potassium
manganate(VII):

HO2C—CO2H 2CO2 + H2O

196
CARBOXYLIC ACIDS
3. Ethanedioic acid is dehydrated by concentrated sulphuric acid:

HO2C-CO2H CO + CO2 + H2O

This is a convenient method for the preparation of carbon monoxide in the

laboratory.

4. Propanedioic acid is dehydrated to form tricarbon dioxide when heated

with phosphorus pentoxide:

Tricarbon dioxide

5. Butanedioic acid forms a cyclic anhydride when heated with ethanoic

anhydride:
HO0C-CH2-CH2-CO2H + CH3-CO-O-CO-CH3

H2C-CH,
/ \ + 2CH3C0,H
q/ \q

Butanedioic anhydride

13.8 Chloroethanoic acid

Substituted carboxylic Chloroethanoic acid is prepared by passing chlorine into hot ethanoic acid
acids in the presence of ultraviolet light:

u.v. light
CH3—CO2H + CI2 -> CH2CI—CO2H + HCl
heat

Further chlorination can occur, giving dichloroethanoic acid and then tri-
chloroethanoic acid (p. 193), and in order to optimise the yield of the mono-
chloro compound the reaction is stopped when there has been the appro¬
priate increase in weight.
Chloroethanoic acid is a deliquescent solid (m.p. 61 °C) which is soluble in
water. It is a stronger acid than ethanoic acid (4.8).
The chlorine atom behaves as in alkyl halides. Thus, it is readily displaced,
as chloride ion, by nucleophilic reagents, for example:
1.. With dilute sodium hydroxide, the sodium salt of hydroxyethanoic acid
is formed:

Cl—CH2—CO2H + 2NaOH ^ HO—CH2—CO2" Na+ + NaCl + H2O

Sodium hydroxyethanoate

2. With sodium cyanide, the sodium salt of cyanoethanoic acid is

formed:
Cl—CH2—CO2H + 2NaCN ^
Nee^C—CH2—C02" Na+ + NaCl + HCN
Sodium cyanoethanoate

3. With ammonia, aminoethanoic acid (glycine) is formed:

Cl—CH2—CO2H + 2NH3 -> H2N—CH2—CO2H + NH4CI

Glycine

197
CARBOXYLIC ACIDS
Hydroxyethanoic acid
Hydroxyethanoic acid is prepared by the hydrolysis of chloroethanoic acid with
sodium hydroxide, followed by acidification:

Cl—CHj—CO2H + 2NaOH ^ HO—CH2—CO2- Na+ + NaCl + H2O

HO—CH2—CO2- Na+ + HCl ^ HO—CH2—CO2H + NaCl
Hydroxyethanoic acid

It is a white solid, readily soluble in water, and is a stronger acid than ethanoic
acid though weaker than chloroethanoic acid.
It exhibits the properties of both a monocarboxylic acid and a primary
alcohol:
1. As an acid, it can be converted into an ester, for example:
H^
HO—CH2—CO2H + CH3—OH ^ HO—CH2—CO—O—CH3 + H2O
Methyl hydroxyethanoate

2. As a primary alcohol, it can be oxidised to an aldehyde by, for example,

acidified sodium dichromate solution; further oxidation occurs readily:
Na2Cr207 Na2Cr207
HO—CH2—CO2H -> 0=CH—CO2H ->■
Oxoethanoic acid
Na2Cr207
HO2C—CO2H -> 2CO2 + H2O
Ethanedioic acid

In addition, it undergoes two reactions which neither a carboxylic acid nor a

primary alcohol can undergo:

3. On being heated, it forms a cyclic ester (an example of a lactide):

.ch.2. /^O
O'
2HO-CH2-CO2H ■> I I + 2H,.0
o
O CH2

4. When warmed with concentrated sulphuric acid, it forms methanal and

carbon monoxide:
cone. H2SO4
HO—CH2—CO2H -> H—CH=0 + CO + H2O

2-Hydroxypropanoic acid
2-Hydroxypropanoic acid (lactic acid) is prepared from ethanal or from propanoic
acid:

OH OH
HCN I HCl I
(1SW) CH.-CH-CN ^ CH,-CH-CO,H

Br OH

CH3-CH2-CO2H CH3-CH-CO2H CH3-CH-CO2H

(11) HCl

198
CARBOXYLIC ACIDS
2-Hydroxypropanoic acid exists as two optically active isomers (p. 238), each
of which has m.p. 26°C. When prepared in the laboratory as above, it is obtained
as a racemic mixture of the two isomers which is optically inactive and has m.p.
18°C; the individual isomers can be obtained by resolution of this mixture
(p. 244). ^
The chemical properties of 2-hydroxypropanoic acid are similar to those of
hydroxyethanoic acid:
1. It has approximately the same acid dissociation constant as hydroxyethanoic
acid.
2. As an acid, it forms esters and acid halides.
3. As an alcohol, the secondary alcohol group is oxidised to a keto group. To
prevent oxidation of the product, 2-oxopropanoic acid, a weak oxidising agent
such as silver oxide must be used:

CHs—CH—CO2H + AgjO ^ CH3—C—CO2H + H2O + 2Ag

OH O
2-Oxopropanoic acid

4. As a 2-hydroxy-acid, it forms a cyclic ester (lactide) on being heated:

CH,
I
OH /CH^ ^O
O C
2CH,-CH-CO.,H I I + 2H.,0
c. .0
"CH'
I
CH3

5. As a 2-hydroxy-acid, it yields carbon monoxide when warmed with con¬

centrated sulphuric acid:
OH
I cone. H2SO4 j
CH3—CH—CO2H-> CH3—CH=0 + CO + H2O

Carboxylic acids react with inorganic bases to form salts, for example:
13.9
Salts of carboxylic acids CH3—CO2H + NaOH CHj—C02‘ Na+ + H2O

Physical properties of salts

Sodium and potassium salts are white, crystalline solids which are soluble
in water but practically insoluble in ether and most other organic solvents.
The calcium salts are much less soluble in water, and the silver salts are
practically insoluble. All the salts have relatively high melting points
(200°-400°C).
The solutions in water have a pH above 7 as the result of hydrolysis:

R—C02" Na+ + H2O ^ R—CO2H + Na+ + OH”

Chemical properties of salts

1. When the anhydrous sodium salt is heated with soda-lime, an alkane is
formed:
r_C02“ Na+ + NaOH ^ R—H + Na2C03

199
CARBOXYLIC ACIDS This is an example of decarboxylation, the term used when the elements
of carbon dioxide are removed from a molecule.
A well-known example of this reaction is the formation of methane from
sodium ethanoate:
soda-lime _
CH3C02“ Na+ + NaOH -> CH4 + Na2C03

These reactions also occur with the acid itself, the sodium salt of the acid
being formed during the reaction, for example when benzoic acid is heated
with soda-lime:

C6H5CO2H + NaOH C6H5C02~ Na+ + H2O

C6H5C02“ Na+ + NaOH CgHg + Na2C03

2. Kolbe's reaction. When a strong aqueous solution of a salt is electro¬

lysed, an alkane is formed:

2R—C02“ Na^ + 2H2O ^ R—R + 2CO2 + 2NaOH + H2

At the anode, the carboxylate ion releases one electron to give a radical,
R—CO—0-. This fragments with formation of an alkyl radical and carbon
dioxide, and two alkyl radicals combine to form the alkane:

R—C02“ ^ R—CO—O- + e

R—CO—O- -> R- + CO2

2R- ^ R—R

At the cathode, there is competition between sodium ions and hydrogen ions
(from water) for discharge. Although the concentration of hydrogen ions
is very low, the discharge potential for hydrogen favours the formation of
hydrogen gas:

H+-f e -> H; 2H ^ H2

3. When the anhydrous sodium salt is heated with an acid chloride, an

acid anhydride is formed:

R—CO2- Na+ + R—CO—Cl R—CO—O—CO—R + NaCl

4. The sodium salts of monocarboxylic acids are stable when heated

alone, except for sodium methanoate which decomposes at 400°C, yielding
disodium ethanedioate and hydrogen:

2H—C02~ Na+ ^ Na+ -O2C—C02“ Na+ + H2

5. The silver salts of carboxylic acids react on heating with alkyl halides
to give esters:

R—C02“ Ag+ + R'—Cl ^ R—CO—O—R' + AgCl

6. When the calcium salt of a carboxylic acid is heated strongly, a ketone

200
CARBOXYLIC ACIDS
is formed, but the yield is usually very low.

(R—C02")2Ca2+ ^ R_cO—R + CaCOj

13.10 Soaps are obtained from natural fats, known as glycerides because they are
esters formed by the trihydric alcohol, propane-1,2,3-triol, which was
Soaps and detergents
formerly known as glycerol, with long-chain carboxylic acids. The glycerides
are hydrolysed by heating with caustic soda (saponification) to form soaps—
the sodium salts of the acids—and propane-1,2,3-triol;
CH2-O-CO-R CH2-OH
I
CH-O-CO-R + 3NaOH CH-OH + 3RC02~Na
CH2-O-CO-R
A glyceride

Sodium chloride is added to precipitate the soap, and this is then pro¬
cessed into bars or soap powder. Glycerides contain saturated carboxylic
acids which have an even number of carbon atoms, generally within
the range 12-20, for example, octadecanoic acid (stearic acid),
CH3-(CH2)i6-C02H.
Soaps act by lowering the surface tension between water and an oil or
other insoluble material. They do so by virtue of containing both a hydro¬
philic (‘water-loving’) group (—C02“) and a hydrophobic (‘water-hating’)
group (the alkyl chain); molecules of water tend to congregate near the
former and molecules of the water-insoluble material congregate around the
latter.
One disadvantage of soaps is that they form insoluble calcium salts with
the calcium ions in hard water and in the clays which are present in dirt;
a good deal of the soap is wasted in this way. This problem is avoided by
the use of synthetic detergents in which a sulphonate group, —SO2—0~,
or sulphate, group, —O—SO2—0“, replaces the carboxylate group as the
hydrophilic component, since the corresponding calcium salts are more
soluble in water than the calcium salts of carboxylic acids.
Until about 1965, the commonest detergents contained alkylbenzene-
sulphonates made from a polymer of propene by a Friedel-Crafts reaction:

CH3 CH3 CH3

CH3-CH-CH2-CH-CH2-CH-CH2-CH-CH2 + CfiHe

AICI3 as cat.
CH3-(CH-CH2)3-CH

followed by sulphonation (p. 105) and neutralisation of the sulphonic acid

with sodium hydroxide:

201
CARBOXYLIC ACIDS However, these detergents suffer from the disadvantage that they are not
degraded by bacteria in sewage plants, which meant that many rivers
suffered from the foam and there was also the danger that detergents could
be ‘recycled’ into the drinking-water supplies. The failure of bacteria to
attack the materials results from the presence of the large number of
branches in their alkyl groups, and the use of such detergents has been
abandoned in the United Kingdom. Reduction in the number of branches
increases their capacity for biodegradation and so most detergents now
contain linear or only singly branched alkyl groups. There are three im¬
portant types; the first two are anionic detergents while the third is non¬
ionic.
All three types are obtained from non-branched alkenes, which them¬
selves are derived partly from the cracking of waxes (19.8), partly from the
polymerisation of ethene with a Ziegler catalyst (20.4) and partly from
alkanes by free-radical chlorination followed by catalytic dehydrochlorina¬
tion : '
Silica gel as cat.
R—CH2—CHj + CI2 ^ R—CH—CHj -R—CH=CH2
j 300°C

Cl

1. Alkylbenzenesulphonates, which are made from non-branched alkenes

(C10-C14):

R-CH=CH,2 + CfiHe -— > R-CH

The alkylbenzene is sulphonated and neutralised. These are used in powder

detergents (e.g. Omo, Surf, Tide), and are slowly biodegradable. The
formation of an alkylbenzenesulphonate is described on p. 205.

2. Alkyl sulphates, which are made from straight-chain alcohols (Cjo-Cu)

derived from alkenes by the 0X0 process (p. 322) or with a Ziegler catalyst
(p. 316):

R—CH2OH r_cH2—O—SO2OH
R—CH2—O—SO2—O- Na+

3. Ethoxylates (which are non-ionic detergents) are made from long-

chain alcohols and epoxyethane, for example:

CH3-(CH2)io-CH2-OH + 8H C-CH
2 2

-> CH,-(CHi)„-CH,-(OCH,CH,)s-OH
• An ethoxylate

These do not contain an ionic group as their hydrophilic component, but

hydrophilic properties are conferred on them by the presence of a number
of oxygen atoms in one part of the molecule which are capable of forming
hydrogen-bonds with molecules of water.

A packet of detergent contains about 20 per cent of active detergent and

an equal amount of sodium sulphate, which increases the bulk of the
powder. A further 30 per cent is made up of inorganic phosphates, which
are added to remove soluble, complex calcium salts by reaction with the

202
CARBOXYLIC ACIDS
calcium ions in the dirt. Other ingredients include sodium peroxoborate
(about 5 per cent), which is a bleaching agent, and fluorescers, which are
organic compounds which absorb ultraviolet light and re-emit the energy in
the blue part of the visible spectrum, thereby making ‘yellow’ clothes appear
white.
The inorganic phosphates which enter lakes and rivers via sewage are
nutrients for algae and are responsible for the proliferation of these plants,
as a green surface sludge, in seas and lakes in various parts of the world.
It is likely that legislation will be introduced to remove, or at least reduce,
the phosphate content of detergents so as to eliminate this source of
pollution.

13.11 Preparation of a carboxylic acid from an ester

Practical work The example given is the preparation of benzoic acid from ethyl benzoate.
Reflux a mixture of 5 cm^ of ethyl benzoate and 25 cm^ of sodium
hydroxide solution (made by dissolving lOg of sodium hydroxide in 25 cm^
of water) for 30 minutes (cf. Fig. 2.1).
Distil the mixture and collect the first 2 cm^ of distillate (cf. Fig. 2.2).
Carry out some of the tests for ethanol (for example. No. 3, p. 161, and
No. 7, p. 161).
Pour the residue from the flask into a beaker and add M sulphuric acid
until the solution is acid to litmus. Filter the crystals, using a Buchner funnel
(cf. Fig. 2.8). Dissolve the crystals in a minimum of boiling water. Allow
the solution to cool and filter the crystals, dry them between pads of filter
paper and determine their melting point.

Small-scale preparation of phenylmagneslum bromide

and benzoic acid
1. Preparation of phenylmagnesium bromide solution
All apparatus and materials used in the preparation of a Grignard reagent
must be thoroughly dry since traces of moisture not only react with the
reagent but also inhibit its formation. Make sure that the apparatus is clean
and that it has been dried in an oven before the experiment.
Diethyl ether must stand over clean sodium wire or pellets and the
bromobenzene over anhydrous calcium chloride in carefully stoppered
tubes or flasks, if possible, overnight before the experiment is to be carried
out.
Place 0-5 g of magnesium turnings in a flask and add one or two crystals
of iodine, followed by 10 cm^ of dry ether. Stand the flask in a cold water-
bath, add 2 cm^ of dry bromobenzene, then fit a dry reflux condenser which
is attached to a calcium chloride drying tube at its open end.
Raise the temperature of the water-bath to 40-45°C, turn out the bunsen
and allow the contents of the flask to reflux for 20-25 minutes. The dis¬
appearance of the colour of the iodine and the formation of a cloudiness
in the reaction mixture are indications that the reaction is proceeding
satisfactorily. Remove the warm water-bath and replace it with a freezing
mixture of ice and salt.

2. Preparation of benzoic acid

Arrange for the carbon dioxide, generated in the Kipp’s apparatus, to be
washed in water to remove traces of hydrochloric acid and to be dried by
passing through concentrated sulphuric acid. A more satisfactory method

203
CARBOXYLIC ACIDS is to place a few pieces of solid carbon dioxide in a conical flask and lead
the carbon dioxide straight into the flask containing the solution, which
should be immersed in ice.
Pass a gentle stream of dry carbon dioxide through the solution of
phenylmagnesium bromide for 5-10 minutes. Decant the contents of the
flask into a small beaker and place the latter in the freezing mixture. Dilute
3 cm^ of concert hydrochloric acid by adding 3 cm^ of water. Introduce
the acid slowly into the beaker, with stirring, in order to liberate the benzoic
acid.
Remove the beaker from the freezing mixture, add 15 cm^ of ether and
stir. Decant the liquid into a separating funnel and return the lower aqueous
layer to the beaker. Repeat the ether extraction twice, using 5 cm^ of ether
each time.
Combine the ether extracts and shake with 10 cm^ of 2M sodium
hydroxide solution in a separating funnel. Remove the stopper from the
funnel occasionally to release the pressure. Most of the benzoic acid enters
the lower aqueous layer as the sodium salt. Transfer the aqueous layer to a
beaker. (If a precipitate of magnesium hydroxide should appear, remove
it by filtration through a Buchner funnel.) Acidify the filtrate with 2M
hydrochloric acid (testing the solution with litmus paper). A white precipi¬
tate of benzoic acid is obtained. Precipitation is hastened by cooling in the
freezing mixture and scratching with a glass rod. Filter off the precipitate;
wash in situ with distilled water. Dry the solid and take its melting point.
If time, purify by recrystallisation from hot water. Dry the crystals in the
oven at about 100°C and redetermine the melting point.

Reduction of a carboxylic acid to an alcohol with

lithium tetrahydridoaluminate
Lithium tetrahydridoaluminate is a specific reagent; for example, it does
not reduce unsaturated carbon-carbon bonds (as in alkenes and alkynes).
The acid chosen for this reaction is 2-chlorobenzoic acid as the product,
2-chlorophenylmethanol, is a solid.
Lithium tetrahydridoaluminate is a dangerous chemical. It reacts violently
with water and only dry solvents and dry apparatus must be used. Experi¬
ments should be done in a fume cupboard with a satisfacotry exhaust for the
hydrogen formed during reactions to be led away. There should be no naked
flames nearby.
Place 30 cm^ of diethyl ether, previously dried over sodium, in a round-
bottom flask, which has been carefully dried, and fit a reflux condenser.
Weigh out 0-5 g of lithium tetrahydridoaluminate on a watch glass and add
it, in very small quantities, to the ether. If there is any effervescence, it means
that the ether is not dry.
Add, in very small quantities at a time, 1-5 g of 2-chlorobenzoic acid to
the solution of lithium tetrahydridoaluminate in ether. After all the acid has
been added, reflux the mixture, using a beaker of hot water, for about
30 minutes.
Then add, dropwise, 5 cm^ of ethyl ethanoate which will be reduced by
the excess of lithium tetrahydridoaluminate and thus decompose it. Allow
the mixture to reflux for a further 10 minutes.
Cool the mixture and then add 20 cm^ of M sulphuric acid. 2-Chloro-
phenylmethanol is liberated from the complex aluminium compound formed
and dissolves in the ether layer.
Place the mixture in a separating funnel and run the ether layer into a

204
CARBOXYLIC ACIDS
conical flask. Add some anhydrous sodium sulphate, fit a stopper to the
flask and swirl the mixture for about 5 minutes.
Decant the solution into a flask and distil off the ether using a beaker of
hot water.
Detach the flask and place it in a beaker of ice. Filter off the crystals of
2-chlorophenylmethanol and dry them between filter papers. Take the
melting point of the crystals, which, before further purification, will be
about 70°C.
If time permits, recrystallise the product from a dilute aqueous solution
of ethanol.

Preparation of detergents
1. To 5 g of dodecanol in a flask, add 2 cm^ of chlorosulphonic acid drop-
wise (CARE), with stirring. Keep the mixture below 35°C by immersing the
flask in a large beaker of cold water. Stir the mixture for a further 10
minutes and then divide into two parts.

C12H25—OH + Cl—SO2—OH C12H25—O—SO2—OH + HCl

Dodecyl sulphate
(a) To one part, add a dilute solution of sodium hydroxide, until the
mixture is neutral. Transfer the solution to an evaporating basin and
evaporate it until the solid detergent is formed. The detergent can be
recrystallised from ethanol.

C12H25—O—SO2—OH + NaOH -> C12H25—O—SO2—0“ Na+ + H2O

(b) To the second part, add a solution of tris-(2-hydroxyethyl)amine

(2 cm^ in 20 cm^ of water):

C,2H25—O—SO2—0H + N(CH2CH20H)3 ^

C,2H25—o—SO2—O- HN(CH2CH20H)3

Finally, make the mixture neutral to litmus by adding dropwise a solution

of sodium hydrogencarbonate.
(c) Examine the lathering properties of the two detergents in both hard
and soft water.

2. Great care must be taken when using oleum. Wear safety glasses and
carry out the experiment in a fume cupboard.
Place 11-5 cm^ of an alkylbenzene in a flask fitted with a thermometer,
and cool the hydrocarbon to about 5°C in an ice-bath. Add 5 cm^ of oleum
(sulphuric acid containing about 20 per cent free sulphur trioxide) using a
dropping-pipette, shaking between each addition. The temperature rises
slowly but must not be allowed to rise above 56°C. The temperature should
be about 55 + 1 °C at the end of the addition.
Replace the ice-bath with a beaker of hot water to keep the mixture at
55 ± 1°C for a further 30 minutes. Cool the mixture.
Place 4 g of crushed ice in a beaker and then surround it with a larger
beaker containing an ice-water mixture. Stir in a solution of 1 - 5 g of sodium
hydroxide dissolved in 6 cm^ of water.
Transfer the reaction mixture containing the alkylbenzenesulphonic acid
to a tap-funnel and add it to the alkali solution, with stirring, making sure

205
CARBOXYLIC ACIDS that the temperature of the mixture does not rise above 50°C. Add the
sulphonic acid until the pH of the mixture in the beaker is between 6'5 and
7-5 (using narrow-range Universal Indicator papers). If the pH becomes
too low, add 2M sodium hydroxide solution to adjust it to 6-5-7-5.
A solid will precipitate out, and the mixture is then heated in an evaporat¬
ing basin over a beaker of boiling water until most of the liquid is removed.
Although it is difficult to purify the detergent any further, its detergent
properties can be studied by transferring a small amount of the mixture in
the evaporating basin to a test-tube and dissolving it in water.
The detergent may be discoloured owing to the formation of ‘hot-spots’
on adding oleum to the hydrocarbon.

Reactions of methanoic acid and sodium methanoate

1. To some solid sodium hydrogencarbonate in a test-tube, add some
methanoic acid.
2. Make up neutral iron(III) chloride solution by adding dilute ammonia
solution to 2 cm^ of iron(III) chloride solution until a precipitate appears.
Add the original iron(III) chloride solution until the precipitate just
disappears.
Place 5 drops of methanoic acid in a test-tube. Add dilute ammonia solu¬
tion until just alkaline and boil off excess of ammonia. To this neutral
solution, add 5 drops of neutral iron(III) chloride solution.
3. To 1 cm^ of methanoic acid in a test-tube, add 1 cm^ of concentrated
sulphuric acid and warm. Test the gas evolved with (a) a lighted splint, (b)
lime-water.
4. Place sodium methanoate crystals in a test-tube to a depth of about 1
cm. Heat vigorously and test the gas evolved with a lighted splint.
Cool the residue and add 1 cm^ of concentrated sulphuric acid. Test the
gas evolved with (a) a lighted splint, (b) lime-water.

The reducing properties of methanoic acid are illustrated by the reactions

5-7.

5. To 0-5 cm^ of methanoic acid in a test-tube, add 1 cm^ of dilute sul¬

phuric acid and warm gently. Add a 1 per cent solution of potassium
manganate(VII) drop by drop, and note whether the permanganate is de¬
colorised.
6. To 1 cm^ of ammoniacal silver nitrate solution (p. 185), add 2 or 3
drops of methanoic acid. A white precipitate of silver methanoate is formed.
If the test-tube is immersed in hot water, a deposit of silver is formed.
7. Add methanoic acid dropwise to 1 cm^ of a solution of mercury(II)
chloride. A white precipitate of mercury(I) chloride is slowly formed. On
warming, the precipitate darkens owing to reduction to mercury.

Reactions of ethanoic acid and sodium ethanoate

8. Repeat experiment 1, using ethanoic acid.
9. Repeat experiment 2, using ethanoic acid.
10. Repeat experiment 5, using ethanoic acid.

11. Mix together some anhydrous sodium ethanoate and soda-lime in a

test-tube. Heat the mixture strongly. Test the gas evolved with a lighted
splint.

206
CARBOXYLIC ACIDS
12. Warm a mixture of 5 drops of ethanol, 5 drops of ethanoic acid
and 1 drop of concentrated sulphuric acid. Note the characteristic odour of
the product.

Reactions of benzoic acid

13. Repeat experiment 1 using a hot solution of benzoic acid, and
compare the result with that obtained when a solution of phenol is added
to sodium hydrogencarbonate.
14. Repeat experiment 5 using a hot solution of benzoic acid.
15. Repeat experiment 11 using benzoic acid in place of sodium ethanoate.
16. Repeat experiment 12 using benzoic acid in place of ethanoic acid.

Reactions of ethanedioic acid and disodium

ethanedioate
17. Repeat experiment 1 using a solution of ethanedioic acid.
18. Repeat experiment 3 using solid ethanedioic acid.
19. Repeat experiment 4 using solid disodium ethanedioate.
20. Repeat experiment 5 using solid ethanedioic acid. Shake until
ethanedioic acid has dissolved. Note whether any reaction occurs (a) in the
cold, (b) when the mixture is warmed.
21. Repeat experiment 11 using disodium ethanedioate instead of sodium
ethanoate.

13.12 Detergents. Elaine Moore (reprinted 1970). Unilever Educational Booklet.

Revised Ordinary Series.
Further reading Theory of Detergency. R. J. Taylor (1969). Unilever Educational Booklet.
Advanced Series.

13.13 Outline of Detergency (F) Unilever.

Film

1 Outline by means of balanced equations and essential reaction conditions (a) two
13.14 general methods for the synthesis of aliphatic carboxylic acids from alkyl iodides,
Questions (b) one method for the synthesis of benzoic acid from benzene.
Give two reactions of methanoic acid which are not shown by other aliphatic
carboxylic acids.
Describe with practical details how you would detect the presence of a
hydroxyl group in benzoic acid. (AEB)
2 Describe, with equations, a chemical test you would employ to distinguish
between the following compounds:
(a) Methanoic acid and ethanoic acid
(b) Methanoic acid and ethanedioic acid
(c) Ethanoic acid and ethanedioic acid
(d) Phenol and benzoic acid.
3 Outline practical reaction schemes to obtain as many compounds as possible
from ethanoic acid. Give the names and formulae of the products and inter¬
mediates, and indicate the reagents which are used.

207
CARBOXYLIC ACIDS 4 Starting with ethanoic acid, by what reactions can the following be prepared: (a)
ethanoic anhydride, (b) ethanamide, (c) methane, (d) chloroethanoic acid, (e)
ethane ?

5 Give three general reactions by which an aliphatic carboxylic acid may be pre¬
pared.
Describe how you would carry out a practical test to show that alcohols and
acids both contain hydroxyl groups.
State briefly how, starting from ethanoic acid, you would prepare (a) methane,
(b) ethanoic anhydride.

6 Describe in outline two methods by which benzoic acid could be made in the
laboratory from benzene, giving the equations for the reactions and conditions
required.
Describe the reactions by which the following could be obtained from benzoic
acid: (a) benzoyl chloride, (b) benzoic anhydride, (c) benzamide.

7 What reactions are characteristic of carboxylic acids? To what extent may

methanoic acid, ethanoic acid and ethanedioic acid be regarded as typical of this
class of compounds?

8 Name five organic substances which can be obtained directly from salts of
ethanoic acid. State what other reagents, if any, would be required. Give the
conditions and equations for the reactions.

9 Starting with carbon monoxide, outline in each case one method by which (a)
methanoic acid and (b) sodium ethanedioate are obtained. What products are
obtained when methanoic acid is treated with mercury(II) chloride solution?

10 The molecular weight of a weak, monobasic, organic acid A was calculated from
(a) the osmotic pressure of its aqueous solution, (b) the depression of the freezing
point of benzene observed when A was dissolved in this solvent. The two methods
gave different values for the molecular weight of A. Suggest an explanation for
this difference.

11 Most of the reactions of carboxylic acids can be classified as belonging to one of

four types: (a) reactions involving cleavage of the O—H bond; (b) reactions at
the carbonyl carbon; (c) reactions at the 2-carbon atom; (d) decarboxylation.
Discuss the chemistry of carboxylic acids under these headings. (W(S))

12 Compare: (a) the properties of the CO group in ethanal, propanone and ethanoic
acid; (b) the properties of the hydroxyl group in phenol and ethanol. What
explanations have been suggested for these differences? How do you account for
the fact that phenylamine is a weaker base than ethylamine? (0(S))

13 A substance A (C4H6O2) rapidly decolourised a solution of potassium per¬

manganate in the cold. On treatment with trioxygen and hydrolysis of the
. products, A gave a neutral substance B (C2H4O) and an acidic substance C
(C2H2O3), both of which gave silver mirrors with ammoniacal silver solutions,
and orange precipitates with 2,4-dinitrophenylhydrazine sulphate in aqueous
methanol (DNPH). B, on oxidation, gave an acid D, whose calcium salt, on
heating, gave E, CaHeO, which gave an orange precipitate with DNPH, but did
not react with the ammoniacal silver solution. Oxidation of C gave an acid F,
which decolourised acidified potassium permanganate on warming, and which’
on heating with concentrated sulphuric acid gave off some gas. This gas was
passed through lime water, which turned cloudy, and the effluent gas was found
to burn in air.
Deduce the structures of .B, C, D, E and F, and outline the course of the
above reactions.

208
CARBOXYLIC ACIDS
14 The following table gives the values of the dissociation constants of ethanoic acid
and some of its related acids:
Acid Dissociation constant {K^
Chloroethanoic 14 X 10-3
Ethanoic 1-86 X 10-5
Phenylethanoic 5-2 X 10-5
Aminoethanoic 1-67 X 10-10

(a) Discuss the theoretical reasons for the differences between the values for
CH3CO2H, CH2CICO2H and CH2NH2CO2H.
(b) Calculate the pH of:
(i) O IM phenylethanoic acid;
(ii) a mixture of equal volumes of 0-2M phenylethanoic acid and 0-2M
sodium phenylethanoate;
(iii) a mixture of equal volumes of 0-2M phenylethanoic acid and 0-2M
ethanoic acid.
(c) Which of the solutions in (b) would change least in pH on dilution ten
times ? Explain your answer. (L(XS))

15 On combustion 0 1575 g of a hydroxy-monocarboxylic acid gave 0-2310 g of

carbon dioxide and 0-0945 g of water. The vapour density of its ethyl ester is 59.
Calculate (a) the percentage composition, (b) the empirical formula, (c) the
molecular weight, (d) molecular formula, of the acid.
Give the names and structural formula of all the hydroxy-acids having this
molecular formula and account for their existence. (AEB)
16 Outline how pure ethanedioic acid crystals can be made, starting from carbon
monoxide. How, and under what conditions, does ethanedioic acid react with
(a) potassium permanganate,
(b) methanol,
(c) sulphuric acid ? (C(N))
17 Outline a synthesis of butanedioic acid,
CH2.CO.OH

CH2.CO.OH
from ethene indicating the reagents and conditions for each reaction you
mention.
By means of equations and brief notes on reaction conditions show how the
following compounds could be prepared from butanedioic acid;
(a) butanedioic anhydride,
CH(OH).CO.OH
(b) 2-hydroxybutanedioic acid, |
CH2.CO.OH
(c) 4-phenyl-4-oxobutanoic acid, CeHj. CO. CH2. CH2. CO. OH,
(d) a mixture of cis- and tra«5-butenedioic acids.
How could you convert tra«^-butenedioic acid into its c/j-isomer? (JMB(S))
18 A substance. A, of molecular formula C3H4OCI2 reacted with cold water to give
a compound, B, C3H5O2CI. A on treatment with ethanol gave a liquid C,
CSH9O2CI. When A was boiled with water, a compound D, C3H6O3 was
obtained. D was optically active and could be ethanoylated.
Deduce the nature of the compounds A, B,C and D, and account for the above
reactions. (C Entrance)
19 Treatment of an aromatic compound A, CsHio, with ethanoyl chloride in the
presence of aluminium chloride gives B, C10H12O. On being warmed with iodine

209
CARBOXYLIC ACIDS and sodium hydroxide, B forms the sodium salt of C, C9H10O2. Both B and C
are converted to D, CgHeO-e, by vigorous oxidation with chromic acid. When
heated, D readily forms E, C9H4OS.
Deduce structures for the compounds Aio E and elucidate the above reactions.
(O Schol.)

20 Three isomeric organic acids have the following composition:

H = 2 09 per cent; C = 43-98 per cent; O = 16-75 per cent; Cl = 37-17 per cent.
When heated with soda-lime, all three isomers are converted to the same
dichlorobenzene.
Give the structural formulae of the dichlorobenzene and each of the three acids
from which it is obtained. (O Schol.)
21 A solid A, m.p. 32-5° and formula CsHgOa, can be isolated from the mixture
formed when starch is boiled with hydrochloric acid. The following reactions
can be carried out with A:
NaiCOg
(i) A -> B (CsHvOgNa) ^ CO2
CH3OH
(ii) A CgHioOj
HCl ^
hydroxylamine
(iii) A > CsHsOjN
NaOH +12
(iv) A > A yellow precipitate C + (C4H404Na2)
oxidation
(v) A > Ethanoic acid + an acid D (C3H4O4)
with HNO3
reduction with
(vi) A > ^(CsHsOjNa).
Na/Hg and water
Elucidate the structure of A, giving your reasoning.
Acidification of F gives a neutral substance CsH802, instead of the expected
acid, and electrolysis of B affords octane-2,7-dione. Can you write formulae to
explain these reactions? (O Schol.)

22 (a) (i) Outline the general structure of a detergent molecule.

(ii) What are the properties of the two parts?
(b) In what way do soaps and sulphonates differ in chemical structure?
(c) By means of equations show how a sulphonate is made from petrochemical raw
materials.
(d) With the aid of diagrams explain how a detergent removes grease from a fabric.
(e) Why do soaps not cause a nuisance in rivers by foaming?
(JMB Syllabus A)

210
Chapter 14
Derivatives of carboxyiic acids

14.1 The hydroxyl group, —OH, in a carboxylic acid can be replaced by other
functional groups, so that there is a series of compounds which contain
Introduction the acyl group, RCO—, which form a parallel series to the derivatives of
the alkanes:

Functional group Acyl derivatives Alkyl derivatives

—OH RCO—OH Acid R—OH Alcohol

—OR' RCO—OR' Ester R—OR' Ether
-X (-F, -Cl, -Br, -I) RCO—X Acid halide R—X Alkyl
halide
—OCOR' RCO—OCOR'Acid R—OCOR' Ester
anhydride
—NH2 RCO—NHj Acid amide R—NHj Amine

In addition, carboxylic acids can be converted into acid nitriles, R—CN.

14.2 General formula

Esters R—C—O—R'

Physical properties
Esters are neutral liquids with pleasant, fruity smells. They are usually
insoluble in water but are soluble in organic solvents.
Their melting points and boiling points are below those of the corre¬
sponding acids because ester molecules, unlike acid molecules, are not
associated by hydrogen-bonding.

Table 14.1. Some esters

NAME FORMULA B.P./°C

Methyl methanoate H—CO2—CH3 32

Ethyl methanoate H—CO2—C2H5 53
Methyl ethanoate CH3—CO2—CH3 56
Ethyl ethanoate CH3—CO2—C2H5 77
Methyl propanoate C2HS—CO2—CH3 79
Ethyl propanoate C2H5—CO2—C2H5 98
Methyl benzoate CsHs—CO2—CH3 200

Preparation
1. By the reaction between an acid and an alcohol in the presence
of a small amount of a strong acid such as sulphuric acid as catalyst

211
DERIVATIVES OF
CARBOXYLIC ACIDS
(esterification), for example:

CHj—CO2H + C2H5—OH T-- CHj—CO—O—C2H5 + H2O

Ethyl ethanoate

The equilibrium constants for esterification are usually close to 1. In

order to obtain a good yield of an ester from a given amount of an acid,
it is necessary to use an excess of the alcohol. Suppose the equilibrium
constant in the above reaction is 1-0. Then, if 1 mol of both ethanol and
ethanoic acid are used, and if the total volume is V cm^ and the amounts of
ester and water at equilibrium are x mol.

Thus, jv: = 0-5 mol, so that the yield of ester, based on ethanoic acid or
ethanol, is 50 per cent. However, if 10 mol of ethanol are used per mol of
ethanoic acid, then

(1 -x)(10-x)/K2

so that X = 0-9 mol; that is, the yield of ethyl ethanoate based on ethanoic
acid is about 90 per cent.
2. By the reaction between an alcohol and either an acid chloride (14.3)
or an acid anhydride (14.4), for example:

CH3—CO—Cl + C2H5—OH -> CH3—CO—O—C2H5 + HCl

Ethanoyl chloride

CH3—CO—O—CO—CH3 + CH3—OH
Ethanoic anhydride
CH3—CO—O—CH3 + CH3CO2H

3. By the reaction between the silver salt of an acid and an alkvl halide
(13.9):

R—CO2- Ag+ + R'—X ^ R_CO—O—R' + AgX

Chemical properties
1. Esters are hydrolysed by heating with mineral acids or alkalis. The
catalysed reaction is reversible, and is the exact opposite of esterification,
for example:

CH3—CO—O—C2H5 + H2O CH3—CO—OH + C2H5—OH

Consequently, because the equilibrium constant is usually close to 1, some

ester always remains at the end of the reaction.
The alkali-catalysed reaction is also reversible:

OH-
R—CO—O—R' + H2O R_cO—OH + R'—OH

However, in this case the carboxylic acid formed reacts with hydroxide ion

212
DERIVATIVES OF
CARBOXYLIC ACIDS
to give the acid anion:

R—CO2H + OH- ^ R—CO2- + H2O

Equilibrium in this step lies almost completely to the right-hand side.

Consequently, as the acid is formed in the first reaction it is removed by
the second, so that eventually practically all the ester is converted into its
hydrolysis products. This makes the alkali-catalysed reaction more efficient
than the acid-catalysed one, and it is normally the process chosen. It is
often referred to as saponification; naturally occurring esters are converted
into soaps in this way (13.10).
In the hydrolysis of an ester, either the bond between the carbonyl group and
the oxygen atom, or the bond between the alkyl group and the oxygen atom,
might be broken.

R-CO- -O-R' R-CO O-R'

I + I
HO H HO H

or R-CO-O R' R-CO-O R'

I + I
H OH

A study of the hydrolysis of esters which are labelled with an *®0 isotope has
shown that it is the former bond that is broken. Thus, when the ester
R—c^®0—**0—R' is hydrolysed, the **0 isotope is found in the resulting
alcohol and not in the carboxylic acid:

R-C-!-i*0-R' + Hji'^O -> R-C-'®OH + R'-i80H

II ; II
160 '®o

The mechanism of alkali-catalysed ester hydrolysis is as follows. The nucleo¬

philic hydroxide ion adds to the carbonyl group of the ester to give an intermediate
like that formed in addition to an aldehyde or ketone (12.5):

OH
I
R-C-OR' + OH R-C-OR'
II I
O 0 “

This intermediate then fragments into the acid and an alkoxide ion:

OH
I
R-C-OR' R-C + R'-0“
I II
0 “ O

The alkoxide ion reacts with the solvent, for example water, to give the alcohol,
and the carboxylic acid dissociates:

R'_0- + H2O ^ R'—OH + OH-

R—CO2H + OH- ^ R—CO2- + H2O

213
DERIVATIVES OF 2. Esters, like acids, can be reduced with lithium tetrahydridoaluminate
CARBOXYLIC ACIDS
to form alcohols;

r_CO-OR' r_CH,-OH + R'-OH

e.g.
CH3-CO-OCH3 CH3CH2OH + CH3OH

Esters cannot be reduced with sodium tetrahydridoborate (p. 177).

Unlike acids, esters can be reduced with sodium in ethanol to form
alcohols:

Na/CjHsOH
R—CO—OR' -^ r_CH2—OH + R—OH

3. Esters react with ammonia, in either concentrated aqueous or alcoholic

solution, to form acid amides:

R—CO—OR' + NH3 ^ R—CO—NH2 + R'-OH

e.g.
CH3—CO—OC2H5 + NH3 ^ CH3—CO—NH2 + C2H5OH
Ethanamide

However, the reaction is much slower than with acid chlorides or an¬
hydrides, and amides are therefore more easily made from these acid
derivatives.

Uses
Esters are used extensively as solvents and plasticisers (p. 327), and some
long-chain esters are used as special lubricants.
Esters are responsible for the smell and flavour of many fruits and
flowers. Hence, artificial flavouring essences are prepared from esters. Ethyl
methanoate is used in raspberry essence and 3-methylbutyl ethanoate in
pear essence. Esters are also used in artificial scents.
Waxes are esters of higher carboxylic acids and higher alcohols. For
example, a constituent of beeswax is C15H33CO2C31H63.
Fats and oils are esters of higher carboxylic acids and propane-1,2,3-triol.
These esters are known as glycerides, and some are used to make soaps
(13.10).

Diethyl propanedioate

Diethyl propanedioate (often called malonic ester) is made from the sodium
salt of chloroethanoic acid by nucleophilic displacement of chloride by
cyanide ion (by heating with potassium cyanide) followed by heating with
ethanol in the presence of sulphuric acid:

Cl—CH2—C02“ Na+ N=C—CH2—C02“ Na+

H2SO4
C2H5O2C—CH2—CO2C2H5
Diethyl propanedioate

214
DERIVATIVES OF
CARBOXYLIC ACIDS Diethyl propanedioate (a liquid, b.p. 198°C) is a valuable reagent in
organic synthesis. Its usefulness stems from the fact that it forms a sodium
salt when treated with sodium ethoxide in ethanol,

CH2(C02C2H5)2 + C2H5O- Na+ ^ Na+ CH(C02C2H5)2 + C2H5OH

the anion of which is a nucleophilic reagent. For example, its reaction with
an alkyl halide gives an alkylpropanedioic ester which, after hydrolysis and
decarboxylation, gives a carboxylic acid:

Na+ CH(C02C2H5)2 + R—X -> R—CH(C02C2H5)2 + NaX

2NaOH 2HC1
R—CH(C02C2Hj)2 -^ R—CH(C02- Na+)2 ->

R—CH(C02H)2 R—CH2—CO2H + CO2

Ethyl 3-oxobutanoate

Ethyl 3-oxobutanoate (often called acetoacetic ester) is made by refluxing

ethyl ethanoate in the presence of a little ethanol over sodium wire. Sodium
ethoxide is formed,

Na + C2H5OH -> C2H5O- Na+ + iH2

and catalyses the reaction:

2CH3—CO2C2H5 CHj—CO—CH2—CO2C2H5 + C2H5OH

Ethyl 3-oxobutanoate is a colourless liquid (b.p. 181°C) which exists as a

mixture of two tautomers (15.2). Like diethyl propanedioate, it is useful in
organic synthesis as a result of its forming a sodium salt,

CH3COCH2CO2C2H5 + C2HJO-Na+ ^

CH3COCHCO2C2H5 + C2H5OH

whose anion is a nucleophilic reagent. For example, its reaction with an

alkyl halide gives a derivative which, after hydrolysis with aqueous acid,
yields a ketone and, after hydrolysis with concentrated alkali, yields a
carboxylic acid:

Na+CHjCOCHCOjQHs + R-X -CH3COCHCO2C2H5 + NaX

R
I
[CHs-CO-CH-COjH] ^ CH3-CO-CH2R + COo

HjO^
R
I
CH,-C0-CH-C0,C,H5
^NaOH

CH3-COr Na+ + RCHj-COr Na+ + QH5OH

215
General formula
R—C—X (X is F, Cl, Br or I)

Physical properties
The lower acid halides are mobile, colourless liquids with pungent odours.
They fume in moist air owing to their ready hydrolysis to the corresponding
halogen acid. Some typical members are in Table 14.2.

Table 14.2. Some acid halides

NAME FORMULA B.P./°C

Ethanoyl chloride ' CH3—CO—Cl 52

Ethanoyl bromide CHj—CO—Br 77
Ethanoyl iodide CH3—CO—I 108
Propanoyl chloride CH3CH2—CO—Cl 80
Benzoyl chloride CfiHs—CO—Cl 197

(Methanoyl chloride has not been isolated.)

Preparation
Acid chlorides are prepared by the reaction between a carboxylic acid and
phosphorus trichloride, phosphorus pentachloride, or sulphur dichloride
oxide:
3R—CO—OH + PCI -> 3R—CO—Cl + H PO
3 3 3

R—CO—OH + PClj R—CO—Cl + POCI + HCl 3

R—CO—OH + SOCI 2 R—CO—Cl + SO -f HCl 2

The choice of reagent is governed by the boiling points of the products.

If the acid halide has a very low boiling point (e.g. ethanoyl chloride), PCI3
is used and the halide is easily separated by distillation from phosphorous
acid (which decomposes at 200°C). If the acid halide has a very high boiling
point (e.g. benzoyl chloride), PCI5 can be ukd; fractional distillation gives
first POCI3 (b.p. 107°C) and then the acid chloride. If it has an intermediate
boiling point (e.g. propanoyl chloride), SOCI2 is suitable; the gaseous SO2
and HCl pass off first.
Acid bromides and iodides are made from carboxylic acids and phos¬
phorus tribromide and phosphorus tri-iodide respectively. These phos¬
phorus halides are prepared.in situ (p. 117).

Chemical properties
The principal reactions of acid halides are with water, ammonia and amines,
in which the overall reaction can be described by the equation:

R—CO—X + H—Y R—CO—Y + H—X

The process is known as acylation. Ethanoylation is the name reserved for the
DERIVATIVES OF
CARBOXYLIC ACIDS substitution of the ethanoyl group, CH3CO—, for example by ethanoyl
chloride or ethanoic anhydride. The introduction of the benzoyl group,
CgHjCO—, by, for example, benzoyl chloride, is known as benzoylation.
Benzoylation generally takes place more slowly than ethanoylation.

1. Aliphatic acid halides are readily hydrolysed:

R—CO—X + H2O -> R—CO—OH + HX

e.g.
CH3—CO—Cl + H2O ^ CHj—CO—OH + HCl

Thus, when the stopper is removed from a bottle of ethanoyl chloride, white
fumes are seen, owing to the interaction of hydrogen chloride with the
moist air.

Reaction occurs by addition of water to the carbonyl group:

HyH

R—C-X + H2O -> R-C-X

II I
o o~

followed by loss of a proton and the halide ion, X”:

OH
I I
R-C-X ■> R-C + H+ + X"
I II
0 “ O

Hydrolysis is far easier than with esters, which are unaffected by water alone
although they react with hydroxide ion. The reason is that a halogen substituent
—X is more strongly electron-attracting than an alkoxide substituent —OR'; the
carbon atom of the carbonyl group is therefore more electron-deficient in an acid
halide than in an ester and reacts with water, H2O, a weak nucleophilic reagent,
whereas for an ester a much stronger nucleophilic reagent, the hydroxide ion,
OH“, is necessary.

2. Acid halides react with alcohols and phenols to form esters, for
example:

CH3—CO—Cl + CH3—OH ^ CH3—CO—O—CHj -b HCl

Methyl ethanoate
CH3—CO—Cl + CgHs-OH ^ CH3—CO—O—CgHj -f HCl
Phenyl ethanoate

CeHj-CO—Cl -b CgHj-OH ^ CeHj-CO—O—CgHj + HCl

Phenyl benzoate

The mechanisms of these reactions are similar to those between acid halides
and water, with the oxygen atom of the alcohol, R—O—H, or the phenol,
CeHs—O—H, acting as the nucleophile.

217
DERIVATIVES OF 3. Acid halides react with ammonia to form acid amides, for example:
CARBOXYLIC ACIDS

CH3—CO—Cl + NHj -> CH3—CO—NH2 + HCi

Ethanamide

NH3 + HCI ^ NH4 cr

Acid halides react with primary amines in a similar way:

CH3—CO—Cl + CH3—NH2 -> CH3—CO—NH—CH3 + HCI

A-M ethylethanamide

CH3—NHi + HCI CH3—NH3 Cl-

The reactions have mechanisms similar to that in hydrolysis, with the nitrogen
atom acting as the nucleophile.

4. Acid halides react with anhydrous sodium salts of carboxylic acids to

form acid anhydrides (14.4), for example:

CH3—CO—Cl + CH3—CO2- Na+ ^

CH3—CO—O—CO—CH3 + NaCl
Ethanoic anhydride
The carboxylate ion acts as the nucleophile and the mechanism of the reaction
is as described above.
The reactions of acid halides with nucleophilic reagents, such as reactions 1-4,
can be compared with those of aldehydes and ketones with nucleophilic reagents
on the one hand and with those of primary alkyl halides with nucleophilic reagents
on the other.
Like aldehydes and ketones, acid halides react by addition of a nucleophile
(symbolised as Y“):
Y
Y" I
R-C-R' -> R-c-R'
II I
o o-

Y
I
R-C-X
II I
o O'
In the case of the acid halide, the halide ion then breaks off:

Y Y
I I
R-C-X R-C + X'
I II
O O

and the overall process is a subMtution reaction. In contrast, in the case of alde¬
hydes and ketones, the group —H or —alkyl does not form a stable anion, so
that instead of one of these groups breaking off, a proton is transferred to the
intermediate adduct from the solvent:

T
R-C-R' + -> R-C-R'

i- iH
and the overall process is an addition reaction.

218
DERIVATIVES OF
CARBOXYLIC ACIDS Primary alkyl halides, like acid halides, undergo substitution with nucleophiles,
but in this case an intermediate adduct cannot be formed since carbon cannot
form 5 bonds; instead, the approach of the nucleophile to the alkyl group is
concerted with the departure of halide ion:

CH,-X Y -CH, -X > Y-CHj + X“

\
R R R

5. Acid halides are reduced by lithium tetrahydridoaluminate to alcohols:

LiAlH4
R—C—X -^ R—CH2—OH + HX

This reduction occurs by way of the aldehyde, RCHO, and in another

method of reduction, due to Rosenmund, it is possible to obtain the
aldehyde as the product. The method is to pass hydrogen into a solution
of the acid chloride in the presence of palladium as catalyst suspended on
barium sulphate; a mixture of sulphur and quinoline is added as a ‘poison’
to prevent further reduction:
Pd as cat.
R—C—Cl + H2 -> R—C—H + HCl
II ‘poison’ II

o o

Uses
Acid halides (normally the chlorides) are used mainly as acylating agents
(i.e. to introduce the group RCO—) for the preparation of esters and acid
amides.

14.4 General formula

Acid anhydrides R—C—O—C—R'
li I
o o

Physical properties
The lower aliphatic members are mobile, colourless liquids with pungent
smells. The simplest aromatic member, benzoic anhydride, is a white solid,
m.p. 42°C. Some typical acid anhydrides are in Table 14.3.

Table 14.3. Some acid anhydrides

NAME FORMULA B.P./°C

Ethanoic anhydride (CHjCOijO 136

Propanoic anhydride (CH3CH2C0)20 168
Benzoic anhydride (C6H5C0)20 360

219
DERIVATIVES
CARBOXYLIC
Preparation
By the reaction between an acid chloride and the anhydrous sodium salt
of a carboxylic acid, for example:

CH3—CO—Cl + CH3—C02“ Na+ ^

CH3—CO—O—CO—CH3 + NaCl
Ethanoic anhydride

‘Mixed’ acid anhydrides can be formed, using an acid chloride and a sodium
salt with different groups, R and R':

R—CO—Cl + R'—C02“ Na+ ^ R—CO—O—CO—R' + NaCl

Manufacture of ethanoic anhydride

Ethanoic anhydride is manufactured by passing ethenone through ethanoic
acid:

CH2=C=0 + CH3CO2H -> CHj—CO—O—CO—CHj

Ethenone is manufactured by the cracking of propanone vapour at

700°C:

CH3—CO—CH3 ^ CH2=C=0 + CH4

Chemical properties
Acid anhydrides react with nucleophilic reagents in the same way as acid
halides except that, because the group R'CO—O— in an anhydride
RCO—OCOR' is less strongly electron-attracting than the halogen X in
an acid halide RCO—X, reaction is slower. Typical examples are:

1. With water, to give the corresponding acid, for example:

(CH3C0)20 + H2O ^ 2CH3CO2H

2. With alcohols or phenols, to give esters, for example:

(CH3C0)20 + C2H5OH ^ CH3CO2C2H5 + CH3CO2H

3. With concentrated aqueous ammonia, to give acid amides, for

example:

(CH3C0)20 + NH3 ^CH3C0NH2 + CH3CO2H

Ethanamide

Uses of ethanoic anhydride

The principal use of ethanoic anhydride is in the manufacture of cellulose
ethanoate (p. 297). In the laboratory, it is used to make esters (ethanoates)
(p. 212) and acid amides (p. 221).

220
14.5 General formula
Acid amides R—C—NHj
II
O

Physical properties
Table 14.4. Some acid amides

NAME FORMULA M.P./°C B.P./X

Methanamide H—CO—NH2 2 193

Ethanamide CH3—CO—NH2 82 222
Propanamide CH3CH2—CO—NH2 79 222

With the exception of methanamide, amides are white crystalline solids.

The lower members are soluble in water; all amides are soluble in organic
solvents. As ordinarily prepared, ethanamide has the characteristic smell of
mice, owing to the presence of the methyl derivative CH3—CO—NH—CH3;
pure ethanamide has no smell.
The high melting points and boiling points of amides are due to the
formation of hydrogen-bonds between the oxygen atom of one molecule
and the amino-hydrogen atom of another:

R
0 = C^ R
\ /
N-H 0=C R
/ \ /
H ^N-H 0=C^
H N-H
/
H

Preparation
1. By the dehydration of the ammonium salt of a carboxylic acid. For
example, ethanamide is generally prepared by refluxing a solution of am¬
monium carbonate in an excess of ethanoic acid for about 4 hours; am¬
monium ethanoate is first formed and then dehydrated:

CH3CO2H CH3CO2- NH4+ -> CH3CONH2 + H2O

2. By the action of ammonia on esters (14.2), acid halides (14.3) or acid

anhydrides (14.4).

Chemical properties
1. Amides are much weaker bases than amines, even though both
contain the group —NH ; thus, amides are neutral to litmus and do not
2

dissolve in hydrochloric acid to form salts.

This is because the orbital on the nitrogen atom which contains the unshared
pair of electrons interacts with the p orbital of the adjacent carbon atom (Fig.
14.1), giving increased delocalisation in an amide as compared with an amine.

221
FIG. 14.1. p-Orbital overlap in an Two electrons
acid amide; the lower diagram
shows one of the delocalised tt
orbitals y—
On&^ectron

One electron

Therefore, relative to an amine, an amide resists reaction with a proton since this
process requires the use of the unshared pair of electrons on nitrogen in the
formation of thfe new N—H bond and so results in the loss of the extra bonding.

2. Amides are hydrolysed by heating with a mineral acid (usually

hydrochloric acid), for example:

CH3—CO—NH2 CH3—CO2H + NH4+

They are also hydrolysed by heating with an alkali such as caustic soda,
for example:

CH3—CO—NH2 CH3—CO2- + NH3

OH

The alkali-catalysed reaction occurs by a similar mechanism to that of the hydro¬

lysis of esters and acid halides. The hydroxide ion adds to the carbonyl group of
. the amide to give an adduct which takes up a proton from the solvent and then

222
DERIVATIVES OF
eliminates ammonia;
CARBOXYLIC ACIDS
OH
I
R-C-NH, + OH -- > R-C-NH2
II L
o o
OH OH OH
1 H2O I + 1
R—C—NH2 R—C—NH3-> R—C + NH3

O- o~ o
R-CO2H + OH“ -^ R-CO.7 + H2O

The group —NH2 is less strongly electron-attracting than —OR', so that amides
are less easily hydrolysed by alkali than esters, RCO2R', and much less easily
than acid chlorides.

The fact that heating an amide with caustic soda liberates ammonia while
an amine does not react enables the two types of compound to be readily
distinguished. Amides can also be distinguished from ammonium salts in
this way, for the latter liberate ammonia in the cold,

R—C 02“ NH + + NaOH ^ R—COi" Na+ + NH + H O

4 3 2

whereas the former do so only on heating.

3. When amides are distilled over phosphorus pentoxide, acid nitriles

are formed, for example:

CH3—CO—NH2 CH3—C=N + H2O

Ethanenitrile

4. Like amines, amides react with nitrous acid to liberate nitrogen, for
example:

CH3—CO—NH2 + HNO2 ^ CH3—CO2H + N2 + H2O

5. Amides react with bromine in a solution of sodium hydroxide to give

amines (Hofmann reaction), for example:

Bfj/NaOH
CH3—CO—NH 2
-> CH3—NH2 + CO2
Methylamine

Carbamide
Although carbamic acid, H N—CO H, has never been isolated, its acid
2 2

amide, carbamide (urea) (H N—CO—NH ), is stable. It can be made by

2 2

heating ammonium cyanate to dryness ( . ) or by the action of ammonia on

1 1

carbonyl chloride:

Cl—CO—Cl + NH 4 3 -> H N—CO—NH + NH CI

2 2 2 4

Physical properties of carbamide

Carbamide is a white, crystalline solid (m.p. 133°C) which is soluble in
water and alcohol but insoluble in most organic solvents.

223
DERIVATIVES OF
CARBOXYLIC ACIDS
Manufacture of carbamide
By heating excess of ammonia with carbon dioxide at 200°C and 200
atmospheres pressure:

CO2 + 2NH3 -> H2N—C02“ NH4+ -> H2N—CO—NH2 + H2O

Chemical properties of carbamide

1. Carbamide is a monoacidic base; it is a stronger base than simple acid
amides like ethanamide but a weaker base than amines. For example, it
forms an insoluble nitrate, H2N—CO—NH3 NO3 .
2. Carbamide is hydrolysed when heated with a solution of alkali:

H2N—CO—NH2 + 2NaOH ^ Na2C03 + 2NH3

3. Carbamide reacts with nitrous acid to form nitrogen:

HjN—CO—NH2 + 2HNO2 ^ CO2 + 3H2O + 2N2

4. When carbamide is heated above its melting point, a compound known

as biuret is formed:

2H2N—CO—NH2 ^ H2N—CO—NH—CO—NH2 + NH3

Biuret

If an alkaline solution of biuret is treated with a drop of copper(II) sulphate

solution, a violet colour appears. This is known as the biuret test, and is
a test for all compounds containing the peptide linkage, —CO—NH—,
such as proteins (18.2).

Uses of carbamide
1. As a fertilizer. Carbamide (sold under its older name, urea) contains
46% nitrogen and is (except for ammonia) the most concentrated nitro-
geneous fertilizer available. It is very satisfactory for feeding quick-growing
crops in hot climates.
2. In the manufacture of carbamide-methanal plastics (p. 333).
3. In the manufacture of melamine, used to make melamine-methanal
plastics (p. 333).
4. In the manufacture of a range of fine chemicals including the bar¬
biturates, a group of drugs with sedative properties of which the parent is
barbituric acid; for example:

0 0

hnA HN ^ ^
1 CH 2 5

O^N'^OH O^N^O
H H
Barbituric Veronal
acid

224
14.6 The compounds are also known as cyanides.
Acid nitriles _
General formula
R—C^N

Physical properties
The lowest members (except for hydrogen cyanide) are colourless liquids
with pleasant smells. They are fairly soluble in water and very soluble in
organic compounds.
Some typical members are in Table 14.5.

Table 14.5. Some acid nitriles

NAME FORMULA B.P./°C

Methanenitrile (Hydrogen cyanide) H—CN 26

Ethanenitrile (Methyl cyanide) CH3—CN 81
Propanenitrile (Ethyl cyanide) CiHs—CN 97
Benzonitrile (Phenyl cyanide) CeHs—CN 190

Preparation
1. By refluxing an alcoholic solution of an alkyl halide and potassium
cyanide, for example:

CHj—I + KCN CHj—CN + KI

Ethanenitrile

Reaction takes place by nucleophilic displacement of halide ion by cyanide ion

(Sn2 reaction; 9.3).

2. By dehydration of the ammonium salt of a carboxylic acid or an amide

with phosphorus pentoxide, for example:

CH3-CO2- NH4^ CH3-CO-NH2 CH3-C^N

Chemical properties
1. Nitriles are hydrolysed, via the amide, by refluxing with either mineral

acid:

CH3-C=N CH3-CO-NH2 CH3-CO2H + NH4+

H H.

or alkali:

CH3—teN CH3—CO—NH2 CH3—cor + NH3

OH OH

2. Nitriles are reduced to primary amines by sodium and an alcohol,

and by lithium tetrahydridoaluminate, for example:

CH3—C=N ^ CH3—CH2—NH2
Ethylamine

CgHj—C=N CgHs—CH2—NH2
(Phenylmethyl)amine

225
J4.7 Isocyano-compounds (isonitriles) have the structure
Isocyano-compounds r—N=C
and are isomers of the nitriles, R—C=N. The lower members are un¬
pleasant-smelling liquids.
The compounds are made by refluxing an alcoholic solution of an alkyl
halide and silver cyanide:

R_X + AgCN R—N=C + AgX

or by heating a primary amine and trichloromethane with an ethanolic

solution of alkali (the carbylamine reaction; 9.4):

r_NH2 + CHCI3 + 3KOH ^ R—N=C + 3KC1 + 3H2O

They are stable to alkali (unlike nitriles) but are readily hydrolysed by
mineral acid to primary amines:

R_N=C + 2H2O R—NH2 + HCO2H

They can thus be distinguished from their isomers, the nitriles, which are
hydrolysed to the ammonium salt of the corresponding carboxylic acid.

14.8 Small-scale preparation of an ester

Practical work in this example, ethyl ethanoate is prepared from ethanoic acid and ethanol.
Set up the apparatus (Fig. 14.2), with 5 cm^ of ethanol and 5 cm^ of

226
DERIVATIVES OF
CARBOXYLIC ACIDS
Heat the oil-bath to 140°C and add the mixture from the funnel at the
same rate as the ester distils over.
When distillation stops, transfer the distillate to a separating funnel and
add 5 cm^ of a 30 per cent solution of sodium carbonate. Shake, removing
the stopper from time to time to relieve the pressure due to carbon dioxide.
Remove the lower (aqueous) layer and add a solution of 5 g of calcium
chloride in 5 cm^ of water to the separating funnel and shake the mixture
(to remove excess of ethanol). Remove the lower layer again.
Pour the ester into a test-tube and add 2 or 3 pieces of anhydrous calcium
chloride. Stopper the tube and shake it. Decant the clear liquid into a flask
and distil it (cf. Fig. 2.2), collecting the fraction boiling between 75 and
79°C.

Preparation of phenyl benzoate

Details of the preparation of phenyl benzoate are given on p. 161.

Reactions of esters
1. Saponification of an ester. Details of the saponification of an ester,
ethyl benzoate, are given on p. 203.

2. Saponification of a fat. Boil for 20 minutes under reflux about 1 g of

lard or olive oil (or any fat), 1 g of potassium hydroxide and 10 crn^ of
ethanol, until no more oil is observed when a few drops of the mixture are
added to water. Distil the reaction mixture to remove the ethanol, and
dissolve the residue in 15 cm^ of hot water.
(a) To 5 cm^ of the solution of the residue, add slowly a saturated solution
of sodium chloride. Filter the precipitate and test the soap for its lathering
properties.
(b) To another 5 cm^ of the solution, add 5 cm^ of water. Shake well.

Reactions of ethanoyl chloride and benzoyl chloride

Take great care when handling these compounds.
1. Carry out the following reactions with both ethanoyl chloride and
benzoyl chloride and compare their reactivities:
(a) To 5 drops of the acid chloride in a test-tube, add 5 drops of water.
Test the gas evolved.
(b) To 5 drops of ethanol, add 2 drops of the acid chloride. Warm the
mixture if necessary. Dilute the solution with 5 drops of water and
note the smell of the organic product.
2. (a) To 5 drops of phenylamine, add 2 drops of ethanoyl chloride
(CARE). A vigorous reaction takes place, and a white precipitate of
N-phenylethanamide is formed.
(b) Schotten-Baumann reaction. To 3 drops of phenylamine in a test-tube,
add 5 cm^ of dilute sodium hydroxide solution. Shake to form a fine oily
suspension and add 5 drops of benzoyl chloride. Fit a cork to the tube and
shake for a minute. Cool the tube and contents under the tap, removing the
cork periodically to release the pressure. Filter the residue of N-phenyl-
benzamide, using a small Buchner funnel and flask.
Transfer the residue to another test-tube and dissolve it in the smallest
possible quantity of hot ethanol. Filter if necessary and allow the solution
to cool. (If the crystals do not appear within 5 minutes, add a few drops

227
DERIVATIVES OF of water, and ‘scratch’ the sides of the test-tube with a glass rod to ‘seed’.)
CARBOXYLIC ACIDS
Filter the crystals, and dry them between pads of filter papers. M.p. 163°C.

Reactions of ethanoic anhydride

1. To 5 drops of ethanoic anhydride in a test-tube, add 5 drops of water.
Shake the mixture and note the rate of reaction compared with that between
ethanoyl chloride and water.

2. Preparation of 'N-phenylethanamide. See p. 266.

3. Preparation of cellulose ethanoate. See p. 299.

Reactions of ethanamide
1. To about 0-1 g of ethanamide in a test-tube, add 2 cm^ of dilute sodium
hydroxide solution. Boil, and test the vapour evolved with moist red litmus
paper.
2. To about OT g of ethanamide in a test-tube, add 3 drops of bromine
(CARE). Add 1-2 cm^ of dilute sodium hydroxide solution; cork the test-
tube and shake for about a minute.
Remove the cork, add one pellet of sodium hydroxide and boil the
solution gently. Test the gas by (a) its smell, (b) its action on moist red
litmus paper.

Reactions of ethanenitrile
1. To 5 drops of ethanenitrile in a test-tube, add 5 drops of dilute sodium
hydroxide solution. Warm gently and test the gas evolved with moist red
litmus paper.
2. To 5 drops of ethanenitrile in a test-tube, add about OT g of zinc dust
followed by 10 drops of concentrated hydrochloric acid. When the effer¬
vescence due to the evolution of hydrogen has more or less stopped, add
sodium hydroxide solution until the mixture is alkaline. Warm the mixture
and test the gas evolved with moist red litmus paper.

Reactions of carbamide
1. Carbamide is a monoacidic base. To 1 cm^ of hot water in a test-tube,
add some crystals of carbamide until the solution is saturated. Decant this
solution into a clean test-tube and add concentrated nitric acid dropwise. A
white precipitate of the nitrate is formed.

2. Hydrolysis of carbamide, (a) To OT g of carbamide in a test-tube, add

1 cm^ of 2M sodium hydroxide solution and boil the mixture. Test the
vapour evolved with moist red litmus paper and note the smell of the gas
evolved.
This reaction shows that the amide group, —CONH2, is present.
(b) Action of the enzyme, urease. Experiment 3(a) in Section 18.4.

3. Dissolve a few crystals of sodium nitrite in water in a test-tube and

cool the solution in a beaker of ice. Add 1 cm^ of dilute hydrochloric acid.
(There may be some effervescence due to reaction between sodium nitrite
and the acid.) Add a few crystals of carbamide. Observe whether any gas is
evolved.

4. Heat about OT g of carbamide in a test-tube until it melts. Test gases

evolved with moist red litmus paper.

228
DERIVATIVES OF
CARBOXYLIC ACIDS Continue to heat the residue gently for a further 3 minutes. Cool it and
add 10 drops of water followed by 2 drops of a dilute solution of copper(II)
sulphate followed by dilute sodium hydroxide solution until the mixture
is alkaline.
A violet coloration confirms that the residue contains a compound which
has a peptide link, —CO—NH—. This compound is called biuret and the
test is named after it, the biuret test.

14.9 1 Name and give the structural formulae of the aliphatic acids and esters which
have an empirical formula C2H4O and a molecular weight of 88.
Questions
Outline the chemical tests that you would apply to enable you to distinguish
between each isomer. (L)

2 Give three general methods for the preparation of esters. State which of them
you would select for the preparation of phenyl benzoate and outline the procedure
you would adopt.
How would you prepare a pure water-free specimen of ethanol from ethyl
benzoate ? Briefly indicate the necessary conditions for each step.
Give a brief account of the constitution of naturally occurring fats, and show
what useful products may be derived from them. (JMB)

3 Describe the preparation from ethanol of pure samples of ethanoic acid and of
ethyl ethanoate.
How, and under what conditions, does ethanoic acid react with (a) thionyl
chloride, (b) methylamine, and (c) soda lime ? (C(T))

4 0-6 g of ethanoic acid were mixed with an equimolecular amount of ethanol

and the mixture was sealed in a small glass tube which was immersed in a bath of
boiling water. When equilibrium had been reached the glass tube was removed
from the water bath and broken open under the surface of about 30 cm^ of water
in a conical flask. The resulting mixture was titrated with OTM sodium hydroxide
using phenolphthalein as indicator and required 33-30 cm^ of the alkali.
Calculate the equilibrium constant for the esterification of ethanol by ethanoic
acid.
Describe in outline how you would modify the above reaction conditions for
the preparation of a pure sample of ethyl ethanoate. (AEB)

5 What do you understand by the equilibrium constant of a reversible reaction ?

What do you need to know if you are to predict correctly the qualitative effect
of temperature on this constant ? State and explain the principle involved.
When one gram-mole each of ethanoic acid and ethanol are mixed together
at 20°C, two-thirds of a gram-mole of ethyl ethanoate is formed. How many
gram-moles of ethyl ethanoate would be present eventually if one gram-mole of
water was added to the above mixture? (O)

6 Give the structural formulae of the functional groups characteristic of (a) car¬
boxylic acids, (b) acid chlorides, (c) acid anhydrides, (d) primary amines, (e)
amides.
Suggest a scheme whereby ethanoic acid might be obtained from methane as
starting material. Indicate by equations how ethanoic acid might be converted
into the corresponding acid chloride, acid anhydride, acid amide and ethyl
ester respectively. (AEB)

7 How would you prepare ethanoyl chloride from ethanoic acid ?

Name the products and write the equations for the reactions of ethanoyl
chloride with: (a) sodium hydroxide, (b) ammonia, (c) ethanol, (d) phenol, (e)
anhydrous sodium ethanoate. (C(N, T))

8 Describe, with essential practical details, the preparation of ethanamide from

ammonium ethanoate.

229
DERIVATIVES OF Describe the reactions which occur between ethanamide and (a) phosphorus
CARBOXYLIC ACIDS pentoxide, (b) nitrous acid, and (c) bromine and sodium hydroxide solution.

9 An organic compound W, on analysis, gave C = 40%, H — 8-5%, N — 23-7%,

O = 27-1% by mass. Refluxing W with dilute hydrochloric acid produced a com¬
pound X which contained 40% carbon by mass and had the general formula

”x was also prepared by the oxidation of a compound Y which had the general
formula C„H2„0.
(a) Calculate the empirical formula of W.
(b) Determine the molecular formula of X and hence deduce the molecular
formula of W.
(c) Write a balanced equation to represent the reaction which occurred when
W was refluxed with dilute hydrochloric acid.
(d) Identify Y giving reasons for your answer.
(e) Give two chemical tests to distinguish between Y and butanone
(CH3CH2COCH3). , (AEB)
10 For each of the following pairs of substances describe one simple chemical test
which would serve to distinguish between them:
(a) ethanoyl chloride and ethanoic anhydride;
(b) ethanal and propanone;
(c) methanoic acid and ethanoic acid;
(d) ethanamide and phenylamine.
In each case state the conditions under which the reaction occurs and give the
equation for it. (L)
11 Compare the hydrolysis of (a) ethyl benzoate, (b) benzoyl chloride, (c) benzamide.
Describe how you would isolate a pure specimen of the common product of
hydrolysis and explain how it may be reconverted into (a), (b) and (c). (W)
12 By means of equations, supplemented by brief notes on relative speeds of reac¬
tion, indicate what reactions occur between ammonia and the following com¬
pounds : (a) chloromethane, (b) ethanoyl chloride, (c) chlorobenzene, (d) ethanal,
(e) ethanoic acid, (f) methyl propanoate.
By what simple chemical means could you quickly distinguish between the
product formed from (b) and that formed from (e) ? (JMB)
13 In this problem the molecular formula of some of the compounds is given in
brackets.
(i) A substance X (C4H80NBr) yielded ammonia when boiled under reflux
with an aqueous solution of sodium hydroxide.
(ii) A portion of the resulting solution was acidified, treated with chlorine,
and then shaken with trichloromethane. Two layers were formed, the
lower of which was orange in colour.
(iii) The remainder of the solution from (i) was evaporated to a solid substance
which on acidification and distillation yielded an acidic substance Y
(C4H8O3).
(iv) On oxidation Y yielded Z (C4H6O3). Z gave a precipitate with 2,4-dinitro-
phenylhydrazine, but not with Fehling’s solution.
Suggest a possible structure for X, Y and Z, and describe what happens in the
reactions outlined above. (L(Nuffield)(S))

14 On analysis a compound X of molecular weight 59 was found to contain 40-67

per cent carbon, 8-5 per cent hydrogen, 23-72 per cent nitrogen, the remainder
being oxygen. Derive the formula of X, write a structural formula and name the
compound.
Stating essential conditions of reaction, describe how the compound you name
«
for X reacts with (a) phosphorus pentoxide, (b) sodium hydroxide, (c) nitrous
acid, (d) bromine and potassium hydroxide. (AEB)

230
DERIVATIVES OF
CARBOXYLIC ACIDS
15 The reaction between ethanoic acid and ethanol is catalysed by hydrogen ions.
Describe how you would attempt to prove this.
16 The percentage composition of an aliphatic compound was found to be C = 20 0,
H = 6'7, O = 26-7, N = 46-6. A solution of 0-25 g of the compound in 20 g of
water froze at -0-39°C. What was the compound and what would be the action
of heat on it ?
(Molecular depression constant for water is 18-6° per 100 g.)
17 How, and under what conditions, does ethanamide react with (i) dilute hydro¬
chloric acid, (ii) bromine and sodium hydroxide, (iii) phosphorus pentoxide,
(iv) nitrous acid (acidified sodium nitrite solution) ?
Give balanced equations and essential conditions for the reactions by which
ethylammonium chloride (ethylamine hydrochloride) could be prepared from
ethanamide.
What volume of 0-50M (0-50N) hydrochloric acid would be required to react
completely with the gas evolved when 1 00 g of ethanamide is boiled with an
excess of sodium hydroxide? (C(T))
18 What general methods are available for the preparation of (a) ethers, (b) acid
anhydrides, (c) esters ?
Compare the structures of diethyl ether, ethyl ethanoate, and ethanoic
anhydride, and their reactions and methods of preparation, so as to bring out the
similarities in structure and the effect of the modifications in structure.
19 What is the importance in organic chemistry of derivatives of hydrogen cyanide?
20 A compound has the structural formula CH3COCH2CH=CHCH2CN. How
would you expect this compound to react with (a) sodium and ethanol, (b)
bromine, (c) sodium hydroxide, and (d) phosphorus pentachloride?
21 A colourless liquid. A, contains 58-54 per cent of carbon, 7-32 per cent of hydrogen
and 34-14 per cent of nitrogen. When boiled with hydrochloric acid, A produced
a compound B. When a pure sample of B was fused with soda-lime, a colourless
inflammable gas was formed. When A was treated with dilute sulphuric acid and
zinc, a colourless liquid, C, was formed, which reacted with iron(III) chloride to
give a brown precipitate.
Identify A, B, and C and explain, giving equations, the reactions which occurred
above.
Describe two reactions by which A could be made.
22 An organic acid P, on treatment with phosphorus pentachloride yielded a sub¬
stance Q, which contained 78-0 per cent of chlorine and had a molecular weight
of 182. Q when heated with dilute sodium carbonate solution yielded a substance
R, which when crystallised and heated with soda lime produced a volatile liquid S.
The product when heated with phenylamine and sodium hydroxide yielded a
substance possessing a highly offensive smell. Derive a formula for P and explain
the course of the reactions described. (L(X,S))
23 2-000 g of a neutral aliphatic compound X, containing carbon, hydrogen and
oxygen only, gave on combustion 3-617 g of carbon dioxide and 1 -233 g of water.
If the vapour density of Jif is 73, calculate its molecular formula.
When X was refluxed with aqueous sodium hydroxide and then distilled, the
distillate contained only one organic compound and this gave a positive response
to the iodoform test. The solution left in the flask was evaporated to give a solid,
which reacted with hot concentrated sulphuric acid to give a mixture of two gases,
one of which turned lime-water milky whilst the other burnt with a blue flame.
Identify X, give its structural formula and explain fully the reactions which
have been used to find out what it is. (O)
24 Describe briefly the preparation of ethanonitrile starting from (a) methanol, and
(b) ethanol.
Ethanonitrile (x g) was boiled under reflux with sodium hydroxide solution
and the ammonia which was evolved was passed into molar hydrochloric acid

231
DERIVATIVES OF solution (50 cm^). The excess of acid required 26 cm^ of molar sodium hydroxide
CARBOXYLIC ACIDS
solution for neutralisation. Calculate x. (O and C)
25 Outline two methods of preparing carbamide. What is the historical importance
of its first synthesis ?
How does carbamide react with (a) nitrous acid solution (sodium nitrite and
hydrochloric acid), (b) concentrated nitric acid, (c) sodium hydroxide solution,
(d) a dilute alkaline solution of bromine? (O and C)

26 When 0-357 g of a colourless liquid. A, which was almost insoluble in water,

was burnt, 0-616 g of carbon dioxide and 0-189 g of water were obtained. When
A was warmed with an excess of aqueous sodium hydroxide and the resulting
solid obtained on evaporation was strongly heated, a gas B was evolved. When
10 cm^ of B was exploded with 30 cm^ of oxygen (an excess) and the resulting gas,
after cooling, was shaken with caustic potash solution there was a diminution
in the volume of the gas by 10 cm^.
When A was warmed with aqueous ammonia, under suitable conditions a
crystalline solid, C, was obtained, which contained 23-75 per cent of nitrogen.
Identify A, B and C and explain the reactions that take place.
What would happen if A was heated with calcium hydroxide in equimolecular
proportions ? (L)

27 A compound A has the molecular formula C6H10O4. It is a sweet-smelling liquid,

sparingly soluble in water.
(a) When A is boiled under reflux with aqueous sodium hydroxide, a clear
solution, B, is obtained.
(b) When solution B is distilled, the distillate, C, turns warm acidified sodium
dichromate solution green.
(c) When a portion of the residual liquid from (b) is neutralised with hydro¬
chloric acid, and calcium chloride solution added, a white precipitate, D,
is observed.
(d) When the remaining liquid from (b) is acidified and then warmed with
potassium permanganate solution carbon dioxide is evolved and a colourless
solution results.
(e) When A is shaken with 0-880 ammonia solution, a white solid, E, is formed.
Suggest the most probable structures for A and explain the reactions described.
How would the result of the reaction of iodine and alkali on distillate C affect
the decision on the structure assigned io Al (SUJB(S))
28 A neutral compound. A, has an empirical formula C8H,02. When 0-4824 g of
A is dissolved in 40-20 g of benzene, the boiling point is raised by 0-119°C (the
boiling point constant, K, for benzene = 2-67°C per 1000 g of benzene). Treatment
of A with an excess of boiling aqueous potash gives a clear solution which when
saturated with carbon dioxide affords a liquid turning to a low-melting solid, B,
molecular formula CeHgO. B gives a purple colour with iron(III) chloride [ferric
chloride] solution, and a white precipitate when treated with bromine water.
Acidification (with mineral acid) of the aqueous solution after removal of B
yields a solid acid, C, empirical formula C2H3O2, and this gives a neutral com¬
pound, Z), when it is boiled with methanol in the presence of a trace of acid. The
empirical formula of D is C3H5O2 and the vapour density, 73. Also when C is
heated it loses water to give E, a neutral substance with molecular formula
C4H4O3.
Deduce the structure of A and explain the formation of the substances B, C,
^ (JMB(S))
29 Give an account of the general properties associated with the unsaturated groups
present in each of the following aliphatic types:
(a) R-CH=CH-R (b)

*
(c) R—C=C—H (d) R—CsN

232
DERIVATIVES OF
CARBOXYLIC ACIDS Call attention to any points of comparison or contrast which you consider of
special interest.

30 When a compound P was heated with concentrated aqueous sodium hydroxide

solution and the reaction product acidified, equimolar amounts of Q, containing
C, 77-8 per cent; H, 7-4 per cent; and R, containing C, 68-8 per cent; H, 4-9
per cent were obtained. Mild oxidation of Q gave P, but more vigorous oxidation
gave R. When 25 cm^ of a solution of R, containing 110 g/dm^, were titrated
with 0-1 ON sodium hydroxide, 22-6 cm^ were required for neutralisation (phenol-
phthalein). When R was heated with soda-lime it gave a hydrocarbon S, contain¬
ing C, 92-4 per cent; H, 7-6 per cent.
Identify P, Q, R and S and give equations for the reactions involved in the
above sequence.
Describe how you would make a pure crystalline derivative from P. (C(S))

31 Three isomeric compounds. A, B and C, have the molecular formula C8H8O2.

(a) When heated for a long time with soda-lime A gave benzene, while both
B and C gave methylbenzene.
,(b) A was a neutral compound, whereas both B and C were monobasic acids.
(c) B and C were both readily chlorinated in sunlight; B gave a compound
containing three chlorine atoms per molecule which was easily hydrolysed
to a dibasic acid D which in turn gave an anhydride on heating. When D
was heated with soda-lime it gave benzene. The chlorinated product from
C contained two chlorine atoms per molecule and was a strong monobasic
acid.
Identify A, B, C and D, write equations for the reactions, and indicate how you
would prepare A from benzoic acid. (C(S))

32 Give examples of (a) addition reactions, and (b) condensation reactions, of the
carbonyl group ^C=0. Account for the difference in reactivity of the carbonyl
group in (a) acids, (b) aldehydes, (c) ketones, (d) esters. (L(S))

33 A neutral white solid, P (C, 40-6 per cent; H, 5T per cent), was refluxed with an
excess of aqueous sodium hydroxide and the reaction mixture was then distilled
to give a distillate, Q, and a residue, R.
No reaction was observed when Q was treated with iodine and alkali, but Q
was oxidised by acid dichromate solution to give a compound S (C, 40-0 per cent;
H, 6-67 per cent) which reduced silver nitrate to silver and mercury(II) chloride to
mercury(I) chloride and mercury.
Careful acidification of R, followed by suitable treatment, gave an anhydrous
crystalline compound, T, 0-90 g of which required 20 0 cm^ of IM (IN) sodium
hydroxide for neutralisation or 40-0 cm^ of OTM (0-5N) acidified potassium
permanganate for oxidation.
Compound P reacted with ethanolic ammonia to give U, empirical formula
CH2NO, and Q.
Identify compounds P to U, and write equations for all the reactions. (C(S))

34 Describe in outline the preparation of diethyl propanedioate from ethanoic acid.

Starting from diethyl propanedioate how would you prepare:
(a) butanoic acid;
(b) 2-methylpropanoic acid.

35 A neutral solid A, C2H3O2, slowly dissolved in water to give a solution which

became progressively more acidic as the substance went into solution. After
boiling A with two molecular proportions of sodium hydroxide, evaporation of
the solution gave B, C02Na, which on heating with soda-lime evolved hydrogen.
On treatment with ammonia, A gave a white infusible solid C, CH2ON, which
gave the biuret reaction. All the formulae given above are empirical.
Elucidate the above reactions.

233
DERIVATIVES OF 36 Discuss from a practical point of view the synthesis and hydrolysis of esters.
CARBOXYLIC ACIDS
What explanation can you offer for the following observations:
(a) Esterification and hydrolysis are equilibrium processes,
RCOOH + R'OH ^ RCOOR' + HiO,
yet in some reactions, nearly quantitative yields of ester may be obtained.
(b) Most esters show a molar freezing point depression of two when dissolved
in concentrated sulphuric acid.
(c) The methyl ester of 2,4,6-trimethylbenzoic acid gives a fourfold freezing
point depression in sulphuric acid ? (C Schol.)
37 Three alternative structures were proposed for a compound A, C16H14O4, namely

(i) O-CO-CHa-CHa-CO-O

(ii) O-CO-CH2-CH2-O-CO

(iii) ((^J^C0-0-CH2-CH2-0-C0

A dissolved completely when it was boiled with aqueous sodium hydroxide

and it was found possible to isolate the organic products liberated when the
solution was acidified, in almost quantitative yields.
Given bromine water and either standard acid or alkali, describe the investiga¬
tions you might make to assign the correct structure to A. (O Schol.)
38 A neutral oil A (C15H26O6) can be extracted from butter. It is immiscible with
water but it dissolves on boiling with aqueous sodium hydroxide. From the
resulting solution a syrupy liquid B (CsHgOa) and a salt C (C4H702Na) can be
isolated. Phosphorus pentachloride converts B into a liquid which contains 24-41
per cent carbon, 3-39 per cent hydrogen and 72-20 per cent chlorine.
Suggest formulae for the compounds A, B and C and if alternatives are possible
state whether it is easy to choose between them in assigning a structure to A.
Give your reasons. (O Schol.)
39 A neutral compound A, C10H18O4, dissolved slowly in boiling sodium hydroxide
solution to give ethanol and, after acidification, a solid B, C6H10O4. The solid B
decomposed smoothly at its melting point (118°) to give a liquid acid C, CsHio02.
The acid C was optically inactive, but could be resolved with an optically active
base. Compound B could not be resolved.
Suggest structures for A, B and C and explain the reactions involved.
(O Schol.)

40 Distinguish between the terms order and molecularity as referred to a chemical

reaction.
During the saponification of ethyl propanoate in alkaline aqueous solution at
295 K, the initial concentrations pf ester and sodium hydroxide were each 0-025 mol
dm“^. The concentration of the ester after various time intervals is given in the table
below.

Time (minutes) 0 5 10 20 40 60 80 100

Ester concentration 25 15-5 11-3 7-3 4-3 3-0 2-3 1-9
(1000 X mol dm-^)
(a) Show that the reaction is second order.
(b) Calculate the reaction velocity constant, stating the units.
(c) Determine the time for 50 % of the reaction to be completed.
(d) Suggest an analytical technique for following the reaction.
«
(O and C (S))

234
Chapter 15
Isomerism

15.1 Isomerism is said to occur when two or more compounds have the same
molecular formula, but have different physical or chemical properties. The
Introduction
compounds that exhibit isomerism are said to be isomers. There are two
principal subdivisions of isomerism—structural isomerism and stereo¬
isomerism.
Structural isomerism occurs when two or more compounds have the same
molecular formula but different structural formulae; that is, at least one
atom is bonded to a different atom in one isomer as compared with another.
Stereoisomerism occurs when two or more compounds have the same
molecular formula and the same structural formula, corresponding atoms
being linked to the same atoms, but different spatial arrangements of their
bonds.

15.2 An outline of structural isomerism was given earlier (1.7) and many
examples of isomerism have been met in the previous chapters. We shall
Structural isomerism here describe briefly the convenient sub-divisions of the subject.
(a) Chain isomerism, concerned with the arrangement of the carbon
atoms in the molecule. For example, butane and 2-methylpropane are
chain isomers:

CHj

CH3—CH2—CH2—CHj CHj—CH—CH3
Butane 2-Methylpropane

(b) Position isomerism, exhibited by isomers in the same homologous

series which have the same carbon skeleton but differ in the position of the
functional group, for example:

CH3—CH2—CH2 CH3—CH—CH3

OH OH
Propan-l-ol Propan-2-ol

Cl Cl Cl

Cl
1,2-Dichlorobenzene 1,3-Dichlorobenzene 1,4-Dichlorobenzene

(c) Functional group isomerism, exhibited by isomers which have the

same molecular formula but contain different functional groups, for

235
ISOMERISM example:
CH3-CH2-CH=0 CH3-C-CH3

0
Propanal Propanone

(d) Tautomerism, exhibited by isomers which are in dynamic equili¬

brium. An example is ethyl 3-oxobutanoate; the two isomers rapidly
interconvert, the equilibrium constant being about 0.11 (i.e. 90 per cent keto
form and 10 per cent enol form):

CH3—C—CH2—CO2C2H5 ^ CH3—C=CH—CO2C2H5
1 1
0 OH
Keto form Enol form

If a reagent is added which reacts with the carbonyl group (e.g. hydroxy-
lamine), it removes the keto isomer and more of the enol form is then
converted into the keto form to re-establish the equilibrium; eventually the
entire mixture reacts with the reagent. The opposite occurs if a reagent is
added which reacts with the enol form, such as bromine:

CH3—C=CH—CO2C2H5 + Br2 ^ CH3—C—CH—CO2C2H5 + HBr

OH 0 Br

Eventually the entire mixture is converted into the bromo-compound.

Stereoisomerism occurs when two or more compounds have both the same
15.3 molecular formula and the same structural formula but differ in the spatial
Introduction to arrangement of the atoms. There are two subdivisions: geometrical iso¬
stereoisomerism merism and optical isomerism.

The essential requirement for the existence of geometrical isomers is the

15.4
prevention of rotation around at least one bond in the compound. The
Geometrical isomerism commonest situation of this sort is when a carbon-carbon double bond is
present; there can then be two geometrical isomers:

X X X Y
\ / \ /
c=c c=c
/ . \ / \
Y Y Y X
cis isomer trans isomer

Note that these isomers have the same structural formula, for in both
isomers each carbon atom is attached to one group X and one group Y;
however, the relative positions in space of these groups are different for
the two isomers. The nomenclature cis is often used when identical substi¬
tuents are on the same side of the double bond, and trans when they are on
«
opposite sides.

236
ISOMERISM
Typical examples of geometrical isomers are:

CHg CH3 CH3 H

\ / " \ /
c=c C=C
/ \ / \
H H H CH3
cw-But-2-ene trans-But-2-ene

HO2C CO2H HO2C H

C=C C=C
/ \ / \
H H H CO2H
m-Butene- tra«5-Butene-
dioic acid dioic aeid

Oximes, which have a C=N b< , also exhibit geometrical isomerism,

for example:

CeHs H
\ /
C
N N
\
OH HO

Rotation is also prevented by the presence of a ring of atoms; for example,

1,2-dimethylcyclobutane exists in cis and trans forms:

cis isomer tram isomer

Geometrical isomers have different physical and chemical properties.

For example, cw-butenedioic acid readily forms a cyclic anhydride on being
heated:

HO2C ,CO,H /Ox

\ / heat o=c c=o
/
C=C
\
\ / + H2O
H H /C=C^
H H

but the tm/75-isomer is unable to do so.

The nomenclature cis and trans is now being replaced by the more
general nomenclature Z (German: zusammen, together) and E (German:
entgegen, across), according to whether the substituents of the higher
priority on each carbon atom are on the same or opposite sides of the
double bond. Priority is decided by the relative atomic mass of the atom
attached to the unsaturated carbon atom: the higher the relative atomic
mass, the higher the priority (e.g. —OH > — CH3). If these atoms are the
same, the atoms to which they are attached are considered (e.g. — CH2CI >
— CH3); and if the atom forms a double or triple bond to another atom, X,

237
ISOMERISM it is regarded as being attached to two or three atoms of X (e.g. — CHO >
-CH20H).Thus,
Cl /CO2H
C
H ^CH3
is the Z-isomer, since Cl is of higher priority than H, and CO2H is of higher
priority than CH3.

15.5 When two compounds have the same molecular and structural formulae
but one is not superimposable upon the other, they are described as optical
Optical isomerism
isomers. The reason is that they differ in their optical properties.
For example, if a compound contains a carbon atom bonded
to four different atoms or groups (e.g. 2-hydroxypropanoic acid,
CH3CH(0H)C02H; p. 198), the construction of models shows that it can

Plate 15.1. Molecular models of

stereoisomers. The carbon atom is
surrounded by four different
groups of atoms. Each group is
represented, for clarity, by a single
coloured ball. The two stereoisomers
are mirror images of one another
which cannot be superimposed

exist in two forms which are not superimposable (Plate 15.1). The two forms
are related as object and mirror image; if a mirror were placed between the
models in the photograph, perpendicular to the page, then the reflection of
the right-hand form in the mirror would be seen as identical with the left-
hand form, and vice-versa. The two forms differ in their behaviour towards
polarised light.

Introduction to polarised light

Light is a form of electromagnetic radiation, like radio waves and Z-rays.
It is a wave-motion, the wavelength of each wave being about 5 x 10“ m,
the exact value being characteristic of the colour of the light. In free
space all these waves, irrespective of colour, travel with a velocity of 3 x
10® m s“ ^ When they impinge upon a transparent homogeneous material.

238
ISOMERISM
however, they are slowed down in a constant ratio, so that:

Velocity in free space

^-j--= a constant.
Velocity m medium

known as the ‘refractive index’, and usually denoted by the symbol /x; its
value depends upon both the medium and the wavelength of the light.
Those who find difficulty in appreciating the properties of electromagnetic
radiation may be helped by likening light waves to the material waves which
can be made by regularly vibrating one end of a long string—which we
shall assume to be stretched horizontally in what follows—although they
must not push the analogy too far, nor expect an analogy to prove anything.
Just as it is possible for the end of the string to oscillate vertically up and
down, or horizontally to and fro, or even in a combination of these direc¬
tions, so also the direction of oscillation of the light waves can be in any
direction which lies at right angles to the direction of propagation of the
light.
Many common media are not homogeneous, in either the optical or in
any other sense; for example, the directional properties of wood are clearly
recognisable in its ‘grain’; the ease with which a paper-knife may be inserted
between the pages of a closed book will depend upon whether its blade is
parallel or perpendicular to the leaves of paper. In the optical analogues of
such ‘anisotropic’ media, the velocity of light which is oscillating in one
plane is not the same as that which is oscillating in the perpendicular plane,
and therefore it will have two different refractive indices. Thus a ray of light
which comprises oscillations in many planes in free space will travel on as
two distinct rays when once it gets inside such a ‘birefringent’ medium. By
subtle optical means it is possible to suppress one or other of these rays in
such devices as Nicol Prisms (two crystals of calcite, CaC03, mounted
together by Canada balsam) or Polaroids. The emergent ray then consists
of vibrations in one plane only and it is said to be ‘polarised’ (Fig. 15.1).

FIG. 15.1. The passage of light

through a Polariser to form plane
polarised light

Incident wave Emergent polarised wave

(Light waves in many (Light waves with one
directions of vibration) direction of vibration)

To the human eye this polarised ray will be indistinguishable from an

ordinary, non-polarised ray vibrating in many planes. But suppose a plane
polarised ray, produced by a ‘Polarising Prism’, impinges subsequently
upon another, similar ‘Analysing Prism’, several possibilities arise. If the
axes of the two prisms are parallel to one another, the light will pass through
them both without diminution in intensity, just as plane polarised waves on
a string would pass unimpeded through parallel slits in two obstructing
screens (Fig. 15.2).

239
 Direction of waves—^
FIG. 15.2. Transmission of plane
polarised light when the axes of
the Polariser and Analyser are
parallel

If, however, the second slit were turned through a right angle by rotating
it about the direction of the plane of the waves in the string, the material
waves would not be able to get through (Fig. 15.3). Light waves are similarly
extinguished when the axes of the Polariser and the Analyser are perpen¬
dicular to one another, or ‘crossed’. It can be shown that if the axes are at
some intermediate angle, 6, then the intensity of the emergent ray is pro¬
portional to cos^ 6.
FIG. 15.3. The extinction of plane Direction of waves
polarised light when the axes of
the Polariser and Analyser are at
right angles to one another

A Polarimeter is an instrument for carrying out precise measurements of

this effect. It consists of a fixed Polarising Prism adjacent to the source of
monochromatic light, followed by a container for the substance under
investigation, and finally an Analysing Prism carried on a circular scale
FIG. 15.4. A Polarimeter which can be rotated about the axis of the whole apparatus (Fig. 15.4).

Source of Polarising Solution of substance Analysing Eye

monochromatic prism to be analysed prism
light

When an optically inactive solution is placed in the containing tube, light

emerges at full intensity from the Analyser when its axis is parallel to that
of the Polariser, and is completely extinguished when these two axes are
perpendicular to one another; the latter situation can be more easily detected
in practice, but even so it is subject to some uncertainty because the cos^ 9
function changes only slowly as it reaches its minimum value.
If, however, a solution of an ‘optically active’ substance is used (this is
explained in detail later), it will be found that the axis of the Analyser is no
longer perpendicular to that of the Polariser when extinction occurs, but

240
ISOMERISM
differs from that position by an angle through which the plane of polarised
light has been rotated while the light has passed through the solution.
Experiment shows that the angle of rotation is directly proportional to the
length of the tube and to the concentration of the solution, for a given
wavelength and at a fixed temperature.
The molar optical rotatory power of a compound, at a given temperature
and for a given wavelength, is the rotation produced by a column of
solution 1 m long and of concentration 1 mol m“^.

Introduction to optical activity

The ability to rotate the plane of polarised light is not confined to calcite
crystals. Other inorganic crystals, such as sodium bromate and sodium
iodate, have the same property. However, they lose the ability when they
dissolve in water; it is the asymmetric arrangement of the atoms or ions in
the crystal lattice which gives these crystals this property.
In contrast, any organic compound which contains a carbon atom
attached to four different atoms or groups rotates the plane of polarised
light both in the crystalline state and in solution; its mirror-image isomer
likewise rotates the plane, but in the opposite direction. Such a carbon atom
is described as an asymmetric carbon atom or chiral carbon atom. Chirality
is the property of ‘handedness’ (Greek: cheir, a hand); the two isomers that
can be formed by attaching four different atoms or groups to an asymmetric
or chiral carbon atom are non-superimposable mirror images just as are a
left and a right hand.
The molar optical rotatory powers of the two isomers are equal but of
opposite sign: the one which rotates the plane of polarised light to the right
is described as dextrorotatory, as giving a positive rotation and as the (+)
form; the other isomer, rotating the plane to the left, is described as
laevorotatory, as giving a negative rotation and as the ( —) form. The two
isomers are described as enantiomers. An equimolar mixture of the two
does not rotate the plane of polarised light and is said to be optically
inactive; it is also known as the racemic form (( + ) form).
It is essential to realise that the phenomenon of optical activity can only
be explained by assuming the tetrahedral arrangement for the molecule.
Such an arrangement was proposed independently by van’t Hoff and Le
Bel in 1874, and is supported by two particular pieces of evidence: (a) there
is only one form of such substances as dichloromethane CH2CI2 (1.2), and
(b) the phenomenon of optical isomerism.

The sign of rotation associated with an optically active compound gives no

information about its absolute configuration—the actual arrangement in space of
the four atoms or groups attached to asymmetric carbon. However, it is possible to
find out what the configuration is, and it is now described in a way related to that for
geometrical isomers (p. 237): the chiral carbon is viewed from the side opposite the
atom or group of lowest priority (often hydrogen). If the other three atoms or groups
then appear in a clockwise order of decreasing priority, the compound is given the
prefix R (Latin: rectus, right); if anticlockwise, the prefix S (Latin: sinister, left). For
example, when viewed in this way.
CH3

would appear as

and is therefore termed S-2-chloropropanoic acid since the three groups in

descending order of priority (Cl > CO2H > CH3) appear in the anticlockwise
order.

241
ISOMERISM
Properties of enantiomers
1. The physical properties of enantiomers, such as melting point, boiling
point and solubility, are identical except for the direction of rotation of
polarised light. Note, however, that it is not possible to predict from the
structural formulae of the enantiomers which will be the (+) and which the
( —) form; this can only be determined by experiment once the actual
structure has been found.
2. Enantiomers give crystals of the same type as each other, but the
crystals of one are the mirror images of those of the other (p. 244), and the
two are described as enantiomorphs.
3. The chemical properties of enantiomers are identical for their
reactions with compounds which are not optically active; for example, ( + )
and (—)2-hydroxypropanoic acid undergo esterification with methanol
at exactly the same rate as each other. However, enantiomers can react at
different rates with an optically active reagent. This has especially
important consequences when biological materials are involved, because
enzymes, which are the catalysts for reactions in living systems, occur in
optically active forms (18.2). Thus enantiomers often behave differently
towards bacteria; for example, ( + )2-hydroxypropanoic acid is consumed
by penicillium glaucum but ( —)2-hydroxypropanoic acid is relatively
unaffected. Again, enantiomers can have different physiological properties;
for example, ( — )adrenalin is more active in contracting the blood
capillaries than ( + )adrenalin, and (—)nicotine is more poisonous than
( + )nicotine.

Optical isomerism of compounds containing two

asymmetric carbon atoms
The 2,3-dihydroxybutanedioic acid molecule contains two asymmetric
carbon atoms:

H H
HO2C-C-C-CO2H
I 1
HO OH

2,3-Dihydroxybutanedioic acid exists in three stereoisomeric forms,

which can be represented in two dimensions as follows;

H ,H H
HO^ ^COgH HOaC^ HO^ ^COaH

i ? .f.
H^/ CO2H HO2C HO'/ CO2H
HO OH H
(I) (2) (3)

Structures (1) and (2) are mirror images of each other but they are not
superimposable on each other. Therefore, each is optically active and the
two constitute a pair of enantiomers; one is dextrorotatory (( + )2,3-di-
hydroxybutanedioic acid) and the other is laevorotatory ((-)2,3-di-
hydroxybutanedioic acid).

242
ISOMERISM
Structure (3) is not superimposable on either (1) or (2), nor is it the
mirror image of either. It is found that it is optically inactive, for the optical
activity due to one of the asymmetric carbon atoms is counterbalanced by
the optical activity due to the other; the top half of structure (3) is the mirror
image of the bottom half, the dotted line representing a plane of symmetry.
[In a sense, (3) has the structure of the top half of (1) and the bottom half
of (2).]
Compounds which contain two or more asymmetric carbon atoms but
are optically inactive are described as internally compensated and defined
by the prefix meso; thus structure (3) is (mei'o)2,3-dihydroxybutanedioic
acid. An equimolar mixture of ( + ) and ( — )2,3-dihydroxybutanedioic acid
is also optically inactive since the specific rotations of the two are equal in
magnitude but opposite in sign; it is described as a racemic mixture, or as
the (± )form, and is said to be externally compensated.
The (•+•) and (— )isomers have identical physical and chemical properties
except in the direction of rotation of polarised light and in their reactions
with other optically active substances However, the (mc5o)isomer has
different properties, for example:

Physical property (+) (-) {meso)

M.p./°C 170 170 140
Density/g cm“^ 1-76 1-76 1-67
Solubility/g 100 cm“^ HjO 139 139 125

When a compound contains two asymmetric carbon atoms of which one is

not bonded to the same three groups as the other, four stereoisomers exist; for
example, for the compound HO^C—CH(OH)—CHfCHa)—CO2H:

H H
HO ^ CO2H HOjC ^COaH HOaC ,^OH
C

'^COaH HO2C ^COjH HO2C

CH3

The first two are non-superimposable mirror-images of each other and so con¬
stitute one pair of enantiomers, and likewise the other two constitute a second
pair of enantiomers, with different properties from the first pair.
In general, a compound with n asymmetric carbon atoms exists in 2" optically
active forms, although when one set of substituents in the compound mirrors
another, internal compensation occurs to reduce this number, as we have seen
for 2,3-dihydroxybutanedioic acid.

Other optically active compounds

As we have seen, compounds containing an asymmetric carbon atom can exist
in optically active forms. However, compounds which do not possess an asym¬
metric carbon atom can also exist in optically active forms provided that the
molecule is asymmetric. Examples occur with certain derivatives of propadiene
(CH2=C=CH2), such as 1,3-diphenylpropadiene: the central atom forms sp-
hybridised a-bonds with the other carbon atoms and provides two p electrons, one
in each of two p orbitals which are mutually perpendicular, for forming w-bonds
with the other carbon atoms. As a result, the substituents at one end of the

243
FIG. 15.5. The stereoisomers of molecule are in a plane which is perpendicular to that of the substituents at the
1,3-diphenylpropadiene other end, so that the compound exists in two forms which are non-superimposable
mirror-images and are optically active (Fig. 15.5).

Substituted biphenyls exhibit optical isomerism when substituents in the

2- positions are large enough to prevent rotation about the bond joining the two
benzene rings. For example, biphenyl-2,2'-disulphonic acid exists in two forms

which are non-superimposable mirror-images but do not interconvert at room

temperature because the energy required to twist one ring through 180° relative
to the other is too high (about SOkJmoP^). This in turn is because, during the
twisting process, the two sulphonic acid groups must come into very close
proximity when the two benzene rings become coplanar and strong repulsive
forces are introduced.

Resolution of enantiomers
The separation of a racemic mixture into the individual enantiomers is
described as resolution. Three methods for resolving enantiomers are:
(a) Crystal picking. When sodium ammonium 2,3-dihydroxybutanedioate
crystallises from solution below 28°C, the (-I-) and ( —)isomers form
crystals which are mirror images of each other (Fig. 15.6). The two types of
crystals can be separated by hand.
This is a tedious method, and is in any case not always applicable (e.g.
when the enantiomers are liquid). It is mentioned mainly for historical
interest, for it was the first method to be employed, by Pasteur. In one sense

FIG. 15.6. The crystal forms of

(+) and {~)sodium ammonium
2,3-dihydroxybutanedioate

( + )Sodium ammonium (-)Sodium ammonium

2,3-dihydroxybutanedioate 2,3-dihydroxybutanedioate

Pasteur was fortunate, because if his solution of sodium ammonium

2,3-dihydroxybutanedioate had crystallised above 28°C, he would have
obtained only one type of crystal, so that no separation would have been
possible. In this crystal, there is an ordered arrangement of the (-I-) and
( —)isomers, and the state is called a racemic compound.

244
ISOMERISM
(b) Chemical method. This is the method of most general use. It is based
on the principle that when each of the two enantiomers reacts with another
compound which is optically active, the products are not mirror images
and do not have identical properties. They are known as diastereoisomers.
Usually one will be less soluble in a particular solvent than the other, and
so the two can be separated by fractional crystallisation and then converted
back into the individual enantiomers.
For example, a ( + )acid (( + )A) can be resolved by making it into a mix¬
ture of two salts with an optically active base (say, (-)B),

( + )A-(-)B
Salt I

(±)A + (-)B

(-)A-(-)B
Salt II

separating them, and then treating each with mineral acid to release (-f )A
and (—)A. A number of optically active bases suitable for the purpose occur
naturally as alkaloids (e.g. quinine) in plants. Similarly, a ( + ) base can be
separated by the use of an optically active acid.
Racemic mixtures which are not acids or bases can often be resolved
by first making them into derivatives with acid groups and resolving the
resulting mixture as above. For example, the enantiomers of butan-2-ol.

CH3—CH(OH)—C2H5, can be resolved by esterification with a dibasic

acid (e.g. benzene-1,2-dicarboxylic acid) to give enantiomeric esters such
as the (±) form of

resolving with an optically active base, and then hydrolysing the individual
(-I-) and (—)esters to give the (-h) and ( —)alcohols.
Diastereoisomers can sometimes be separated by chromatographic tech¬
niques. For example, it has been found that (± )amino-acids can be
resolved by converting them into diastereoisomeric esters with an optically
active secondary alcohol, separating these by gas-liquid chromatography,
collecting the fractions and hydrolysing each separately to give the (-I-) and
( —)amino-acids.
(c) Biochemical method. Bacteria will sometimes grow in solutions of
racemates, and may feed on them by consuming one of the forms. Thus,
(— )2-hydroxypropanoic acid can be prepared by allowing penicillium
glaucum to feed on (±)2-hydroxypropanoic acid; it destroys the (-I-) acid.

Racemisation
Racemisation is the opposite of resolution; that is, it consists of the forma¬
tion of equal amounts of a pair of enantiomers from either of the two.
It can occur when a reaction takes place which breaks one of the bonds to
the asymmetric carbon atom. For example, if a solution of one of the

245
ISOMERISM enantiomers of 2-iodobutane, say the (-)isomer, is treated with a solution
of sodium iodide, an iodide ion in the solution displaces an iodide ion from
the organic compound by attacking from the opposite side:

/QHs
I" VH
CH3

Consequently, the (+)isomer is formed. However, this too can react, in the
reverse manner, to regenerate the (-)isomer, and eventually a point is
reached when equal amounts of the two isomers are in dynamic equilibrium.

15.6 ISOMERISM
(Same molecular formula)
Summary

STRUCTURAL ISOMERISM STEREOISOMERISM

(Different structural formula) (Same structural formula)

CHAIN POSITION FUNCTIONAL TAUTOMERISM

ISOMERISM ISOMERISM GROUP
ISOMERISM

GEOMETRICAL OPTICAL
ISOMERISM ISOMERISM

15.7 Optical isomerism

Practical work Make ball and stick models of the following compounds, using different
coloured balls to represent different functional groups:
(a) The stereoisomers of 2-hydroxypropanoic acid:

OH
I
CH3-C-CO2H
H

(b) The stereoisomers of 2,3-dihydroxybutanedioic acid:

OH
H-i-COoH
I
H-C-COoH
I '
« OH

246
ISOMERISM
Make sure that you have constructed a model of (wc5'o)2,3-dihydroxy-
butanedioic acid and that you can see why it is internally compensated.

(c) The stereoisomers of the acid:

OH

H-C-COoH
I
H-C-CO,H
I
Br

Show whether or not it is possible for this compound to have a (meso) form.

Geometrical isomerism
Dissolve 10 g of butenedioic anhydride in 12 cm^ of boiling water in a test-
tube to form a solution of cw-butenedioic acid ;

O
H II /CO2H
C
II o + H20 -> II

H H CO2H
II
o
Cool the solution by placing the test-tube in a beaker of cold water. Filter
the solid acid using a Buchner funnel (Fig. 2.8), but do not attempt to wash
the solid. Dry it between pads of filter papers and determine its melting
point.
Collect the filtrate, add 20 cm^ of concentrated hydrochloric acid and
reflux the mixture for about 20 minutes. Crystals are formed which can
be filtered, washed with water and dried between pads of filter paper.
Determine the melting point. [M.p. of m-butenedioic acid 130°C; m.p. of
/ra«5-butenedioic acid 287°C (sublimes).]
Hydrogen chloride adds on to the c/5-butenedioic acid molecule to form
an intermediate in which there is unrestricted rotation about the C—C
bonds:
Cl
H^ .CO2H I HO,C. H
C HCl H-C-CO 2 H HCI C'
II I
H-C-CO2H c
H CO2H I H^ '^CO,H
H
restricted unrestricted restricted
rotation rotation rotation

1 Explain the terms (a) Empirical formula, (b) Molecular formula, (c) Structural
15.8 formula, (d) Isomerism.
Questions Three unsaturated dichloroalkenes of molecular formula C2H2CI2 exist and
also two unsaturated dicarboxylic acids of molecular formula C4H4O4. Account
for the existence of these isomers discussing the stereochemical principles.
Indicate one method by which you could assign to each its structure if you
were given pure specimens of the two unsaturated acids. (SUJB)

247
ISOMERISM 2 (a) Distinguish between the terms empirical, molecular and structural formulae.
(b) Discuss the various types of isomerism which occur in organic chemistry,
illustrating your answer by reference to the isomerism of the following com¬
pounds: (i) CjHfiO, (ii) C2H2CI2, (iii) CaHsOj (only acids).
There are no isomers of compounds of the type CH2X2 where X = Cl, Br,
etc.; what does this show ?
3 (a) What do you understand by (i) structural isomerism, (ii) optical isomerism
and (iii) geometrical isomerism? Illustrate your answers with examples chosen
from the chemistry of aldehydes and ketones and their derivatives.
(b) Three isomeric mononitrobenzoic acids are known to exist. Write down
their structural formulae, and state, giving reasons, which of these would normally
be formed on the nitration of benzoic acid. Suggest how you might attempt to
prepare the other isomer(s). (L(X))
4 Discuss the various types of isomerism which occur in organic chemistry, illustrat¬
ing your answer by reference to the isomerism that is shown by the following
compounds: (a) C2H2Br2; (b) CH4N2O; (c) C3H6O3 (only acids); (d) C3H5N
(no ring structures); (e) C6H3CI3 (only derivatives of benzene).
Describe two tests by which you would distinguish between two isomers in (d).
(0(S))

5 Explain as fully as you can what is meant by the term ‘isomerism’, illustrating
your answer by examples of your own choice.
How would you distinguish by not more than two chemical tests in each case
between the substances in each of the following pairs ?

(a) (CH3)3C-0H and CH3-CH2-0-CH2-CH3,

CH3
/
(b) CH3-CH=CH-CH2-CH3and CH3-CH=C^
CH3
(c) CH3-C0-0-C2H5and C2H5-C0-0-CH3. (L)

6 Describe, with diagrams, the type of isomerism shown by cis- and /ra«j-butene-
dioic acids. Give examples of the differences in physical properties of these acids.
How can these acids be distinguished by chemical methods ?
The addition of hydrogen bromide to each of the two acids gives the same
pair of isomers, while the addition of bromine to the two acids gives a total of
three isomeric dibromo-acids. Draw the structures of the five brominated com¬
pounds and describe the type of isomerism involved. How might the two mono-
bromodicarboxylic acids be separated ?

7 An optically active compound A when oxidised with chromic acid yields a

substance B which gives a condensation product with phenylhydrazine but no
precipitate when warmed with a solution of potassium iodide in sodium hypo¬
chlorite. The empirical formulae of A and B are CeH^O and C6H12O respectively.
When A is warmed with concentrated sulphuric acid it gives a hydrocarbon C,
the percentage composition of which is C, 85-7; H, 14-3; and the vapour density,
42.
Treatment of a trichloromethane solution of C with trioxygen yields a com¬
pound which on decomposition with water yields two compounds, D and E,
both having an empirical formula, CsHeO. Both D and E give precipitates with
phenylhydrazine, but only E yields a yellow precipitate with a solution of iodine
in sodium hydroxide. On the other hand the compound D is easily oxidised to an
acid, C3H6O2.
Deduce the structure of A, explaining briefly why it is optically active, and
write equations for the formation of B, C, D and E.

8 What do you understand by the terms structural isomerism, cis-trans {geometrical)

* isomerism and optical isomerismi Illustrate your answer with examples of isomers

248
ISOMERISM
drawn from as wide an area of organic chemistry as you can and explain how each
type of isomerism arises.
To what extent are the properties of corresponding isomers similar? (C)

9 Give an account of structural, geometrical and optical isomerism, illustrating

your answer with two examples in each case.
(a) The physical and chemical properties of geometrical isomers often differ
greatly. Choose suitable examples to illustrate this.
(b) Some compounds have several asymmetric carbon atoms. Discuss the
relationship between the number of asymmetric carbon atoms and the number of
isomers which result. Illustrate your answer by reference to

H H H H

(i) HO2C—C—C—CO2C2H5 (ii) HO2C—C—C—CO2H

OH OH OH OH
(L(S))

10 (a) Give two factors which may restrict rotation about carbon-carbon bonds, and
indicate the consequences which this restriction may lead to in the field of
isomerism. Illustrate your answer with specific compounds.
(b) What is meant by the term chiral molecule and to what extent is the term
confined to organic chemistry? Illustrate your answer with diagrams of specific
molecules.
(c) Illustrate the importance of stereochemistry as an approach to understand¬
ing organic reaction mechanisms. (O and C)

11 Describe the different types of isomerism which occur in carbon compounds.

Illustrate your answer by reference to isomers of each of the following: C^Hjo,
C3H7CI, C^Hg and CH3CH(OH)COOH. State what differences (if any) in chemical
behaviour and in physical properties you would expect the various isomers of each
formula to show.
A hydrocarbon C5H10 reacts with sulphuric acid and the product is hydrolysed to
give an alcohol C5Hi20. This alcohol can be separated into two enantiomeric forms
(optical isomers). Suggest one possible structure for the hydrocarbon. (JMB)

12 Define isomerism. What isomers exist for the following compounds:

RiR2R3SiCl (Ri, R2, and R3 represent different alkyl or aryl groups)

HOOCCH=CHCOOH
Pt(NH3)2Cl2
RiR2R3R4N+r (Ri, R2, R3, and R4 represent different alkyl or aryl groups)?
(C Schol.)

13 Show by means of examples what types of isomers are encountered in organic

chemistry. Write the structures of the isomers of formula C3H5CI. Indicate briefly
how you might distinguish between them.

14 A compound is found to have the molecular formula C4Hg02. Write down as

many possible structures for this compound as you can c/a55//y/«g them according
to the types of functional group present. Indicate briefly how you might distinguish
between the various classes by chemical tests. (O Schol.)

15 Optically active 2-iodobutane

(CH3.CH2.CHI.CH3)
racemises in acetone solution containing sodium iodide. When the experiment
is carried out with labelled sodium iodide (prepared from “‘I, the radioactive
isotope of iodine), the alkyl iodide is found to lose optical activity and to exchange

249
ISOMERISM its ordinary iodine for radioactive iodine. The rates of each of these reactions
are proportional to
[alkyl iodide] [I“]
but racemisation is exactly twice as fast as isotopic exchange.
Discuss the stereochemical implications of these experiments.
What conclusions would you have drawn if the rates of racemisation and
isotopic exchange had been independent of the concentration of iodide ion and
exactly equal 1 (O Schol.)
16 Describe the different types of isomerism exhibited by organic compounds,
illustrating your answer with specific examples.
Vigorous oxidation of a compound X (C2H4)„ gives only ethanoic acid. With
bromine X forms Y (C2H4Br)„ and with hydriodic acid it forms Z, C4H9I.
Draw possible structures for compounds, X, Y and Z, and indicate the stereo-
isomeric forms in which they could exist. (O Schol.)

250
Chapter 16
Amines

16.1 Amines can be regarded as organic derivatives of ammonia. There are three
Introduction classes of amines:
R R R
1 1 1

H H R^ H R R
Primary Secondary Tertiary
amine amine amine

where R can be an alkyl group or an aromatic ring. The prefixes, primary,

secondary and tertiary, define the number of carbon groups attached to
the nitrogen atom and not (as with alkyl halides and alcohols) the number
of such groups attached to the carbon atom which bears the functional
group.

Aliphatic amines
Simple amines are usually named by adding the word amine to the names
of the groups to which the nitrogen atom is attached (Table 16.1).

16.2 Table 16.1. Some aliphatic amines

Nomenclature NAME B.P./°C
FORMULA CLASS

Methylamine CH3—NH2 Primary -7

Ethylamine CH3CH2—NH2 Primary 17
Dimethylamine CH3—NH—CH3 Secondary 7
Methylethylamine CH3CH2—NH—CH3 Secondary 35
Diethylamine CH3CH2—NH—CH2CH3 Secondary 56
CH,
Trimethylamine N-CHo Tertiary 3
/ '
CH3
CH3CH2
\
Triethylamine N-CH2CH3 Tertiary 89
CH3CH2

An alternative nomenclature employs the prefix amino; for example:

CHg
I
CH3-C-CH2-CH3

NH, NH2

2-Aminobutane 2-Amino-2-methylbutane

251
AMINES Aromatic amines
The simplest, phenylamine, C6H5-NH2, is sometimes called by its original
name, aniline. Examples of substituted phenylamines are:

2-Bromo-5-
4-N itrophenylamine chlorophenylamine

There are also amines which contain more than one aryl group, for
example:

CeHs CfiHs
\ \
NH N-QHs
/ /
C6H5
Diphenylamine Triphenylamine

As will be seen, there are significant differences both in methods of

preparation and in properties between aliphatic amines and those aromatic
amines in which the nitrogen atom is attached to the benzene ring (aryl
amines, e.g. phenylamine). (Phenylmethyl)amine, C6H5CH2NH2, although
it contains the benzene ring, behaves like an aliphatic amine since the
nitrogen atom is not attached directly to the ring.

16.3 The lower aliphatic amines are gases or low-boiling liquids, their boiling
points being lower than those of the corresponding alcohols. They have
Physical properties of
a smell rather like bad fish; indeed, decaying fish produce various amines.
amines They are readily soluble in water and in organic solvents.
Aromatic amines are liquids or solids with high boiling points. They have
a characteristic smell and are soluble in organic solvents but almost in¬
soluble in water.

16.4 Laboratory methods

Methods of preparation 1. Aliphatic amines can bp prepared by the reaction between an alkyl

of amines halide and ammonia (9.3). However, the method is rarely used because a
mixture of primary, secondary and tertiary amines and quaternary
ammonium salts is obtained:

R—Br + NHj ^ R—NH2 + HBr

R—Br + R—NH2 -> R2NH + HBr

R—Br + R2NH -> R3N + HBr

R—Br + R3N ^ R4N+ Br-

252
AMINES
(The hydrogen bromide evolved reacts with ammonia and amines to form
salts.)
Aryl amines cannot be made in this way because aryl halides are
unreactive towards ammonia.
2. By the reduction of a nitro compound, giving a primary amine. The
reducing agent can be hydrogen, catalysed by nickel;
cd-t
R—NO2 + 3H2 -^ R—NH2 + 2H2O

or lithium tetrahydridoaluminate:

LiAlH4
R—NO2 -^ R—NH2

Tin and hydrochloric acid are used to reduce aromatic nitro compounds;
the amine is produced as a complex salt from which it is liberated with
alkali, for example:

2C6H5—NO2 + Sn + 6HC1 ^ (QHj—NH3)2SnCl62- + 2H2O

(C6H5NH3)2SnCl62" + 8NaOH ^
2C6H5NH2 + Na2Sn03 + 5H2O + 6NaCl

3. Primary amines are also formed by the reduction of nitriles and

amides with lithium tetrahydridoaluminate:

R—CN ----> R—CH2—NH2

R—CO—NH2 --—> R—CH2—NH2

4. By the Hofmann degradation reaction, giving a primary amine (14.5).

An amide is treated with bromine and alkali, and the resulting amine is
distilled off:
R—CO—NH2 + Br2 + 4NaOH ->
R—NH2 + Na2C03 + 2NaBr
+ 2H2O

It should be noted that an amide, RCONH2, is reduced to the amine with

the same number of carbon atoms, RCH2NH2, with lithium tetrahydrido¬
aluminate but gives the lower homologue RNHj in the Hofmann reaction.

Manufacture
Primary aliphatic amines are usually produced by heating the appropriate
alcohol with ammonia under pressure, in presence of a catalyst.
Phenylamine is manufactured by the hydrogenation of nitrobenzene in
the gas-phase over copper, or in the liquid phase with nickel as the catalyst.

16.5 Reactions at the nitrogen atom

Chemical properties of 1. Amines, like ammonia, give alkaline solutions in water as a result of
the equilibrium:
amines
R—NH2 + H20^R—NH3 + OH-

253
AMINES The equilibrium constant for this reaction is given by

[R-NH3KOHT]
[R-NH2][H,0]

As with carboxylic acids, it is customary to assume [H2O] is constant, and

the base strength of an amine, is defined as:

[R-NH3][QH-]
[R-NH2]
Again, as with carboxylic acids, Kf, is usually defined as a dimensionless quantity
in advanced texts (p. 62). Moreover, base strength is often described in terms of the
dissociation constant of the conjugate acid of the base; for example, RNHj’’’ is the
conjugate acid of the base RNH2:

R—NHj R—NH2 + H +
[RNH2][Hn
and
[RNH3]
Since [H] [OH ] = (the ionic product of water), it follows that Xj = KJK^.

Amines are weak bases; values of Kf, are in Table 16.2.

Table 16.2. Dissociation constants of

ammonia and some amines

COMPOUND Xfc AT 25°C

NH3 1-8 X 10'^

CH3—NH2 4-4 X 10"^
(CH3)2NH 5-9 X 10“^
(CH3)3N 6-3 X 10“^
QH5~-NH2 4-2 X 10“^°
QH5CH2-NH2 2-2 X 10“^

The aliphatic amines are approximately as basic as ammonia. However,

the aryl amines such as phenylamine are much weaker bases.

This is because the unshared pair of electrons on the nitrogen atom in phenyl¬
amine is in an orbital which overlaps with the adjacent carbon p orbital, giving
increased delocalisation in the aromatic, as compared with an aliphatic, amine.
One of the delocalised n molecular orbitals is shown in Fig. 16.1.
FIG. 16.1. An molecular orbital
in phenylamine

254
AMINES
Therefore, relative to an aliphatic amine, phenylamine resists reaction with a
proton since this process requires the use of the unshared pair of electrons on
nitrogen in the formation of the new N—H bond and so results in the loss of the
extra delocalisation {cf. the weaker basicity of an amide compared with an amine,
p. 221).
On the other hand, an aromatic amine such as (phenylmethyl)amine does not
contain a nitrogen atom adjacent to a carbon atom with a p orbital and is approxi¬
mately as strong a base as an aliphatic amine.

The salts formed by amines with acids are analogous to ammonium salts. For
example, amine hydrochlorides, like methylammonium chloride, CFljNHj Cl^,
are white crystalline solids with high melting points (above about 200°C) which are
soluble in water.

2. Primary and secondary amines react with acid chlorides and acid
anhydrides to form substituted amides, for example:

CH3—NH2 + CH3—CO—Cl CH3—NH—CO—CH3 + HCl

N- M ethylethanamide

CfiHs-NH2 + CH3—CO—O—CO—CH3 ^
CgHs-NH—CO—CH3 + CH3CO2H
jV-Phenylethanamide

In these reactions the amine acts as a nucleophile:

H H R'
I Cl I + I C' Hd
N R-N-C-Cl —> R-NH-CO-R'
I I ^
H H O

3. Amines react with nitrous acid in a way which depends on whether

they are primary, secondary or tertiary amines.

Primary aliphatic amines react to yield gaseous nitrogen. The reaction

is complex; for example, propylamine gives nitrogen almost quantitatively,
but a mixture of other compounds is also formed, among them being
propene, propan-2-ol and a trace of propan-l-ol.
Primary aryl amines react to give aromatic diazonium salts, providing
that the temperature is below about 10°C. For example, when phenylamine
is treated with hydrochloric acid and sodium nitrite, it reacts with the
nitrous acid formed to give benzenediazonium chloride:

CeHs—NH2 + HNO2 + HCl ^ CgHs-NeeeN CF + 2H2O

Benzenediazonium
chloride

Aromatic diazonium salts are of value in making other aromatic com¬

pounds and are discussed separately (16.8).

Secondary amines, both aliphatic and aromatic, react with nitrous acid
to give nitroso compounds, which are yellow oils :

R2NH + HNO2 -> R2N—N=0 + H2O

A nitrosamine

255
AMINES Tertiary amines react with nitrous acid to give solutions containing
substituted ammonium nitrites. Since these are salts formed by a weak acid
and a weak base, they are extensively hydrolysed; that is, the equilibrium

RjN + HNO2 ^ R3NH N02“

lies to the left-hand side.

4. Primary amines react with trichloromethane and a solution of potas¬

sium hydroxide in ethanol to form isocyano-compounds (carbylamine
reaction; 9.4);

R—NH2 + CHCI3 + 3KOH ^ R—N=C + 3KC1 + 3H2O

For example, phenylamine forms isocyanobenzene, CgHjNC.

Substitution in the aromatic ring of aryl amines

The amino group in an aryl amine directs electrophilic reagents mainly to
the 2- and 4-positions and renders the compound far more reactive than
benzene towards these reagents. For example, whereas benzene requires a
halogen carrier to react with bromine to form bromobenzene (8.3), phenyl¬
amine reacts rapidly with bromine water in the absence of a carrier at all
three 2- and 4-positions:

The reason is that the amino group stabilises the adducts formed by
reaction at the 2- and 4-positions by sharing the positive charge (8.5).
This can be represented by the structures:

NH2 NH2
X H
A
V H Br

In order to obtain a monobromo derivative, it is necessary to reduce

the reactivity of the aromatic ring. This can be achieved by ethanoylation
of the amino group with ethanoic anhydride (p. 255); the substituent
—NH—CO—CH3 is 2-,4-directing, but much less strongly activating
than —NH2, and reaction with bromine gives N-2- and A^-(4-bromophenyl)
ethanamide. The ethanoyl group can then be removed by hydrolysis with a
dilute solution of sodium hydroxide:

256
AMINES
NHCOCH,

NaOH

NHCOCH, Br
Major product
Br,

NHCOCH,
Br
NaOH

Minor product

This use of the ethanoyl group is an example of protection; the aromatic

ring is protected from the extensive reactions which occur with phenylamine
itself. Protection is also required for the nitration of phenylamine, in this
case not only because otherwise reaction would occur at all the 2- and 4-
positions but also because the amino group is itself readily oxidised by
nitric acid. Nitration of 7V-phenylethanamide gives the 2- and 4-nitro
derivatives from which 2- and 4-nitrophenylamine can be obtained by
hydrolysis:
NHCOCHg NH,

NaOH
->

NO2 NO,
NHCOCH,
Major product
HNOj/HjSOi

NHCOCHg NH,
.NOg NO,
NaOH
->

Minor product

1. Plastics. 1,6-Diaminohexane, HgN—(CHgje—NHg, is used in the

16.6 manufacture of nylon-6.6 (p. 336). Other amines are used in the production
Uses of amines of isocyanates for polyurethane plastics (p. 334).

2. Inhibitors. Amines are effective at preventing the deterioration of

rubber through oxidation by atmospheric oxygen.

3. Dye-stuffs. Primary aromatic amines are used to make azo-dyes (16.8).

4. Medicines. Amines are used in the manufacture of many pharma¬

ceuticals (e.g. Paludrine, an antimalarial drug).

257
16.7 Amino-acids contain at least one amino and one carboxyl group. The most
important are the 2-amino-acids, which are traditionally referred to as
Amino-acids a-amino-acids and retain their original names, which will be used in this
book; examples are aminoethanoic acid (glycine), 2-aminopropanoic acid
(a-alanine) and 2,6-diaminohexanoic acid (lysine). Their importance stems
from their being the constituents of the proteins (18.2). There are also
3- amino-acids (jS-amino-acids), such as 3-aminopropanoic acid (;S-alanine),
4- amino-acids (y) and so on.

H2N—CH2—CO2H H2N—CH—CO2H H2N—CH2—CH2—CO2H

Glycine a-Alanine j8-Alanine

NH2 CO2H

(CH2)4 CH2
I
H2N—CH—CO2H H2N—CH—CO2H
Lysine Aspartic acid

Physical properties
Amino-acids are high-melting crystalline solids (e.g. glycine has m.p. 235°C)
which are usually readily soluble in water but insoluble in organic solvents.
In these respects, they resemble salts, and this is because they exist as
internal salts, known as zwitterions, in which both cation and anion are
held together in the same unit, for example:

H3N—CH2—C02“

Amino-acids with an equal number of amino and carboxyl groups (e.g.

glycine) are neutral in solution, but when the solution is acidified the
carboxyl group is protonated,

H3N—CH2—CO2" + H+ -> H3N—CH2—CO2H

and when it is basified the amino group is freed:

H3N—CH2—CO2- + OH- -> H2N—CH2—CO2- + H2O

If the amino-acid contains more carboxyl groups than amino groups (e.g.
aspartic acid), its solution in water is acidic. Conversely, if it contains an
excess of amino groups (e.g. lysine), its solution in water is basic.
Most a-amino-acids have at least one asymmetric carbon atom and exist
in optically active forms (e.g. a-alanine).

Preparation of a-amino-acids
1. By reaction of a 2-chloro-acid with concentrated ammonia solution,
for example:
2NHi wr'i
Cl—CH2—CO2H -> H2N—CH2—CO2- NH4+ —>
H2N—CH2—CO2H + NH4CI
Glycine

258
AMINES
The disadvantage of this method is that the amino group which is introduced
can react with a second molecule of the chloro-acid:

H2N-CH2-CO2H + CI-CH2-CO2H

CH2-CO2H
> HN + HCl
\
CH2-CO2H

A mixture of products is therefore formed. This problem is overcome by

using potassium benzene-1,2-dicarboximide as the source of the nitrogen
atom:

K+ + CI-CH2-CO2H

The resulting derivative is hydrolysed with acid:

+ 2H2O

O
+

H2N-CH2-CO2H
2. By reaction of an aldehyde with a mixture of potassium cyanide and
ammonia:
OH
NH, I - H,0
R-CH=0 R-CH [R-CH=NH]
I
NH2

CN CN
CN- I I
R-CH R-CH
I I
NH NH,

followed by hydrolysis of the nitrile:

CN CO2H
I I
R-CH + 2H2O — R-CH + NH3
I
NH2 NH2

Chemical properties of a-amino-acids

a-Amino-acids show reactions of both acids and amines individually, as
well as some reactions dependent on the presence of both groups. Typical

259
AMINES reactions, illustrated for glycine, are:

Reactions of the carboxyl group

1. Formation of esters, for example:

HzN—CHa—CO2H + CHj—OH t-^

H2N—CH2—CO—0—CH3 + H2O

2. Decarboxylation, when heated with soda-lime (cf. the decarboxylation

of ethanoic acid, p. 199):

H2N—CH2—CO2H + 2NaOH ^ H2N—CH3 + Na2C03 + H2O

Methylamine

Reactions of the amino group

1. Formation of acyl derivatives, for example:

H2N—CH2—CO2H + CH3—CO—Cl ->

CH3—CO—NH—CH2—CO2H + HCl

2. Reaction with nitrous acid:

H2N—CH2—CO2H + HNO2 ^ HO—CH2—CO2H + N2 + H2O

Hydroxyethanoic acid

Reactions dependent on the presence of both groups

1. When heated, a cyclic compound (diketopiperazine) is formed:

H2N-CH2-CO2H Q
HN^
+ -> 1 1 + 2H2O
/NH
HO2C-CH2-NH2 0^
H2

2. When treated with copper(II) ion in water, a deep blue colour is

produced due to the compound, copper(II)-glycine:

0 ^ H2
^C-O N_cH2
2H2N-CH2-CO2H + Cu=*+-> Ci!f -f 2H +
^ \
H2C-N O-C^
H2 0

. Copper(II)-glycine

16.8 Primary aromatic amines react with nitrous acid in acid solution below
about 10°C to form aromatic diazonium salts, for example:
Aromatic diazonium
salts C^Hj-NH2 + HNO2 + HCl -> CeHj-N^N CF + 2H2O
Benzenediazonium
chloride

«
The process is known as diazotisation and was discovered by Griess in 1858.

260
AMINES
Diazonium salts are stable in solution provided that the temperature is
kept low. However, most of them are explosive in the solid state, and so
they are usually not isolated from the aqueous solutions in which they are
made. They are useful in synthesis because the substituent —N=N can
be replaced by a variety of other groups by treating the aqueous solution
of the salt with an appropriate reagent.
Their reactions can be divided into two groups: (a) those in which the
two nitrogen atoms are replaced, and (b) those in which the nitrogen atoms
are retained.

(a) Reactions in which the nitrogen atoms are replaced

1. Replacement by the hydroxyl group. If the solution of the diazonium
salt is warmed, a phenol is formed, for example:

CeHs—N=N Cr + H2O ^ CgHj—OH + Nj + HCl

Phenol

2. Replacement by a halogen atom.

(i) Iodine. When the solution of the diazonium salt is warmed with an
aqueous solution of potassium iodide, an iodo compound is formed, for
example:

CeHj—N=N cr + KI ^ CgH;—I + N2 + KCl

lodobenzene

(ii) Chlorine and bromine. A solution of a copper(I) halide dissolved in

the concentrated halogen acid is added to the solution of the diazonium
salt, for example:

CeHs—N=N cr . CuCl/HCl^ CgHj—Cl + N2

Chlorobenzene

+ CuBr/HBr
CeHj—N=N cr + HBr -^-> CeHj—Br + HCl + N2
Bromobenzene
These are known as Sandmeyer reactions.
(iii) Fluorine. When a solution of potassium tetrafluoroborate, KBF4, is
added to the solution of the diazonium salt, a precipitate of a diazonium
fluoroborate is formed, for example:

CfiHs—N=N cr + KBF4 ^ CeHj—feN BF4- + KCl

This salt, which is more stable than diazonium chlorides, is filtered and
dried. On being heated carefully to about 120°, the aryl fluoride is formed:

CgHj—N^N BF4- -> CeHj—F + N2 + BF3

Fluorobenzene

3. Replacement by the nitrile group. Copper(I) cyanide dissolved in

aqueous potassium cyanide is added to the solution of the diazonium salt,
for example:
+ KCN
CeHj—N=N cr + CuCN -> C^Hj—CN + N2 + CuCl
Benzonitrile

261
AMINES 4. Replacement by a hydrogen atom. Hypophosphorous acid is added
to the solution of the diazonium salt, for example:

CfiHs—NeeeeN Cl- + H3PO2 + H2O ^ CeHg + N2 + H3PO3 + HCl

(b) Reactions in which the nitrogen atoms are retained

1. Diazonium salts react with phenols and tertiary aromatic amines to
form bright-coloured azo-compounds.
The reaction with phenols is carried out in alkaline solution (that is, with
the phenoxide ion), for example:

The products formed with naphthalen-2-ol are insoluble, mostly red com¬
pounds. Thus, the formation of a red precipitate when an amine is treated
with nitrous acid and the solution is poured into an alkaline solution of
naphthalen-2-ol shows that the amine is a primary aromatic one; this
provides a useful test.

The reaction of diazonium salts with tertiary aromatic amines is carried out
in neutral solution. An example is:

Azo-compounds are coloured because the group —N=N— absorbs

light; the group is known as a chromophore. The particular colour depends
on what other substituents are present; these are called auxochromes. Many
of the compounds are used as dyes; an example is Orange II, which is made
by diazotising the sodium salt of 4-aminobenzenesulphonic acid and
coupling the product to naphthalen-2-ol:

262
AMINES

Orange II

2. Diazonium salts are reduced by a solution of sodium sulphite to

arylhydrazines, for example:

CgHs—N=N Cr + 2Na2S03 + 2H2O ->

CgHj—NH—NH2 + 2Na2S04 + HCl
Phenylhydrazine

16.9 Small-scale preparation of methylamine and

Practical work methylammonium chloride: The Hofmann
degradation
To 2 g of ethanamide and 2 cm^ of bromine in a boiling-tube, add 2 g of
sodium hydroxide dissolved in 10 cm^ of water, while shaking the tube
under a stream of cold water. A solution of sodium A^-bromoethanamide is
formed, which is pale yellow.
In a separate boiling-tube, dissolve 4 g of sodium hydroxide in 10 cm^
of water, and pour this solution into the flask (Fig. 16.2). Place the solution
of sodium A-bromoethanamide in the dropping funnel.
Heat the flask to 70°C in a water-bath and add the sodium A-bromoethan-
amide solution dropwise. Keep the mixture at this temperature for a
further 10 minutes. Remove the beaker of water and boil the solution, using
a bunsen burner, to drive over methylamine. The base will be absorbed in
the acid to yield methylammonium chloride, a salt.
Transfer the solution of the salt to an evaporating basin and evaporate
the solution to dryness, using a water-bath.
Transfer a few crystals of methylammonium chloride to a test-tube and
add dilute sodium hydroxide solution. Warm and test the gas evolved by
(a) smell, (b) moist red and blue litmus papers.
If there is time, transfer the rest of the methylammonium chloride to a
boiling-tube, add 5 cm^ of ethanol and warm the mixture in a beaker of
boiling water. Filter (to remove any solid ammonium chloride which may
have been formed from the alkaline hydrolysis of ethanamide), and allow
the filtrate to cool. Filter oflF crystals of methylammonium chloride and dry
them between pads of filter papers. Find the m.p.

263
FIG.16.2. Preparation of
methylammonium chloride by the
Hofmann reaction

Small-scale preparation of phenylamine

To 2 cm-^ of nitrobenzene and 4 g of tin in a flask, add 10 cm^ of concentrated^
hydrochloric acid from the dropping funnel (Fig. 16.3). Shake the mixture,'
immersing the flask in cold water if the reaction becomes too vigorous.
Remove the flask-head and heat the flask in boiling water for 30 minutes
(to complete the reduction of nitrobenzene). If some tin remains, add
concentrated hydrochloric acid dropwise until it has dissolved. Phenylam-
monium chloride and phenylammonium hexachlorostannate(IV) are
soluble in water, but when excess of concentrated sodium hydroxide
solution is added, phenylamine is liberated as an oil and the tin salts are
precipitated and then dissolve to sodium stannate(IV).
Set up the flask for steam distillation (Fig. 2.6), keeping a low bunsen
flame below the centre flask to prevent undue condensation of steam from
the steam flask. Distil until no more oily drops of phenylamine can be seen
distilling over. Transfer the distillate to a separating funnel and add about
2 g of sodium chloride and shake. (The salt reduces the solubility of phenyl¬
amine in water). Run off the organic layer into a test-tube and add some
potassium carbonate to remove water. After standing for a few minutes,
the turbidity should disappear.

264
FIG. 16.3. Preparation of
phenylamine

Redistil phenylamine, using an air condenser (cf. Fig. 2.3), collecting the
fraction boiling between 180 and 185°C.

Reactions of ammonia, methylamine,

phenylamine and (phenylmethyl)amine
To compare their reactions as bases
1. (a) Place 2 cm^ of an aqueous solution of ammonia, 2 cm^ of an
aqueous solution of methylamine, 3 drops of phenylamine and 3 drops of
(phenylmethyl)amine in separate test-tubes, and test the solutions with
Universal Indicator solution.
(b) To 3 drops of phenylamine and 3 drops of (phenylmethyl)amine in
separate test-tubes, add 5 drops of water. Shake to see whether the amine
dissolves. Then add dilute hydrochloric acid dropwise, with shaking.

2. To 1 g of methylammonium chloride in a test-tube, add 5 cm^ of dilute

sodium hydroxide solution. Boil gently. Test the vapour with moist red
litmus paper. Note the smell of the gas.
Repeat the experiment with phenylammonium chloride.

265
AMINES
Formation of amides
3. (a) A test-tube experiment to demonstrate the formation of N-
phenylethanamide is described on p. Ill, and a small-scale preparation is
described below.
(b) The Schotten-Baumann reaction to form N-phenylbenzamide is des¬
cribed on p. 111.

Reactions with nitrous acid

4. Make up a solution of about 3 g of sodium nitrite in 10 cm^ of water,
and cool it to about 5°C.
In separate test-tubes, make up solutions of a few crystals of methyl-
ammonium chloride in water, a few drops of phenylamine in concentrated
hydrochloric acid and of (phenylmethyl)amine is concentrated hydrochlo¬
ric acid. Cool these solutions and add the cool solution of sodium nitrite
to each. Observe what happens (a) in the cold, (b) when the phenylamine
solution is warmed.
The reaction between phenylamine and nitrous acid is studied further
below.

Reaction of the phenyl group

5. To 5 drops of phenylamine in a test-tube, add concentrated hydro¬
chloric acid dropwise until the base dissolves. Add bromine water carefully
until nothing more is seen to occur.
Compare the rate of, and the products formed by, the reaction of bromine
and phenylamine with those of bromine with (a) methylbenzene (p. 113),
(b) phenol (p. 162).

Small-scale preparation of Af-phenylethanamide

Cool a flask containing 4 cm^ of glacial acetic acid and 4 cm^ of ethanoic
anhydride in a beaker of cold water, and add 4 cm^ of phenylamine dropwise
with gentle shaking.
Reflux the mixture for 30 minutes (cf. Fig. 2.1) and then pour the liquid
into a beaker containing 100 cm^ of water. Filter the crystals of fV-phenyl-
ethanamide using a Buchner funnel and flask, and wash them with cold
water.
Transfer the crystals to a boiling-tube, dissolve them in the minimum
quantity of boiling water and allow the solution to cool. Filter the crystals
again and dry between pads of filter paper. M.p. 114°C.

Small-scale preparation of benzenediazonium

chloride solution
To 3 cm^ of phenylamine and 10 cm^ of water in a boiling tube, add 8 cm^ of
concentrated hydrochloric acid. Cork and shake the tube until the amine
has dissolved.
Cool the solution in a beaker containing ice to about 5°C, and add a
solution of sodium nitrite (3 g in 8 cm^ of water), previously cooled to 5°C.
Make sure that the temperature of the mixture does not rise above 10°C.

Reactions of benzenediazonium chloride solution

(a) Replacement reactions
1. Boil 2 cm^ of the diazonium solution. Note the odour of phenol
which separates as an oily liquid.

266
AMINES
2. To 2 cm^ of the diazonium solution at 5°C, add, drop by drop, 1 cm^
of a 10 per cent solution of potassium iodide, previously cooled to 5°C.
Allow to stand for 5 minutes and then gently boil. Observe oily drops of
iodobenzene.

3. Sandmeyer reaction. Dissolve 1 g of copper(I) chloride in concentrated

hydrochloric acid in a test-tube. Put the test-tube in a beaker of water at
60°C, and add 2 cm^ of benzenediazonium chloride solution. Note whether
the product is soluble in water and its smell.

(b) Coupling reactions

4. To 2 cm^ of benzenediazonium chloride solution in a test-tube, add
phenylamine (cooled to below 5°C) dropwise.
5. Dissolve 2 or 3 crystals of phenol in 2 cm^ of dilute sodium hydroxide
solution. Cool the solution in ice, and add the diazonium solution drop
by drop. A yellow precipitate of a dye, the sodium salt of 4-hydroxyazo-
benzene, is obtained.
6. Repeat experiment 5 using napthalen-2-ol instead of phenol.

Reactions of glycine
1. Test the solubility of glycine in (a) water, (b) ethanol, (c) ether.

(a) Properties of the amino group

2. Dissolve about 1 g of glycine in the minimum quantity of concentrated
hydrochloric acid. Cool the mixture and observe whether a white crystalline
solid is formed.
3. To about 1 cm^ of an ice-cold solution of sodium nitrite in a test-tube,
add 1 cm^ of dilute hydrochloric acid. Some decomposition of the nitrous
acid formed will occur. When the effervescence has subsided, introduce a few
crystals (or a few drops of a concentrated aqueous solution) of glycine.
Effervescence occurs again as nitrogen is evolved.

(b) Properties of the carboxyl group

4. To a solution of glycine in water, add some solid sodium hydrogen-
carbonate.

(c) Properties due to both groups

5. To a solution of 1 g of glycine in 10 cm^ of water, add copper(II)
carbonate until it is in excess. Filter the mixture using a Buchner f^unnel,
and transfer the filtrate to a boiling-tube and allow it to stand. Observe
whether crystals are formed, their colour and shape. Suggest what reaction
has taken place.

Azodyes (F,V) Open University. S246/10F.

Film and Videotape

1 Give two methods by which pure ethylamine may be prepared. How does
16.11 ethylamine react with (a) iodoethane, (b) sodium nitrite and dilute hydrochloric
Questions acid, (c) ethanoic anhydride, (d) trichloromethane and ethanolic potash?

267
AMINES 2 Outline two general methods which could be used to prepare a pure primary
aliphatic amine, and state whether they could be applied to the preparation of
phenylamine.
Give three types of reaction which both ethylamine and phenylamine undergo.
Describe two tests which may be used to distinguish between solutions of
ethylamine and phenylamine in dilute hydrochloric acid.

3 An organic base A contains 61 01 per cent C, 15-25 per cent H, and 23-73 per
cent N. When treated with nitrous acid A yields an alcohol B, and nitrogen is
evolved. B contains 60-00 per cent C, and 13-33 per cent H, and on careful
oxidation yields C, which has a vapour density of 29. C forms an oxime
and an addition compound with sodium hydrogen sulphite, but does not react
with Fehling’s solution. Suggest structures for A, B and C, and indicate the course
of the above reactions. (L)

4 A compound A gave on analysis C, 61-0 per cent; H, 15-2 per cent; N, 23-7 per
cent. Treatment of A with acid and sodium nitrite yielded a compound B of
molecular formula CsHgO. Oxidation of B with chromic acid gave C with a
molecular formula CjHgO. The product C gave a positive iodoform reaction
2_^l| - and formed a crystalline derivative with hvdroeen sulphite but did not
react with ammoniacal silver nitrate. The compound B when treated with
ethanoic anhydride gave a product D corresponding to C5H10O2.
Deduce the identity of A, B, C and D. Explain the sequence of reactions by
means of equations involving structural formulae for the organic molecules. (W)

5 A pungent-smelling liquid A was analysed and found to contain carbon, hydrogen,

chlorine and possibly oxygen. Upon reaction with aqueous ammonia a neutral
compound B was formed. When this was treated with bromine and potassium
hydroxide a base C resulted. In acidic solution substance C reacted with sodium
nitrite, giving nitrogen and an alcohol D. Upon mild oxidation, the alcohol D
was converted into compound E of molecular formula C2H4O which gave a
positive test with ammoniacal silver nitrate solution.
Describe the chemistry involved in these reactions and identify the compounds
A, B,C,D and E. (L)

6 How would you prepare a specimen of phenylamine in the laboratory starting

from nitrobenzene ? Compare the reactions, if any, of phenylamine with those of
methylamine towards the following reagents: (a) nitrous acid, (b) ethanoic
anhydride, (c) water. (L)

7 (a) How and under what conditions does butylamine react with
(i) concentrated hydrobromic acid;
(ii) aqueous copper(II) sulphate;
(iii) ethanoyl chloride (acetyl chloride);
(iv) nitrous acid (nitric(III) acid)?
In each case, indicate the experimental conditions for the reaction, describe what
happens, write an equation and give the names and formulae of the reaction
products.
(b) Describe and explain the tests you would perform in order to demonstrate the
presence of nitrogen in butylamine. (SUJB)

8 State, with equations, three methods by which primary amines can be prepared.
How, and under what conditions, do primary amines react with (a) nitrous
acid, (b) ethanoic anhydride, and (c) dilute sulphuric acid?
Describe the chemical properties and reactions of glycine.

9 Phenylamine (b.p. 184°C) is prepared in the laboratory by the reduction of nitro¬

benzene with tin and concentrated hydrochloric acid. When the reduction is com¬
plete, this mixture is made alkaline with an excess of sodium hydroxide and steam
distilled. Salt is added to the distillate, and then the resulting solution is extracted
twice with ether.
Explain fully the reasons for the instructions in the above preparation which
are printed in italics. (O)

268
AMINES
10 What happens when an organic compound containing nitrogen is heated with
copper(II) oxide?
Calculate the percentage of each of the elements in phenylamine, C6H5NH2.
How does phenylamine react with (a) hydrochloric acid, (b) ethanoyl chloride,
(c) bromine water, (d) nitrous acid ? (AEB)

11 Define the term base. Arrange the following compounds in order of increasing
base strength: ammonia, phenylamine, ethanamide, methylamine, ethylamine.
Compare the action of nitrous acid on phenylamine and ethylamine. Outline
how ethanamide may be converted into (a) methylamine, (b) ethylamine (W)

12 (a) Describe briefly how ethane could be converted into (i) ethylamine and
(ii) ethanol. In each case give the necessary conditions, reagents and mecha¬
nistic equations and comment on any side reactions which may occur.
(b) Explain why ethylamine and ethanol are not prepared in these ways on an
industrial scale.
(c) Account for (i) the relative acidity of ethanol, phenol and ethanoic (acetic)
acid and (ii) the relative basicity of ethylamine aniphenylamine (aniline).
(JMB)

13 How would you prepare, in the laboratory, a pure specimen of phenylamine from
nitrobenzene ?
Compare and contrast the behaviour of phenylamine and methylamine towards
(i) nitrous acid, (ii) bromine, (iii) sulphuric acid, (iv) ethanoyl chloride.

14 A is a gaseous organic compound containing carbon and hydrogen only. A volume

of A requires an equal volume of hydrogen for complete reduction to B, the
volumes of gases being measured at the same temperature and pressure. A reacts
with hydrogen chloride to produce a very volatile liquid C, with a composition C,
37-2 per cent; H, 7-75 per cent; Cl, 55-0 per cent. Creacts with potassium cyanide
to produce D.
D reacts with sodium hydroxide to evolve an alkaline gas; and with dilute
sulphuric acid to produce E, of which 1 -00 g dissolved in water requires 27-0 cm®
of 0-5M (0-5N) sodium hydroxide for complete reaction.
D reacts with zinc dust and an excess of concentrated hydrochloric acid to
produce a solution, which, when made strongly alkaline with sodium hydroxide
and warmed, evolves an alkaline gas F.
Identify the compounds A to F and write equations for the reactions in the
above scheme. Outline the preparation of a sample of A in the laboratory.
(C(T))

15 Read the following instructions for the preparation of phenylamine. Explain, as

fully as possible, the reasons for using the apparatus, techniques, materials and
conditions printed in italics.
‘Into a 250 cm® wide-necked flask, fitted with an air condenser, place 8-4 cm®
of nitrobenzene and 18 g of granulated tin. Pour about 6 cm® of concentrated
hydrochloric acid down the condenser. Shake thoroughly for 5 minutes. Continue
to add the acid in 5 cm® portions at 5 minute intervals with continued shaking
until 40 cm® has been added. During the addition of acid the flask can be immersed
in a cold water bath but this should not be done more than is judged to be necessary.
Heat the mixture on a boiling water bath for 30 minutes. Cool. Add gradually
a solution of 30 g of sodium hydroxide in 50 cm® of water until the precipitate
dissolves and the mixture, after shaking, is alkaline. Cool. Steam distil until the
distillate is no longer turbid. Saturate the distillate with salt, transfer to a separating
funnel and add about 20 cm® of ether. Shake, releasing the tap momentarily.
Allow to stand, separate the upper layer and place it in a corked conical flask
with some anhydrous magnesium sulphate. Shake for several minutes. Filter and
distil the lower fraction of the filtrate using a warm water bath and water condenser.
With the lower fraction now removed, distil the higher fraction by direct heating
over a wire gauze using an air condenser. The yield of phenylamine is 7-0 g.’
(SUJB)

269
AMINES 16 Outline the preparation of a solution of a benzenediazonium salt. (Full practical
details are not required.)
How does benzenediazonium chloride react with (i) copper(I) chloride, (ii)
phenol, (iii) potassium iodide, and (iv) water ? What is the industrial importance
of diazonium salts ?

17 A compound Z is boiled with excess alkali. Ammonia is expelled. The resulting

mixture is evaporated to dryness and on prolonged heating, benzene is evolved.
Acidification of the cold residue causes an effervescence of carbon dioxide.
Suggest a structure for Z to account for the reactions described and name the
compounds involved.
Outline briefly reactions for converting Z into (a) phenylamine, (b) isocyano-
benzene, (c) (phenylmethyl)amine, C6H5.CH2.NH2.
Explain why ammonia is a weaker base than trimethylamine. (SUJB)

18 What is the diazo reaction? Illustrate the use of this reaction to prepare each
of the following compounds starting from phenylamine: (a) benzoic acid, (b)
2,4,6-tribromobenzene, (c) methyl phenyl ether, (d) phenylhydrazine, (e)
4-hydroxyazobenzene. (W(S))

19 When the neutral compound A, C10H13NO, was refluxed with dilute acid it formed
two products B, C2H7N and C.
On analysis, C was found to contain 70-59% carbon, 23-53% oxygen and 5-88%
hydrogen by weight. The relative molecular mass of C was found to be 136.
On reaction with alkaline potassium manganate(VII) (permanganate) solution, C
was oxidized to D, C8H6O4.
D, which was acidic, was readily dehydrated to the neutral substance E, CgH^Oj.
B reacted with gaseous hydrogen chloride to form the ionic solid, F, C2H8NCI.
When F was dissolved in dilute hydrochloric acid and sodium nitrite solution
added, a yellow oil, G, was formed and no effervescence occurred.
(a) What is the empirical formula of C?
(b) What is the molecular formula of C?
(c) Write the structural formulae for substances A to G.
(d) What are the names of substances B, C and F? (SUJB)

20 A compound X is believed to have the structure

H2N.CH2.CONH.CH3.

How would you:

(a) show that X contains nitrogen;
(b) show that X contains an amine group ?

How would X react with:

(i) soda-lime;
(ii) sulphuric acid;
(iii) ethanoyl chloride?
Suggest a series of reactions by which X could be prepared from ethanoic acid.
(C(S)j
21 Illustrate, with examples, the difference in:
(a) the acidity of the —OH group in ethanoic acid, ethanol and phenol;
(b) the basicity of the —NH2 group in ethanamide, phenylamine and methyl-
amine.
How do you account for such differences ?
How would you expect the basicity of (phenylmethyl)amine C6H5CH2NH2 to
compare with that of phenylamine and why? (L(S))

22 A compound was found to have the structural formula illustrated below. From
your knowledge of the typical groups it contains give an account of the principal
chemical properties which you would expect it to possess.

270
AMINES
o H

HaC ^ C ^C-H
1
H^C NH,

CH
LHg i
J_J
(L(X, S))

23 Hydrolysis of a compound L, C15H15NO with 50% sulphuric acid gave two pro¬
ducts, an acid M, CgHgOj, and a base N, C7H9N. Reaction of N with nitrous acid
at 5°C followed by warming the solution on a hot water bath at 50°C gave 4-
hydroxymethylbenzene. Addition of phenol to the solution obtained from reaction
between N and nitrous acid at 5°C gave an orange solid O, C13H12N2O. Oxidation
of the acid M with alkaline potassium manganate(VII) gave P, C8H6O4, which
reacted with methanol in the presence of concentrated sulphuric acid to afford Q,
C10H10O4. Under suitable conditions P reacts with ethane-1,2-diol to produce
poly(ethylene terephthalate)*.
Identify the compounds M, N, O and Q, and suggest a structure for L. Write
down equations for the reactions involved.
* terephthalic acid = benzene-1,4-dicarboxylic acid. (C(S))

24 Describe degradative, analytical, and synthetic methods you would employ to

establish the structure of 3-methylphenylamine. (W(S))

25 (a) Compare the physical properties of aminoethanoic acid with those of methyl-
amine and of ethanoic acid.
Explain what is meant by the term zwitterion, and draw a diagram to show the
variation of pH with the amount of added hydroxide ion when hydroxide ion is
gradually added to a solution containing the ion ^NH3CH2C02H until the
solution has become strongly alkaline.
(b) How does urea react with four of the following reagents:
(i) aqueous sodium hydroxide;
(ii) aqueous sodium bromate(I) (sodium hypobromite);
(hi) concentrated nitric acid;
(iv) an aqueous solution containing urease;
(v) aqueous nitrous acid? (0(S))

26 An optically active compound M, C3H7O2N, forms a hydrochloride, but dissolves

in water to give a neutral solution. On heating with soda-lime, M yields N,
C2H7N; both M and N react with nitrous acid, the former yielding a compound
P, CaHgOs, which on heating is converted to Q, C6H8O4. Account for the above
reactions and suggest how M may be synthesised.

27 (a) Starting from ethanal, suggest a reaction scheme for the preparation of
-aminopropanoic acid (alanine) CH3CH(NH2)COOH stating the necessary
2

reagents and conditions.

(b) Give the structure of the products obtained by reacting 2-aminopropanoic acid
with
(i) aqueous sodium hydroxide solution,
(ii) dilute hydrochloric acid,
(iii) ethanoyl chloride (acetyl chloride),
(iv) an acidified aqueous solution of sodium nitrite.
(c) 2-aminopropanoic acid is said to exhibit optical isomerism. What is meant by
this statement? Explain why this term can be applied to this amino acid. Give
the name and structure of an alkane which also exhibits optical isomerism.
(C)

28 A compound A, Ci HioN
4 20, when heated with dilute sulphuric acid gave
ammonium sulphate, a compound B, C8H6O4, and a compound C, C6H7N (as
its sulphate). Compounds B and C behaved as follows:

271
AMINES heat
B C8H4O3 {D) C

heated with dil H2SO4

soda-lime 4- NaN02 in
Y the cold, and
then heated
benzene
T

CfiHeO {E)

heated with
Zn dust

benzene

Suggest a formula for A and account for the above reactions. (C Schol.)

29 Explain the following changes, and deduce the nature of the compounds A to F:
Cl3HgN206
A

acid
hydrolysis

f ]
C7H5NO4 C6H5NO3
B C
gives a colour with FeCl3, and
{a) SOCI2 is non-volatile in steam
{b) NH3

C7H6N2O3
D

Br2 -t- aqueous KOH

"" (a) cold NaN02 + HCl

C, r
gives a colour with FeCl3, and is
volatile in steam

(O and C(S))

30 What are the principal reaction^ of the primary amino-group in an amine such
as ethylamine? How do the properties of this group differ when it is present
in an amide, e.g. ethanamide?
Is the structural formula NH2.CH2.COOH, satisfactory for glycine (amino-
ethanoic acid) which is soluble in water, very sparingly soluble in ether and
benzene and which is still solid at 200°C? Give your reasons. (O Schol.)

31 An amino-acid NH2RCOOH functions both as an acid and as a base in its

reactions with water, according to the following equations.
(a) as an acid:
NH2RCOOH + H2O = NH2RCOO- + H3O+ Ka = 10-‘0M.

272
AMINES (b) as a base:
NHjRCOOH + H2O = +NH3RCOOH + OH- Kt = 2x lO-'"* M.
At the isoelectric point the concentrations of the cationic and anionic forms
of the amino-acid are equal. Calculate the pH of an aqueous solution of the amino-
acid at its isoelectric point from the acidic and basic dissociation constants given
above and the fact that the ionic product of water is 10-^‘*M^.
The amino-acid is a crystalline solid which melts at 250°C. What do you deduce
about its chemical structure in the crystal ? (O Schol.)

273
Chapter 17
Nitro compounds

17.1 General formula

Introduction R—NO2

Nitro compounds have the structure

R-N
O
where R is an alkyl group or an aromatic ring. They are isomeric with
nitrites, R—O—N=0.

17.2 Nitroalkanes are named by combining the prefix nitro with the name of the
Nomenclature corresponding alkane, together with a number to indicate the position of
the nitro group in the carbon chain where more than one position is possible.
Aromatic nitro compounds which contain the benzene ring are named as
derivatives of nitrobenzene, for example:

Nitrobenzene Methyl-3-nitrobenzene Chloro-2-nitrobenzene

Table 17.1. Some nitro compounds

NAME FORMULA B.P./°C

Nitromethane CH3—NO2 101

Nitroethane CH3CH2—NOa 115
1-Nitro propane CH3CH2CH2—NO2 132
2-Nitro propane (CH3)2CH—NO2 120
Nitrobenzene CsHs—NO2 210

17.3 The lower nitroalkanes are colourless liquids which are sparingly soluble
Physical properties of in water. Most aromatic nitro compounds are yellow crystalline solids
(except for nitrobenzene and methyl-2-nitrobenzene which are yellow
nitro compounds liquids) and are insoluble in water and may be purified by steam distillation.

17.4 Laboratory methods

Methods of preparation 1. Nitroalkanes are prepared by the action of a solution of silver nitrite

of nitro compounds in ethanol on an alkyl halide, for example:

C2H5—Br + AgN02 C2H5—NO2 + AgBr

Nitroethane
Ethyl nitrite (C2H5—ONO) is also formed (p. 123).

274
NITRO COMPOUNDS
2. Aromatic nitro compounds are prepared by the nitration of the
aromatic compound with nitric acid. The choice of conditions depends on
the reactivity of the aromatic compound. For benzene, a mixture of con¬
centrated nitric acid and concentrated sulphuric acid is necessary (8.3):

CgHe + HNO3 QHs—NO2 + H2O

Nitrobenzene
For compounds which are much less reactive than benzene (e.g. 1,3-dinitro¬
benzene), fuming nitric acid and fuming sulphuric acid are required,
whereas for compounds which are much more reactive than benzene (e.g.
phenol), dilute nitric acid is suitable.

Manufacture
Nitroalkanes are obtained by the reaction of alkanes with nitric acid in
the vapour phase at about 350°C. Alkanes with more than two carbon
atoms give mixtures of products which are separated by fractional distilla¬
tion, for example:
CH3-CH2-CH2-NO2 + H2O
1-Nitropropane
CH3-CH2-CH3 + HNO3
CH3-CH-CH3 + H2O
NO2
2-Nitropropane

Aromatic nitro compounds are obtained industrially by the same methods

as are used in the laboratory.

1. The most important general property of nitro compounds is their

17.5 reduction to primary amines. This can be effected with hydrogen on nickel:
Chemical properties of
nitro compounds SS CH.t
R—NO2 + 3H2 -^ R—NH2 + 2H2O

and with lithium tetrahydridoaluminate:

LiAlH4
R_N02 -^ R—NH2

Aromatic nitro compounds are conveniently reduced with tin and hydro¬
chloric acid (16.4), for example:

C6H5-NO2 (C6H5-NH3)2SnCl62- - QH3-NH2

Phenylamine

2. Nitroalkanes are hydrolysed by mineral acids to form a carboxylic

acid and hydroxylamine:
H SO
r_CH2—NO2 + H2O —R—CO2H + NH2OH

3. Aromatic nitro compounds undergo electrophilic substitution in the

275
NITRO COMPOUNDS aromatic ring. The nitro group reduces the reactivity of the ring and is
3-directing; for example, nitrobenzene undergoes nitration less readily
than benzene and gives 1,3-dinitrobenzene (p. 110).

17.6 Aliphatic nitro compounds

1. Solvents in industry, particularly for plastics and dyes. They are very
Uses of nitro compounds
useful as they have medium boiling points and do not have obnoxious
smells.
2, Specialist engine fuels and rocket propellants.

Aromatic nitro compounds

1. In the preparation of aromatic amines, used for the production of
dyes (16.8).
2. A particularly important aromatic nitro compound is methyl-2,4,6-
trinitrobenzene (trinitrotoluene, T;N.T.), a powerful explosive made by the
nitration of methylbenzene with a mixture of fuming nitric and fuming
sulphuric acids:

NOa

T.N.T.

Small-scale preparation of 1,3-dinitrobenzene

To 8 cm^ of concentrated nitric acid in a flask, add slowly 10 cm^ of con¬
centrated sulphuric acid, shaking and cooling the flask under a stream of
running water.
Arrange the apparatus (Fig. 17.1) and add 6 cm^ of nitrobenzene drop-
wise from the dropping funnel, gently shaking the flask.
Boil the water in the beaker, and heat the mixture for about 40 minutes.
Pour the mixture into 150 cm^ of cold water in a beaker and stir the
contents from time to time for about 15 minutes. Filter off the solid and
wash it with distilled water (Fig. 2.8).
Squeeze the crude product between filter papers to absorb any liquid
impurities, and then place it in a boiling tube. Add about 20 cm^ of ethanol
(or methylated spirit) and heat in a beaker of boiling water. (If there are
any solid impurities, filter.) Allow the solution to cool and filter the product.
Dry the crystals between pads of filter paper (and if possible in an oven
at 70°C). Find the m.p. of 1,3-dinitrobenzene.

17.7 ^
Small-scale preparation of methyl 3-nitrobenzoate
Practical work Cool 10 cm^ of concentrated sulphuric acid in a test-tube, surrounded by
ice, to 5°C. Add 5 cm^ of methyl benzoate with shaking, keeping the tem¬
perature at 5°C.
To a separate test-tube, pour in 3-5 cm^ of concentrated nitric acid and
3-5 cm^ of concentrated sulphuric acid. Swirl the two acids to mix them and
cool this nitrating mixture to 5°C.
Add, using a dropper, the nitrating mixture to the solution of the ester,
keeping the temperature to between 20 and 25°C. After the final addition,
stand the test-tube for about 15 minutes at room temperature.

276
FIG. 17.1. Preparation of
1,3-dinitrobenzene

Half-fill a 100 cm^ beaker with crushed ice and pour the mixture onto it.
Swirl and filter, using a Buchner funnel (Fig. 2.8). Wash the solid with water
and then twice with 5 cm^ portions of methanol.
Recrystallise a small portion of the nitro compound with methanol, dry
it, and determine the melting point (76-77°C).
If time, transfer about 2 g of the ester to a 100 cm^ beaker and add
20 cm^ of 2M sodium hydroxide solution. Boil the mixture for about 5
minutes. Then add 2M hydrochloric acid until the solution is just acid (test
with litmus paper).
Filter the solid, using a Buchner funnel, and wash thoroughly with water.
Transfer the solid to another 100 cm^ beaker and recrystallise it using
water. Determine the melting point of the dry product (132°C). [The
melting point is depressed significantly by water. Ensure that the product is
dry].

Comparison of the rates of nitration of

aromatic compounds
1. Nitration of methylbenzene
(a) Dissolve about 2 g of sodium nitrate in 10 cm^ of dilute sulphuric acid
in a test-tube. Add 5 drops of methylbenzene, shake well and pour the

277
NITRO COMPOUNDS mixture into a beaker containing about 10 cm^ of cold water. Observe
whether nitration has occurred (a dense pale yellow liquid with a
characteristic smell of almonds should be formed).
(b) Place 10 drops of concentrated nitric acid in a test-tube, and add
10 drops of concentrated sulphuric acid, shaking and cooling the test-tube
under a stream of cold water. Add the mixed acids to 5 drops of methyl-
benzene in another test-tube. Shake the mixture under a stream of cold
water and then pour it into a beaker containing about 10 cm^ of cold water.
Note the colour and smell of the organic compound.
2. Nitration of chlorobenzene
(a) Repeat experiment 1 (a), using chlorobenzene instead of methyl-
benzene.
(b) Repeat experiment 1 (b), using chlorobenzene instead of methyl-
benzene. The mixture of chlorobenzene and the mixed acids should be gently
warmed for 2-3 minutes, and then cooled before being poured into the
beaker containing the cold water.
3. Nitration of phenol
Dissolve about 2 g of sodium nitrate in 10 cm^ of dilute sulphuric acid
in a test-tube. Cool the solution by placing the test-tube in a beaker of ice.
In a second test-tube, dissolve 1 g of phenol in 2 cm^ of water (it may be
necessary to warm the mixture). Add the solution of phenol dropwise to
the solution of sodium nitrate, making sure that the temperature does not
rise above 15°C. Allow the reaction mixture to stand for about 1 hour,
decant the solution from the dark brown (black) solid formed, and wash it
twice, in a separating funnel, with small amounts of water, discarding the
aqueous layers.
Transfer the organic liquid to a small beaker and remove the last drops
of water with a dropping pipette or the edge of a filter paper.
Identify the products by thin-layer chromatography (p. 31).

Reduction of aliphatic and aromatic nitro compounds

1. To 3 drops of nitrobenzene in a test-tube, add 1 cm^ of water, 1 cm^ of
concentrated hydrochloric acid and 2 or 3 small pieces of tin. Warm. The
phenylamine formed goes into solution in the form of its cation
NH3C
Make alkaline with dilute sodium hydroxide solution, to liberate the
phenylamine as an oil. Add enough alkali to dissolve the tin(IV) oxide
formed as sodium stannate(IV).
Test for phenylamine by adding 1 drop of the oil to an aqueous suspen¬
sion of bleaching powder. If phenylamine is present, a blue colour will
appear.
2. To 1 cm^ of nitroethane in a test-tube, add 1 cm^ of dilute sodium
hydroxide solution and a small quantity cm on the end of a wooden
splint) of powdered aluminium. Warm gently to start the reaction.
When the evolution of hydrogen ceases, warm the solution, note the
characteristic smell of ethylamine and test the vapour with moist red litmus
paper.

17.8 1 Describe the preparation of a pure sample of 1,3-dinitrobenzene from nitro¬

benzene.
Questions How would you distinguish between the members of the following pairs by

278
NITRO COMPOUNDS
one chemical test in each case:
(i) chlorobenzene and bromobenzene,
(ii) nitrobenzene and phenylamine,
(iii) bromoethane and 1,1-dibromoethane?

2 Describe in detail two experiments which you have seen or performed to illustrate
the nitration of the benzene ring under varying conditions, and outline the
purification of the products.

3 Give an account of the nitration of benzene. Your account should include the
essential conditions for the reaction as well as an indication of the mechanism of
the reaction.
In which two ways does the nitration of methylbenzene (toluene) differ from
the nitration of benzene? (Mechanisms are not required here.)
Giving the reaction sequences required in the form of equations and the essen¬
tial conditions and reagents for each, indicate how benzene may be converted into
(a) phenylamine (aniline),
(b) benzonitrile,
(c) (phenylmethyl)amine, C6H5CH2NH2. (O and C)
Write structural formulae for the isomers corresponding to the molecular formula
CsHio which contain a phenyl group. Give structural formulae for the different
main mononitration products of each isomer. Assign possible structural formulae
to C8H8(N02)2 which can yield only two different nuclear monobromosubstitu-
tion products. (W)
,N02
A compound has the structural formula
CH=CH-COOH
Outline experiments to
(a) confirm its molecular weight;
(b) identify the functional groups present;
(c) show how many acidic hydrogens are present;

(d) convert the compound to (JMB Syllabus B)

6 Four bottles contain benzene, ethanonitrile, phenylamine and nitrobenzene,

respectively, but the labels have fallen off and have become mixed up. What
physical and chemical tests would you apply to these substances so that the bottles
may be correctly re-labelled ? (L)

7 Illustrate the directive influence of groups in aromatic nuclear substitution by

giving the names and structural formulae of the main organic products formed
by the reaction of a mixture of concentrated nitric and sulphuric acids on (a)
methylbenzene, (b) benzoic acid, (c) methyl-2-nitrobenzene, (d) chlorobenzene.
What is the active nitrating species in a mixture of concentrated nitric and
sulphuric acids, and how may the reactivity of the acid mixture be increased ?
What procedure is adopted to protect the amino-group when phenylamine is
nitrated with the concentrated mixed acids and what is the main product of
nitration under such conditions ? (W)

8 Nitrobenzene may be prepared in the laboratory by treating benzene with a mixture

of concentrated sulphuric and nitric acids. The reaction occurs in several stages.
(i) The reaction of nitric and sulphuric acids to form, firstly, protonated nitric
acid, and second, nitronium ions.
(ii) Electrophilic attack of the benzene ring by nitronium ions to form
C6H6NO2".
(iii) Loss of a proton to form C6H5NO2.
(a) Explain the term electrophilic. Give the structures of the intermediates men¬
tioned in (i) and (ii) above, and write equations for the stages involved in the
formation of nitrobenzene.

279
NITRO COMPOUNDS (b) In a typical experiment 39 g of benzene produced 50 g of nitrobenzene.
Calculate the percentage yield of nitrobenzene. Suggest TWO reasons why
the yield was not 100%.
(c) Nitrobenzene contains a delocalized system of electrons. What do you
understand by this statement? Cite THREE pieces of evidence for delocali¬
zation in the benzene ring.
(d) Outline the essential practical details of the preparation and purification of
an aromatic nitro-compound of your choice.
(Relative atomic masses: H = 1, C = 12, N = 14, O = 16) (L)

280
Chapter 18
Naturally occurring compounds

18.1 A wide variety of organic compounds can be isolated from living organ¬
isms. They include the pigments of birds and flowers, the scents and odours
Introduction of plants and animals, and compounds in plants such as strychnine and
morphine that have powerful physiological effects on animals. From this
huge range, four groups of compounds stand out as being common to all
forms of life: the proteins, carbohydrates and lipids (fats), which have be¬
come household words owing to their importance in nutrition, and the
nucleic acids DNA and RNA which are heard about increasingly owing
to their importance in genetics (the science of inheritance) and genetic
engineering. This chapter is specifically concerned with proteins, nucleic
acids and carbohydrates.

Protein accounts for about 80 per cent of the dry mass of all the soft parts
18.2
of an animal body (i.e. excluding the skeleton). Plants contain a lower
Proteins proportion.
Proteins are derived from a-amino-acids which are joined together by
the elimination of a molecule of water from the carboxyl group of one
molecule and the amino group of the next, so that they contain peptide
bonds, —CO—NH—. When only two amino-acids are joined in this way,
as in

CH3

H2N—CH2—CO—NH—CH—CO2H

from glycine and a-alanine, the compound is called a dipeptide. A tripeptide

is made up from three amino-acid molecules, a tetrapeptide from four and
so on.
The name protein is given to naturally occurring polypeptides contain¬
ing more than about 40 amino-acid residues (the term ‘residue’ is used for
an a-amino acid which has lost the elements of water in forming a peptide
bond). The number of potentially different proteins is virtually infinite;
20 a-amino-acids are used in their formation, and they can, in theory, be
linked in any possible permutations of sequences and total number. In
practice, not all possibilities occur, but the total number is nevertheless
enormous.
Living organisms need to synthesise new protein continuously, partly to
support growth and partly to replace proteins which are broken down
during the process of living. Some bacteria can use air as a source of nitro¬
gen for amino-acid, and thence protein, synthesis; this ‘nitrogen fixation’
involves its reduction to ammonia and occurs at room temperature and
pressure (compare this with the Haber process). Some of these bacteria
are found in nodules in the roots of plants belonging to the pea family
(Leguminosae). The excess of fixed nitrogen is available to the host plant
which, in return, supplies the nitrogen-fixing bacteria with other necessary
nutrients. Other plants and bacteria need a supply of nitrate ions or
ammonia from which they make the amino groups of their amino-acids. An

281
NATURALLY OCCURRING interesting area of current research is to try to develop, possibly by genetic
COMPOUNDS
engineering (p. 345), a variety of bacterium which would form root nodules
in cereals and other commercially important crops and thereby reduce the
need to supply these plants with nitrogen-containing compounds.
In contrast to plants and bacteria, animals must obtain their a-amino-
acids from proteins in their diet. The proteins are first hydrolysed to their
constituent amino-acids, in processes catalysed by the enzymes pepsin,
in the stomach, and chymotrypsin, trypsin and other enzymes (p. 284) in
the intestine. The constituent amino-acids pass into the blood stream and
then to the liver and other tissues where, under the influence of nucleic
acids (18.3), they are converted into the proteins required by the body.
There are certain clinical conditions, such as diabetes, which are related to the
body’s inability adequately to synthesise a required protein (insulin in the case of
diabetes). These conditions can only be alleviated by injection of the protein, for if it
were taken by mouth it would simply be hydrolysed.
%
Of the 20 a-amino-acids that constitute naturally occurring proteins,
12 can be synthesised in the human body from other amino-acids, for
example:

NH, NHa
/ ‘O’ /
CHo-CH -> HO CHa-CH
' \ Enzyme \
COgH COgH
Phenylalanine Tyrosine

However, eight are described as essential a-amino-acids; their residues must

be present in the protein diet since they cannot be synthesised in the human
body. Examples of a-amino-acids present in proteins, those which are
underlined being essential, are:

CH XH.
CH, CH
I I
H2N-CH2-CO2H H 2N-CH-CO2H H2N-CH-CO2H
Glycine a-Alanine Valine

^6^5 SH NH2
I
CH2 CH2 (CH2)4
I I
H2N-CH-C02H H2N-CH-CO2H H2N-CH-CO2H
Phenylalanine Cysteine Lysine

Isolation and purification of proteins

Most proteins occur in mixtures with other proteins of closely similar
properties and their separation is therefore difficult. Some purification can
usually be achieved by selective precipitation of the protein of interest by
careful adjustment of either the pH or the concentration of added salts.
Other methods are based on column chromatography (2.6). Crystallisation
is normally only attempted if 2f-ray diffraction studies are to be carried
out for, although many proteins have now been crystallised, it may take
weeks, months or even years to find how to grow satisfactory crystals of
a newly isolated protein.

282
NATURALLY OCCURRING
COMPOUNDS
Testing the purity of a protein is also difficult, since they do not have
sharp melting-points but decompose on strong heating. Tests are therefore
based on other physical differences between proteins. The most reliable
method involves electrophoresis, in which the protein is placed as a band
on a column of a suitable solid support (for example, polyacrylamide gel)
and a voltage is applied to the column. Different proteins move at different
rates along the column and can be identified (after staining) as bands on the
column. A pure protein produces a single band, and a further advantage
of this method is that it can also be used to estimate, with reasonable pre¬
cision, the relative molecular mass of the protein.

Structure of proteins
Each protein is defined by the number and nature of its constituent amino-
acid residues and the sequence in which these are arranged. In its natural
environment each protein folds up into a specific well-defined shape, known
as its native structure, which is held together by a combination of different
kinds of interaction between atoms and groups in the molecule. One of
the most important of these interactions is the hydrogen bond which occurs
between the N—H group of one residue and the C=0 group of another;

^N—H—0=C\

This can lead to the twisting of the protein chain into a helix. Several model
helical structures can be built, but the one which occurs most frequently
in proteins is the a-helix in which the amino group of one residue is bonded
FIG. 18.1. a-Helix of a protein. to the carbonyl group of the fourth residue along. In Fig. 18.1 the spacing
The change in angle between one between the turns of the helix is 540 pm.
unit and the next occurs at the
carbon atom to which the side
group R is attached. The helix is
held rigid by hydrogen-bonding

Another important factor determining the shape of a protein is the in¬

ability of some groups on the protein (the —CH(CH3)2 group of valine
is an example) to interact with water. This leads to a tendency for these
non-polar groups to clump together in the centre of a protein from which
water is excluded.
Covalent bonds (other than those defining the sequence of amino-acids)
are not needed to make a protein fold up. However, in some folded proteins,
it happens that two cysteine residues are brought close enough to each

283
NATURALLY OCCURRING other for them to be linked by oxidation:
COMPOUNDS

CO CO CO CO

CH-CH2-SH + HS-CH2-CH -> CH-CH2-S -S-CH2-CH ( + 2H)

NH NH NH NH

Once these covalent bonds have been formed, they add greatly to the
stability of the folded structure.
Folded proteins can be assigned to one of two groups, the fibrous proteins
and the globular proteins, which can be distinguished by various physical
properties. In fibrous proteins each molecule is folded to form a long, thin
shape. These proteins are usually insoluble in water and form important
structural features. An example is keratin (in hair and feathers) in which
the basic structure is the a-helix; several helical molecules coil together
rather like a rope and the elastic nature of these structures results from the
ability of the protein chains to stretch out from their helices into extended
chains. A more universal fibrous protein is collagen, the material which
makes up tendons, ligaments and the sheets of connective tissue which
separate the individual muscles of a joint of meat. In collagen, three mole¬
cules coil round each other to form a triple-stranded helix.
In globular proteins each molecule is folded into an approximately
spherical shape, giving a compact structure; the proteins are mostly soluble
in water.
Enzymes are a particularly important group of globular proteins. They
are the catalysts which enable living organisms to bring about necessary
reactions at body temperature. Some consist solely of protein and others
of a protein joined to another molecule (a prosthetic group).
Enzymes are more specific than artificial catalysts, being able to catalyse
only the making and breaking of one type of bond, and usually that bond
must be located in one of a very limited range of compounds. This speci¬
ficity arises from the requirement that the molecules in which reaction is
to occur (the substrates) must fit exactly into the contours of the enzyme
surface to which it must be attached by non-covalent bonds (such as hydro¬
gen bonds).
Since the forces that determine the shape of a protein are relatively weak
(e.g. a hydrogen bond is far weaker than a covalent bond), the shape can
readily be disrupted, and this is known as denaturation. It occurs, for ex¬
ample, when an aqueous solution of a protein is warmed, or when the pH
is altered, and it is accompanied by changes in physical characteristics and
the loss of biological activity. Denaturation can usually be reversed, and
biological activity then returns, showing that a protein folds spontaneously
into its native state.
However, do not expect to be .able to ‘unboil’ an egg. Although boiling the egg
brings about denaturation, the high temperature also causes disulphide bonds to
break and reform in the wrong places, and these covalent changes cannot be easily
reversed.

Determination of protein structure and shape

The problem is usually approached by finding (a) which amino-acids are
present and in what relative amounts, (b) what their sequence is, and (c) the
detailed shape of the folded protein.
(a) The protein is hydrolysed with mineral acid and the resulting amino-
acids are separated by chromatography on an ion-exchange resin. With

284
NATURALLY OCCURRING
COMPOUNDS modern methods, this analysis can be done with a fraction of 1 mg of
protein within 24 hours. (The older method, using paper chromatography,
is much harder to make quantitative.)
(b) For a polypeptide, one approach is to treat the compound with
phenylisothiocyanate, CgHj—N=C=S. This reacts with the —NH2
group at the end of the chain, but not with the —NH— groups in the pep¬
tide bonds. The product is then hydrolysed with hydrogen chloride in an
organic solvent, conditions which remove the terminal amino-acid as a
cyclic derivative but are not so vigorous as to hydrolyse the peptide bonds;

-CO—CH—NH—CO—CH—NH2 >

R^ Ri S

-CO—CH—NH—CO—CH—NH—C—NH—CgHs

R2 R'
^CH
-CO—CH—NH2 + N—C«H
NH

The cyclic product can be identified by comparison of its behaviour on

chromatography with that of known compounds, and the degraded poly¬
peptide, with a new terminal —NH2 group, can be subjected to the
sequence again. The yields are good enough for a skilled scientist to deter¬
mine a sequence of about 30 amino-acid residues. With larger molecules,
this method must be combined with hydrolysis of the protein to poly¬
peptides. It is usually possible to find conditions in which a protein is
hydrolysed in different ways (for example, by the use of two different en¬
zymes). The natures of the resulting shorter sequences are then determined,
and the original full sequence pieced together. For example, if one method
of hydrolysis yields, amongst others, the polypeptides, A—B—C—D—E
and F—G—H—I, and a second yields C—D—E—F—G and H—I—J,
then the full sequence is A—B—C—D—E—F—G—H—I—J.
(c) Detailed knowledge of the shape of a protein molecule can only be
obtained by A-ray diffraction studies on protein crystals. The first structure
to be worked out in sufficient detail for individual atoms to be resolved was
that of myoglobin in 1962. This was a remarkable feat, for the relative
molecular mass is nearly 17 000 and there are about 2500 atoms in the
molecule. With modern methods, including fast computers, it is now
possible to determine the structures of even more complex molecules
rapidly.

A cell which synthesises proteins needs to be able to store information

18.3 about the sequences of amino-acids in those proteins and to translate the
Nucleic acids coded information into a real sequence. Both these properties are conferred
by nucleic acids.

285
NATURALLY OCCURRING Nucleic acids are polymers formed from nucleotides. A nucleotide is
COMPOUNDS
composed of a carbohydrate, a phosphate group and a nitrogen base, and
in the nucleic acids the polymer chain consists of alternating carbohydrate
and phosphate units; this is shown schematically in Fig. 18.2. There are only
FIG. 18.2. Alternating phosphate
two general types of nucleic acid: ribonucleic acid (RNA), in which the
and carbohydrate residues in a
carbohydrate is ribose and the bases are cytosine and uracil (members of
nucleic acid. A base is attached to
each carbohydrate. In {b), the the pyrimidine group) and adenine and guanine (members of the purine
bonding between the group); and deoxyribonucleic acid (DNA), in which the carbohydrate is
carbohydrate deoxyribose and the bases are the same as in RNA except that thymine
residues and the phosphate replaces uracil (Fig. 18.3). However, there is an enormous number of in¬
residues is shown. X represents H dividual compounds in each of these two categories because of the variety
in deoxyribose and OH in ribose of possible sequences for the four bases in each case.

In DNA, two nucleic acid chains, each in the form of a helix, are inter¬
twined. The two strands are held together by hydrogen bonds between the
bases. This arrangement is only possible when the right pairs of bases are
opposite each other: Fig. 18.4 shows the geometrical fit which occurs when
adenine and thymine come together and when guanine and cytosine come
together. The pairs have exactly the same overall dimensions, even though
neither the two purines nor the two pyrimidines are identical, so that when

286
FIG. 18.3. The bases present in NH2 o
RNA and DNA. The bond that 1
attaches the base to the c
carbohydrate is shown -N
-N
\CH \CH
HC /C. /

/
\
-N

Adenine Guanine

NH2 O O

H,C.
HC' N HC' 'NH 'C' 'NH

HC,
N ^N' O

Cytosine Uracil Thymine

the bases are paired in this way, but in no other, they can be stacked neatly
on top of each other and the chains of alternating phosphate and carbo¬
hydrate can take up a regular helical structure irrespective of the sequence
of bases along it (Fig. 18.5). Because of this matching geometry the pairs
A;T and G:C are known as complementary base pairs. In DNA the se¬
quences of the two strands are always complementary to each other; for
example, if the sequence of one strand is AGTCG then the sequence of the
other will be TCAGC.
The double-helical structure of DNA fits it exactly for its role as a store
of information, since each strand of the DNA carries enough information
for the complementary strand to be synthesised on it. When a cell repro¬
duces itself, the DNA molecules first separate into their individual strands
and each then acts as a template for the synthesis of a new strand. The syn¬
thesis is carried out by an enzyme which moves along the single strand of
DNA, selecting the nucleotide with the appropriate complementary base
and linking them together to form a new complementary chain.
The translation of this coded information for specifying the sequence
of amino-acids in a protein involves RNA. RNA molecules have only a
single strand, and they are synthesised, using one of the two strands of
DNA as a template, in an exactly analogous way to the synthesis of a com¬
plementary strand of DNA. In RNA, uracil replaces the thymine of DNA,
but again the dimensions are right for a geometrical fit corresponding to
that of Fig. 18.4.
The relative molecular mass of an RNA is much less than that of a DNA,
so that only a comparatively short length of a DNA molecule is needed to
make an RNA molecule, and each DNA carries enough information to
make several different RNA molecules.
Two types of RNA molecules are synthesised: a smaller type (80-100
nucleotides long) known as transfer RNA (tRNA), each cell having at least
as many types of tRNA as there are amino-acids; and a larger type (up to

287
FIG. 18.4.The hydrogen-bonding
between thymine and adenine and
between cytosine and guanine

FIG. 18.5. Part of the double helix

of DNA. The helix consists of two
strands of alternate carbohydrate
(□) and phosphate (•) residues. To
each carbohydrate residue, a base
is attached. Hydrogen-bonding
between two bases, one from each
strand, holds the helix together.
This is represented by the rungs of Strand 1 Strand 2
the ladder, and two of these rungs
are shown in more detail
(A = Adenine, T = Thymine,
G = Guanine, C = Cytosine)
10,000 nucleotides long) known as messenger RNA (mRNA). They have dif¬
ferent roles in protein synthesis: mRNA specifies the sequence of amino-
acids in the protein, and for this reason the piece of DNA from which a
particular RNA is copied (transcribed) may be called a gene; and tRNA
translates the message in the mRNA by ensuring that a particular amino-
acid recognises the appropriate sequence of three bases in the mRNA.
These roles are achieved as follows.
Each amino-acid forms an ester bond, through its carboxyl group, to
a specific tRNA. Each tRNA is folded into a particular shape so that a
unique sequence of three bases.(a triplet) is positioned on the surface where
it can interact with a complementary set of bases on a mRNA molecule;
for example, if a mRNA sequence has a sequence of six bases, AGUCGA,
it will interact with the exposed triplets UCA and GCU of two tRNA
molecules. This brings the two amino-acids carried by the two tRNA
molecules into close proximity to each other, and an enzyme then breaks
the ester bond of one of them and forms instead a peptide bond between it
and the amino-group of the other amino-acid. Other amino-acids are
brought successively into position in this way and further peptide bonds
are formed so that eventually the coded message in the RNA has been
completely translated and the new protein is released from the template.

288
18.4 Carbohydrates are so called because of the historical observation that
Carbohydrates they have the empirical formula C^(H20)j,—that is, they correspond to
‘hydrates of carbon’. However, the name gives a misleading impression
of their true molecular structure, and furthermore several compounds (such
as deoxyribose found in nucleic acids) have been discovered which do not
have this general formula but which it is convenient to classify as
carbohydrates.
Plants are the main source of carbohydrates for both human food and
other commercial uses. They synthesise carbohydrate from carbon dioxide
and water (photosynthesis) and use it as an oxidisable fuel (source of energy
to the plant), store it for later use as a fuel (for example, starch in potatoes
or grain and sucrose in sugar-beet or sugar-cane) or convert it into struc¬
tural material (for example, cellulose).
Animals can synthesise carbohydrate from excess of amino-acids in their
diet, but most obtain their carbohydrate by eating plant material directly
or indirectly. Vertebrate animals use carbohydrate primarily as an oxidis¬
able fuel (to provide the energy for life) and the form in which they store it
for this purpose is glycogen. In many invertebrates, chitin, which is closely
related to cellulose, forms a structural material.
The monomeric units of carbohydrates are called monosaccharides; most
of the naturally occurring ones have the formula C H O (the pentoses)
5 10 5

or Cf,Hi20e, (the hexoses). When two monosaccharides are condensed

together by the elimination of a molecule of water, a disaccharide is formed;
naturally occurring disaccharides are made up from two hexoses and there¬
fore have the formula C12H22O11. Together the monosaccharides and the
disaccharides are known as the sugars. They are crystalline solids which
dissolve readily in water.
Polysaccharides are formed when many hexose units are joined together
to give compounds of formula (C6Hio05)„ where n can range from about
200 to several thousands (naturally occurring polysaccharides based on
pentoses are unknown). Polysaccharides may be extremely insoluble in
water (such as cellulose) or rather soluble (such as glycogen). In summary,
carbohydrates can be classified in the following way:

CARBOHYDRATES

SUGARS POLYSACCHARIDES
(CeH.oOj)^

MONOSACCHARIDES DISACCHARIDES
I C12H22O11

PENTOSES HEXOSES
CjHjoOs

289
NATURALLY OCCURRING
COMPOUNDS
Monosaccharides
The most commonly occurring group of monosaccharides is the hexoses,
with general formula C H O , and of these the most important are
6 12 6

glucose and fructose.

In aqueous solution, glucose consists of a mixture of three structures
in equilibrium:

OH CH=0 OH
I
CH2
O.
HO ,OH
HO
OH

CHpOH

The left-hand structure corresponds to the major component and the

non-cyclic structure is present in the smallest proportion. However, equi¬
librium between the three is fairly rapidly established, so that if a reagent
is introduced which reacts with aldehydes, for example Fehling’s solution,
it removes the non-cyclic isomer but more is then formed from the cyclic
isomers to maintain the equilibrium; eventually the entire quantity of
glucose reacts.

Preparation of glucose and fructose

Glucose is obtained by the hydrolysis of starch with dilute sulphuric acid
at high temperatures under pressure:

(QHioOs)^ + nH^O
Starch Glucose

There are four asymmetric carbon atoms in glucose so that (since there
is no internal compensation) it is optically active and has 2"^ - 1 = 15
stereoisomers (p. 243), some of which also occur naturally.
Glucose can be obtained in the solid state in two crystalline forms, which
correspond to the two cyclic structures. When either form is dissolved in
water, equilibrium between all three structures is soon established. The
process, known as mutarotation, can be followed with a polarimeter (p. 240)
since the isomeric forms have different molar optical rotatory powers.
Fructose, too, occurs in solution as an equilibrium mixture of cyclic and
non-cyclic structures; the non-cyclic structure is:

CH2OH

c=o
HO—C—H

H—C—OH

H—C—OH

CH2OH

290
NATURALLY OCCURRING
COMPOUNDS
Fructose is obtained by the hydrolysis of sucrose (p. 294) with dilute
sulphuric acid:

C12H22O11 + H2O + QH12O6

Sucrose Glucose Fructose

Properties of glucose
Glucose is a white crystalline solid, soluble in water but insoluble in most
organic solvents. It behaves like an aliphatic aldehyde in the following
respects:

1. It reduces Fehling’s solution to copper(I) oxide and ammoniacal silver

nitrate to silver.
2. With weak oxidising agents, such as bromine water, it forms gluconic
acid:

CHO C02H

(CH0H)4 (CH0H)4

CH20H CH20H
Gluconic
acid

With strong oxidising agents, such as nitric acid, the primary alcohol group
is also oxidised:

CHO CO2H

(CH0H)4 (CH0H)4

CH20H C02H
Saccharic
acid

3. It forms a cyanohydrin:

HO CN
CHO ^CH
I OH ” ^
(CH0H)4 + HCN-> (CH0H)4

CH2OH CH2OH

4. It undergoes condensation reactions with hydroxylamine, phenyl-

hydrazine, etc, for example:

CHO CH=N—NH—CgHj

(CH0H)4 + CgHs—NH—NH2 ^ (CH0H)4 + H2O

CH2OH CH2OH

291
NATURALLY OCCURRING With excess of phenylhydrazine, it forms an osazone;
COMPOUNDS
CH=N—NH—CfiHs
CHO I
I C=N—NH—CfiHs
(CHOHU + 3C6H5—NH—NHj ->• | + CgHj—NH2 + 2H2O + NH3
' (CH0H)3
CH2OH
CH2OH
Glucosazone
5. It is reduced, for example by sodium amalgam and water, to a poly-
hydric alcohol:
CHO CH2OH

(CH0H)4 (CH0H)4

CH2OH s CH2OH

Glucose as a source of energy

Glucose is commonly used as a source of energy by plants, animals and
bacteria. Animals normally ingest it in the form of starch and sucrose (cane
sugar) which are hydrolysed to glucose by enzymes. Mammals do not
possess an enzyme capable of hydrolysing cellulose to glucose, which is
why cellulose is useless as a foodstuff.
Energy is obtained from glucose by oxidation. Complete oxidation is
summarised by:

C6H12O6 + 6O2 6CO2 + 6H2O + Energy

The amount of energy released is 2870 kJ moG^ of glucose.

Oxidation occurs in the cell by a sequence of enzyme-catalysed reactions.
Glucose is first phosphorylated (i.e. converted into a phosphate ester) to
give glucose-6-phosphate by reaction at its —CH2OH group. Glucose-6-
phosphate then isomerises to fructose-6-phosphate, and a second phos¬
phorylation occurs to give fructose-l,6-diphosphate. This compound forms
two 3-carbon molecules which, by oxidation, give 2-oxopropanoic acid,
CH3—CO—CO2H (which, at cellular pH, is ionised).

HO—CH2—(CH0H)3—CH(OH)—CHO
Glucose

P—O—CH2—(CH0H)3—CH(OH)—CHO ->
Glucose-6-phosphate
p—o—CH2—(CH0H)3—CO—CH2OH ^
F ructose-6-phosphate
p—o—CH2—(CH0H)3—CO—CH2—O—P ^
F ructose-1,6-diphosphate

P—O—CH2—CH(OH)—CHO + HO—CH2—CO—CH2—O—P
CH3—CO—C02“
2-Oxopropanoate ion
O

(P is —P—0“)

o-

292
NATURALLY OCCURRING
COMPOUNDS Oxidation and decarboxylation of this leaves an ethanoyl group,
CH3CO , which is attached via a sulphur atom to a large organic
molecule known as Coenzyme A; this coenzyme itself contains an —SH
group, so that the ethanoyl derivative is a thiol ester containing the group
CH3 CO—S—. This is then oxidised by a pathway known as the Krebs
cycle:

CH3—CO—CO2 ^ CH3—CO—Coenzyme A CO2 + H2O

The Krebs cycle is also involved in the oxidation of fats, and this link
between the oxidation pathways of the body’s two chief fuels plays an
important part in the control of their use.
Human beings obtain most of their energy from oxidising fat rather than
carbohydrates. But the use of carbohydrate gives added flexibility since
it can provide a supply of energy during particularly intense muscular
activity when oxygen cannot be obtained rapidly enough. In this situation
2-oxopropanoate (formed as described earlier) is reduced to 2-hydroxypro-
panoate to give an overall equation for glucose breakdown

C6H12O6 -> 2CH3—CH(OH)—CO2H + Energy

This process produces much less energy than the oxidation of glucose, but
it does avoid the need for oxygen. The 2-hydroxypropanoate is later re¬
oxidised to 2-oxopropanoate which is either oxidised through the Krebs
cycle or resynthesised into glycogen.

Disaccharides
Disaccharides have the molecular formula C12H22OH, and consist of two
monosaccharide molecules, C6H,20g, joined together with the loss of a
molecule of water. Three disaccharides occur naturally; these are maltose,
lactose and sucrose. They all have similar physical properties, being white
crystalline solids which are soluble in water.

Manufacture of sucrose
Sucrose is obtained from either sugar-cane or sugar-beet. The cane is cut
into small pieces and crushed, and the juice is pressed out. The juice is made
alkaline with calcium hydroxide, impurities being precipitated and filtered
off. The proteins are precipitated by passing steam through the liquid, and
the clear juice is concentrated by evaporation under reduced pressure. The
syrup is cooled and some sugar crystallises out. The residual sugar remains
in the thick liquid, known as molasses, and sucrose is recovered by dilution
and recrystallisation.
The brown sugar obtained is dissolved in water, and is treated with
calcium hydroxide and carbon dioxide, and more impurities are precipitated.
The filtrate is decolorised by boiling with charcoal, and the solution is
filtered and concentrated by vacuum distillation. The sugar is allowed to
crystallise out and may be granulated or moulded into cubes. The molasses
are used in the manufacture of ethanol by fermentation or for cattle foods.

293
NATURALLY OCCURRING
COMPOUNDS
Chemical properties of sucrose
1. Sucrose, like all disaccharides, is hydrolysed by dilute mineral acids
to two monosaccharide molecules. Sucrose yields glucose and fructose:

C,2H220„ + H2O QH,206 + QH,206

Glucose Fructose

Sucrose is dextrorotatory, and glucose and fructose are dextro- and laevo-
rotatory, respectively. Although an equimolar mixture is formed, the re¬
sulting solution is laevorotatory as fructose has a higher rotatory power
than glucose. This hydrolysis is known as the inversion of sucrose, and the
resultant mixture is known as invert sugar. The reaction may also be effected
by the enzyme invertase, present in yeast.
2. Concentrated sulphuric acid deliydrates sucrose, leaving almost pure
carbon, known as sugar charcoal:

cone. H2SO4
C12H22O11
-^ 12C+ IIH2O

3. Sucrose is a non-reducing sugar; it does not reduce Fehling’s solution

to copper(I) oxide or an ammoniacal solution of silver nitrate to silver.

Structure of sucrose
An examination of the chemical properties above suggests that sucrose is
made up of a molecule of glucose and a molecule of fructose, with the
elimination of a molecule of water. Further, the absence of reducing
properties suggests that the glucose and fructose ‘residues’ cannot equili¬
brate with non-cyclic carbonyl-containing structures with which the
reducing properties of sugars are associated and that therefore these
residues are joined through the carbon atoms which, in the corresponding
monosaccharides, form carbonyl groups. The structure of sucrose has been
deduced, from this and other evidence, as:

Chemical properties of maltose and lactose

1. They are hydrolysed by dilute mineral acids to two monosaccharide
molecules:

C12H22OU + H2O 2QH12O6

Maltose Glucose

Cl2H 220n + H2O QH12O6 + C6H12O6

Lactose Glucose Galactose

294
NATURALLY OCCURRING
COMPOUNDS 2. Concentrated sulphuric acid dehydrates both disaccharides to char¬
coal.

3. Both disaccharides reduce Fehling’s solution to copper(I) oxide and

an ammoniacal solution of silver nitrate to silver. They are known as
reducing sugars.

Structures of maltose and lactose

An examination of the chemical properties above suggests that both are made up of
two monosaccharide ‘residues’. However, their reducing properties suggest that one
or both residues can equilibrate with a non-cyclic, aldehydic structure. The
structures have been deduced, from this and other evidence, as:

OH
I

OH
I

Polysaccharides

Polysaccharides are polymers, made up of monosaccharide units, which

occur in both animals and plants. They are hydrolysed by mineral acid to
monosaccharides:

(C6Hio05)„ + «H20 > «C6Hi206

The two most widely occurring polysaccharides are starch and cellulose.

Starch
Starch occurs in wheat, barley, rice, potatoes and all green plants. It is the
main carbohydrate reserve of plants. It is also an important ingredient of
animal foods since it provides a source of glucose; it is hydrolysed to
glucose by enzymes in saliva.
Starch has two components: a-amylose and ^-amylose (amylopectin).
a-Amylose is composed of long chains of glucose units:

295
NATURALLY OCCURRING Its formula weight is between 10,000 and 60,000, depending on the degree
COMPOUNDS
of polymerisation. It is water-soluble and is used for making starch solu¬
tions.
P-Amylose also contains long chains of glucose units, but these are
joined together at various points by other glucose chains and complex
glucose derivatives, so that the structure is a complex three-dimensional
network. Its formula weight is in the range 50,000—100,000 and it is insoluble
in water.

Hydrolysis of starch
If a solution of starch is warmed to about 70°C in the presence of the
enzyme amylase (present in human saliva), it is hydrolysed to the disac¬
charide, maltose:

2(C6H,o05)„ + «H20 ^ nCi2H220n

Maltose

The hydrolysis is also catalysed by a mixture of enzymes known as diastase

(present in malt) (see below).
However, if starch is boiled with dilute sulphuric acid, it is hydrolysed to
the monosaccharide, glucose:

(QH,o05)„ + nH20 -> nC,n,20e

Glucose

Presumably, starch is first hydrolysed to maltose, which, in turn, is hydro¬

lysed to glucose.

Fermentation of starch
Wheat or barley is ‘mashed’ with hot water and then filtered to extract
a solution of starch. The aqueous solution is heated to about 55°C with
malt, which is germinated barley and contains a mixture of enzymes known
as diastase. Starch is hydrolysed to maltose.
The liquid is cooled to 35°C, and yeast, which contains the enzyme
maltase, is added to catalyse the hydrolysis of maltose to glucose:

C12H22O11 +H20^2C6H2206

Glucose is then broken down to 2-oxopropanoate exactly as it would be by

any other organism using glucose as a fuel. However, in the absence of
oxygen, 2-oxopropanoate is not reduced to 2-hydroxypropanoate in pres-
sence of yeast, unlike the processes that take place in mammalian muscle;
instead, it is decarboxylated to give ethanal which is then reduced to
ethanol, giving the overall equation:

C6H12O6 2C2H5OH -f- 2CO2 -I- Energy

When the concentration of ethanol in the fermented liquor reaches about

10 per cent it is toxic to the yeast so that further fermentation is not pos¬
sible. The resulting liquor can be used for alcoholic drinks (for example,
beer from barley and malt) or purified to obtain pure ethanol. Purification
by distillation is discussed on p. 146.

296
NATURALLY OCCURRING
COMPOUNDS Most strains of yeast contain enzymes which catalyse the oxidation by
molecular oxygen (in air) of 2-oxopropanoate to carbon dioxide and water
via the ethanoyl derivative of Coenzyme A (p. 293), which explains the
necessity for keeping oxygen out of fermenting liquors, and when storing,
for example, beer, cider and wines.

Cellulose
Cellulose is the principal constituent of the cell walls of plants. It consists
of a three-dimensional network of chains of glucose units and some com¬
plex glucose derivatives.
Cotton is almost pure cellulose, but cellulose is usually manufactured
from wood, which is a mixture of cellulose and a material known as lignin.
In this process, wood shavings are heated with calcium hydrogensulphite
to dissolve the lignin and the cellulose is removed by filtration. It can be
purified by dissolving it in a solution of ammonia containing a copper(II)
salt and then adding mineral acid to precipitate the pure cellulose.

Rayon
Rayon is the name given to cover all fibres manufactured from cellulose.
Cellulose ethanoate, celanese silk, is made by ethanoylating cellulose with
ethanoic anhydride. Cellulose ethanoate does not burn readily, and is used
to make films, lacquers and varnishes.
Viscose rayon is manufactured by treating cellulose with sodium hydrox¬
ide and carbon disulphide, and the reaction can be represented thus:

/O—X
X—OH + NaOH + CS2 -> S=CC" + H2O
S“Na +
Cellulose Cellulose
xanthate
The viscose solution is forced through fine jets into a bath of dilute sulphuric
acid, giving a fine thread of viscose rayon. The thread is spun and made
into fabrics which are lustrous and supple, and can be easily dyed. Although
rayon is cheaper than natural silk, it is not as strong or durable.
Cellophane is manufactured by forcing the viscose solution through slits
into the acid.
If cellulose is treated with dilute nitric acid, cellulose mononitrate and
dinitrate are formed; the mixture is known as pyroxylin. A solution of
pyroxylin in ethanol, ether or propanone is known as collodion, and is used
as an adhesive. If pyroxylin is heated with ethanol and camphor, celluloid is
formed.

18.4 Preparation of a protein soiution

Practical work with Beat an egg-white with about five times its volume of water, adding about
proteins 1 g of sodium chloride to help the albumen to dissolve. Filter carefully
through a muslin cloth.

Reaction of the protein soiution

1. Denaturing. Test 2 cm^ portions for precipitation by (a) boiling,
(b) adding M sulphuric acid, (c) adding 2M sodium hydroxide solution,
(d) adding ethanol.

297
NATURALLY OCCURRING 2. Biuret test. To a 2 cm^ portion of protein solution, add 2 cm^ of 2M
COMPOUNDS
sodium hydroxide solution, followed by 2 or 3 drops of copper(II) sulphate
solution. Note the violet colour.
3. Reactions catalysed by enzymes. \
(a) The hydrolysis of carbamide {urea) catalysed by the enzyme urease.
The hydrolysis of carbamide in neutral solution is catalysed by the
enzyme, urease. Dissolve about OT g of urea in 2 cm^ of water in a test-tube, *
and add a few grains of urease powder.
Stand the test-tube in a beaker of water at 40°C, and test the gas evolved
by (a) smell, (b) moist red litmus paper. It may take about 5 minutes to
obtain enough gas to test with the litmus paper.
(b) The hydrolysis of starch by enzymes present in saliva (p. 299).
(c) The decomposition of hydrogen peroxide catalysed by the enzyme,
catalase.
The decomposition of hydrogen peroxide solutions is catalysed by the
enzyme, catalase. Catalase can be obtained as a pure powder and is also
present in blood and in potatoes.
Set up an apparatus similar to that in Fig. 7.1, except with a boiling-tube
as the receiver. The boiling-tube should first be calibrated in 10 cm^
portions, by running in water from a burette and marking the heights for
10, 20, 30 and 40 cm^ with a crayon or dab of paint.
Place 1 cm^ of catalase solution (made up by dissolving about 0-01 g of
catalase in 100 cm^ of water) in the test-tube, and 2 cm^ of 20-volume
hydrogen peroxide in the dropping pipette. Squeeze the rubber teat so that
all the hydrogen peroxide solution is added and collect the gas evolved in
the boiling-tube.
Repeat the experiment using (a) a solution of 1 small drop of blood in
2 cm^ of water, (b) a mixture made by chopping up a small piece of potato
and water, instead of the solution of catalase in water.
If time, repeat the experiment using a small amount (ca. 0-1 g) of solid
manganese(IV) oxide in place of the enzyme. By diluting the solution of
catalase used above, see how active the enzyme is as a catalyst compared
with manganese(IV) oxide.

N.B. If blood is to be drawn, it is important that the correct procedure is

used. A useful guide is ‘Human blood sampling: recommended procedures’
Education in Science, No. 82, p. 27 (1979).

18.5 Reactions of glucose (a monosaccharide)

Practical work with 1. Heat about 1 g of glucose in a test-tube. It first melts, then gives off
carbohydrates water of crystallisation and finally chars, smelling of burnt sugar.
2. Heat about 0’5 g of glucose with 2 cm^ of concentrated sulphuric acid.
Warm. Note charring with formation of carbon.
3. Fehling's test (p. 185). Use about 0-1 g glucose in place of the aldehyde.
4. Silver mirror test (p. 185). Use about OT g glucose in place of the
aldehyde.
5. Formation of an osazone. Dissolve 0-5 g of glucose in 5 cm^ of water.
Dissolve 1 g of phenylhydrazine in 1 g of ethanoic acid and dilute to 10 cm^.
Mix the solutions, and warm on a waterbath. Filter the crystals, wash with
water and with ethanol. Recrystallise from ethanol. Study the crystalline
structure of the osazone under a microscope. M.p. of the osazone is 204°C.

298
NATURALLY OCCURRING
COMPOUNDS Reactions of fructose (a monosaccharide)
6. Repeat the experiments above with fructose in place of glucose.

Reactions of sucrose (a disaccharide)

7. Repeat experiments 1 and 2 above.
8. Dissolve 2 g of sucrose in 10 cm^ of water and divide into two portions.
With one, repeat experiment 3. Acidify the second portion with 5 drops
of M sulphuric acid and boil for a few minutes. Neutralise by adding 2M
sodium hydroxide solution dropwise and then test the solution with
Fehling’s solution.

Reactions of starch (a polysaccharide)

9. Make a paste with about 0-1 g of soluble starch and about 1 cm^ of
water. Pour 10 cm^ of boiling water into the paste and shake the solution.
Divide the solution into three parts.
To the first part, add 2 drops of iodine solution. Note the blue coloration.
Add 5 drops of M sulphuric acid to the second portion and boil for a
few minutes. Neutralise the solution by adding 2M sodium hydroxide
solution drop by drop. Divide the solution into two parts. Test one with
iodine solution to show the absence of starch. Test the second with Fehling’s
solution to show the presence of a reducing agent (glucose).
Add a little saliva to the third portion and warm to about 70°C for a few
minutes. Test the solution as in the previous experiment with iodine and
with Fehling’s solution.

Small-scale preparation of cellulose ethanoate

Mix 20 cm^ of ethanoic acid, 5 cm^ of ethanoic anhydride and 2 drops of
concentrated sulphuric acid in a beaker. Then add, with stirring, 0-5 g of
shredded cotton wool.
Continue to stir so that the cotton wool is evenly distributed throughout
the mixture and there are no air bubbles. Leave the mixture for about a
day, stirring occasionally. The mixture gradually becomes less viscous,
ending in a clear liquid.
Pour the liquid slowly into a large beaker containing 500 cm^ of water.
A precipitate of cellulose ethanoate separates out, which should be filtered
using a small Buchner funnel (Fig. 2.8). Wash it with water and dry it
between pads of filter paper.
Take about a quarter of the ester and dissolve it in about 15 cm^ of
trichloromethane in a boiling tube. If the solution is turbid, place some
anhydrous calcium chloride in the tube and shake the solution. Decant the
solution into a basin and allow it to evaporate in a fume cupboard. Remove
the film of cellulose ethanoate.

The Chemistry of Proteins. R. J. Taylor. Unilever Educational Booklet, Advanced

18.6 Series.
Further reading Carbohydrates. Unilever Educational Booklet, Advanced Series.
A Guidebook to Biochemistry. 4th ed. (1980). M. Yudkin and R. Offord. Cambridge
University Press.
The Double Helix. James D. Watson. (1968). Weidenfeld and Nicholson. Paperback,
Penguin 1970.

299
NATURALLY OCCURRING Introducing Biochemistry (1982) E. J. Wood and W. R. Pickering, John Murray.
COMPOUNDS
Molecules to Living Cells. (1980) A collection of papers from Scientific American.
Introduction by P. C. Hamawalt. W. H. Freeman and Co.

18.7 The Structure of Protein (F) Unilever.

'Pruteen' Food for the Future (V) ICI
Film and Videotape

18.8 1 What is meant by the term sugar! Give a brief account of the sources of sucrose
and glucose, and their relationships with one another and with starch and ethanol.
Questions How would you obtain a sample of ethanol from glucose and show it to be
ethanol ?

2 Give an account of the natural occurrence and properties of two types of poly¬
saccharide. Write down structural formulae for glucose and fructose, and describe
the principal reactions of these substances. Describe the preparation and chemical
properties of sucrose.

3 How, if at all, do (a) glucose and (b) sucrose react with (i) dilute sulphuric acid,
(ii) concentrated sulphuric acid, (iii) phenylhydrazine, (iv) hydrogen cyanide.

4 (a) Distinguish between an aldose and a ketose by drawing structural formulae for a
named example of each, and show how each may be represented by both open
chain and ring structures.
(b) Explain why such compounds may be optically active. Suggest why the optical
activity of a solution of dextro-rotatory sucrose alters during hydrolysis to
monosaccharides.
(c) How might starch be converted into monosaccharides?Jllustrate this change by
a block diagram or an equation. Describe and explain TWO instances of
difference in chemical behaviour between starch and its constituent
monosaccharides. (L)

5 (a) '(x-amino acids are the final products of the hydrolysis of proteins.’
Explain what you understand by the terms a-amino acid and protein. Describe
how the hydrolysis may be effected.
(b) State how an a-amino acid may be formed from propanoic acid, and explain
why an aqueous solution of a pure a-amino acid prepared in this way is neutral
to litmus, and optically inactive.
(c) By what chemical reaction has the chemist synthesized nylon and in what way
are proteins and nylon structurally similar?
(Note: a-amino acids are also called 2-amino acids.) (L)

300
Chapter 19
Petroleum

19.1 The term petroleum is used to describe the mixture of hydrocarbons in oil,
including the gases above the liquid in oil wells and the gases and solids
Introduction
which are dissolved in the liquid. Well over 200 different hydrocarbons can
be identified in a sample of crude petroleum.
Petroleum was formed in remote periods of geological time. Some
was formed over 500 million years ago. Even the newest deposits are over
50 million years old. It was produced from the remains of living organisms.
It is, therefore, a fossil fuel. Weathered rock material eroded from land
masses and carried to the sea accumulated in layers over millions of years
in subsiding basins, and the remains of large quantities of marine plant and
animal organisms became incorporated in the sediment (Fig. 19.1). Owing
to the great thickness of the sediments, high pressures built up which,
probably in conjunction with biochemical activity, led to the formation
of petroleum, although the detailed mechanism is obscure. It is probable,
though, that anaerobic microbes lowered the oxygen and nitrogen content
of what had been living matter.
Subsequent earth movements which caused uplift of the sedimentary
basins also caused migration of the petroleum through pore spaces in the
FIG. 19.1. (a) Debris accumulates rocks, sometimes to areas distant from the formation zones. In the course
by erosion of the mountains, of migration, some of the petroleum came to accumulate in traps where
together with the bodies of marine the porous rock was bounded by impermeable rock. The principal types
organisms, (b) Pressure from the
of trap in oil fields such as those in the Middle East, North America and
debris on the organisms contributes
to the formation of oil

(a)

301
 the North Sea are the anticline (an upfold in the strata), the fault trap and
the salt dome (Fig. 19.2).
The gas is principally methane, with smaller amounts of other alkanes.
The liquid contains mainly alkanes (with up to about 125 carbon atoms
in the chain), with smaller amounts of cycloalkanes and aromatic hydro¬
carbons. The relative amounts of the three classes of compound vary with
the oil-field; for example, petroleum from the Middle East and Pennsyl¬
vania contains a high proportion of alkanes, whereas that from Venezuela
is relatively rich in cycloalkanes and that from California in aromatic
hydrocarbons. The length of the carbon chains also varies. For example,
the petroleum from Mexico has a high proportion of heavy oils and resi¬
dues, while the oils from the Middle East and the North Sea are much
lighter.
Crude oil does not just consist of hydrocarbons. Also present are a
variety of nitrogen- and sulphur-containing compounds which must be re¬
moved during its refining.

FIG. 19.2 Structural traps, (a)

Salt from an evaporated sea has
been forced upwards. The
resulting salt dome seals the
oil-bearing porous rocks, (b) The
crust of the earth may buckle.
The up-folds are known as
anticlines, and these may lead
to an impermeable seal, thus
keeping the oil in a confined
space. On the right, the crust
has been stressed causing a
fault, thus trapping the oil

19.2 The discovery of natural gas fields under the North Sea in 1967 has focused
Natural gas attention on this very important material which serves both as a fuel
and as a source of chemicals. Many fields all over the world are now ex¬
ploited commercially, including those in Libya (from which gas is trans¬
ported to other countries in the liquid state in specially constructed
refrigerated tankers), Italy, Holland and the U.S.A.

302
Plate 19.1. An aerial photograph
of the San Andreas fault in
California. There is a sharp line
between mountains and farmland
{Esso Petroleum Co. Ltd.)

Plate 19.2. An aerial photograph

of an anticline in Iran {Shell
Petroleum Co. Ltd.)

303
PETROLEUM It is believed that natural gas is formed from the decomposition of
petroleum or coal deposits. It contains principally methane (if it is more
than 95 per cent methane it is known as ‘dry’ natural gas). It may contain
larger amounts of ethane, propane and butane (‘wet’ natural gas). The large
North Sea natural gas fields are ‘dry’, but those in the U.S.A. are ‘wet’, and
the higher alkanes are recovered and used as fuels (‘bottled’ gases) and to
make alkenes (20.4).
Natural gas may also contain inert gas and, occasionally, hydrogen sul¬
phide. Gas from the Lacq field in France contains about 15% of hydrogen
sulphide, providing an important source of sulphur. Certain gas fields in
the United States contain up to 5% helium and these have become a major
source of the noble gas. On the other hand, the gas from the Groningen
field in the Netherlands contains 14% nitrogen which merely reduces its
calorific value.
Natural gas from the North Sea contains over 95% methane. Its sulphur
content is so low that organic sulphur compounds are added by law before
it is pumped into the grid so that it can be detected by a very unpleasant
smell.

19.3 When crude petroleum is underground, it is under pressure. Hydrocarbon

gases, the lower members of the alkane family (methane, ethane, propane
Distillation of
and the butanes), are both above and dissolved in the liquid. When the
crude petroleum liquid is obtained, it is allowed to degas at the oil field and the gas is either
‘flared’ or is used for generating power or as a feedstock for organic chemi¬
cals. For example, it is pumped ashore from the North Sea and is generally
liquefied. In this form it is known as liquefied petroleum gas (LPG). It
is a very useful source of ethane and propane (5.5 and 20.3) for chemical
feedstocks.
Once the gas is removed, the crude petroleum is said to be ‘stabilised’
and is transported to refineries, which are generally located in the countries
where the products are to be used. The crude petroleum is distilled into
fractions with different boiling ranges (Fig. 19.3).

CRUDE PETROLEUM

1 1 1 “n
GAS GASO¬ NAPHTHA KEROSINE LIGHT HEAVY RESIDUE
LINE GAS-OIL GAS-OIL (Residual
crude)
B.P,/“C <40 40-100 100-160 160-250 250-300 300-350 >350
7„wt 3 7 7 13 9 9 52
V___t
Numbers of
C atoms in <4 4-10 10-16 16-20 20-25 >25
the alkanes

The values for percentage composition are approximate and vary widely,
depending on where the crude petroleum is found (19.1). The residue,
known as residual crude, is used both as a fuel for large industrial furnaces
and as a source of lighter fractions on cracking. Some is distilled under
vacuum to yield fractions suitable for lubricating oils and waxes. The resi¬
due is bitumen (19.8).

304
19.4 Petrol (gasoline) is a mixture of volatile liquid hydrocarbons which is
vaporised before entering the cylinder of the engine. The straight-chain
The gasoline and alkanes, as obtained from crude petroleum, are not ideal as engine fuels
naphtha fractions because they do not burn uniformly in the cylinder, the process being known
as ‘knock’. This leads to both wear in the engine and wastage of petrol.
However, the highly branched alkanes, as well as the cycloalkanes and the
aromatic hydrocarbons, are less susceptible to knock. It is convenient to
have a measure of the suitability of petrols as fuels, and for this purpose
each compound is given a rating known as the octane number, which is
determined by experiment. Two arbitrary reference points are used in the
scale: heptane, octane number 0 (a poor fuel), and an isomer of octane,
2,2,4-trimethylpentane, octane number 100 (a good fuel). The fraction
obtained by the distillation of crude petroleum has an average octane
number of less than 60, whereas modern car engines, which have high com¬
pression ratios, require petrol with an octane number between 90 (2 star)
and 98 (4 star).
It is therefore necessary to enrich the petrol obtained from crude pe¬
troleum with branched-chain alkanes, cycloalkanes and aromatic hydro¬
carbons, and these compounds are themselves made from other fractions
obtained by the distillation of crude petroleum. The methods for doing this
include the isomerisation and alkylation of the smaller alkanes obtained
from the gas fraction, the reforming of straight-chain alkanes obtained from
the gasoline and naphtha fractions, the cracking of long-chain alkanes

305
Plate 19.3. An aerial photograph
of a salt dome in Louisiana. The
salt has been forced upwards
under high pressure and is now
acting as a seal, preventing the
escape of oil upwards from the
porous rocks (Esso Petroleum
Co. Ltd.)

obtained from the kerosine and gas-oil fractions, and the polymerization of
small alkenes.

Isomerisation
Isomerisation is said to occur when a molecule undergoes a rearrangement
to give an isomer. This process is used to convert straight-chain into
branched alkanes over an acid catalyst. For example, butane is isomerised
to 2-methylpropane when it Is passed over a catalyst:

CH3
Pt on AI2O3 withf,
CH3—CH2—CH2—CH3 A1cT3;100“C ^^H3-C-H

CH3

Reaction occurs via carbonium ions: initially, a secondary carbonium ion

is formed and this, by rearrangement, gives the more stable tertiary car-

306
PETROLEUM
bonium ion (6.4):

CH3
- I
CH3—CH2—CH—CH3 CH3—C+
I
CH3

This reaction is important because it gives a supply of 2-methylpropane

for the alkylation process (below).
Isomerisation of a mixture of pentanes and hexanes, from naphtha, can
increase the octane number of a fuel from 70 to over 90.

Alkylation
As the use of lead compounds to improve the octane number of fuels
becomes less, methods of increasing the proportion of branched-chain
alkanes in the fuel become more important. One way is by alkylation.
Alkenes (obtained by cracking, p. 309) react with alkanes in the presence
of acid to form larger alkanes. For example, 2-methylpropene reacts with
2-methylpropane (obtained by isomerisation as above) to give 2,2,4-
trimethylpentane:

CHg CH, CH,

CH3^ I cone. H2SO4
C=CH2 + H-C-CHg » CH,-C-CH,-C-CH,
25 °C
CH3 H CH,

2,2,4-Trimethylpentane

Reforming
Reforming is another important way in which the octane number of a fuel
can be increased. If the gasoline and naphtha fractions are passed over
a catalyst at 500°C, the straight chain alkanes undergo cyclisation and
dehydrogenation to form a mixture of cycloalkanes and aromatic
hydrocarbons. For example:

CHg
I CHg
CHg I
CH^
HaC CHg H,C CHa
I I
HaC CHa HgC. ^ CHa
^CHa ^CHa"

The process is known as catalytic reforming and the catalyst is generally

platinum suspended on aluminium oxide. During the process some of the
alkane decomposes to carbon. However, if an excess of hydrogen is mixed
with the gasoline and naphtha vapours before they are passed over the
catalyst, formation of carbon is suppressed. Platforming is thus sometimes
referred to as hydroforming. If carbon is formed, the catalyst is purified by
removing it and burning off the carbon. In recent processes, the catalyst
is continuously repurified and regenerated.

307
PETROLEUM The catalyst, which contains a metal and an oxide acting as an acid, is
known as a bifunctional catalyst: the platinum catalyses the dehydrogena¬
tion reaction while the aluminium oxide catalyses the rearrangement of
the skeleton of carbon atoms. Recently, rhodium has also been added to
promote the yield of aromatic hydrocarbons in place of cyclohexanes.
As well as being employed for the enrichment of petrol, the product from
reforming, known as the reformate, is the major source of aromatic hydro¬
carbons such as benzene and methylbenzene for the petrochemical industry
(20.5). The reformate contains over 60% of aromatic hydrocarbons.

Polymerization
Propene in the presence of traces of an acid (e.g. sulphuric acid) forms a
mixture of branched chain alkenes, containing 3 or 4 propene units; these
are known as trimers and tetramers and have the molecular formulae
C9H18 and C12H24. On hydrogenation, they form alkanes which, because
of the branched chain, are adde'd to petrol with a low octane number to
improve its rating.

Tetraethyllead
The octane number of petrols can be improved significantly by additions
of small amounts of chemicals known as ‘anti-knocks’. Tetraethyllead,
Pb(C2H5)4, is the most effective.
In recent years, there has been considerable concern that the exhaust
gases of cars, which contain volatile lead compounds, could be a serious
hazard to health. Although there is no direct proof, lead is certainly a
cumulative poison; for this reason, the permitted level in the U.K. has been
reduced from 0'84 g dm"^ in 1972 to 0T5 g dm“^ in 1985. The debate on
whether the addition of lead compounds to petrol leads to a health hazard
or not has been overtaken in the United States by another factor. In order
to remove carbon monoxide and unchanged hydrocarbons from car ex¬
hausts, many cars are now fitted with a converter, under the car, between
the engine and the exhaust pipe. Some of the most advanced exhaust
systems contain two converters. One (with a rhodium—platinum alloy)
catalyses the reduction of oxides of nitrogen by the excess of hydrocarbons
and carbon monoxide in the exhaust gases. The second (based on
palladium), using the excess of air which is pumped in, catalyses the
oxidation of the remaining hydrocarbons and carbon monoxide. Lead
compounds poison the catalyst and so there is a considerable demand for
lead-free petrol.
There is therefore more demand for branched-chain, cyclic and aromatic
hydrocarbons, so that isomerisation, alkylation, reforming and poly¬
merisation are becoming increasingly important. An interesting develop¬
ment has been the use of methyl t-butyl ether, which is sometimes added
to improve the octane number.
An alternative strategy is to build engines that can run efficiently on
lower grade petrols, and this too is being pursued vigorously by engine
manufacturers in collaboration with the oil companies.

19.5 The kerosine fraction boils between 160 and 250°C and contains alkanes
with 10-16 carbon atoms. Kerosine is used in space- and radiant-heaters;
The kerosine fraction the ‘blue flame stoves’ burn the fuel without smell or smoke. The kerosine
is marketed under such names as Aladdin Pink and Esso Blue.

308
PETROLEUM
Kerosine is the principal constituent of jet (gas-turbine) fuels. The fuel
must have the correct viscosity (so that the fuel spray is atomised), volatility
and freezing point, for the temperature of the air at 10,000 metres is below
-90°C.
When there is an excess of kerosine available from the distillation of
petroleum, it is cracked (19.6).

19.6 The light gas-oil fraction has a boiling point range of 250-300°C; the
number of carbon atoms in the alkane molecules varies from 16 to 20. Much
The light gas-oil of the fraction is used as a heating oil and in high-speed diesel engines
fraction (DERV; Diesel Engine Road Vehicle). In the diesel engine, the fuel is
injected as a fine spray; the engine, which does not have a sparking plug,
relies on the heat of compression to ignite the mixture of fuel and air.
That part of the fraction not needed as a fuel is heated in the presence
of a catalyst, at high temperature; the large molecules are thereby broken
into two or more smaller molecules. The process is known as catalytic
cracking or ‘cat-cracking’. The gas-oil or kerosine vapour is passed through
a fine powder made of silica and aluminium oxide (sometimes containing
small quantities of nickel or tungsten) at 400-500°C. The powder acts as a
fluid and continuously flows out of the reactor into a second chamber
through which air is passed. In this way, any carbon deposited on the
catalyst is burnt off, so reactivating the catalyst which then flows back to
the reactor (Fig. 19.4).
The products from cat-cracking are: (a) a gas, known as refinery gas,
of which the alkenes, ethene and propene, are major constituents; this is
used to make many chemicals (20.4); (b) a liquid containing a high yield of
branched-chain alkanes, cycloalkanes and aromatic hydrocarbons, which
can be used as high-grade petrol; (c) a residue of high boiling point, used
as a fuel-oil.
A variant of the process is called hydrocracking. The catalyst is a sodium
aluminosilicate in which some of the sodium ions are replaced by platinum.
In the presence of excess of hydrogen, no alkenes can be formed and all the
products are saturated. For example, the gases produced contain a high
proportion of 2-methylpropane used to make high-grade petrol by
alkylation (p. 307).

The heavy gas-oil fraction has a boiling-point range of 300-350°C, and the
19.7 number of carbon atoms in the alkanes varies from 20 to 25. It is used as
The heavy gas-oil a fuel for slow speed (e.g. stationary or marine) diesel engines.

fraction

The residue from the primary distillation of petroleum is known as residual

19.8 crude (p. 304). As this consists of over half of the crude petroleum and has
The residue often been transported over very large distances from the oil field to the
refinery, it is important that as great a use is made of it as possible.
Much of the residual crude is burnt as a fuel, widely used to generate
steam in the production of electricity. This is of vital importance to coun¬
tries which do not have the other main fossil fuel, coal. Modern ships also

309
FIG. 19.4. A 'cat-cracker’’ using a
fluid catalyst

Plate 19.4. A cat-cracker {a fluid

catalytic cracking unit). The
regenerator (A) and stripper (B)
are in front of the reactor (C),
which is partly hidden. On the left
is a fractionating column {D)
(Shell Petroleum Co. PLC.)

310
 use residual crude as a fuel oil and indeed the technology of modern ship
design now depends on its use. Some is distilled in vacuum to yield light
and heavy lubricating oils, greases, waxes and a residue, bitumen.
The viscosity of lubricating oils changes with temperature; the oils are
too thick when cold and too thin when hot. Recent developments in
producing multigrade oils (for example, BP Viscostatic and Super Shell
multigrade) have ensured that the viscosity changes relatively little with
temperature. In particular, a polymer of 2-methylpropene is added whose
viscosity is almost constant over a wide temperature range. Detergents are
also added to keep any sludge formed as a fine dispersion.
The effect of engine wear has been studied with pistons which have been
irradiated in an atomic pile and so contain a radioisotope of iron; the
occurrence of wear leads to the enrichment of the oil with the radioactive
iron and so can be followed by ‘monitoring’ the circulating oil. It has been
found that most wear occurs while the engine is at rest; it results from attack
on the metal by acids formed in the oxidation of oils. Chemicals, such as
substituted phenols, are therefore added to inhibit the oxidation of oil.
Paraffin wax, deposited on distillation of the residue, is used for water¬
proofing paper cartons and in the manufacture of candles and polishes.
By cracking the wax in the presence of excess of steam at 500°C, straight-
chain alkenes with terminal double bonds (i.e. RCH=CH2) and 5-18
carbon atoms are formed; they are separated by fractional distillation. The
lower members are used to make branched-chain alkanes for high-grade
petrol (p. 307), and the higher members are used in the manufacture of
detergents by a number of routes (13.10).
Bitumen (the residue from the vacuum distillation of residual crude) is
mainly used in making the surfaces for roads and in part for coating
materials such as cables to give them water and electrical insulation.

19.9 Distillation of crude oil

Practical work Pour in some crude oil to a depth of about 2-5 cm in a test-tube with a
side arm and push down a plug of Rocksil to soak it up. Set up the apparatus
as in Fig. 19.5 and distil the crude oil very slowly, collecting the fractions in
dififerent test-tubes:

Fraction 1—up to 70°C

Fraction 2—70-130°C
Fraction 3—130-180°C
Fraction 4—180-240°C
Fraction 5—the residue.

Pour each fraction on to a separate watch glass and light it with a burning
match or splint.

Cracking of oil to form alkenes

Pour some paraffin oil in a test-tube to a depth of about 2-5 cm. Add
Rocksil until the oil has been soaked up, and then fill the tube to a
depth of about 5 cm with porous pot chips (Fig. 19.6). Heat the chips
and collect the gases in test-tubes, placing corks in them when they are full
of gas.

311
FIG. 19.5. Distillation of crude oil

Rocksil and

FIG. 19.6. Cracking of oil to form

alkenes

Tests for alkenes

1. To one test-tube, apply a lighted splint. Note the colour of the flame
and compare it with the colour of the flame of an alkane.

2. To another test-tube, add 2 drops of a solution of bromine in tetra-

chloromethane and shake.

3. To a third test-tube, add 2-3 drops of an alkaline solution of potas¬

sium manganate(VII) (made by dissolving about 0-1 g of anhydrous
sodium carbonate in 1 cm^ of 1 per cent potassium manganate(VII) solu¬
tion). Shake the contents of the test-tube. A green solution may be obtained

312
PETROLEUM
first [potassium manganate(VI)]. Subsequently, a brown precipitate of
mangananese(IV) oxide is formed.

19.10 Our Industry, Petroleum. (1977). The British Petroleum PLC.

North Sea Heritage. D. Scott Wilson (1974). British Gas.
Further reading The Oil Venture. Shell Education Service.
Energy from synthetic fuels. C. A. McAullife. Education in Chemistry, Vol. 15, p. 21
(1978).
Energy from oil and gas. C. A. McAuliffe. Education in Chemistry, Vol. 15, p. 24
(1978).
Energy from coal. Journal of Chemical Education, Vol. 56, p. 186 (1979).

19.11 A wide range of films on prospecting for petroleum and natural gas can be obtained
from Shell Film Library, British Petroleum PLC and from the Gas Council.
Films and Videotapes Of particular interest are
Location North Sea (F, V) B.P. Education Service
Flags under the North Sea (F, V) Shell Film Library
The Origins of Oil (F, V) Shell Film Library
Films covering the distillation of oil can also be obtained from these sources. One
such film is: Oil Refinery (F, V) Shell Film Library
Another interesting film describing our finite fuel resources is Energy in Perspec¬
tive (F) B.P. Education Service.

North Sea Challenge. A resource pack including audio-casettes. B.P. Education

19.12
Service.
Other resource
material

313
Chapter 20
The petrochemical industry

20.1 In the United States and Western Europe, about half-a-tonne of organic
chemicals is produced every year for every member of the population. It
Introduction is difficult to imagine how we consume such an enormous amount. But we
do—in the clothes we wear, the furniture in our homes, in medicines, in
foods, in our cars and planes. It is interesting to spend a few minutes
checking on the materials we use in our homes, in the places where we work
and in the sports we play.
More than 90% of these organic compounds come from natural gas
and petroleum, and the petrochemical industry is one of the vital industries
in our developed societies. Yet, to put this in perspective, this only accounts
for about 5% of the petroleum used each year—the remainder being used
as fuels in one form or another. This ratio will change as the oil deposits
become scarcer and this will provide chemists with one of the greatest
challenges in the next 20 or 30 years.
As this chapter develops, you will see that the myriad of chemicals is
produced from only seven raw materials—methane, ethene, propene, a
mixture of C4 unsaturated hydrocarbons, benzene, methylbenzene, and a
mixture of dimethylbenzenes. Indeed, the chemicals we make from ethene
account for nearly half the entire petrochemical industry.

20.2 There are four important primary sources for the manufacture of petro¬
leum chemicals: natural gas, gas dissolved in oil deposits (LPG), refinery
Primary sources gas and naphtha. The first three have been discussed in the preceding
chapter. Refinery gas, which contains hydrogen, and alkanes and alkenes
containing up to four carbon atoms, is obtained from the distillation of
petroleum (19.3), catalytic cracking (19.6) and catalytic reforming (19.4).

20.3 (a) Methane

Chemicals from alkanes Methane is obtained from natural gas. The sulphur compounds are first
removed (20.8) and then the natural gas is fractionated to yield pure
methane.
The principal use of methane is the formation of a mixture of carbon
monoxide and hydrogen (known as synthesis gas) by reaction with steam
(5.4). The uses of synthesis gas are discussed in Section 20.7. Suffice to say
that this is the main use of methane because the hydrogen co-product is
needed for the Haber Process.
Chloromethane and dichloromethane are formed when an excess of
methane is heated with chlorine at 400°C. Chloromethane is used in the
manufacture of silicones (p. 339), and dichloromethane is an important
solvent.
Methane is the principal source of hydrogen cyanide;

2CH, + 2NH, + 30,—. 2HCN + 6H,0

which is used in the manufacture of hexanedinitrile (p. 320) and methyl

2-methylpropenoate (p. 331).

314
THE PETROCHEMICAL
INDUSTRY A more recent and increasingly important use of methane is in the manu¬
facture of ethyne (5.4).
The uses of methane as a source of chemicals are summarised in a chart
given in Appendix I.

(b) Ethane and propane

Ethane and propane, constituents of ‘wet’ natural gas (19.2) and liquefied
petroleum gas (LPG) (19.3) are important sources of ethene and propene,
which are used to make a very wide range of chemicals (20.3 and 20.4).
Some ethane is chlorinated to form chloroethane, used to make tetra¬
ethyllead (19.4). Some propane is used in bottled fuel and also to make
nitroalkanes. When it is heated with nitric acid vapour at 350°C, a mixture
of nitromethane, nitroethane, 1-nitropropane and 2-nitropropane is
formed; the compounds are separated by distillation and used as solvents
and as fuels for small engines.

(c) Butane
Butane is generally obtained from refinery gas (20.2). Much is liquefied,
bottled and sold as a liquid, portable, fuel (for example, Calor Gas).
Butane is a major source of ethanoic acid (13.4).
2-Methylpropane is used in the manufacture of 2,2,4-trimethylpentane
for high-grade petrol (19.4).

(d) Higher alkanes

The naphtha fraction (C4-Cio alkanes) is cracked to form'alkenes (20.4).
It is also reformed to produce a mixture of aromatic hydrocarbons (19.4).
It is an important source of ethanoic acid (13.4).
2-Methylbuta-1,3-diene, required for the synthetic rubber industry (21.4),
is obtained in part from 2-methylbutane.
The kerosine fraction (Cjo-Cie alkanes) contains both straight-chain
and branched alkanes. These are separated by passing them through syn¬
thetic zeolites which ‘sieve’ the molecules, the straight-chain compounds
(average diameter of 500 pm) being absorbed and the larger branched-
chain compounds (diameter of about 550 pm) passing through. The
straight-chain alkanes are then desorbed by washing the column with
pentane, and these are converted into detergents (13.10).

20.4 Production of ethene and propene

Chemicals from alkenes Ethene and propene are the two most important compounds in the petro¬
chemical industry. As we have already seen, ethene and its products ac¬
count for about half the tonnage of the industry.
In the UK and in other Western European countries, they are produced
principally by the thermal cracking of the naphtha fraction (containing
C5-C10 hydrocarbons), obtained from the distillation of crude oil. Naph¬
tha is vaporised, mixed with steam, and passed, very quickly, through tubes
made of a special alloy, at 700-900°C. The time taken for the heating of the
mixture of naphtha and steam is a few thousandths of a second.

315
THE PETROCHEMICAL After condensation of the liquid products, the gases are compressed and
INDUSTRY
separated by fractional distillation. The final result is to give the following
products:

NAPHTHA

HYDRO- ETHENE ETHANE PROPANE C4 HIGH- RESIDUE

GEN PROPENE HYDRO- GRADE (Fuel-oil)
METHANE CARBONS PETROL

At 700°C, about 15 per cent of the feedstock is converted into ethene and
a further 15 per cent into propene. When the temperature is raised to 900°C,
the conversion into ethene increases to 30 per cent, but the amount of high-
grade petrol (19.4) decreases from 30 to 25 per cent.
In the United States, ethene ahd propene are generally manufactured by
cracking the ethane and propane obtained from ‘wet’ natural gas (p. 302).
In the United Kingdom, liquefied petroleum gas (LPG (19.3)), a mixture of
the smaller alkanes, is becoming available in very large quantities from the
stabilisation of North Sea oil sent for export. ‘Steam cracking’ LPG gives a
high yield of ethene, and this source may well become more important than
naphtha by the late 1980s.
Most of the ethene produced, and the chemicals derived from it, end
up as polymers—poly(ethene), poly(chloroethene), poly(phenylethene),
poly(ethenyl ethanoate), and the polyesters, which are discussed in Chap¬
ter 21. In addition, the Ziegler process for the manufacture of poly(ethene)
(p. 328) has been adapted to make longer-chain alkenes and alcohols from
ethene. The ethene is heated with triethylaluminium:

A1(C2H5)3 + nC2H4 ^ AlRiR^R^

For the formation of alkenes, the resulting trialkylaluminium (in which

R\ R^ and R^ are alkyl groups larger than ethyl) is heated at 300°C under
pressure, for example:

A1(CH2CH2CH2CH2R)3 ^ A1(C2H5)3 + 3RCH=CH2

For the formation of alcohols, the aluminium trialkyl is oxidised in air to

an aluminium alkoxide which is subsequently decomposed with dilute
sulphuric acid:

R^ R^ RiQ OR2
\ /
\./ air \
\ /
/ HoSO,
A1 -> A1 -^ R^OH + R20H -f R^OH
1 1
R3 OR3 -1-

Al2(SO,)3

The alcohols are used in the manufacture of detergents (13.10).

Ethanol is made from ethene, either by direct hydration or via ethyl
hydrogensulphate (10.4), and is used to make ethanal. Ethanol is used in the
manufacture of esters and as a solvent for lacquers and varnishes, cellulose
nitrate (used to make cellophane) and in many cosmetic and toilet pre¬
parations.
Ethanal is made from ethene by the Wacker process (12.3), and can then
be converted into ethanoic acid and ethanoic anhydride.

316
THE PETROCHEMICAL
INDUSTRY Ethene is oxidised directly to epoxyethane (11.6), which is used to make
detergents (13.10) and ethane-1,2-diol (10.7). The diol is used as an anti¬
freeze and also to make Terylene (p. 337).
Chloroethane, used to make tetraethyllead (9.3), the antiknock agent for
gasoline, is manufactured from ethene in the gas phase:

CH,=CH, + HCI CH,-CH,C1

and by the chlorination of ethane (9.3), while 1,2-dichloroethane is made

in the liquid phase by the addition of chlorine to ethene, with 1,2-dichloro¬
ethane itself as the solvent:

CH2=CH2 + CI2 CI-CH2-CH2-CI

Chloroethene is manufactured from ethene (p. 85) and is the source of

poly(chloroethene) (21.2).
The uses of ethene in the chemical industry are summarised in a chart
in Appendix I.

Chemicals from propene

By far the greatest use for propene is in the manufacture of raw materials
needed to make plastics, the major single outlet being polymerisation to
poly (propene) (21.2).
Propene is the starting material for propenenitrile. This important
chemical is formed by passing propene, ammonia and oxygen over a
molybdenum-based catalyst at 450°C:

Bismuth
molybdate „ „
CH —CH=CH2 + NH + f02 -^ CH2=CH—CN + 3H2O
3 3

Newer catalysts, based on antimony(V) and uranium(VI) oxides, are being

formulated and used.
Propenenitrile is used for the manufacture of synthetic fibres (for
example, Acrilan) (21.3) and nitrile rubber (21.4). A very useful by-product
of the process is hydrogen cyanide.
If only propene and oxygen are in the reactant mixture which is passed
over bismuth molybdate, propenoic acid (acrylic acid) is produced. This
can be esterified and polymerised to form the ‘polyacrylates’ used as a base
in emulsion paints.
Propene is one of the starting materials in the Cumene process which is
used to manufacture propanone and phenol (p. 156). Propanone is also
manufactured from propene via propan-2-ol (10.4 and 12.4).
Although propanone is used both as a solvent and in the manufacture of
a variety of other products, one of the most important uses is in the manu¬
facture of the thermosoftening plastic. Perspex (p. 331).
Large amounts of propene are now converted by the 0X0 process into
butan-l-ol. This is described later (20.7) and is an example of how recent
advances in catalysis have enabled us to produce ‘tailor-made’ molecules.
Other uses of propene include the manufacture of propane-1,2,3-triol
(10.7) for glyptal resins (p. 335) and epoxypropane for polyurethane foams
(p. 334).
The uses of propene in the chemical industry are summarised in a chart
in Appendix I.

317
THE PETROCHEMICAL
INDUSTRY
Butenes and pentenes
The butenes are obtained from refinery gas. 2-MethyIpropene is converted
into polymers to improve the viscosity characteristics of lubricating oils for
car engines (p. 311). It is used, too, to make branched alkanes, for example,
2,2,4-trimethylpentane, to improve the octane number of petrol (p. 307).
Pentenes are used to make 2-methylbuta-l,3-diene, the monomer for an
artificial rubber (p. 339).

Higher alkenes
Higher alkenes are made by polymerisation of ethene (20.4) or the cracking
of waxes (19.8). They are used principally for the manufacture of detergents
(13.10) and long-chain alcohols [via the 0X0 process (20.7)] for making
esters used as plasticisers (21.2).

20.5 Production of aromatic hydrocarbons

Chemicals from The traditional source of benzene, methylbenzene, the dimethylbenzenes
and the polycyclic aromatic compounds such as naphthalene was coal-tar.
aromatic compounds However, by far the most important source of these hydrocarbons is now
by reforming the gasoline and naphtha fractions from the distillation of
petroleum (19.4). Over 90% of benzene is now produced from petroleum.
In order to obtain pure aromatic hydrocarbons from the reformate, a
number of physical processes are used. First, the aromatics are separated
from other hydrocarbons by solvent extraction (bis-2-hydroxyethyl ether,
HOCH2CH2—O—CH2CH2OH, is often used since aromatic hydrocar¬
bons, unlike aliphatics, dissolve readily in it). Then, the aromatic hydrocar¬
bons are carefully distilled. The first fraction contains benzene. The second
contains methylbenzene, which is purified by further distillation. The third,
containing a mixture of the dimethylbenzenes and ethylbenzene, is purified
by both fractional distillation and fractional recrystallization.
More methylbenzene and dimethylbenzenes are formed during the
reforming processes than is required, and they are converted into benzene
by a process known as hydrodealkylation. The vapours of the aromatics are
mixed with hydrogen and passed over a catalyst under pressure at about
600°C. For example:

Chemicals from benzene

Most of the benzene which is manufactured is converted into materials
needed for the plastics industry; examples are the formation of phenylethene
and thence poly(phenylethene) (p. 329), and cyclohexane (p. 320), an
intermediate in the manufacture of nylon (p. 336).
Benzene is also used to make phenol via (l-methylethyl)benzene (p. 156).
The principal uses of phenol are in the manufacture of cyclohexanol (p. 159)

318
THE PETROCHEMICAL
INDUSTRY used to make nylon (p. 336), thermosetting plastics such as Bakelite (p. 333),
substituted phenols for epoxy resins (p. 335) and selective weed-killers such
as 2,4-D (2,4-dichlorophenoxyethanoic acid) (p. 160).
The uses of benzene are summarised further in Appendix I.

Chemicals from methylbenzene

Some methylbenzene is converted into benzene by hydrodealkylation, some
is used in fuels for aeroplanes and cars and some is employed in the manu¬
facture of the polyurethane plastic foams (p. 334).

Chemicals from dimethylbenzenes

1,2-Dimethylbenzene is used for makingbenzene-l,2-dicarboxylic anhydride,
which is required for plasticisers (p. 327) and glyptal resins (p. 335):

o
II
V2O5 as cat. \
O + 3H,0
500 °C ,/

O
Benzene-1,2-dicarboxylic
anhydride

1,4-Dimethylbenzene is oxidised to benzene-1,4-dicarboxylie acid,

required for Terylene manufacture (p. 337). One of the most recent methods
for the oxidation is by passing air into the liquid hydrocarbon under
pressure in the presence of a cobalt salt as catalyst:

rn H

Co salt as cat.
■> + 2H2O
200°C, 20 atm.

CO2H
Benzene-1,4-dicarboxylic
acid

The most important alicyclic compounds used in industry are cyclohexane,

20.6
cyclohexanol and cyclohexanone, all of which are made from benzene.
Alicyclic compounds
OH 0
1 II
H2C^ ^CH2 H2C CH2
1
1
H,C ^CH, H2C. /CH2 H2C^ /CH2
^CH2'^

Cyclohexane Cyclohexanol Cyclohexanone

319
THE PETROCHEMICAL Cyclohexane is manufactured by passing hydrogen through liquid
INDUSTRY
benzene in the presence of a nickel catalyst under pressure:

Ni as cat.
->
200°C, 40 atm.

Cyclohexane is oxidised by passing air through the liquid under pressure, in

the presence of a catalyst (often a cobalt salt):

OH

The mixture of cyclohexanol and cyclohexanone (known as ‘mixed oil’ or ^

K.A—ketone/alcohol) is oxidised in the liquid phase to hexanedioic acid ’
using moderately concentrated nitric acid and a copper salt as catalyst:

CH2-CH2-CO2H
CH2-CH2-CO2H

Part of the hexanedioic acid is converted into hexanedinitrile which is

reduced by hydrogen over nickel to 1,6-diaminohexane. Hexanedioic acid
and 1,6-diaminohexane are then used to make nylon-6.6 (p. 336).

A more recent method for making 1,6-diaminohexane starts with buta-l,3-diene.

It is an intriguing process for it would not have been predictable from our basic
knowledge of organic reactions.
Hydrogen cyanide is added to buta-l,3-diene to produce an unsaturated nitrile,
as expected:

CH2=CH—CH=CH2 + HCN -> CH3—CH=CH—CH2—CN.

The product is then treated with more hydrogen cyanide in presence of a catalyst to
produce hexanedinitrile direct:

CH3—CH=CH—CHj—CN + HCN ^ NClCHjl^CN

A bifunctional catalyst (p. 308) *is used. The catalyst is made up of a compound
containing a transition element (e.g. Pt, Pd, Ru) and an acid. Apparently the
transition element isomerises the unsaturated nitrile to a compound with a terminal
double bond
H2C=CH—CH2—CH2—CN
and the catalyst also encourages a molecule of HCN to add in the anti-
Markownikoff manner (p. 86). Hexanedinitrile is then hydrogenated to 1,6-
diaminohexane.

Another important polymer is nylon-6, for which pure cyclohexanone is

required. When the mixed oil is heated under pressure with copper(II) and

320
THE PETROCHEMICAL
INDUSTRY chromium (III) oxides, the cyclohexanol, which is a secondary alcohol, is
dehydrogenated to the corresponding ketone:

OH

CuO + CrjOj as cat

+ > 2 + H,
200°C, pressure

Cyclohexanone is then converted into caprolactam, from which nylon-6 is

made (p. 336).

Cyclohexanone Caprolactam
oxime

The isomerisation of the oxime to caprolactam by sulphuric acid is an example of

the Beckmann rearrangement in which an oxime is transformed into an amide in the
presence of acid:

R. .R + /R
C
II
= I N^ + .N
"^OH (yu. R'
HO^ ^R
C
,N ,NH
R' R"

20.7 Manufacture of synthesis gas

Synthesis gas ‘Synthesis gas’ is a mixture of carbon monoxide and hydrogen. It is
manufactured by heating methane or naphtha with steam on a nickel
catalyst, for example:

Ni as cat.
CH4 + H2O -> CO + 3H2
900X, 30 atm.

Since methane contains a higher ratio of hydrogen to carbon than naphtha,

it is the preferred feedstock when a high proportion of hydrogen is required
in the product.

Uses of synthesis gas

1. Methanol is manufactured from carbon monoxide and hydrogen.
Water gas has traditionally been the source of these gases (10.4). However,
synthesis gas is purer than water gas and is now used for manufacturing

321
THE PETROCHEMICAL methanol with an active catalyst, based on copper:
INDUSTRY

CO + 2H2 CH3OH

A temperature of 250°C and 70 atmospheres pressure can be employed, and

these conditions are milder and therefore more economical than those
of the older process using water gas in which the catalyst of zinc and
chromium(III) oxides requires a higher temperature and a pressure of 300
atmospheres. The uses of methanol are outlined in Section 10.6. They
include the manufacture of methanal and thence Bakelite, Perspex, and
methanoic and ethanoic acids.

2. Higher alcohols are manufactured from alkenes by the 0X0 process.

This is an example of a reaction known as hydroformylation (the addition
of CH2O).
The alkene, together with synthesis gas, is heated in presence of a catalyst
and an isomeric mixture of aldehydes is produced. However, it has been
shown recently that if the catalyst is either cobalt carbonyl, Co(CO)8, or a
complex of rhenium and triphenylphosphine, only the more useful straight-
chain aldehyde is formed. This is hydrogenated to the primary alcohol. For
example:

CH3—CH=CH2 + CO + H2 ^ CH3CH2CH2CHO
Propene Butanal
CH3CH2CH2CHO + H2 -> CH3CH2CH2CH2OH
Butan-l-ol

Butan-l-ol is an important solvent. If longer chain alkenes are used, the

primary alcohols produced become a basis for detergents (13.10).

3. Carbon monoxide is produced by cooling synthesis gas, liquefying the

carbon monoxide and pumping off the hydrogen. It can be converted into
carbonyl chloride, COCI2, which is used to make polyurethane foam plastics
(p. 334).

An important new use of carbon monoxide is in carbonylation reactions

(the addition of carbon monoxide to a molecule). The principal route for the
manufacture of methanoic and ethanoic acids is by the reaction of methanol
with carbon monoxide (13.4).

4. Hydrogen is manufactured by modifying the process used for the

production of synthesis gas. Excess of steam is used, encouraging the
formation of carbon dioxide, rather than carbon monoxide, and hydrogen:

CH4 T 2H20^C02 + 4H2

However, some carbon monoxide is still present and so the gases are
successively passed over iron at 400°C and a more active copper catalyst at
200°C. Carbon monoxide is oxidised by the steam, still present in the
mixture:

CO + H20->C02 + H2

The gases are passed through an alkali (for example, a solution of

potassium carbonate) to remove carbon dioxide. Traces of carbon

322
THE PETROCHEMICAL
INDUSTRY monoxide can be removed by passing the mixture over nickel:

Ni as cat.
CO + 3H2 -CH4 + H2O
300°C,20atm. ^ ^

Hydrogen is used to make ammonia:

Fe as cat.
N2 + 3H2 -2NH3
500°C, 150 atm.

Apart from countries where oil supplies are considered insecure, all
ammonia is now manufactured from naphtha or methane. It is used to
make nitric acid and inorganic fertilisers such as ammonium nitrate.
However, an organic nitrogen compound, carbamide (urea), is now a very
important fertilizer for use in warm climates. It has a high nitrogen content
(46%) and it is made by heating ammonia and carbon dioxide under
pressure:

200°C, 200 atm.

2NH3 + CO2 -^ H2N—CO2" NH4+
H2N—CO—NH2 + H2O

Carbamide is also used to make plastics (p. 333).

20.8 Crude oil contains both hydrogen sulphide and organic sulphur com¬
pounds (which during the various processes in the refinery are converted
Sulphur into hydrogen sulphide). The refinery gas is passed through a solution of a
weak base (an amine) which absorbs hydrogen sulphide, which is an acid.
The solution is later boiled to release hydrogen sulphide in a concentrated
form. Some natural gas also contains hydrogen sulphide and this is
removed in the same way.
The removal of sulphur compounds is necessary to avoid pollution.
However, it is also economically important since over a quarter of the
sulphur now used in industry is recovered from petroleum sources.
It is most convenient to convert the hydrogen sulphide into solid sulphur,
which is then in turn converted into sulphuric acid by the Contact Process.
The hydrogen sulphide is burnt in air to form sulphur dioxide:

2H2S + 302-^2802 + 2H2O

Sulphur dioxide is mixed with excess of hydrogen sulphide and passed over
aluminium oxide at 400°C:

2H2S + S02^3S + 2H2O

Sulphuric acid is used in many ways, for example, in the manufacture of

fertilisers (superphosphate and ammonium sulphate), titanium (IV) oxide,
rayon and cellophane (p. 297), explosives (p. 276), and detergents (p. 201).

If you had been studying chemistry 30 years ago, you would have been
20.9 using a book in which a great deal of space was devoted to the use of coal
When the oil both as a fuel and as a feedstock for the chemical industry. During the 1930s
runs out and 1940s there was a revolution in the industry and, in order to give a

323
THE PETROCHEMICAL picture of the organic chemicals industry today, we have concentrated, in
INDUSTRY
Chapters 19 and 20, exclusively on petroleum. This revolution has been
remarkable. Just consider its growth. In 1920, less than 100 tonnes of
chemicals manufactured in the United States were derived from petroleum.
By 1950 the output had grown to 5 million tonnes and, by 1970, to about 30
million. In the 1950s, there was also a swing from coal to oil as an important
source of electricity generation.
But petroleum is a non-renewable resource, in the sense that we are using
it up faster than it is being made. Sometime in the future—and the estimates
of when this will happen vary widely—there will be a major shortage of oil
in the world.
Industrial chemists are meeting this new and exciting challenge in two
ways. One way is to develop methods for using the petroleum more
effectively. For example, ethanoic acid can be made more cheaply by a
catalytic process based on synthesis gas than by the oxidation of naphtha.
At the same time, methods are being sought for saving energy, for
example, by developing catalysts that enable chemical processes to be
operated at lower temperatures and pressures.
But the major question still remains: what will happen when the oil runs
out? There are many carbon-containing materials other than oil. Coal is an
obvious one and many countries still have vast untapped reserves. Biomass
is another, and tropical countries, in particular, are very suitable for the
rapid growth of wood and plants like cane sugar.
Much work is being done now to find effective ways of converting coal
and biomass into synthesis gas, from which, as we have seen in Section 20.7,
a wide range of chemicals such as plastics and detergents can be produced.
Petrol, too, can be made from synthesis gas, but at present it is too expensive
for most countries to replace oil in this way. However, the economics of the
process will become more favourable as oil becomes increasingly scarce and
its price rises.
Historians may well see the era of oil-based materials in which we now
live as a brief interlude in which man used a valuable and unique material
extravagantly and thoughtlessly. But the organic chemist will still provide
us with the materials upon which we depend for comfort and our way of life
by adapting the feedstocks away from oil to the older feedstock, coal, and a
newer one, biomass.

20.10 Cracker. I.C.I. Educational Publications.

Chemicals from coal J. Gibson, Chemistry in Britain, Vol. 16, p. 26 (1980).
Further reading There is a series of articles from 1981 in Journal of Chemical Education, entitled
‘Real World of Industrial Chemistry’, by H. A. Wittcoff, which is a very useful
source of information.

ADVANCED READING
Basic Organic Chemistry. Part 5. Industrial Products. J. M. Tedder, A. Nechvatal
and A. H. Jubb (1975. Reprinted 1979.). John Wiley and Sons.
Industrial Organic Chemicals in Perspective. Parts I and 2. H. A. Wittcoff and B. G.
Reuben (1980). John Wiley and Sons.

324
20.11 A large number of films and videotapes can be obtained from the Shell Film Library
and from the British Petroleum Company Ltd.
Films and Three videotapes which illustrate important aspects are;
Videotapes Catalysis (V) ICI
Ammonia (V) ICI
Organics by the Ton (V) Open University S246/15V v
Wherefrom Next (V) Open University P.V. 10 illustrates some developments of the
future as oil resources dwindle.

20.12 1 Most widely used organic compounds are now made directly or indirectly from
petroleum. Select THREE important such compounds, preferably of different types.
Questions Describe how they are made and indicate briefly what they are used for.
For ONE of the compounds, suggest how it might be made if we had no petroleum.
(L)

2 What is the chemical nature of petroleum? Show how petroleum can be processed
to give
(a) fuels and
(b) pure compounds, having two or three carbon atoms in their molecules. (NI)

3 In the United States, the simple gaseous hydrocarbons are all readily available.
Suggest routes by which certain of these gases could be used for the preparation of
propanone, benzene, ethanol, ethanoic acid and poly(ethene).

4 Write an essay on petroleum as a raw material in the production of simple organic

compounds of low relative molecular mass.

5 Describe, with the aid of a diagram, the primary refining process used in the
treatment of crude petroleum.
Explain, quoting one example in each case, the meaning of: (a) catalytic cracking;
(b) alkylation. Say why these processes were introduced into the petroleum industry.
Describe how the following are made from refinery sources: (i) ethanol, (ii) ethane-
1,2-diol, (iii) methylbenzene.

6 Write an account of the production of organic chemicals from crude petroleum.

(SUJB(S))

7 Outline the means by which the following are industrially produced from
petroleum: (a) petrol, (b) benzene, (c) propan-2-ol.
By what means, in the petrochemical industry, are hydrocarbons containing
about twelve carbon atoms changed into lower molecular weight homologues?
Indicate all the steps involved in the production of benzene from a different
natural source. (SUJB)

8 Ethene is made on an industrial scale by cracking naphtha in the presence of steam

at 900°C at just above atmospheric pressure.
(a) (i) State what you understand by the term cracking.
(ii) What is the function of the steam in this process?
(iii) Why is the process operated at this comparatively low pressure?
(iv) Show by means of an equation the first chemical step in the purely
thermal cracking of octane.
(v) Thence show how ethene is formed.
Ethene is frequently called the building block chemical.
(b) (i) Show by means of equations and conditions how ethene can be
converted into either chloroethene {vinyl chloride) or phenylethene
(styrene).
(ii) Name a material in everyday use made from chloroethene or from
phenylethene.
(iii) Give the structural formula for the material in (ii). (JMB)

325
THE PETROCHEMICAL 9 Describe the production of the following, writing equations for important chemical
INDUSTRY
processes.
(a) polyethene (polythene) from petroleum;
(b) high-grade petrol (gasoline) from petroleum;
(c) aromatic hydrocarbons from coal.
Candidates should aim to submit clear, concise accounts, rather than voluminous,
rambling descriptions. (SUJB)

10 (a) Write a comparative account of the processes involved in obtaining the chief
products of commercial importance from coal and petroleum. Details of the
industrial plant are not required.
(b) Comment on the relative importance of coal and petroleum as raw materials
in the world today.
(c) Explain, with an illustrative equation, in each case, what you understand by
the terms:
(i) catalytic cracking,
(ii) reforming,
and why these processes are important in the production of large volumes of
high-quality petrol. (SUJB)

11 Explain how ethene, trichloroethene, propene and styrene can be obtained from
petroleum. How are ethene, propene and styrene converted into polymers of in¬
dustrial importance? (NI)

12 (a) Discuss the relative importance and availability of coal, oil and natural gas
as raw materials in the world today and in the future.
(b) ‘Organic chemists, working in industry, consume scarce natural resources at
an alarming rate and produce materials which pollute our environment.
Living would be cleaner and more enjoyable without their activities.’ Discuss
this statement. (SUJB)

13 Alkenes react with alkanes in presence of acid catalysts; the process is known
as alkylation. What product(s) would you expect to obtain from propene and 2-
methylbutane? Give reasons.

14 Crude petroleum contains over 200 different hydrocarbons and the ratio of short
chain to long chain and the proportions of alkanes, cycloalkanes, and aromatic
hydrocarbons also vary.
There is a possibility that oil may be discovered near the islands of Egg and
Muck, and you, as a member of the oil company exploring for it, are asked what
composition you hope that it will have. Giving reasons, outline this composition.

326
Chapter 21 Polymers

21.1 When two or more molecules of a simple compound (a monomer) join

together to form a new compound (a polymer), the process is described as
Introduction polymerisation. In addition polymerisation, the polymer has the same
empirical formula but a higher formula weight than the monomer; an
example is the formation of poly(ethene) from the monomer ethene:

«CH2=CH2 ->-CH2—CH2—CH2—CH2—CH2—CH2—•••

In condensation polymerisation, polymerisation of the monomer is accom¬

panied by the elimination of small molecules such as water or ammonia;
an example is the formation of nylon-6.6:

I2H2N—(CH2)6—NH2 + «H02C—(CH2)4—CO2H
H-E-NH—(CH2)6—NH—CO—(CH2)4—CO-j^OH + {In - 1)H20

Natural polymers such as proteins (18.2) and carbohydrates (18.4) are

discussed elsewhere. This chapter is concerned with synthetic polymers.

Many synthetic polymers can be moulded into required shapes and are
21.2 useful as plastics for the manufacture of a wide range of articles. The
Plastics properties of plastics can be modified by the addition of compounds known
as plasticisers. For example, esters of benzene-1,2-dicarboxylic acid, made
from benzene-1,2-dicarboxylic anhydride (p. 319) with long-chain alcohols
such as octanol, are added to PVC (poly(chloroethene)) (p. 330) to produce
a softer and more easily worked material. Dyes and pigments are added to
give the plastic colour, and it is sometimes possible to add a cheap material
known as a filler to increase the bulk of the plastic and make it less expensive,
without altering its desirable properties.
Plastics can be subdivided into two groups: the thermosoftening and
thermosetting plastics.

Thermosoftening plastics

The characteristic of these plastics is that they become soft when heated and
can then be moulded or remoulded. They are linear polymers; that is, they
are of the general structure:

_x—X—X—X—X—X—X—
where X represents the monomer.
Many of the thermosoftening plastics are formed by the polymerisation
of monomers which contain a C=C bond; examples are poly(ethene) from
ethene, poly(propene) from propene, poly(chloroethene) from chloroethene
and Perspex from methyl 2-methylpropenoate.

327
POLYMERS Poly(ethene) (polythene)
Two types of poly(ethene) are manufactured. One, with a low density and
a formula weight ranging from 50,000 to 300,000, softens at a comparatively
low temperature (about 120°C). It is made by compressing ethene under
very high pressure at about 200°C, in the presence of a very small amount
of oxygen:

O2 as cat.
nCH2=CH2 -(-CH2 CH2^
200°C, 1500-2000
atm.

The second form is a high density material with a formula weight in the
range 50,000 to 3,000,000 and a higher softening point (about 130°C). It
is generally manufactured by a process developed by the Swiss chemist,
Ziegler, in which ethene is passed under pressure into an inert solvent (an
aromatic hydrocarbon) containing as catalyst triethylaluminium and
titanium(IV) chloride. After polymerisation, the catalyst is decomposed by
adding dilute acid, and the crystalline polymer is filtered off. Poly(ethene)
produced this way has a greater rigidity and higher softening point than
the low density polymer.
Poly(ethene) is used without a plasticiser or filler and can be readily
coloured. The plastic is an insulator and is acid-resistant. The low density
polymer is used as a film for packaging and for coating and as a covering
for cables. The high density polymer is particularly suitable for good quality
mouldings including bottles.

Poly(propene)
While poly(ethene) has been manufactured since 1939, poly(propene) is a
comparatively new material. Its manufacture did not begin in Great Britain
until 1962, but the amount produced then increased very rapidly. It can be
used in place of poly(ethene), over which it has the advantages of being
stronger and lighter and having a higher softening point. Its high tensile
strength allows it to be pulled to produce tough fibres which can be used to
make ropes and carpets. Carpets made from polypropene are popular as it
is possible to mop up spillages from them, so that they are particularly
useful in kitchens. Polypropene’s impact strength makes the polymer very
useful for mouldings, too, and it is used, for example, to make bottles, often
replacing metal and glass.
It is manufactured by a method invented by the Italian chemist, Natta,
in which propene is passed under pressure into an inert solvent (heptane)
which contains a trialkylaluminium and a titanium compound;

AIR3 + TiCb
CH3 CH3
cat.
2nCH3— CH=CH2 -f-CH—CH2—CH—CH2 h.
100°C, 10 atm.

This method produces a polymer in which the methyl groups on the

alternate carbon atoms all have the same orientation. This can be repre¬
sented as in Fig. 21.1. Such polymers are described as isotactic, and it is
this regularity of structure which allows neighbouring molecules of poly-
(propene) to pack closely together to give a crystalline structure. In atactic
polymers, the orientations of the side-chains are random and the com¬
pounds are non-crystalline.
Ziegler and Natta were awarded a Nobel Prize in 1963 for their work
on polymerisation.

328
FIG. 21.1. A molecular model
of polyipropene), an isotactic
polymer

(HI CH3 Q c Oh

Plate 21.1. The manufacture of

polyipropene). Following
polymerisation, many polymers
are too finely divided and are
first melted. In this photograph,
molten polyipropene) is being
forced through a die into water.
The 200 strands are then chopped
into lengths of about 0-3 cm, known
as moulding powder. This can be
used in injection mouldings and for
extruders iShell Petroleum Co.
PLC.)

Poly(phenylethene)
Phenylethene, —CH=CH2, is being produced on a rapidly expanding
scale in order, principally, to make poly(phenylethene) and synthetic
rubbers, especially SBR (p. 338). It is made largely by the reaction of
benzene with ethene in the presence of aluminium chloride:

AICI3 as cat.
QHe + CH2=CH2 -> CeHs—CH2—CHj
300°C, 40 atm.
Ethylbenzene

followed by dehydrogenation of the resulting ethylbenzene at 600°C over

a catalyst such as zinc, iron(IIl) or magnesium oxide supported on charcoal
or alumina:
FejOa as cat.
C6H5—CH2—CH3-^ CgHj—CH--CH2 + H2
600°C

329
POLYMERS The method by which phenylethene is polymerised is typical of that for a
number of other monomers. It is treated with a compound, an initiator,
which readily decomposes to form free radicals, for example di(benzoyl)
peroxide on being heated:

CgHs—CO—O—O—CO—CgHs 2C6H5—CO—O-

One of the resulting radicals, R*, adds to the alkene, giving a new radical,

R- + CH2=CH—CeHs -> R—CH2—CH—CgHj

which in turn adds to another molecule of phenylethene:

CgHs CgH; CfiHs

r_CH2—CH- + CH2=CH—CeHs R—CH2 -CH—CH2—CH-

Further additions of this sort occur, giving long-chain radicals, but

occasionally the chain is interrupted and polymerisation stopped by the
chance collision of two radicals:

CfiHs CfiHs CsHs CftHs

r_(CH2—CH)„,—CH2—CH. + -CH—CH2—(CH—CH2)„—R ^

CgHs CgHj C^Hs CfiHs

I 11 1
R—(CH2—CH)„—CH2—CH—CH—CH2—(CH—CH2)„—R

Collisions between radicals are very effective at yielding final products, but
the concentration of radicals in the solution is very small, and it is much
more likely that one radical will meet a molecule of phenylethene than
another radical; in this way, long chains are built up. Note that the length
that the growing polymer chain will have reached by the time that collision
with another radical brings the growth to an end will vary from one chain to
another, since it depends on chance collisions in the solution; therefore,
the polymer does not have an exact formula weight, and formula-weight
measurements give an average for all the chains. Note also that the phenyl
groups occur at regular intervals in the chain except when two chains couple
together in the example above. The regularity arises because the growing
radical tends to add to the CH2 group of phenylethene and not to the
CHfC^Hs) group.
The groups at each end of a polymer depend on the nature of the initiator.
For simplicity, the end groups are often omitted in representing the poly¬
mers.
The main uses of poly(phenylethene) are in making light-weight packaging
materials and a wide variety of household goods such as egg boxes and the
lining material for refrigerators.

Poly(chloroethene) (polyvinyl chloride, PVC)

Chloroethene is manufactured from ethene (p. 85) and from ethyne (p. 95),
with the former route now becoming the more important. It is converted
into poly(chloroethene) by heating in an inert solvent with di(benzoyl)

330
POLYMERS
peroxide to initiate polymerisation:

Cl Cl
I
2«CH2=CHC1 -f CH2—CH—CH2—CH^

PVC is a very tough polymer, and a plasticiser is added to soften it; esters
of benzene-1,2-dicarboxylic acid are used for this purpose, sometimes
accounting for up to 50 per cent of the total weight. PVC is easy to colour, it
is resistant to weathering, fire and chemicals, and it is also a good electrical
insulator. It is used as the insulator for cables and in the manufacture of
artificial leather (e.g. car upholstery), household goods such as curtains and
table cloths, gramophone records and floor coverings.
A more rigid type of PVC is used for guttering and water down-pipes,
used frequently in houses.

Poly(ethenyl ethanoate)
Ethenyl ethanoate is manufactured by passing a mixture of ethene,
ethanoic acid vapour and oxygen over heated palladium(II) and copper(II)
chlorides:

CH3CO2H + ch2=ch2 + i02 ^ CH3C0—o—ch=ch2 + H2O

This is related to the Wacker process for ethanal (12.4).

Ethenyl ethanoate is polymerised in the same way as phenylethene:

CH3 CH3

CO CO

o o
2nCH3—CO—O—CH=CH2 -> -(CH2—CH—CH2—CH)^r

Poly(ethenyl ethanoate) is employed mainly as the essential constituent in

plastic emulsion paints. Other uses are as “chewing gum base” and, in
solution in propanone, to greaseproof paper.

Polyacrylic esters
Methyl 2-methylpropenoate is manufactured from propanone by addition
of hydrogen cyanide in the presence of alkali to give the cyanohydrin (p. 174)
followed by reaction with methanol in the presence of sulphuric acid;

OH
HCN
(CH3)2C=0 > (ch3)2c:
NaOH CN
98% H2SO4
XH^OH

OH CO2CH3
I
CH3-C-CO2CH3 > CH2=C + H2O

CH3 CH3

331
POLYMERS Its polymer is formed in the same way as poly(phenylethene) and is sold
under the name of “Perspex” (blocks and sheets) and “Diakon (powders):

CO2CH3 CO2CH3
/CO2CH3
2nCH2=C -(CH2-C-CH2-C)^
^CH3
CH3 CH3

The plastics are light, strong and transparent. Sheets are used where
transparency is important (packaging, aeroplane windows, lenses, corru¬
gated roof lights).

Polyoxymethylene
Polyoxymethylene is formed by the polymerisation of methanal; gaseous
methanal is passed into an inert solvent (a hydrocarbon) which contains an
amine as catalyst:

2uCH2=0 + H2O HO—(CH2—O—CH2—0)„—H

To prevent depolymerisation, the —OH groups at the end of each chain

are esterified.
The plastic can be readily moulded, and is very hard, resistant to
chemicals and remarkably resistant to abrasion. For example, a shaft
running at 110 m.p.h. against a steel gear was found to have no wear after
10 000 miles. However, it tends to decompose above about 100°C.

Poly(tetrafluoroethene) (PTFE)
PTFE is made by heating tetrafluoroethene (p. 132) under pressure in the
presence of ammonium peroxosulphate as catalyst:

2nCF2=CF2 _(cF2—CF2—CF2—CF2)^

The polymer is exceptionally resistant to chemical attack. It also has a high

softening point (above 320°C). These properties make it suitable for making
seals and gaskets which are subject to heavy wear at high temperatures.
Its ‘anti-stick’ properties make it useful as a surface coating for cooking
equipment.
The anti-stick properties of PTFE are probably due to its structure, which
is a helix, with the fluorine atoms on the surface of an inner chain of carbon
atoms. There is a very smooth surface, and molecules can readily flow
over it.

Thermosetting plastics

These plastics contain three-dimensional networks of bonds and are

moulded during the polymerisation stage of their manufacture. Unlike
thermosoftening plastics, they cannot be remoulded.

332
 Phenol-methanal plastics (Bakelite)
In 1910, Baekeland patented a process for producing resins from phenol
and methanal which are now known as Bakelite. The resins are moulded,
together with a filler (such as wood shavings) and a pigment, to form a wide
range of articles. The electrical resistance of Bakelite makes it especially
useful for electric plugs, switches and tools.

Carbamide-methanal and melamine-methanal

plastics
Carbamide and methanal, when heated in a dilute solution of acid, give a
resin;

I
CH2
I I
-CH2 CH2-N-CO-N-
\ /
A7H2N-CO-NH2 + wCHaO N-CO-N
/ \
-CH2 CH2-N-CO-N-
I I
CH2
I

Melamine can be used instead of carbamide to make a resin. Melamine is

manufactured from carbamide;

NH.,

6 C0(NH2)2 ^ N + 6NH3 + 3CO2

HM
AN NH,

Both carbamide-methanal and melamine-methanal polymers absorb

dyes easily, and the powders can be moulded under pressure to form table¬
ware (e.g. Melaware), trays and many household utensils. They are used, in
solution, to strengthen paper and to improve the shrink resistance of cotton,
wool and rayon. Both are used as ion-exchange resins to demineralise
water, as the free NH2 groups combine with the acidic groups in the water.
Carbamide-methanal polymers find an importance in the production
of chipboard and in cavity-wall insulation.

333
POLYMERS Polyurethanes
Polyurethane resins are made by polymerisation of a mixture of a di¬
isocyanate and a molecule containing three hydroxyl groups (a triol);

A triol often used is made from propane-1,2,3-trioI and epoxypropane;

CHoOH
I
CHOH -I- On -h SfCHo-CH-CHa
I \ /
CH^OH O

CH3 CH,
I I
CHaO-fCHaCHOl^-CHaCHOH

CH3 CH3
I I
CHO—(CHaCHOl^-CHaCHOH

CH3 CH3
I I
CHaO-fCHaCHOl^-CH.^CHOH

The di-isocyanate is made from methylbenzene:

334
POLYMERS
The polymers can be made into the well-known foam-polyurethane
plastics by treatment with water. This converts some of the terminal
isocyanate groups into amino groups,
_N=C=0 + H2O —NH2 + CO2
these react with other isocyanate groups to extend the polymer chain,
—NH2 + 0=C=N-> _NH—CO—NH—
and the carbon dioxide liberated in the first step is embedded in the polymer
and causes the characteristic ‘foam’, as used in making cushion pillows and
padding. Polyurethane fibres (such as Lycra) are used in stretch fabrics.

Epoxy resins
The epoxy resins are made from the epoxide derived from 3-chloro-
propene and substituted phenols, for example:

CH3

HoC-CH-CHa C-^^)^0-CH2-CH-CH2)„-
\ /
'O' CH3 ^
The value of n can be controlled so as to give a range of resins varying
from viscous liquids to solids with high melting points. The resins are used
as adhesives (e.g. Araldite), surface coatings and electrical insulators.

Glyptal resins
The glyptals are polyesters formed from propane-1,2,3-triol (glycerol)
and benzene-1,2-dicarboxylic (phthalic) anhydride. Each of the three
hydroxyl groups in glycerol forms an ester linkage with the anhydride,
giving a three-dimensional, thermosetting polymer:
O
II

+ rtHO-CHo-CH-CHa-OH
I
OH

335
21.3 Natural fibres include carbohydrates such as cotton (which contains more
than 90 per cent of cellulose) and proteins such as wool; the former are
Synthetic fibres obtained from plants and the latter from animals. Synthetic linear polymers
can also be made into fibres, and the following are important examples.

Nylon
Nylon was the first synthetic fibre to be prepared by polymerisation. It was
discovered by Carothers in the U.S.A. in 1935 and first manufactured in
1940.
The nylon made by Carothers is now called nylon-6.6 because it is made
from two components each of which contains six carbon atoms. The com¬
ponents are hexanedioic acid and 1,6-diaminohexane (p. 320), which are
heated together to give the polymer:

«H02C—(CH2)4—CO H + «H2N—(CH2)6—NH ^
2 2

H0+C0-(CH2)4—CO—NH~-(CH2)6—NH+H + {2n - 1)H20

There are other forms of nylon, each of which contain the peptide link
—CO—NH— as in the naturally occurring protein fibres such as wool.
Nylon-6.10, prepared from decanedioic acid [HO2C—(CH2)8—CO2H]
and 1,6-diaminohexane, has similar properties to nylon-6.6. Its preparation
from decanedioyl chloride is described on p. 341.
Nylon-6 is softer and has a lower melting point than either nylon-6.6 or
nylon-6.10. It is prepared from caprolactam, whose manufacture has been
discussed (p. 321):
O
W H
C-N
H2C 'tH, > +NH-(CH2)5-C0+„
n \ /
H2C. CH2
C^
H2

Plate 21.2. Molten terylene being

forced through a metal disc. The
filaments are then stretched to give
them greater strength {Imperial
Chemical Industries PLC)

336
POLYMERS
The method for making the crude nylon polymer into fibres is to melt
it and then force it through fine jets. It can be bleached with a dilute solution
of peroxoethanoic acid. The finer threads are woven for clothing and the
thicker ones are used for articles such as brushes, tarpaulins and tyre cord.

Polyester fibres
Polyester fibres are formed by condensation of polyhydric alcohols and
polybasic acids. A particularly important one is Terylene (called Dacron
in the U.S.A.) which is manufactured by the polymerisation of the dimethyl
ester of benzene-1,4-dicarboxylic acid (p. 319) and ethane-1,2-diol (p. 152):

n CH3-O-CO + n HO-CH2-CH2-OH

CH3-O- CO-O-CH2-CH2-O -H + (2«-1)CH30H

The molten polymer is extruded to form fibres which are used for making
material for clothes. It has the useful property of being able to form per¬
manent creases for trousers and skirts.

Propenenitrile fibres
Propenenitrile is mainly manufactured from propene (p. 317). It is poly¬
merised to form a fibre which is sold under the name Orion for making
clothes:

CN CN
I I
2«CH2=CH—CN -^CH2—CH—CH2—CH^

Propenenitrile is also made into a copolymer by polymerisation together

with ethenyl ethanoate. The copolymer contains a more or less regular
alternation of the individual monomers:

2«CH,=CH-CN + 2« CH2=CH-0—CO-CH3

CH3
CO
I
CN o CN

CH2-CH-CH2-CH-CH2-CH-CH2-CH^

The copolymer is used as a fibre for making materials under the name
Acrilan.

337
21.4 Natural rubber is obtained from latex, which is an emulsion of rubber
particles in water found in the bark of many tropical and sub-tropical trees.
Synthetic rubbers The latex slowly extrudes from the bark when the tree is cut, and it is
coagulated by the addition of ethanoic acid.
Rubber has the structure

CH. H
/CH2-
X=c
CH3 H CH, "h

<- 810 pm ->

and can be regarded as a polymer of 2-methylbuta-1,3-diene,

CH2=C(CH3)—CH=CH2. In its crude form, it is not a strong enough
or elastic enough material for use, but these properties are improved by
heating it with a few per cent of sulphur (vulcanisation). The exact mechanism
of vulcanisation is not known, but it probably consists of the introduction of
links between neighbouring chains of the polymer, corresponding to
introducing the rungs into a ladder.
Synthetic rubbers mostly resemble natural rubber in having a series of
double bonds in their polymer chain. For this reason, buta-1,3-diene is
usually used in their manufacture; when it is incorporated into a polymer,
one of the two double bonds is retained.
Buta-1,3-diene is made by the dehydrogenation of butane and butenes.
They are passed over a catalyst consisting of aluminium and chromium(III)
oxides at 600°C, for example:

AI2O3 + Cr203
as cat.
CH3—CH=CH—CH3 -CH2=CH—CH=CH2 + H2
600°C 2 ' 2

The most widely used synthetic rubbers are made by the co-polymerisa¬
tion of buta-1,3-diene with another monomer, phenylethene (p. 329).
The polymer, known as SBR, is used in the manufacture of car tyres,
hoses, shoes soles and waterproof boots; it is vulcanised with sulphur and
carbon black is added to strengthen it.
Copolymerisation of buta-1,3-diene with propenenitrile gives nitrile
rubber:

2«CH2=CH—CH=CH2 + «CH2=CH—CN

CN

X(CH2—CH=CH—CH2)2—CH2—CH^

Nitrile rubber is very resistant to chemicals and so is used in oil seals and
gaskets and for making flexible fuel and storage tanks.
Another rubber, known as ABS, is made by copolymerising propeneni¬
trile (30 per cent), buta-1,3-diene (20 per cent) and phenylethene (50 per
cent), and is used in car bodies.
Derivatives of buta-1,3-diene are often used to make rubbers of special

338
POLYMERS
qualities. One, known as Neoprene rubber, is resistant to organic solvents
and is strong. It is used to make hoses and gaskets where oil resistance
is needed. Neoprene is made from 2-chlorobuta-1,3-diene (chloroprene):

Cl Cl
I SjOo*” as cat. - I
n CH,=CH-C=CH, -CH2-CH=C-CH2

Neoprene
2-Chlorobuta-l,3-diene is made from buta-1,3-diene:

—HCl
CH2=CH—CH=CH2 ^ CH2=CH—CHCl—CH2CI
CH,=CH—CC1=CH,

Polymerisation of 2-methylbuta-1,3-diene with a Ziegler catalyst gives a

polymer which has very similar properties to natural rubber. The catalyst
(triethylaluminium and titanium(IV) chloride) produces a regular molecu¬
lar pattern similar to that of natural rubber, the distance being 810 pm
between each recurring unit (p. 338).
Competition between natural rubber and man-made poly(2-methylbuta-
1.3- diene) (polyisoprene) is one of economics. In recent years, rubber trees
have been bred which give a very high yield of latex, while 2-methylbuta-
1.3- diene is still expensive to make. 2-Methylbuta-1,3-diene at present is
generally manufactured by the dehydrogenation of 2-methylbutane, which
is present in naphtha (p. 315), or 2-methylbut-1 -ene and 2-methylbut-2-ene,
which are formed during the cracking of naphtha (p. 318). For example:

CH 3
AI2O3 + Cr203 CH 3

as cat.
CH3—CH2—C=CH2 CH2=CH—C=CH2 + H2
600°C ^

Silicones are polymers with alternating silicon and oxygen atoms and alkyl
21.5 or aryl groups attached to the silicon atoms:
Silicones
R R

—Si—O—Si—O—

R R

This —Si—O— framework gives the polymers stability towards heat, as in

silica, while the nature of the organic groups determines other properties
such as solubility in organic solvents, water-repellency and flexibility. For
example, phenyl groups give more flexible polymers than alkyl groups.
Silicones are manufactured from chlorosilanes, e.g. R SiCl and RSiCl ,
2 2 3

where R is an alkyl or aryl group. The chlorosilanes are made in a variety

of ways. For example, if chloromethane is passed through heated silicon,
with copper as catalyst, a volatile mixture of chlorosilanes distils over,
which can be purified by fractionation. For example:

Cu as cat. ___ ^
Si + 2CH3C1 (CH3)2SiCl2

339
POLYMERS The dichlorosilanes are hydrolysed to form dialkylsilanediols, R Si(OH) ,
2 2

which polymerise spontaneously to form long linear chains. Cross-linking

can be effected at a later stage by addition of an organic peroxide, to yield
silicone rubbers.
Silicones can be sub-divided into three classes: (a) silicone fluids, (b)
silicone rubbers, (c) silicone resins. Their physical form depends on the
structure of the polymer.
(a) Silicone fluids are long-chain polymers of the alkylsilanediol, which
are very stable. They have the structure:

R R R
I I I
HO-Si- -O-Si- -O-Si-OH
I I I
R R 'I
R
or

R R R
I I I
R-Si- -O-Si- O-Si-R
I I I
R R R

Those with short chains are oils, which have a more or less constant viscosity
over a wide temperature range (—50 to 4-200°C). They also have very low
vapour pressures. Thus, fluids with phenyl groups attached to the silicon
atom are used as oils for vacuum pumps. Fillers are added to silicone fluids
to form heavy greases used when changes in viscosity or a high vapour
pressure are undesirable.
The fluids are also used in polishes (a mixture of wax and a silicone fluid
dissolved in an organic solvent), in paints and for water-proofing fabrics,
paper and leather. They also have anti-foaming properties and have been
used sometimes to suppress the foaming of detergents in sewage disposal
plants, although the fluids are very expensive.
(b) Silicone rubbers are made by introducing some cross-linking into long
chain linear polymers. Thus the structure is somewhat similar to natural
rubber. The cross-linking is effected by addition of catalysts (e.g. di(benzoyl)
peroxide).
Although their strength at normal temperatures is inferior to that of
natural rubber, silicone rubbers are more stable at low temperatures
(-70°C) and at high temperatures (200-300°C) and are generally more
resistant to chemical attack. They are thus used for specialised purposes
when ordinary rubber would be useless.
(c) Silicone resins have a three-dimensional structure similar to that of
silica:

R R
O O I I
I I -O-Si-O-Si-O-
-Si-O-Si-O-
I 1 R
o o O
R
I I
-Si-O-Si-O-
1 I -O-Si-O-Si-O-
o o I I
o o
I I
Silica Silicone resin

340
POLYMERS
Note that the structure is three-dimensional and that the atoms are tetra-
hedrally arranged about the silicon atoms. The resins are usually applied
as a solution in an organic solvent, and are used as an electrical insulating
varnish or for application to surfaces where water repellency is desired.
They are also used to give an ‘anti-stick’ surface to materials coming into
contact with ‘sticky’ materials such as dough and other foodstulfs.
Thus the word ‘silicones’ is a general term used for long-chain polymers
(fluids), two-dimensional structures (rubbers) and three-dimensional
macromolecules (resins). Silicones have several distinctive and valuable
properties: (i) constancy of physical properties over a wide temperature
range, (ii) water-repellency, (iii) electrical insulation, (iv) ‘anti-stick’, (v)
‘anti-foam’.

21.6 1. Preparation of poly(phenylethene)

Practical work Mix thoroughly 5 cm® of phenylethene and O'3 g of di(dodecanoyl) peroxide
in a test-tube. Place the test-tube in a beaker of boiling water for about 15
minutes, and then cool.
The solid formed, poly(phenylethene), can be dissolved in methylbenzene
and then precipitated from this solution by adding ethanol.

2. Preparation of a polyacrylic ester

Fill a test-tube about 1/3 full of small Perspex chips and heat them, collecting
the distillate, which is methyl 2-methylpropenoate, in a test-tube surrounded
by cold water (Fig. 21.2).
FIG.21.2. Preparation of methyl
2-methylpropenoate from
Perspex

To 5 cm^ of the monomer in a test-tube, add about OT g of di(dodecanoyl)

peroxide and shake the mixture until the catalyst has dissolved. Stopper the
tube and place it in a beaker of water. Bring the water to the boil (take care,
as the stopper may be expelled with considerable force). The liquid will
polymerise after about 10 minutes.
The monomer may be discoloured (owing to oxidation during its prepara¬
tion).

3. Preparation of nylon-6.10
In a 250 cm^ beaker, dissolve 1-5 cm^ of decanedioyl chloride in 50 cm^ of
tetrachloromethane. In a second beaker make up a solution of 2-2 g of

341
POLYMERS 1,6-diaminohexane and 4 g of sodium carbonate in 50 cm^ of water.
Add the aqueous solution of 1,6-diaminohexane from a pipette, with a
pipette bulb, making sure that there is a minimum of mixing of the two
layers.
Then, using a wire loop, draw a thread of nylon from the liquid interface.
A glass rod can also be used, with the advantage that the nylon fibre can
be wrapped around it as the fibre is drawn from the interface.

4. Preparation of nylon-6.6
Nylon-6.6 can be prepared with hexanedioyl chloride in place of decanedioyl
chloride.

5. Properties of nylon-6
Warm, very gently over a small bunsen flame, about 1 g of nylon-6 pellets
in a test-tube. Draw some fibres from the melt, using a thin wire.
(a) Test the fibres for strength and elasticity by pulling them.
(b) Examine a fibre using a polaroid, before and after stretching.

6. Preparation of a phenol-methanal resin

To 10 cm^ of formalin in an ‘old’ boiling-tube (which can be thrown away
after being used), add about 4 g of phenol. Then add about 1 cm^ of con¬
centrated sulphuric acid dropwise, with stirring. Keep the mixture for two
or three days, when the condensation polymerisation shduld have taken
place and the solid resin formed.

7. Preparation of a carbamide-methanal resin

Place about 0-5 g of carbamide in a test-tube, and add about 1 cm^ of con¬
centrated \iy dr ochlor'ic acid and 5 cm^ of water. Add 1 cm^ of formalin and
shake the mixture. Allow the mixture to stand and a white powder is
deposited. This is a thermosetting plastic.

8. Preparation of an epoxy resin

To 10 cm^ of Epikote 815 in a small tin, add, with stirring, about 1-5 cm^
of bis-(2-aminoethyl)amine. After about 30 minutes, a hard mass of epoxy
resin is formed. Observe whether heat is evolved during the reaction.
Bis-{2-aminoethyl)amine has a harmful vapour and safety glasses should be
worn.

9. Preparation of a polyurethane foam

To about 5 cm^ of a triol (Caradol GXR 13) in a small tin or old beaker,
add 10 cm3 Qf ^ di-isocyanate (Caradate 30). Mix the two liquids together
with a spatula for about 15 seconds. Clean the spatula at once. The foam is
expanded by the carbon dioxide produced during the reaction, because
some water, already added to the triol by the manufacturer, reacts with
some of the isocyanate groups.

10. Preparation and properties of a silicone

Put a few drops of dichlorodimethylsilane (CARE) in a shallow dish.

342
POLYMERS
Allow the vapour of the chlorosilane (CARE) to come into contact with
a filter paper, by placing the paper on top of the dish for a few minutes.
Although the paper may appear to be dry, there is a considerable number
of layers of water molecules adsorbed on to the surface. These react with
the dichlorodimethylsilane vapour to form diols, which then polymerise
to yield a silicone.
Then pour water gently on to the treated paper, and contrast its water-
repelling properties with the properties of an untreated filter paper.

21.7 ‘Technology of Plastics’. Penguin Technology Survey 1967. N. Denton, ed. A.

Garratt. Penguin Books Ltd.
Further reading

The Polyolefins (F) Shell Film Library.

21.8 Catalysis (V) I.C.I.
Film and Videotapes Polymers (V) I.C.I.

21.9 1 Give an account of the preparation, structure, properties and uses of high
polymers. (0(S))
Questions
2 Explain using suitable examples the principles of addition and condensation
polymerisation.
What is the effect of chain-length and cross-linking on the physical properties
of polymers? (L(X,S))

3 The structure drawn below is that of one end of a molecule of a compound, the
total molecular weight of which is of the order of 10‘*.
H H H
1 1 1
H,N-CH-C-N-CH-C-N-CH-C-N-CHo-C—
1 II 1 II 1 II II
(HaO. O CH O CHo O O
1 / \ 1
H3N CH3 CH3

(a) Name two classes of compound to which the large molecule could belong.
(b) If the compound were hydrolysed by boiling with hydrochloric acid, four
smaller molecules would be formed from that part of the structure shown
above. Draw the structural formula of each of these molecules.
(c) To what general class of compound do these smaller molecules belong?
(L(Nuffield))

4 Give (a) two general methods for increasing the length of a carbon chain and
(b) two general methods for diminishing the length of a carbon chain. Illustrate
by means of specific examples, with accompanying formulae, names and reagents.
How is poly(ethene) made? Give two reasons for its widespread use as a
container material. Show by reference to the respective chemical structures how
poly(ethene) differs from the polyester Terylene. (JMB)
5 Name (a) one commercial plastic formed by addition polymerisation, (b) one
commercial plastic formed by condensation polymerisation, (c) one naturally
occurring polymer. Give the monomers from which these three polymers are
formed and give an account of the new chemical bonds formed in polymerisation
and of the conditions necessary for the formation of the polymers named in (a)
and (b).
What features of molecular structure should be present in polymers which are
required to be (i) elastic, (ii) rigid ? (C(T))

343
POLYMERS 6 Explain the meaning of (i) addition polymerisation, (ii) condensation polymerisa¬
tion. Give one large-scale use of each of these processes. What is the effect of
‘cross-linking’ on the physical properties of a polymer?
Write the structural formulae for two different polymers you might expect to
be formed from glycine, (H2N. CH2. CO. OH). (C(T))

7 What is meant by a ‘condensation reaction’ ? Illustrate your answer with examples

drawn from the chemistry of aldehydes and ketones. Explain why the condensa¬
tion reaction between HO. OC(CH2)4CO. OH and H2N. (CH2)6. NH2 is of
commercial importance. Comment on the relative merits of various methods
which you could employ to determine the molecular weight of the product of
this reaction. (W(S))

8 (a) Define what is meant by each of the following terms:

(i) a random copolymer,
(ii) an alternating copolymer,
(iii) a block copolymer.
(b) (i) Define what is meant by the glass transition temperature, Tg.
(ii) State three structural factors which determine this temperature.
(iii) Is Tg for poly(ethanediyl benzene-1, 4-dicarboxylate) (Terylene) higher
or lower than that for nylon 6?
(iv) Give a reason for your answer in (iii).
(v) How do you account for the fact that pleats in a polyester (Terylene)
garment are not removed during washing of the garment?
(JMB Syllabus A)

9 (a) Give the structural formula of hexanedioic {adipic) acid.

(b) Give the structural formula of 1,6-diaminohexane.
(c) What feature of the molecules in (a) and (b) enable them to be used to make
polymeric material?
(d) Indicate by means of equations how the compounds in (a) and (b) interact to
form a polymer.
(e) What is the name of the chemical group formed in (d)?
(f) Give the structural formula of the repeating unit of this polymer.
(g) What is the common name of this polymer?
(h) What type of polymerisation process did you show in (d)?
(j) How is the material made in (d) converted into a fibre?
(k) What determines the strength of a fibre?
(l) Which particular physical property of the fibre in (j) makes it especially suitable
for use in ropes and ladies’ stockings?
(JMB Syllabus A)

10 (a) What are the chemical structures of (i) nylon 6, (ii) polyester (Terylene) and
(iii) poly(propene)?
(b) In what major chemical respect does silk or wool differ from nylon 66?
(c) Why does wool absorb moisture more readily than nylon?
(d) Describe briefly what is meant by melt-spinning.
(e) What is the effect of the process known as drawing?
(JMB Syllabus A)

11 (a) What do you understand by the term thermoset as used to describe polymers?
(b) Describe very briefly the experimental conditions for the production of (i)
phenol-methanal (phenol-formaldehyde) resins and (ii) urea-methanal (urea-
formaldehyde) resins.
(c) State one major use for each of the resins in (b).
(d) In what two respects is urea-methanal superior to phenol-methanal?
(e) How is carbamide (urea) made on the industrial scale?
(JMB Syllabus A)

344
Chapter 22
Looking to the future

Organic chemistry is a constantly evolving subject. As we remarked in the

Preface, industrial methods for the production of organic compounds have
changed profoundly since we prepared the first edition of this book some
ten years ago; moreover, the basic principles of the subject have also
developed significantly. What may happen in the next decade? We shall
draw attention to some likely lines of development.
One is biotechnology, which has been practised for centuries in the pro¬
duction, for example, of cheese, beer and wines, and which is now taking
off in new directions. It is organic chemistry that is effected by the enzymes
in micro-organisms instead of by the more usual reagents and catalysts
of the laboratory. Enzymes invariably act far more rapidly than laboratory
catalysts, and they are highly specific in the reactions they bring about.
A recent example is the manufacture of protein mainly from methanol
and ammonia, with small amounts of other materials such as sodium and
potassium ions. The bacterium that effects this conversion reproduces with
such rapidity that a few dm^ of its culture yield 40 tonnes of protein in
36 hours. The product is marketed as “Pruteen”, and is just as nutritious
as the more familiar protein from animals since it contains all the necessary
amino-acids.
Genetic engineering is a form of biotechnology of immense potential. It
is based on using specific hydrolytic (bond-breaking) and condensation
(bond-making) methods, usually involving enzymes, to insert a section of
a DNA molecule into an organism that does not normally possess it. In
this way, a gene is introduced that causes the organism to synthesise a type
of protein unnatural to it. For example, one development is the introduc¬
tion of the gene that codes for human insulin into a bacterium: the bacteria
reproduce rapidly to provide a supply of human insulin which can augment
the pig and cow insulin used in the treatment of diabetes, and indeed may
prove to have advantages over them.
Secondly, new speciality polymers will be developed. One—“super¬
glue”—is already well established; it is based on a disubstituted alkene
monomer which polymerises when it is exposed to the air to give a polymer
with very strong adhesive qualities. Others will be developed for medical
prostheses and implants to replace parts of the cardiovascular system, for
which they have the advantages of easy fabrication, high strength per unit
mass and chemical inertness, so that there is minimum response from
neighbouring tissues. Still other polymers will probably be obtained, in
the longer term, which have catalytic properties similar to those of en¬
zymes, so that fast and efficient production methods can be developed; one
has only to recall the high pressures and high temperatures required for
many of the processes in Chapter 20 to realise the cost advantage of
employing a catalyst which could operate at much lower pressures and
temperatures—ideally under normal atmospheric conditions.
Thirdly, methods will be explored for making petrol, or its equivalent,
from sources other than oil. One is already well developed and is operated
in South Africa: the gasification of coal brought about with air and steam.

345
The process can be considered in terms of these simple equations.

C + 02^C02
C + H O —> CO + H
2 2

C + CO ^ 2CO
2

The second and third reactions are endothermic, and the heat to bring them
about is provided by the (exothermic) first reaction. The mixture of carbon
monoxide and hydrogen—synthesis gas is then converted, with a zeolite
catalyst, into a mixture of alkanes and aromatics which is suitable as petrol.
Other approaches for the production of fuel already include the fermen¬
tation of sugar or starch to give ethanol. This is being successfully used in
Brazil, where cane and cassava are the crops employed; the principal fuels
for cars there are either ethanol or conventional petrol to which ethanol
is added. This innovation will gather momentum in other countries.
The digestion of cellulose to give methane is another way in which
chemists will rely on the use of natural products to generate petrol and
other important industrial chemicals.
It is, incidentally, notable that these processes correspond to converting
biomass into energy, whereas the Pruteen plant essentially brings about the
opposite, since the methanol and ammonia required as feedstocks are
manufactured by energy-consuming methods. This reflects the flexibility
of organic chemistry, an aspect that gives the industry based on it an ad¬
vantage over many other industries; one can choose either to build large
molecules up from small ones or degrade large molecules to smaller ones.
Which one chooses to do depends on factors such as location (Brazil is
ideal for the rapid growth of biomass. South Africa is rich in coal and so on),
and the prevailing needs, and so economics, of the time.
It is worth noting the central place taken by synthesis gas in this discus¬
sion. We saw earlier that it is formed from various fractions of oil and from
natural gas (Chapter 20). In this tailpiece, we have described how it can also
be formed from coal and from biomass. When oil and natural gas become
more expensive as supplies dwindle, the chemical industry, for many of
its large-bulk materials, will switch to these other sources.
Fourthly, new and often complex organic compounds will be synthesised
or obtained from natural sources which could have important biological
properties, as drugs, herbicides and so on. Particular attention will be given
to tailor-making compounds for specific uses, for example, developing
methods for insect control to which only one chosen species of insect is
susceptible.
Finally, there are bound to be significant developments in elucidating
how organic reactions occur. In this book, we have often described the
present theory of how a particular reaction is thought to occur—its
mechanism—but there has been no space to give the detailed evidence.
Nonetheless, this is an important branch of organic chemistry and one in
which scientists have applied considerable ingenuity.
One particularly fascinating line related to this is opening up: the attempt
to understand how molecules vital to life were first synthesised. In this
pre-life stage, the earth’s atmosphere contained mainly methane, ammonia
and water, with small amounts of phosphine and hydrogen sulphide.
Various sources of energy were available that could have induced reactions
between them: ultraviolet light, electrical discharges, radioactive emana¬
tions from the crust of the earth and heat from volcanoes. Laboratory
experiments have shown that, when a mixture of methane, ammonia and

346
water vapour is subjected to an electrical discharge, several of the bio¬
logically important amino-acids are produced, and these observations have
provided a clue and a starting point. Now attempts are in progress to find
how other small molecules may have been made, and to find, too, how
these combined to form the large biomolecules like proteins on which life
depends.
It has sometimes been said that the researches of the ‘pure’ chemist in
areas such as reaction mechanism and synthesis have no relevance to the
important applications of chemistry. Nothing could be further from the
truth. For example, if we can understand how a particular compound acts
as a pain-killing drug, we may be in a better position to design improved
drugs for we should know what the key functional groups and their rela¬
tionships are. When Kipping, in Britain in the 1930s, began his studies of
organic compounds of silicon, preparing as many as he could, seemingly
for the sake of it, who could have predicted the wide industrial applications
his work was to have? Some of his compounds turned out to yield polymers
with valuable properties, and so laid the basis for the manufacture of
silicones.
Whether the predictions we have made in our tailpiece prove to be right
or wrong, we have no doubt that the next decades will provide develop¬
ments in organic chemistry that are both fascinating and important.

347
Appendix I
Summary of industrial
processes

Charts 1-4 summarise the uses of four major organic starting materials:
methane, ethene, propene and benzene.
CHART 1 Uses of methane

Silicones

349
CHART 2 Uses of ethene

350
CHART 3 Uses of propene

ethanoate
Cellulose
Perspex

351
CHART 4 Uses of benzene

Nylon

Antiseptics
Herbicides

X
z
K
U

352
Appendix II
Questions

1 Suggest the identity of compounds A, B and C from the following data, and
explain the reactions which are described.
/I is a crystalline solid. When a little water is added to A two liquid layers are
formed; addition of more water gives a homogeneous aqueous solution. This
liquid
(i) has no reaction with sodium carbonate solution but indicators show that
it has a pH value of less than 7;
(ii) gives a purple coloration on the addition of a few drops of iron(IIl)
chloride solution;
(iii) gives a white precipitate on the addition of bromine water.
is a white crystalline solid which, on heating with an excess of soda-lime,
evolves a gas producing an alkaline solution in water. An aqueous solution of B
(i) deposits a white crystalline precipitate on the addition of concentrated
nitric acid;
(ii) evolves carbon dioxide and nitrogen on the addition of sodium nitrite
solution followed by dilute hydrochloric acid;
(iii) evolves nitrogen on the addition of alkaline sodium hypochlorite solution.
C is a gas which dissolves in water to give a strongly alkaline solution. This
solution, when neutralised by hydrochloric acid, gives a white crystalline com¬
pound on evaporation.
When C is burnt it produces twice its own volume of carbon dioxide, the gas
volumes being measured at the same temperature and pressure. (C(T))

2 Identify the following compounds, explaining your reasoning and writing equa¬
tions where possible for the reactions which are described.
(a) A colourless solid. A, dissolves in water to give a neutral solution. When
heated with aqueous sodium hydroxide an alkaline gas is expelled and the
residual solution effervesces when it is treated with an excess of a dilute
acid. A white crystalline precipitate is formed when an aqueous solution
of A is treated with concentrated nitric acid.
(b) A liquid B is miscible with water in all proportions to give solutions with
a pH of less than 7. When B is warmed with concentrated sulphuric acid
a gas is given off which burns with a blue flame. B reduces hot alkaline
permanganate and ammoniacal silver nitrate.
(c) A colourless liquid C, boiling at 184°C, is sparingly soluble in warm water
to which it gives a feebly alkaline reaction. When it is treated with sodium
nitrite, in the presence of an excess of dilute hydrochloric acid at 5°C, it
yields a solution which reacts with an alkaline solution of phenol to give
an orange yellow precipitate.
(d) Z) is a blue solid which gives off an inflammable vapour and leaves a bright
metallic residue when it is heated in a test tube. It dissolves in water to
give a light blue solution which (i) turns to a deep blue colour with an
excess of ammonia, and (ii) gives a reddish brown colour with a few drops
of iron(III) chloride solution. When D is heated with concentrated sulphuric
acid a sharp smell is produced, but if alcohol is also present a sweet fruity
odour results.
(e) £■ is a white solid which is almost insoluble in cold water but dissolves
quite readily in hot water to give an acidic solution. When it is heated
with soda-lime it gives off a vapour which burns with a luminous and smoky
flame and is 39 times denser than hydrogen under the same conditions. (O)

353
QUESTIONS 3 Identify the organic compounds described below and where possible write
equations to represent the reactions which occur.
(a) A colourless solid, /I, has a characteristic smell. Its aqueous solution is
very feebly acidic and gives a white precipitate with bromine water and a
violet coloration with aqueous iron(III) chloride. It reacts with phosphorus
pentachloride to give a derivative with a vapour density of 56-3.
(b) A white solid, B, has a high melting point and dissolves in water to give
a neutral solution. It forms salts with acids and bases and yields a gas when
heated with soda-lime which is alkaline to litmus paper but, unlike
ammonia, burns in air.
(c) A colourless oil, C, has a very characteristic smell. On exposure to the
air it slowly forms a colourless solid which dissolves very sparingly in cold
water to give an acidic solution. C reduces ammoniacal silver nitrate but
not Fehling’s solution, and reacts with phenylhydrazine to give a crystalline
derivative.
(d) A colourless fuming liquid, D, reacts violently with water to give two acids
in equal molar proportions. Its vapour density is 39.
(c) A white solid, E, dissolves in water to give a solution which is acidic and
which decolorises a hot acidified solution of potassium permanganate.
When heated with concentrated sulphuric acid it is completely decomposed
without blackening and yields a mixture of gases, one of which turns lime
water milky and one of which burns with a blue flame. (O)

4 Suggest, with reasons, the identity of the organic compounds A, B and C.

(i) A colourless liquid hydrocarbon A has no reaction with bromine in the
dark until iron powder is added, when a colourless gas, fuming in moist
air, is evolved.
When A is heated under reflux with alkaline potassium permanganate,
and then an excess of sulphur dioxide is bubbled through the mixture,
colourless crystals separate on cooling.
(ii) A colourless liquid B reacts violently with water to give a strongly acidic
solution. A portion of this solution on treatment with dilute nitric acid
and silver nitrate solution gives a white precipitate. A further portion,
after neutralisation with ammonia, reacts with ‘neutral’ iron(III) chloride
(ferric chloride) solution to give a deep red brown solution. This solution
gives a brown precipitate when boiled.
1 00 g of 5 was allowed to react with an excess of water. The resulting
solution required 21 -6 cm^ of 1M (IN) sodium hydroxide for an end point
with phenolphthalein indicator.
(iii) A colourless crystalline compound C gives an acidic solution in water.
This solution gives a white precipitate on treatment with calcium chloride
solution.
When treated with hot concentrated sulphuric acid, C reacts to evolve
a gas which burns with a blue flame together with a gas which gives a
white precipitate with lime water. (C(T))

5 An optically inactive acid A, CsHgOs, on being heated lost CO2 to give an acid
B, C4H8O3, capable of being resblved.
On action of sulphuric acid, B gave an acid C whose ethyl ester gave D on
the action of hydrogen and platinum.
D, with cone, ammonia gave E, C4H9ON, which with bromine and potassium
hydroxide solution gave F, C3H9N. Fwith nitrous acid gave G.
G on mild oxidation gave H. Both G and H gave the iodoform reaction.
Elucidate the reaction scheme and suggest a synthesis of C.

6 Explain the following observations, and identify all the compounds mentioned.
(a) Three isomeric compounds of formula (C3H9N) react differently with
nitrous acid.

354
QUESTIONS
(b) Compound J (C4H10O) gives a mixture of three compounds K, L, M (C4Hg)
when passed over alumina at 300°C. Compound N (C4H10O) gives only
one product P (C4H8) under the same conditions. Comment on the types
of isomerism shown by compounds K, L, M and P. (C(N, S))

7 A compound A contains C, 66-4 per cent; H, 5-5 per cent; and Cl, 28-1 per cent.
Show how these figures are used to derive the empirical formula C7H7CI.
When A is treated with aqueous potassium hydroxide it is converted into a
hydroxy compound B. Mild oxidation of B gives a compound which yields a
white precipitate with hydroxylamine (H2N. OH). Further oxidation of B gives
a white crystalline solid C which liberates carbon dioxide from aqueous sodium
carbonate. Benzene is obtained on heating C with soda-lime.
From these data deduce the structure of A and by means of equations trace
the course of the above reactions.
How may A be synthesised from the parent hydrocarbon ? (JMB)

8 Suggest a possible structural formula for each of the compounds Aio D inclusive
and explain the reactions involved, giving equations where possible:
A, molecular formula C3H6O2, gives an effervescence with sodium hydrogen
carbonate solution and reacts with phosphorus pentachloride to give a
compound which contains 38-4 per cent by weight of chlorine.
B, empirical formula CH2Br, on refluxing with aqueous sodium hydroxide
gives a compound which reacts with sodium to give hydrogen, one mole
of the compound giving one mole of hydrogen.
C, molecular formula C4H10O, is readily oxidised to give a compound C4H8O
which can be further oxidised to give a compound C4H8O2.
D, molecular formula C7H7CI, on refluxing with aqueous sodium hydroxide
and subsequent mild oxidation gives a compound C7H6O, which readily
undergoes further oxidation to give a compound C7H6O2. (L(X))

9 Account for the following observations, and identify the compounds G-S.
(a) The chlorine in compound G (C7H7CI) is readily displaced by treatment
with dilute aqueous sodium hydroxide. The chlorine in the isomeric
compound H is unaffected by this treatment.
(b) Compound J (C4H10O) gives a mixture of three compounds K, L, M
(C4H8) when passed over alumina at 300°C. Compound N (C4H10O) gives
only one product P (C4H8) under the same conditions. Comment on the
types of isomerism shown by the compounds K, L, M and P.
(c) If compound Q is treated first with bromine and then with ethanoyl
chloride, compound R (CsHgBrsNO) is obtained. If Q is treated first with
ethanoyl chloride and then with bromine, compound S (CsHsBrNO) is the
product. (C(T, S))

10 Give the structures of A, B and C in the following:

(i) Compound A, C7H8O, which is soluble in aqueous sodium hydroxide, is
no more soluble in aqueous sodium carbonate than it is in water. If forms
C7H50Br3 with bromine.
(ii) Compound B, C6H4Br2, which is unaffected by boiling with aqueous
sodium hydroxide, gives two isomers of formula C6H3Br2N02 with a
mixture of concentrated nitric and sulphuric acids.
(iii) Compound C, C8H6O4, dissolves in aqueous sodium carbonate but not
in cold water. When heated, the compound is converted into C8H4O3.
(O Schol.)

11 A compound A, CnHioBrNO, which is sparingly soluble in cold water, dissolved

on boiling with concentrated hydrochloric acid. When cooled the resulting
solution deposited a solid B, C7H6O2, which displaced carbon dioxide from
sodium carbonate solution. Basification of the solution which remained yielded
a solid C, which contained C, 41-9 per cent; H, 3-5 per cent; Br, 46-5 per cent

355
QUESTIONS and N, 81 per cent. Treatment of an ice-cold solution of C in hydrobromic acid
with sodium nitrite followed by copper® bromide gives compound D. The
reaction of D with fuming nitric acid and concentrated sulphuric acid gives only
one compound E.
Suggest possible structures for the compounds A, B, C, D and E. (O Schol.)

12 Treatment of 2-bromobutane (CH3.CHBr.CH2.CH3) with hot, alcoholic

potassium hydroxide gives a mixture of three isomeric butenes, C, D, and E.
Reaction with trioxygen and then water of the minor product, C, gives methanal
and another aldehyde in equimolar amounts. Both D and E give the same single
product, F, with trioxygen and then water.
Write down the structures of the compounds C, D, and E. Describe the type
of isomerism shown by D and E, and explain how it arises.
What reactions would you carry out to identify F?
Draw clear, structural diagrams to represent the two molecular species obtained
when either D ox E h treated with hydrogen chloride. How would these species
differ in properties ? What difficulties would be encountered in their separation ?
(C(T,S))

13 How, by a chemical test, would you distinguish

(i) between CH . C H . NH and CsHj. CH . NH ;
3 6 4 2 2 2

(ii) between CH . C H . OH and CeHs. CH . OH;

3 6 4 2

(iii) between CH . C H . Cl and CeHs. CH . Cl;

3 6 4 2

(iv) between CeHe (benzene) and C H (hexene)?

6 12

In each case, indicate which member of the pair is identified and give the
reaction, if any, of the other compound with the reagent used. (C(T))

14 Describe two chemical tests which you would carry out in each case to distinguish
between the following pairs of substances:
(a) ethene and ethyne;
(b) methylamine and ammonia;
(c) ethanal and propanone;
(d) ammonium ethanoate and ethanamide;
(e) methanoic acid and ethanoic acid.
Wherever possible give equations for the reactions. (O)

15 Describe, with equations, a chemical test you would employ to distinguish

between members of each of the following pairs of compounds:
(a) ethyl ethanoate and ethyl benzoate;
(b) chlorobenzene and (chloromethyl)benzene;
(c) ammonium ethanoate and ethanamide;
(d) ethanoyl chloride and chloroethanoic acid;
(e) propanone and pentan-3-one. (W)

16 (a) Give one chemical test in each case to distinguish

(i) between benzoic acid and phenol;
(ii) between benzaldehyde and phenylethanone;
(iii) between nitrobenzene and phenylamine.
(b) Outline the essential practical steps that you would take to obtain pure
samples of both substances from a mixture of phenol and phenylamine. (C(T))

17 For each of the following pairs of compounds describe one simple chemical test
that would distinguish between its members. State exactly what you would do
and what you would expect to see and write equations for all the reactions you
describe.

356
QUESTIONS (a) Propanone and methanol;
(b) phenol and benzoic acid;
(c) carbamide and ammonium ethanoate;
(d) hexane (CH3.CH2.CH2.CH2.CH2.CH3) and benzene;
(e) propene (CH .CH=CH ) and propyne (CH C=CH);
3 2 3

(f) stearin (a fat) and casein (a protein). (JMB)

18 Describe one chemical test in each case to distinguish:

(a) between methanoic acid and ethanedioic acid;
(b) between phenol and benzenesulphonic acid;
(c) between propanone and ethanal;
(d) between methylbenzene and benzene;
(e) between trichloromethane and 1,2-dichIoroethane. (C(N))

19 Describe how you would distinguish between the following pairs of isomers by
two simple chemical tests in each case. Give equations for the reactions involved.
(a) CH =CH.CH=CH andCH .CH .C^H;
2 2 3 2

(b) CH . CH . CH . OH and (CH ) CH. OH;

3 2 2 3 2

(c) -CH . C H . NH and CsHs. CH . NH ;

0 3 6 4 2 2 2

(d) HO.CH .CO.NH andNH .CH .COOH.

2 2 2 2 (C(S))

20 With the aid of a named example in each case, outline, giving conditions and
reagents, how you would bring about the following change of functional group
in aliphatic compounds:
(a) —CH Hto—CO.OH;
20

(b) —CO.Clto—CHO;
(c) —NH to—OH; 2

(d) —CN to—CO.OH;

(e) —CO.OHto—CN.
Name the product formed in each case. (W)

21 Describe briefly how you would carry out the following conversions, giving
essential conditions for the reactions but no details of the apparatus:
(a) CH3I C2HSNH2;
(b) CfiHsCHs ^ CsHsCHO;
(c) CsHe C H NH ; 6 5 2

(d) CH3COOH ^ CH3NH2. (O)

22 Give conditions and equations to show how the following conversions may be
made:
(a) ethylamine to ethanol;
(b) ethanoyl chloride to ethanal;
(c) ethanamide to methylamine;
(d) ethanonitrile to ethanoic acid;
(e) ethanoic acid to aminoethanoic acid. (W)

23 Indicate the steps by which you would carry out the following conversions:
(a) CtoCHjCOOH;
(b) either D O to CD 2 4

or CH3COOH to CH2NH2COOH;
(c) (C H to HCOOH;
00 )2

(d) C6HsN02 to CsHfi. (0(S))

357
QUESTIONS 24 Indicate the steps by which you would bring about four of the following
conversions:
(a) CHjCOOHtoCjHsNHz;
(b) CsHfitoCfiHsNH.NHj;
(c) CH3OHtoCH3.CO.CH3;
(d) C2H5OH to CH3. CHOH. COOH;
(e) C2H5OH to H2N. CH2. CH2. CH2. CH2. NH2.
Give equations for the reactions which take place. (0(S))

25 Write down one reaction scheme for each of the following conversions, indicating
reagents and conditions for each step:
(a) ethyne ^ ethanoyl chloride;
(b) phenylamine phenyl benzoate;
(c) ethanol -> chloroethanoic acid;
(d) benzene phenylmethanol
(e) propan-2-ol ^ 2-aminopropane. (O and C(S))

26 Write down one reaction scheme for each of the following conversions:
(a) ethyne ^ ethanoic acid;
(b) benzene —> 2-nitrophenol;
(c) ethanal ^ ethylamine;
(d) diethyl propanedioate ^ propanonitrile;
(e) nitrobenzene benzoic acid. (O and C(S))

27 Write formulae to show the structures of the products and comment briefly on
the reactions between the substances mentioned for five of the following cases:
(a) ethanol and sulphuric acid;
(b) ethanol and sodium hypochlorite;
(c) benzene and fuming sulphuric acid;
(d) propanone and sodium hydrogen sulphite;
(e) phenylamine, hydrochloric acid and sodium nitrite at 0°C;
(f) phenol and ethanoic anhydride. (O Schol.)

28 How, and under what conditions, does sodium hydroxide react with
(i) 1,2-dibromoethane;
(ii) ethanal;
(iii) benzaldehyde;
(iv) carbamide;
(v) ethyl ethanoate?
What chemical test will distinguish between ethanamide and carbamide ? (C(T))

29 How, and under what conditions, does sodium hydroxide react with the following
compounds:
(a) chlorobenzene;
(b) chloroethanoic acid;
(c) carbamide;
(d) tristearin (a fat);
(e) sodium 2-hydroxypropane-2-sulphonate

(O and C)

358
QUESTIONS
30 Describe, with the aid of an illustrative example in each case together with any
necessary conditions, one use of each of the following reagents in organic
chemistry:
(a) phosphorus pentachloride;
(b) phosphorus pentoxide;
(c) trioxygen;
(d) lithium aluminium hydride;
(e) hydrogen. (W)

31 Give an account of the uses in organic chemistry of:

(a) sodium;
(b) silver nitrate;
(c) benzoyl chloride;
(d) copper(II) sulphate;
(e) phosphorus pentachloride;
(f) nitric acid. (C Entrance)

32 Discuss the use of the following in the synthesis of organic compounds:

(a) sulphuric acid;
(b) nitric acid;
(c) sodium;
(d) aluminium chloride. (0(S))

33 How and under what conditions does sulphuric acid react with the following
compounds:
(a) propanone;
(b) propene;
(c) phenylamine;
(d) 2-hydroxypropanoic acid;
(e) A^-phenylethanamide? (O and C(S))

34 How, and under what conditions, does sulphuric acid react with:
(i) ethanamide;
(ii) phenol;
(iii) ethanedioic acid;
(iv) ethanol;
(v) phenylamine? (C(T))

35 Describe the reactions which can take place between sulphuric acid and each of
the following substances; ethyne, ethanol, ethanal and benzene.
Give the conditions under which the reactions take place. (O)

36 What is the action of reducing agents on the following compounds:

(a) ethyne;
(b) ethanoyl chloride;
(c) benzenediazonium chloride;
(d) phenol;
(e) bromoethane?
State the reducing agents and the conditions necessary in each case.
(O and C(S))

37 By means of specific examples, illustrate the uses of the following as oxidising

agents in organic chemistry: potassium permanganate, potassium dichromate,
trioxygen, ammoniacal silver nitrate.

359
QUESTIONS How would you convert compound (a), below, into compounds (b) and (c) ?
(a) CH3.CH0H.CH2.CH2.CH=CH2
(b) CH3.CO.CH2.CH2.CO2H
(c) H0.C0.CH2.CH2.CH=CH2. (OSchol.)

38 By means of equations and brief notes on conditions of temperature and

concentration, indicate the various ways in which sulphuric acid can react with
the following compounds:
(a) ethanol;
(b) ethene;
(c) benzene;
(d) glycine;
(e) carbamide.
How and under what conditions does sodium hydroxide react with (i) the
products from ethene and (ii) the products from benzene? (JMB)

39 Explain concisely the meaning of each of the following in organic chemistry,

illustrating your answer with one example in each case:
(a) ethanoylation;
(b) unsaturation;
(c) nitration;
(d) polymerisation;
(e) sulphonation. (AEB)

40 Explain, with the aid of appropriate reactions and necessary conditions in each
case, the significance of the following terms: dehydration, decarboxylation,
alkylation, acylation, condensation. (W)
41 Explain and illustrate with an appropriate example, each of the following terms:
(i) alkane;
(ii) alkene;
(iii) ethanoylation;
(iv) saponification;
(v) polymerisation;
(vi) Cannizzaro reaction. (AEB)

42 Explain carefully the following terms illustrating each by one example of your
own choice:
(a) photochemical chlorination;
(b) reaction with trioxygen;
(c) hydrogenation;
(d) ‘cracking’ of alkanes;
(e) saponification.
Describe very briefly the commercial application of any three of these processes.
. (JMB)
43 Describe, with the necessary conditions, the hydrolysis of five named compounds
each selected from a different homologous series. (W)

44 Explain, and illustrate with one example in each case, the meaning of five of
the following: unsaturation, polymerisation, homologous series, saponification,
nitration, ethanoylation. (O)
45 Indicate, giving one example in each case, what you understand by the following
terms: nucleophile, electrophile, free radical, homolysis, and heterolysis.
Give the mechanisms of THREE of the following reactions:
(a) 1-chlorobutane with aqueous sodium hydroxide;

360
QUESTIONS (b) benzene with a mixture of concentrated nitric and sulphuric acids;
(c) methane with chlorine;
(d) 2-iodo-2-methylpropane with aqueous sodium hydroxide. (L)

46 ‘The carbonyl group, ^C=0, modifies the properties of the group to which it
is attached.’ Comment on this statement by comparing:
(a) ethanoic acid with ethanol in (i) the degree to which ionisation occurs in
water, (ii) the reaction with phosphorus pentachloride;
(b) the action of cold dilute acids on ethanamide and ethylamine;
(c) the action of cold water on ethanoyl chloride and chloroethane.
Show how the properties of the carbonyl group are modified by comparing
the behaviour of propanone and ethanoic acid towards 2,4-dinitrophenyl-
hydrazine, and interpret the result in terms of electronic theory. (SUJB)

47 Carbon compounds are often classified according to the functional groups they
contain. Examples of such groups are given below:
(i) >C=C< (v)

(ii) -c=c- (vi) -C-O-C-

I
(iii) -CHj-OH (vii) -C-NHj

(iv) >C=0

(a) Name the class of compound you associate with each functional group.
(b) Give one named example from each class and write its structural formula.
(c) Give one typical reaction for each of the seven compounds you have
chosen in part (b).
Write an equation for each reaction. (L(X))

48 In what types of organic compounds do the following functional groups occur:

(ii) —COCl, (ii) —NHj, (iii) —COOH, (iv) —O—, (v) —CONH2 ?
Describe two reactions characteristic of each group. (AEB)

49 For each of the following pairs of compounds give:

(a) one reaction in which the specified group behaves similarly in both;
(b) two reactions in which the specified group behaves differently in each.
(i) the ^C=0 group in CH3. CH2. CHO and
CH3.CH2.CO.CH3;
(ii) the —NH2 group in CH3. CH2. CH2. NH2 and
C6H5.CO.NH2;
(ii) the —Br atom in CH3. CH2. Br and CeHs. Br.
Give brief practical details of chemical tests for distinguishing between any
two pairs of the components in (i), (ii), and (iii).
(The results of negative tests must be made clear.) (SUJB)

50 ‘A benzene ring influences the chemical behaviour of functional groups attached

to it.’ Illustrate and discuss with suitable examples. (O Schol.)

361
51 (a) For each of parts (i) to (x) below ONLY ONE of the alternatives A, B, C, D,
E is correct. Answer each part by giving the appropriate letter.
A CH3.CH2.NH2 B CH3.CO.NH2 C C6H5NO2
D C6H5.NH2 E C6H5N2CI
(i)
Which is a strong electrolyte?
(ii)
Which dissolves in dilute hydrochloric acid, but not in water?
(iii)
Which is insoluble in water, acid and alkali?
Which is a colourless liquid at 20°C when pure?
(iv)
Which has the highest vapour pressure at 15°C?
(v)
(vi)
Which is explosive when pure?
(vii)
Which yields a product with one less carbon atom when treated with
bromine and potassium hydroxide solution?
(viii) Which is the most ready to combine with a proton?
(ix) Which gives ammonia on warming with aqueous sodium hydroxide
solution?
(x) Which evolves nitrogen on treatment with nitrous acid at 5°C?
(b) How and under what conditions, does ethanamide (acetamide) react with
(i) bromine and sodium hydroxide
(ii) dilute hydrochloric acid? (SUJB)

52 For each of the following, give the name and structural formula of one organic
compound which contains the type of linkage mentioned:
(a) a triple bond between carbon and carbon;
(b) a triple bond between carbon and nitrogen;
(c) a triple bond between nitrogen and nitrogen;
(d) a double bond between carbon and oxygen;
(e) a double bond between carbon and nitrogen;
(f) a double bond between nitrogen and oxygen.
Specify the reagents and conditions necessary to reduce the substances you
mention giving the appropriate equations and naming the products. {Note. The
reagents should be different in each case.)
Describe one other characteristic reaction of each of the compounds with triple
bonds. (JMB(S))

53 A compound X is shown to have the following structure

OHC. CH2. CH=CH. CH2. CONH2
How would you expect X to behave towards the following reagents under varying
conditions:
(a) sodium hydroxide;
(b) phosphorus pentachloride;
(c) potassium permanganate;
(d) bromine;
(e) sodium and ethanol ? (L)

54 From your knowledge of the reactions of particular groups predict the reactions
of the compound

H0-(^(^^CH=CH-CH2-0H

with:
(a) bromine water;
(b) hydrogen and finely divided nickel;
(c) sodium hydroxide;
(d) ethanoic acid;
(e) dilute, alkaline potassium permanganate. (C(T))

362
QUESTIONS
55 Name the organic compounds produced, write their structural formulae, and
state the conditions under which they are formed when:
(a) phenol, ethanamide, and ethyne are each treated with bromine;
(b) phenylamine, ethanol and propene are each treated with hydrogen bromide.
(W)

56 (a) Monochlorination of benzene in the presence of iron(III) chloride (ferric

chloride), and treatment of the product with a mixture of concentrated nitric and
sulphuric acids gives two isomers of formula C6H4CINO2. Write structures for
these isomers, and suggest a method by which a third isomer could be prepared
from benzene.
(b) A compound is believed to have structure X.

By what chemical reactions would you show the presence of:

(i) the benzene ring;
(ii) the amino group;
(iii) the ester group ?
By what reactions would you distinguish between isomers X, Y and Z?

NH2
I

(C(T, S))

57 This question involves practical procedures in the chemistry of carbon com¬

pounds. Where you have direct experience describe the practical details as fully
as you can, otherwise give an account of the reagents and conditions which you
think would be appropriate.
(a) Suppose you were given a substance reputed to have the following formula;

CH-CH-COCHj

CHO

Describe how you would attempt to confirm its formula. You should
assume that the constituent elements and the molecular weight of the
compound are known.
(b) Describe how you would attempt to prepare a substance of the following
formula:

Assume that the only starting material containing the benzene ring which

363
QUESTIONS you are allowed is phenol. You are allowed to use any other material which
you would normally expect to find in a school laboratory. (L(Nuffield,S))

58 Using your knowledge of simpler substances, set out the differences which you
could expect to find in the properties and reactions of the compounds in the
following pairs of substances:

(b)(i) and (ii)

(c) CHa-CHO and CHg

Starting from methyl-2-nitrobenzene, outline a possible synthesis of amine, (b)

(ii), and show how this, in turn, could be converted into the hydroxy-derivative
(b) (i). (JMB(S))

59 An understanding of the ‘nature of the chemical bond’ allows one to explain

some of the properties of known compounds and make predictions about the
properties of unknown ones.
Discuss the chemical bonding in the following compounds and explain how
chemical properties of each depend on the types of bonds they contain;
CH4, CH2=CH—CH=CH2, (CH3)3N, CH3CHO.
What predictions would you make about the properties of cyclopropane,

CHg
/\
H2C-CH2 (CSchol.)

60 How, and under what conditions, do the following pairs of substances react:
(a) propene and hydrogen iodide;
(b) ethanol and sulphuric acid;
(c) nitrobenzene and nitric acid;
(d) chlorine and ethanoic acid ?
Explain the underlying chemical principles in (a) and (c).
What light does reaction (d) throw on the structure of ethanoic acid ? (AEB)

61 Give names and structural formulae for the isomers of compounds having
molecular formulae (a) C2H4Br2, (b) C4H9I.
State what happens when each of the isomers you name reacts with potassium
hydroxide, mentioning any essential conditions of reaction. In the cases of any
hydroxy compounds formed, what happens to these on oxidation ?
Outline how the isomers of C2H4Br2 may be prepared. (AEB(S))

62 Suggest reasons why

(i) ethers do not react with sodium, but dissolve in concentrated acids and
possess lower boiling points than the isomeric alcohols;

364
QUESTIONS
(ii) methanoic acid shows reducing and acidic properties, but ethanoic acid
shows only acidic properties;
(iii) the compounds represented by CH3.CHOH.COOH may or may not be
optically active;
(iv) nitrous acid gives an alcohol on reaction with a primary aliphatic amine
but not with a primary aromatic amine. (AEB(S))

63 Discuss the following statements:

(a) Benzoic acid is soluble in aqueous sodium hydrogen carbonate whereas
phenol is not.
(b) The carbon-oxygen bond lengths in dimethyl ether are T43 A; in propan-
one the carbon-oxygen bond length is 1 -24 A, and in carbon monoxide it
is M3 A.
(c) The addition of bromine to ethene is a very fast reaction, whereas the
addition of bromine to propenonitrile (CH2=CH—CN) proceeds slowly.
(d) Ethyne has a pKa of 26 and forms salts with sodium, copper and Grignard
reagents. Ethene does not form salts with these reagents. (C Schol).

64 Explain:
(a) The carbon-carbon bond lengths in the benzene nucleus are all identical.
(b) Nitric acid alone has no reaction with benzene but a reaction occurs if
concentrated sulphuric acid is present.
(c) Propanone, but not methyl ethanoate, forms a phenylhydrazone.
(d) Chloroethanoic acid is stronger than bromoethanoic acid and both are
stronger than ethanoic acid itself.
(e) Hydrogen bromide reacts with propene to give two isomeric derivatives
with one isomer in much the greater proportion. (S(S))

65 Give a classified account of the addition of simple molecules to unsaturated

linkages in organic compounds. Your account should include, if possible, any
suggested mechanisms to explain the additions. (L(S))

66 (a) Friedel—Crafts reactions involve a halogen carrier.

(i) Give two examples of such carriers and explain how they work.
(ii) Give three different types of reactions in which the halogen carrier is
used to bring about substitution in the benzene nucleus.
(iii) State the conditions required in order to obtain good quantitative yields
of product in Friedel—Crafts reactions.
(b) The halogenation of alkanes takes place in the presence of ultraviolet
radiation.
(i) What type of reaction is this?
(ii) What part does the radiation play?
(iii) What are the products of the chlorination of ethane using this process?
(c) Give the name or structure of the major organic product formed when 2-
chloropropane reacts with
(i) sodium hydroxide in alcoholic solution,
(ii) silver ethanoate in alcoholic solution,
(iii) dry silver oxide. (AEB(I))

67 Comment on four of the following observations:

(a) Treatment of tra«j-butenedioic acid with dilute potassium permanganate
solution gives (±)2,3-dihydroxybutanedioic acid.
(b) Bromobenzene is stable to dilute aqueous sodium hydroxide solution
whilst 1-bromobutane is hydrolysed.
(c) Reaction of 1-chloropropane with benzene and aluminium chloride gives
(l-methylethyl)benzene.

365
QUESTIONS (d) When a limited amount of hydrogen is passed into benzene containing a
nickel catalyst a mixture of cyclohexane and benzene is obtained, but no
intermediate products can be isolated.
(e) When propanone is dissolved in heavy water containing sodium carbonate
in solution hexadeuteropropanone is formed. (O Schol.)

68 Explain three of the following observations:

(a) On treatment with hot chromic acid ethanal and propanone both yield the
same compound which is soluble in sodium carbonate solution with the
evolution of carbon dioxide; but with cold chromic acid only ethanal
yields such a compound.
(b) Both ethanol and propanone yield trichloromethane when they are heated
with aqueous sodium hypochlorite.
(c) A compound, BrCH=CHBr, on being heated yields another compound
of the same molecular formula.
(d) Optically inactive 2-hydroxypropanoic acid reacts with a laevorotatory
base to give a mixture of two salts which can be separated by fractional
crystallisation. (O Schol.)

69 Comment on three of the following:

(a) The values for propanoic, ethanoic, and fluoroethanoic acids are 4-88,
4-80 and 2-66, respectively.
(b) When treated with deuterium oxide containing a trace of sodium hydroxide,
pentan-3-one incorporates four atoms of deuterium per molecule.
(c) Optically active butan-2-ol (CH3.CH2.CHOH.CH3) retains its optical
activity indefinitely in aqueous solution, but racemises rapidly in aqueous
sulphuric acid. The alcohol undergoes no chemical change in aqueous
sodium bromide solution, but is rapidly converted into 2-bromobutane
upon addition of sulphuric acid to the solution.
(d) In addition of bromine to alkenes, the following order of reactivity is
observed:
CH2 = CH2 < CH3 .CH = CH2 < (CH3)2C = CH2.
(e) The heats of combustion of cyclopropane, cyclobutane, cyclopentane, and
cyclohexane are 500, 656, 793-5, and 944 kcal/mole, respectively.
(O Schol.)

70 Comment on four of the following observations:

(a) Phenylamine dissolves in dilute hydrochloric acid solution to a much
greater extent than it dissolves in water.
(b) While benzaldehyde reacts smoothly with boiling sodium hydroxide
solution to give phenylmethanol, C6H5CH2OH, and sodium benzoate,
ethanal is polymerised by cold sodium hydroxide solution.
(c) Phenol dissolves in sodium hydroxide solution but not in sodium carbonate
solution.
(d) Benzene tends to undergo substitution reactions (with bromine for example)
rather than addition reactions.
(e) Phenol is brominated much more rapidly than benzene. (C Schol.)

71 Write balanced equations for the reactions which occur, name the organic products
formed, and state what would be observed when:
(a) benzoyl chloride is added slowly to cooled concentrated aqueous ammonia,
and the product is isolated and distilled with phosphorus pentoxide;
(b) benzene is added slowly to a mixture of fuming nitric acid and concentrated
sulphuric acid, and the resulting mixture is poured into water;
(c) propan-2-ol is mixed with a little potassium iodide solution and a solution
of sodium hypochlorite is added;

366
QUESTIONS (d) ethanamide is treated with bromine, followed by dilute potassium hydroxide
solution, and the resulting mixture is run into hot concentrated potassium
hydroxide solution;
(e) benzaldehyde is shaken with concentrated potassium hydroxide solution
and, after standing overnight, water is added. The mixture is then extracted
with ether and the aqueous solution is acidified. (JMB)

72 You are given unlabelled bottles of the following substances: propan-2-ol (iso¬
propanol),.methanoic acid, benzoyl chloride, methanal, phenylamine, ethanoni-
trile.
In each case give a single positive chemical test (six texts in all) which would
enable the bottle to be labelled correctly. (O and C)

73 For each of the following cases give the structural formula of a compound which
fulfils the stated conditions:
(a) an aldehyde, CsHioO, which shows optical activity;
(b) a compound, C6H5NO3, which is volatile in steam;
(c) a hydrocarbon, C4H6, which gives a silver derivative;
(d) an amine, C3H9N, which cannot be ethanoylated;
(e) a dibasic acid which on heating gives a compound, C5H10O2, which could
show optical activity;
(f) an aliphatic aldehyde, C5H10O, which is not polymerised by alkali;
(g) an aldehyde, CgHsO, which polymerises with alkali. (O and C(S))

74 Explain what* impurities are likely to be present in crude samples of iodoethane,

benzoic acid, ethyl ethanoate, and phenylamine prepared in the laboratory.
Describe, in each case, how such impurities may be removed and what criterion
of purity you would employ for the final product.
A yield of 13-04 g of phenylamine was obtained from 2T30 g of nitrobenzene.
Calculate the percentage of the theoretical yield for the reaction. (W)

75 Crude A^-phenylethanamide is purified by washing successively with water,

aqueous sodium hydrogen carbonate, dilute hydrochloric acid and water.
Explain the purpose of each wash.
Why is it incorrect to attempt to dry (a) ethanol with calcium chloride, (b)
ethanoic acid with potassium carbonate, (c) iodoethane with sodium ? What alter¬
native procedure would you adopt to dry ethanol, ethanoic acid and iodoethane?
What principle underlies the use of a mixed melting point as a criterion of
purity? (W)

76 (a) Mention, with brief comment, one example of each of the following:
(i) A pure compound which cannot be adequately represented by a single
structural formula.
(ii) A reversible isomeric change.
(iii) A reaction which is catalysed by acids and by bases.
(b) Predict the outcome of the following experiments:
(i) 2-Methylpropene (Me2C=CH2) is treated with concentrated sulphuric
acid and then with water.
(ii) A mixture of 1 mole of ethyl benzoate and 1 mole of ethyl 2,4,6-trimethyl-
benzoate is heated with water containing 1 mole of potassium hydroxide.
(iii) Phenyl ethyl ether is treated with concentrated hydriodic acid. (C Schol.)

77 Explain five of the following observations:

(a) Glycine, C2H5O2N, is soluble in water and insoluble in ether.
(b) Ethanal and methanal react differently with aqueous sodium hydroxide.
(c) From a mixture of ethene and ethyne, ethene may easily be separated.

367
QUESTIONS (d) A ketone, C5H10O, is known which does not give the iodoform reaction.
(e) The reaction between iodoethane and silver cyanide yields two isomeric
compounds.
(f) A compound, C2H3CIO, reacts violently with ethanol, evolving an acidic
gas. (O and C)

78 Explain:
(a) The function of sulphuric acid in the nitration of benzene.
(b) The ease with which phenol is nitrated compared with benzene.
(c) The increase in acid strength as the hydrogen atoms of the methyl group
in ethanoic acid are successively replaced by chlorine atoms.
(d) The low yield of ethanal when a mixture of calcium methanoate and
ethanoate is heated.
(e) The reluctance of bromine to add on to ethene if the walls of the yessel
containing the gases are covered with inert wax. (SUJB(S))

79 Suggest explanations for the following observations:

(a) The apparent molecular weight of ethanoic acid calculated from physical
measurements was found to be 55 using a solution of the acid in water
and 120 using a solution of the same concentration in benzene.
(b) A solution of 2-hydroxypropanoic acid isolated from a natural source
rotated polarised light to the right, whereas a solution of 2-hydroxypropanoic
acid synthesised from ethanal did not rotate polarised light.
(c) Phenylethene, C6H5.CH:CH2, changed into a hard, transparent solid on
standing in air. (C(S))

80 Describe briefly how you would obtain one constituent in a pure condition from
each of the following mixtures (a chemical method is required in each case):
(a) phenol and benzenesulphonic acid;
(b) ethanol and propanone;
(c) ethanoic acid and methanoic acid;
(d) phenylamine and chlorobenzene;
(e) benzene and methylbenzene. (C(N))

81 By using simple laboratory procedures indicate how you would obtain a pure
specimen of the compound with the first structure from each of the following
mixtures:

(a) CH3.CH2.CH2.CH3 CH3.CH2.CH=CH2 CH3.C=C.CH3

(b) OH COOH

(c) CH3-CO-CH3 CHg-CHa-OH

Physical methods alone will not be accepted and all chemical reactions must be
fully explained. (SUJB(S))

368
82 Describe and explain how you would obtain a sample of the first named substance
from each of the following mixtures:
(a) ethanol and water;
(b) phenol and ethanoic acid;
(c) ethane and ethene;
(d) chloroethane and ethylamine. (C(N))

83 Give one example in each case of a reaction involving

(a) homolytic fission of a C—H bond;
(b) heterolytic fission of a C—halogen bond;
(c) formation of a C—C bond.
Suggest a possible mechanism for each reaction that you mention.
(O Schol.)

84 Write short notes on the types of reactions which establish new carbon-carbon
bonds. (C Schol.)

85 (i) In two of the following cases, write formulae to illustrate reactions in which
(a) a new carbon-carbon bond is made;
(b) a new carbon-nitrogen bond is made;
(c) an existing carbon-oxygen bond is broken.
[Give two or three examples in each case, with the names of the substances, the
reagents and the essential conditions. No further account is required.]
(ii) Show which bonds in the following molecule could be broken (a) by
oxidation, and (b) by hydrolysis. Write the formulae of the products.

CH=CH-C0-NH-CH3

Cl
(O Schol.)

86 Give a well-ordered account of methods of forming carbon-carbon bonds in

organic chemistry.
What would you expect the yield of propane to be in the Wurtz reaction be¬
tween sodium and an equimolar mixture of iodomethane and iodoethane?
(O Schol.)

87 Suggest syntheses for five of the following compounds, starting from readily
available chemicals containing not more than 3 carbon atoms for aliphatic
compounds, or 6 carbon atoms for aromatic compounds:

CHgCHaCHCHaCHs

OH

CH2
/\
H2C-CH2

CH3(CH2)2CH20CH2(CH2)2CH3 (C Schol.)

88 Answer three of the following. On an examination paper, a student offered the

suggestions given below for carrying out a series of conversions of organic
compounds. Point out the errors made by this student, and where the method was
not correct, offer suggestions by which the desired conversion could be carried

369
QUESTIONS out;

NaCl (i) Mg, ether

(a) CH3CH2OH -^ CH3CH2CI CH3CH2CHO;
(ij) hCHO

NaOH CH3OH
(b) CH3CH2C(0)CH3 » CH3CH2C(0)0H
NaOH^
CH3CH2C(0)0CH3;
H2O NaOH
(c) CH CH=CH -> CH3CH(0H)CH3 ->
3 2

CH I
CH3CH(ONa)CH3 -^ CH2CH(OCH3)CH3;

C(0)0H

(C Schol.)

89 Write feasible schemes (formulae and reagents, and conditions where relevant)
for four of the following transformations:

(a) CH3.CH2.OH ^ HC=CH;

(b) CfiHs ^ CsHsCN;
(c) CH3.CO.CH3 CH3.CH(C02H).CH3;

(e) CH2=CH2 ^ CH3.CO.OCH2.CH2.OCO.CH3;

(f) NH3 H2N.CO.NH.CO.NH2. (O Schol.)

90 In each of these questions a statement is followed by five alternative responses.

Only one of these alternatives is correct. Indicate your choice of A, B, C, D or E.
(i) Which one of the following is a polyester?
A 1,2-diaminohexane; B poly(chloroethene);
C Terylene; D Nylon 6.6;
E poly(phenylethene).
(ii) Which one of the following statements is necessarily an inaccurate
description of the homolytic fission of a chemical bond?
A One of the atoms leaves with both of the electrons forming the bond.
B The fission is promoted by ultraviolet light.
C The fission gives atoms or radicals.
D The fission is assisted by heat.
E The fission can occur in either the gaseous or the liquid phase.
(iii) When equal volumes of ethanoyl chloride and ethanol are mixed and the
mixture is then poured into excess cold dilute alkali, which one of the
following statements best describes the change which takes place?
A A white precipitate is thrown down.
B A vinegar-like smell is discernible.
C A pleasant fruity smell is discernible.
D A pungent smell of hydrogen chloride is discernible.
E A vigorous effervescence ensues.

370
QUESTIONS
(iv) Which one of the following groups of products is formed when methane
and chlorine are mixed in the dark ?
A chloromethane and hydrogen chloride;
B chloromethane, dichloromethane and hydrogen chloride;
C trichloromethane, tetrachloromethane and hydrogen chloride;
D no products are formed;
E hydrogen chloride and a deposit of carbon.

(v) Which one of the compounds having the following structural formulae
can be resolved into optical isomers ?
A NH2.CH2COOH;
B NH2.CH(CH3).C00H;
C CH3.CH=CH.C2H5;
D H00C.CH2.CH(CH3).CH2.C00H;
E H0.CH2.CH2.C(CH3)2.CH2.NH2.
(vi) Which of the following reagents does not react with ethanal ?
A NH3; B CN-; C CH3.CO2C2H5;
D BH4“; E acidified Mn04“.

(vii) The compound CH3—CH—CO2H was produced by oxidation. Which

I
CH3
one of the following compounds was the most likely starting material ?
A butan-l-ol;
B 2-methylpropan-l-ol;
C butan-2-ol;
D 2-methylpropan-2-ol;
E propan-2-ol. (JMB (Specimen paper))

91 For each of the following reactions choose from the list A-E the most appropriate
reaction type.
A nucleophilic addition; B electrophilic addition;
C nucleophilic substitution; D electrophilic substitution;
E redox
AlBrj
CfiHfi + Br2 -> CsHjBr
Fehling’s
CfiHs.CHO -% CfiHs.COOH
solution
CH2=CH2 + HBr -^ CH3—CH2Br
CfiHs. CHO + NaHSOs -^ CeHs. CH(OH). S03Na

The following formulae represent the structure of five organic compounds.

H H H H H H
I I I I I I
H-C-C-C-H H-C-C-C-OH
I I I I I I
H H H H H H

A B

H H
I I O
CH,-C=C-C QH5NH2 CHo-CHo-C-Cl
"OH
O
D E

371
questions By using the appropriate letter for the compounds answer the questions which
follow and give the equation' where indicated.
(a) Which compound will form a salt with mineral acids ?
(b) Which compound will react with an organic acid in the presence of a
mineral acid to form an ester ?
(c) Which compound will decolorise both bromine water and dilute acidified
potassium permanganate solution ?
(d) Which compound would form an aldehyde on oxidation ?
(e) Which compound would you expect to react vigorously with water ?
(JMB (Specimen paper))

92 (a) Give the structural formula for each of the following compounds. (The
formulae should be abbreviated to the type: CH3. CHCl. CH2. CH3).
A sodium hexanoate
B 3-methylpenta-2,4-diene ^
C 4-bromo-2-methylpentan-l-ol
D 2-methylpentane-l,4-diol
(b) Which one of the compounds would be the most likely starting point for
the development of a new polymer ?
(c) Which one of the compounds would be most likely to leave a solid residue
when it is strongly heated in air?
(d) State briefly how C could be converted into D (the isolation of the product
is not required).
(e) (i) Which one of the compounds is most likely to be a solid at room
temperature?
(ii) State briefly the reasons for your answer.
(f) If you were given unlabelled samples of B, C and D, state briefly how you
would quickly distinguish between them using the least number of tests and
observations. (L(Nuffield))

93 Given samples of D2O and CD3OH as the only deuterium-containing compounds,

but having available any other organic or inorganic compounds, how would you
prepare

CD3CO2H, CH3CO2D, CH3CHDCH2D, CH3CHBrCH2D,

CD3CH2OH?
(O and C(S))

94 Compare and contrast the reaction of benzene with concentrated sulphuric acid
at 80°C with that of ethene and concentrated sulphuric acid at the same tem¬
perature.
How can the formation of a new carbon-sulphur bond in one case, but not in
the other, be explained in mechanistic terms, and to what extent do the results
justify the inclusion of benzene in a class of hydrocarbons separate from the
alkenes?
Dimethyl sulphate, CH3O.SO2.OCH3, an ester of sulphuric acid, is a methyla¬
ting agent. It reacts with sodium etHoxide to give methoxyethane. On the other hand
the ester methyl ethanoate does not give methoxyethane on reaction with sodium
ethoxide. Both of the esters are hydrolysed by aqueous alkali to methanol. How
can these results be explained? 0(S)

372
Appendix III
Apparatus and chemicals

Apparatus with ground-glass joints is described throughout this book. In this

way, preparations and reactions can be studied more rapidly than with older
apparatus.
Sensible precautions ensure a long life for the apparatus. The joints must be
clean and the flasks must be heated over a gauze, or in a water- or oil-bath, by
means of a small flame from a microburner or bunsen burner.
The same apparatus as described in Organic Chemistry through Experiment
(D. J. Waddington and H. S. Finlay, Bell and Hyman Ltd) is used, namely
that produced in conjunction with Philip Harris Ltd. The number of pieces has
been reduced to a minimum and the set consists of:

1. Stillhead;
2. Adaptor for thermometer, steam lead or air leak;
3. Stopper;
4 and 5. Adaptors for collection of gases;
6. Steam lead which can also be used as an air leak;
7. Dropping funnel, which can also be used as a separating funnel;
8. Water condenser, which can also be used as an air condenser;
9. Flask head;
10. 50 cm^ pear-shaped flask. A 100 cm^ round-bottomed flask may be
used instead;

An air condenser, fractionating column and an air leak (for vacuum distillation)
can also be obtained.
Test-tube preparations of gases and liquids are used throughout the book for
they can be done quickly, and many of the properties of compounds are studied
by experiments using test-tubes and other simple equipment. Standard sized
Pyrex test-tubes (150 x 16 mm) and delivery tubes made from soda-glass tubing
(4 mm i.d.) are convenient to use. Dropping pipettes may be made from soda-glass
tubing (6 mm i.d.) and using cylindrical rubber teats. These should be calibrated
in 0'5 cm^ portions with a small dab of paint, the calibration saving much time
during experiments.
A list of organic and inorganic reagents is given in this Appendix and a list
of suppliers in Appendix IV.

373
APPARATUS AND
CHEMICALS
Organic reagents
Alkylbenzene (Appendix IV) Ethanoyl chloride
Ethyl benzoate

Benzaldehyde
Formalin, 40 per cent solution ot
Benzoic acid
Benzoyl chloride methanal in water
Bis-(2-aminoethyl)amine Fructose
(diethylene triamine) Fuchsin
(Appendix IV)
Bromobenzene Glucose
1- Bromobutane Glycine
2- Bromobutane
2-Bromo-2-methylpropane Hexane (or pentane)
Butan-l-ol Hexanedioyl chloride (adipyl
Butenedioic anhydride (maleic chloride) (Appendix IV)
anhydride)
1- Iodobutane
Caradate 30 (Appendix IV)
Caradol C2 (Appendix IV) Lard (or olive oil)
Carbamide (urea) Light petroleum, b.p. 60-80°C
Chlorobenzene
2-Chlorobenzoic acid Malachite green
1- Chlorobutane Methanoic acid (formic acid)
2- Chlorobutane Methanol
(Chloromethyl)benzene (benzyl Methylamine, 33 per cent solution
chloride) in water
Cotton wool Methylammonium chloride
Cyclohexane Methylbenzene (toluene)
Cyclohexanol 2- Methylbutan-2-ol
Cyclohexene Methylene blue
2-Methylpropan-2-ol
Decanedioyl chloride (sebacoyl
chloride) (Appendix IV) Naphthalen-2-ol (/S-naphthol)
1,6-Diaminohexane Ninhydrin
Dichlorodimethylsilane Nitrobenzene
(Appendix IV) Nitroethane (Appendix IV)
1,1 -Dichloroethane 2-Nitrophenol
1,2-Dichloroethane 4-Nitrophenol
Di(dodecanoyl) peroxide (lauroyl Nylon-6, pellets (Appendix IV)
peroxide) (Appendix IV)
Diethyl ether Oil, crude
2,4-Dinitrophenylhydrazine. 1 g in Olive oil (or lard)
50 cm^ of methanol to which
2 cm^ of concentrated sulphuric
Paraffin oil
acid is added. Filter if necessary
Dodecanol (Appendix IV) Pentane (or hexane)
Pentan-3-one (diethyl ketone)
Pent-l-ene (Appendix IV)
Epikote 815 (Appendix IV) Perspex, chips
Ethanal (acetaldehyde) Phenol
Ethanamide (acetamide) Phenylamine (aniline)
Ethanedioic acid (oxalic acid) Phenylethanone (acetophenone)
Ethane-1,2-diol (ethylene glycol) Phenylethene (styrene)
Ethanoic acid (acetic acid) (Appendix IV)
Ethanoic anhydride (acetic Phenylethyne (phenylacetylene)
anhydride) (Appendix IV)
Ethanol Phenylhydrazine
Ethanenitrile (acetonitrile) Phenylmethanol (benzyl alcohol)

374
APPARATUS AND
CHEMICALS
(Phenylmethyl)amine Sodium methanoate (sodium
(benzylamine) formate)
Proline Sodium potassium
Propane-1,2,3-triol (glycerol) 2,3-dihydroxybutanedioate
Propan-2-ol (sodium potassium tartrate)
Propanone (acetone) Starch, soluble
Sucrose
SchifF’s reagent. 01 per cent
solution of fuchsin in water T etrachloromethane
through which sulphur dioxide Trichloromethane (chloroform)
is passed until the solution is T riphenylchloromethane
colourless (Appendix IV)
Sodium ethanedioate (sodium Tris-(2-hydroxyethyl)amine
oxalate) (triethanolamine)
Sodium ethanoate (sodium acetate),
anhydrous Urease (Appendix IV)

Inorganic reagents
Alumina, for chromatography Magnesium, turnings for Grignard
Aluminium, powder reactions
Aluminium chloride, anhydrous Magnesium sulphate, anhydrous
Ammonia solution, (a) concentrated, Mercury(II) chloride
(b) 2M solution
Nitric acid, (a) concentrated, (b) 2M
Bleaching powder
solution
Bromine, (a) liquid, (b) saturated
aqueous solution
Phosphoric acid, concentrated
Calcium dicarbide Phosphorus pentachloride
Calcium chloride, anhydrous Platinum, wire (about 28 s.w.g.)
Calcium hydroxide, saturated solution Potassium bromide
(lime-water) Potassium carbonate, anhydrous
Chlorosulphonic acid Potassium dichromate
Copper(II) carbonate Potassium hydroxide, pellets
Copper (I) chloride Potassium manganate(VIl)
CopperdI) oxide
Copper(II) sulphate, anhydrous Rocksil

Disodium pentacyanonitrosylferrate- Silica gel (Appendix IV)

(III) (sodium nitroprusside) Silver nitrate
Soda-lime
Fehling’s solution I. 7 g of copper(II)
Sodium
sulphate pentahydrate in 100 cm^ of
Sodium carbonate, anhydrous
water
Sodium chloride
Fehlihg’s solution II. 12 g of sodium Sodium dichromate
hydroxide and 5 g of sodium Sodium hydrogencarbonate
potassium 2,3-dihydroxybutane- Sodium hydroxide, (a) pellets, (b) 2M
dioate (tartrate) in 100 cm^ of water solution
Sodium metabisulphite
Hydrochloric acid, (a) concentrated, Sodium nitrate
(b) 2M solution Sodium nitrite
Sodium sulphate, anhydrous
Iodine, (a) solid, (b) 1 per cent
Sulphur dioxide. Syphon
solution in 20 per cent solution of
Sulphuric acid, (a) oleum, (b)
potassium iodide
concentrated, (c) M solution
Iron, filings
Iron(III) chloride
Tin, granulated
Iron(II) sulphate

Lithium tetrahydridoaluminate Zinc, dust

375
Appendix IV Suppliers of apparatus
and chemicals

Most of the chemicals and apparatus can be obtained from all laboratory suppliers.
If the following prove to be difficult to find, they may be obtained from the suppliers
below:

Supplier
Alkylbenzene 4,6
Bis-(2-aminoethyl)amine (diethylene triamine) 1
Caradate 30 5
or Polyurethane foam, polymer B 2
CaradolGXR13 5
or Polyurethane foam, polymer A 2
Decanedioyl chloride (sebacoyl chloride) 1, 2, 3
Decanedioyl chloride (sebacoyl chloride, 5% in
tetrachloromethane) 2,3
Dodecanol 1,3
Epikote 815 4
Hexanedioyl chloride (adipoyl chloride) 1, 2, 3
Hexanedioyl chloride (adipoyl chloride in ampoules.
5% in tetrachloromethane) 2, 3
Nitroethane 1,2
Nylon-6 pellets 1, 2, 3
Nylon tubing, for column chromatography 7
Phenylethene (styrene) 1, 2, 3
Phenylethyne (phenylacetylene) 1
Silica gel G for thin-layer chromatography 1, 2, 3
Triphenylchloromethane 1

Suppliers
(1) B.D.H. Chemicals Ltd., Poole, Dorset, BH12 4NN
(2) Griffin and George Ltd., Ealing Road, Alperton, Wembley, Middlesex, HAO
IHJ or Braeview Place, Nerston, East Kilbride, Glasgow G74 3XJ or Ledson
Road, Wythenshawe, Manchester, M23 9NP
(3) Philip Harris Ltd., Lynn Lane, Shenstone, Staffordshire, WS14 OEE
(4) Shell Chemicals U.K. Ltd., Public Relations Department, Downstream
Building, Shell Centre, York^Road, London SEl 7PG
(5) Strand Glassfibre Ltd., Brentway Trading Estate, Brentford, Middlesex
TW8 8ER
(6) Education Section, Unilever PLC, P.O. Box 68. Unilever House, London
EC4P 4BQ
(7) Walter Coles and Co. Ltd., Plastic Works, 47/49 Tanner Street, London
SEl3PL

376
Appendix V
Teaching aids and materiais

The following are the addresses of some of the major companies and organisations
producing teaching aids and materials, some of which have been specifically
mentioned in the text.

Education Liaison Officer

Room 414
British Gas Corporation
326 High Holborn
London WCIV 7PT
BP Educational Services
Britannic House
Moor Lane
London EC2Y 9BU
Education Department
Esso Petroleum Company Limited
Esso House, Victoria Street
London SWIE 5JW
Schools Liaison Officer
I.C.I. Educational Publications
P.O. Box 96
1 Hornchurch Close
Coventry CVl 2QZ
Shell Education Service
Shell U.K. Limited
PO Box 148, Strand
London WC2R ODX
Unilever Education Section
Unilever PLC
Unilever House, PO Box 68
London EC4P 4BQ
Schools Information Centre on the Chemical Industry
The Polytechnic of North London
Holloway Road
London N7 6DB
Public Relations Department
National Coal Board
Hobart House
Grosvenor Place
London SWIX 7AE
Information Officer
Institute of Petroleum
61 Cavendish Street
London WIM 8AR

Films (F) and videotapes (V) are suggested in the text. Those obtainable free are
marked with an asterisk (*).
British Petroleum: application to borrow BP films should be made to the BP
Film Library, 15 Beaconsfield Road, London NW10 2LE. Enquiries concerning the
purchase of films should be addressed to Films/TV Branch, Information Depart¬
ment, The British Petroleum Company Limited, Britannic House, Moor Lane,
London EC2Y 9BU.

377
 Educational Foundation for Visual Aids, National Audio-Visual Aids Library,
Paxton Place, Gipsy Road, London SE27 9SS.
Gas Council Film Library, 59 Bryanston Street, London WlA 2AZ.
Guild Organisation Ltd., Woodston House, Oundle Road, Woodston,
Peterborough PE2 9PZ.
Shell Film Library, 25 The Burroughs, Hendon, London NW4 4AT.
Unilever films are handled by the National Audio-Visual Aids Library (see above)
but special enquiries may be addressed to The Film Librarian, Unilever Films,
Unilever House, PO Box 68, London EC4P 4BQ. In Scotland, Unilever films are
obtainable from the Scottish Central Film Library, Glasgow G3 7XN.
ICI videotapes may be purchased from Argus Film and Video Library,
15 Beaconsfield Road, London NWIO 2LE.
Open University films may be bought from Open University Educational Enter¬
prises Ltd., 12 Cofferidge Close, Stony Stratford, Milton Keynes MKll IBY, or
hired from Guild Organisation Ltd. (see above). Open University videotapes may be
bought from Guild Organisation Ltd.
Appendix VI Physical constants

International relative atomic masses

= 12 000000)

Element Relative atomic mass Element Relative atomic mass

Aluminium 26-9815 Nitrogen 14-0067

Bromine 79-909 Oxygen 15-9994
Calcium 40-08 Phosphorus 30-9738
Carbon 12-01115 Platinum 195-09
Chlorine 35-453 Potassium 39-102
Copper 63-54 Silicon 28-086
Fluorine 18-9984 Silver 107-870
Hydrogen 1-00797 Sodium 22-9898
Iodine 126-9044 Sulphur 32-064
Iron 55-847 Tin 118-69
Lead 207-19 Zinc 65-37
Magnesium 24-312

Masses of isotopes relative to

1-0078246 35C1 34-9688531

14n 14-0030738 3"C1 36-9659034
160 15-9949141 ’’Br 78-91839
31p 30-973764 8iBr 80-91642
127J 126-90466
32S 31-9720727

Some aldehydes and ketones and the

2,4-dinitrophenylhydrazone derivatives
For Experiment 12, Section 12.7, p. 186.

B.p. (°C) of the M.p. (°C) of the

aldehyde or 2,4-Dinitro-
ketone phenylhydrazone

Methanal -21 166

21 168
Ethanal
49 155
Propanal
75 123
Butanal
2-Methylpropanal 64 187
104 98
Pentanal

Benzaldehyde 179 237

2-Hydroxybenzaldehyde 197 252
4- Methoxybenzaldehyde 248 254
4-Methylbenzaldehyde 204 233

379
PHYSICAL CONSTANTS Propanone 56 128
Butan-2-one 80 115
Pentan-2-one 102 144
Pentan-3-one 102 156
Heptan-4-one 144 75

Phenylethanone 202 250

4-Methylphenylethanone 224 258
Diphenylmethanone 306 238
(m.p. 49°C)

380
 Index
Page numbers in bold type refer to experimental work

Absolute ethanol, 146 nomenclature, 189 Alkanes,

ABS rubber, 338 physical properties, 190 nomenclature, 69
Acetaldehyde—see Ethanal preparations, 134, 190, 203-4 physical properties, 69-71
Acetamide—see Ethanamide reactions, 191-4, 204-7 reactions 72-6, 314-5
Acetanilide—see N-Phenylethanamide structure, 193-4 uses, 314-5
Acetic acid—see Ethanoic acid uses, 194 Alkenes,
Acetic anhydride—see Ethanoic salts of—see Salts manufacture of long chain, 316
anhydride sulphonic, 64, 105-6, 157 nomenclature, 79
Acetoacetic ester—see Ethyl Acrilan, 337 physical properties, 79
3-oxobutanoate Acrolein—see Propenal preparations, 88-9
Acetone—see Propanone Acrylonitrile—see Propenenitrile reactions, 80-8, 316-18
Acetonitrile—see Ethanenitrile Activation energy, 119-20 uses, 85, 88, 316-18
Acetophenone—see Phenylethanone Acylation, 216-17 Alkylation, 105, 307
Acetylation—see Ethanoylation Acyl chlorides—see Acid chlorides Alkyl bromides.
Acetyl bromide—see Ethanoyl bromide Acyl group, 211 nomenclature, 116-17
Acetyl chloride—see Ethanoyl chloride Acyl halides—see Acid halides physical properties, 116-17
Acetylene—see Ethyne Addition polymerisation, 327 preparations, 117-18,134-5
Acetyl iodide—see Ethanoyl iodide Addition reactions, reactions, 118-25,137
Acid amides, of aldehydes, 174-8 Alkyl chlorides,
physical properties, 221 of alkenes, 80-3, 90-1 manufacture, 118, 314
preparations, 214, 218, 220, 221 of alkynes, 95, 97-8 nomenclature, 116-17
reactions, 221-3, 228 of aromatic hydrocarbons, 106, 108 physical properties, 116-17
structure, 221-2 of ketones, 174-8 preparations, 117-18,135-6
Acid anhydrides, Adenine, 287-8 reactions, 118-25,137
physical properties, 219 Adipic acid—see Hexanedioic acid uses, 125, 339
preparations, 200, 218, 220 Adipyl chloride—see Hexanedioyl chloride Alkyl cyanides—see Acid nitriles
reactions, 220, 228, 266, 299 Adrenalin, 242 Alkyl fluorides, 131-2
uses, 212, 220, 297 a-Alanine, 258, 282 Alkyl halides,
Acid bromides, 216-19 /j-Alanine, 258 manufacture, 118, 314
Alcohol, absolute, 147 nomenclature, 116-17
Acid chlorides,
Alcohols, dihydric—see Diols physical properties, 116-17
physical properties, 216
monohydric, preparations, 117-18, 134-6
preparations, 216
classes, 143-4 reactions, 118-25, 137
reactions, 216-19, 227
distinguish between primary, uses, 125, 339
uses, 219
secondary, tertiary, 151 Alkyl iodides,
Acid halides,
hydrogen bonding, 144-5 nomenclature, 116-17
physical properties, 216
manufacture, 146-7, 316, 321-2 physical properties, 116-17
preparations, 216
nomenclature, 143-4 preparations, 118
reactions, 216-19, 227
physical properties, 144-5 reactions, 118-25, 137
uses, 219
preparations, 122, 133-4, 145-6 Alkyl isocyanides, 226
Acid isonitriles, 226
reactions, 147-51,160-2 Alkyl nitrites, 123
Acid nitriles,
uses, 151-2 Alkyl radical (group), 5
physical properties, 225
polyhydric, 152-5,162, 227 Alkynes,
preparations, 123, 223, 225, 261
Aldehydes, manufacture 73, 94
reactions, 225, 228
identification, 186 physical properties, 94
Acids, amino—see Amino-acids
manufacture, 173-4 preparations, 96
dicarboxylic,
nomenclature, 171 reactions, 95, 97-8
nomenclature, 195
physical properties, 171-2 uses, 96
preparations, 195-6
preparations, 172-3, 219 Allenes—see Propadienes
reactions, 153, 196-7
reactions 174-83,184-6 Allyl chloride—see 3-Chloropropene
monocarboxylic,
uses, 183 Alumina,
dimerisation, 190
Aldol—see 3-Hydroxybutanal for chromatography, 18, 20,
dissociation constants, 62-5, 191-2
Alicyclic compounds, 7, 76-7 29-30
distinguish from phenols, 192
Aliphatic compounds, 7 Aluminium lithium hydride—see
hydrogen bonding, 190
lithium tetrahydridoaluminate
manufacture, 191

381
 Benzal chloride—see (Dichloromethyl) Bitumen,
Aluminium oxide,
benzene manufacture, 304
for chromatography, 18, 20, 29-30
Amides—see Acid amides ’ Benzaldehyde, uses, 311
physical properties, 171-2 Biuret, 224
Amines,
preparations, 109, 172-3 Biuret test, 224, 228-9, 298
basic strength, 65
classification into primary, reactions, 174-83,184-6 Blasting gelatin, 155
secondary, tertiary, 251 Benzaldehyde oxime, Bond energy, 4, 54-5
isomerism of, 237 Bonding, 1-4, 50-62, 100-1
manufacture, 253, 320
nomenclature, 251 Benzaldehyde phenylhydrazone, 178 covalent, 1-4, 53
physical properties, 65,252 Benzanilide—see iV-Phenylbenzamide delocalised, 59-61,101
preparations, 122, 223, 225, 252-3, Benzene, electrovalent, 53
263-5 bonding, 59-61, 99-101 hydrogen—see Hydrogen bonding
reactions, 253-7, 265-6, 341-2 heat of hydrogenation, 100 ionic, 53
uses, 257, 320, 334, 336 manufacture, 101, 308, 318 localised, 59,101
Amino-acids, physical properties, 102 pi (tc), 58
essential, 282 reactions, 102-6 sigma (ct), 58
nomenclature, 258, 282 stabilisation energy, 101 tetrahedral, 1
physical properties, 258,267 structure, 59-61, 99-101 Bond length, 54
preparations, 258-9 uses, 156, 318-19 Bromination,
proteins, 281-5 Benzenediazonium chloride, of benzene, 104
reactions, 259-60, 267 preparation, 260-1, 266 of cyclohexene, 90-1
2-Aminobutane, 251 reactions, 261-2, 266-7 of ethene, 80, 82, 89
Aminoethane—see Ethylamine Benzenediazonium tetrafluoroborate, 261 of phenol, 159,162
Aminoethanoic acid—see Glycine Benzene-1,2-dicarboximide, 259 of phenylamine, 256, 266
Aminomethane—see Methylamine Benzene-1,2-dicarboxylic acid, 195 Bromine,
2- Aminopropanoic acid—see a-Alanine Benzene-l,4-dicarboxylic acid, 195, 319 qualitative analysis of, 46-7
3- Aminopropanoic acid—see ;8-Alanine Benzene-1,2-dicarboxylic anhydride, 319, quantitative analysis of, 35
Ammonia, 335 Bromobenzene,
manufacture, 323 Benzene hexachloride—see physical properties, 130
uses, 314, 317, 323, 345 Hexachlorocyclohexane preparations, 104, 129-30,136
Ammonium carbamate, 224, 323 Benzenesulphonic acid, reactions, 130-1,137, 203
Ammonium cyanate, 1 preparation, 105-6 1- Bromobutane,
Amylase, 296 reactions, 156 physical properties, 117
a-Amylose, 295 sodium salt of, 156 preparation, 134-5
^-Amylose, 296 uses, 156 reactions, 137
Analysis, Benzoic acid, 2- Bromobutane, 137
qualitative, 46-7 dissociation constant, 65, 192 1 -Bromo-1 -chloro-2,2,2-trifluoroethane,
quantitative, 33-5 manufacture, 191 132
Aniline—see Phenylamine physical properties, 190 Bromoethane,
Anisole—see Methyl phenyl ether preparations, 109,191, 203-4 NMR spectrum, 44
Anti-freeze, 154 reactions, 191-3, 207 physical properties, 117
Anti-knock, 308 Benzoin—see 2-Hydroxy-l,2- preparations, 82, 117-18,134
Araldite, 335 diphenylethanone reactions, 122-5
Aromatic halides, Benzonitrile, 261 Bromoethanoic acid,
manufacture, 130 Benzophenone—see Diphenylmethanone dissociation constant, 64
physical properties, 130 Benzotrichloride—see (Trichloromethyl) 2-Bromoethanol, 82
preparations, 129-30, 261 benzene Bromoform—see Tribromomethane
reactions, 130-1 Benzoylation, 217, 227-8 Bromomethane, 44, 117
uses, 131 Benzoyl chloride, 2-Bromo-2-methylpropane, 137
Aromatic hydrocarbons, 7-8, 99-109,113 physical properties, 216 Bromophenylamines, 257
Aryl compounds—see under Aromatic preparation, 216 /V-(Bromophenyl)ethanamides, 257
compounds (e.g. Aromatic halides) or reactions, 161, 217-19, 227-8 1- Bromopropane, 86-7
under the title of the homologous series Benzoyl group, 217 2- Bromopropane, 86
(e.g. Aldehydes) Benzyl alcohol—see Phenylmethanol Buchner funnel, 17
Aspartic acid, 258 Benzylamine—see (Phenylmethyl)amine Buta-1,3-diene, 338, 339
Asymmetric carbon atom, 241-3 Benzyl bromide—see (Bromomethyl) Butane,
Atomic orbitals, 50-2 benzene manufacture, 315
Auxochromes, 262 Benzyl chloride—see (Chloromethyl) physical properties, 69
Azines, 178 benzene preparation, 125
Azo compounds, 262-3, 266-7 Berzelius, 1 reactions, 306-7
Azo dyes, 262-3, 266-7 Bifunctional catalyst, 308 uses, 306-7, 315
Bimolecular reaction, 120 Butanedioic acid, 195, 197
Bakelite, 333, 342 Biomass, 324, 346 Butanedioic anhydride, 197
Barbiturates, 224 Biotechnology, 345 Butanoic acid, 189, 190
Barbituric acid, 224 Biphenyl-2,2'-disulphonic acid, Butan-l-ol, 29,31, 144,322
Base peak, 38 isomerisim of, 244 Butan-2-ol, 144
Beckmann rearrangement, 321 Bis-(2-hydroxyethyln ether, 318 Butanone, 171

382
But-2-enal, 179 Cellulose xanthate, 297 high pressure liquid, 20-2
But-l-ene, 79 Chain isomerism—see Isomerism gas, 24-6,32, 245
ds-But-2-ene, Chain reaction, 75 paper, 23-4,31-2
isomerism of, 237 Chiral carbon, 241 thin-layer, 22-3, 30-1
physical properties, 79 Chirality, 241 Chromophore, 262
trans-But-2 ene, Chitin, 289 Coal, 324
isomerisiii of 237 Chloral—see Trichloroethanal Co-enzyme A, 293
physical properties, 79 Chlorination, Collagen, 284
cts-Butenedioic acid, 237, 247 of benzene, 104 Collodion, 297
trans-Butenedioic acid, 237, 247 of carboxylic acids, 193 Condensation polymerisation, 327
Butenedioic anhydride, 237, 247 of ethane, 76 Condenser,
Butyl alcohol—see Butan-l-ol of ethene, 80, 85 air, 13-14
t-Butyl alcohol—see of ethyne, 95 reflux, 12
2-Methylpropan-2-ol of methane, 73, 75 water, 13-14
Butyl bromide—see 1-Bromobutane of phenol, 159 Contact process, 323
t-Butyl bromide—see 2-Bromo-2-methyl- Chlorine, Co-polymerisation, 337, 338
propane qualitative analysis of 46-7 Copper(I) dicarbide, 95, 96
Butyl chloride—see 1-Chlorobutane quantitative analysis of 35 Copper(II)-glycine, 260, 267
t-Butyl chloride—see 2-Chloro-2-methyl- Chlorobenzene, Cordite, 155
propane manufacture, 130 Cotton, 297
Butyl iodide—see 1-Iodobutane physical properties, 130 Cracking,
But-l-yne, 94 preparations, 104,129-30, 261, 267 catalytic, 101, 309,311-12
But-2-yne, 94, 96 reactions, 110-11,130-1, 278 of gas oil, 309
uses, 131 of kerosine, 309
Cannizzaro reaction, 180,182,186 2-Chlorobenzoic acid, of naphtha, 316
Caprolactam, 321, 336 reduction, 204-5 of paraffin oil, 311-12
Carbamic acid, 223 2-Chlorobuta-1,3-diene, 339 thermal, 315
Carbamide, 1- Chlorobutane, 117,137 Crystallisation, 17-18
manufacture, 224, 323 2- Chlorobutane, 117 Crystal picking, 244
physical properties, 223 Chloroethane, Cumene, 156
preparations, 1, 223 manufacture, 118, 317 Cumene hydroperoxide, 156
reactions, 224, 228-9, 298, 342 physical properties, 117 Cumene process, 156-7
synthesis, 1 preparations, 76, 82,117 Cyanohydrins, 174-6,198, 291, 331
uses, 224, 333 reactions, 118-25 Cycloalkanes, 7, 76-7,77-8, 106, 319-20
Carbamide-methanal plastics, 333,342 uses, 125, 317 Cyclobutane, 77
Carbanions, 127 Chloroethanoic acid, Cyclohexane,
Carbenes, 127-8 dissociation constant, 64 manufacture, 320
Carbohydrates, 289-98, 298-9 preparation, 193, 197 reactions, 77, 77-8, 320
Carbon, reactions, 197 structure, 77
NMR, 45 2-Chloroethanol, 82, 169 uses, 320
qualitative analysis, 46 Chloroethene, Cyclohexanol,
quantitative analysis, 33-5 manufacture, 85, 95, 317 manufacture, 320
Carbonium ions, 66-7 physical properties, 117 uses, 320
Carbon monoxide, polymerisation, 330-1 Cyclohexanone,
manufacture, 321, 322 reactions, 128-9 manufacture, 320
uses, 191, 321 structure, 128-9 uses, 320-1
Carbon suboxide—see Tricarbon dioxide Chloroethylbenzenes, 109-10 Cyclohexanone oxime, 321
Carbon tetrachloride—see Chloroform—see Trichloromethane Cyclohexene,
Tetrachloromethane Chloromethane, physical properties, 79
Carbonyl group, 171 manufacture, 118 preparation, 88
Carboxylic acid group, 189 use, 339 reactions, 89,90-1
Carboxylic acids—see Acids Chloromethylbenzenes, 108, 129 Cyclopentane, 7, 77
Carbylamine reaction, 127-8, 226, 256 (Chloromethyl) benzene, 109, 129 Cyclopropane, 76-7
Carius’s method, 35 2-Chloro-2-methylbutane, 135-6 Cysteine, 282, 283-4
Carotenes, 19 2-Chloro-2-methylpropane, 117 Cytosine, 287-8
Carothers, 336 Chlorophylls, 19, 29
Carrier gas, 24 Chloroprene—see 2-Chlorobuta-l,3-diene Dacron, 337
Catalase, 298 1- Chloropropane, 117 D.D.T, 131
Catalysis, bifunctional, 308 2- Chloropropane, 117,118 Decane, 69
Catalytic cracking, 101, 309, 311-12 3- Chloropropene, 154 Decanedioyl chloride, 341-2
Catenation, 4 Chlorosilanes, 339 Decarboxylation,
Celanese silk, 297 CHN analyser, 33-5 of benzoic acid, 207
Cellophane, 297 Chromatography, of disodium ethanedioate, 207
Celluloid, 297 of amino-acids, 23,31-2, 284-5 of ethanedioic acid, 196, 207
Cellulose, 297 column, 18-20, 29-30 of glycine, 260
Cellulose ethanoate, 297, 299 dry-pack method, 20 of propanedioic acid, 196
Cellulose trinitrate, 155 gel permeation, 21 of sodium ethanoate, 199-200, 206

383
 Dehydration, 150 Diethyl ketone—see Pentan-3-one promotion, 54
Delocalisation, 59-61 Diethyl malonate—see Diethyl stabilisation, 101
Delocalisation energy, 101 propanedioate Energy level, 51
Denaturation, 284, 297 Diethyl propanedioate, 214-15 Enol group, 159,162, 236
Deoxyribonucleic acid, 286-7 ^em-Dihalides, 126,137-8, 173 Enzymes, 284
DERV, 309 ric-Dihalides, 125-6,137-8 Epoxides—see Epoxyethane and Epoxy¬
Detergents, 2,3-Dihydroxybutanedioic acid, propane
alkylbenzene sulphonates, 202, 205-6 isomerism, 242-3 Epoxyethane,
alkyl sulphates, 202, 205 meso-form, 243 manufacture, 83, 168
ethoxylates, 202 physical properties, 243 physical properties, 168
preparations, 205-6 Di-isocyanates, 334-5 preparation, 83
Dettol, 160 Diketopiperazine, 260 reactions, 168-9
Dextrose—see Glucose Dimethylacetylene—see But-2-yne uses, 169, 208
Dialkylsilanediols, 340 Dimethylamine, 251 Epoxypropane, 317, 334
1,6-Diaminohexane, 320, 336 4-Dimethylaminoazobenzene, 262 Epoxy resins, 335, 342
Diastase, 296 Dimethyl benzene-1,4-dicarboxylate, 337 Essential amino-acids, 282
Diastereoisomers, 245 Dimethylbenzenes, 318, 319 Esterification, 211-12
Diazonium compounds, 1.2- Dimethylcyclobutane, Esters,
preparation, 260, 266 isomerism, 237 physical properties, 211
reactions, 261-3, 266-7 Dimethyl ether, 166 preparations, 122, 192,211-12
Diazotisation, 260 2.2- Dimethy!propane, 71 reactions, 212-14
Di(benzoyl) peroxide, 330, 331, 340 1.3- Dinitrobenzene, saponification, 201, 205, 213, 227
1.2- Dibromoethane, preparation, 110, 275, 276 uses, 214
physical properties, 117 2.4- Dinitrophenylhydrazine, 178,184-6, Ethanal,
preparations, 80, 125 374 manufacture, 95, 173-4
reactions, 125-6 2.4- Dinitrophenylhydrazones, physical properties, 171-2
1.2- Dibromoethene, 95 physical properties, 379-80 preparations, 172,183
1.3- Dibromopropane, 77 preparations, 178,186 reactions, 174-82,184-6
Dichlorobenzenes, 129, 235 Diols, uses, 183, 316
Dichlorocarbene, 127-8 manufacture, 152 Ethanal cyanohydrin, 174-5, 198
2.4- Dichloro-3, 5-dimethylphenol, 160 nomenclature, 152 Ethanal 2,4-dinitrophenylhydrazone,
1,1 -Dichloroethane, physical properties, 152 184-5
manufacture, 76, 95 reactions, 153,162 Ethanal hydrogensulphite, 184
physical properties, 117 uses, 154, 337 Ethanal oxime, 178
preparations, 126, 178 Dipeptides, 281 Ethanal resin, 179,185
reactions, 126,137-8 Diphenylamine, 252 Ethanal tetramer, 182,186
1.2- Dichloroethane, Diphenyl ether, 166 Ethanal trimer, 182
manufacture, 76, 85, 317 Diphenylmethanone, 171, 173 Ethanamide,
physical properties, 117 Dipolar ions, 258 basic strength, 221-2
preparations, 76, 80, 125 Dipole moment, 61 physical properties, 221
reactions, 85, 125-6,137-8, 317 Disaccharides, 289, 293-5, 299 preparations, 214, 218, 220, 221
uses, 85, 317 Disodium ethanedioate, reactions, 221-3, 228
Dichloroethanoic acid, 64 preparation, 196, 200 structure, 221-2
1.2- Dichloroethene, 95 reactions, 207 Ethane,
Dichloromethane, Distillation, 13-16 manufacture, 76, 315
physical properties, 117 fractional, 15 physical properties, 69
preparation, 73 steam, 16 preparations, 80, 95
structure, 2-4 vacuum, 14-15 reactions, 76
(Dichloromethyl)benzene, 109, 129 DNA 286-8 uses, 76, 315
2.4- Dichlorophenol, 160 Dodecane, 69 Ethanedioic acid,
2.4- Dichlorophenoxyethanoic acid, 160 Dodecyl sulphate, 205 manufacture, 196
2.2- Dichloropropane, 178 Dyes, 262-3, 267 physical properties, 195
Di(dodecanoyl) peroxide, 341 Dynamite, 155 preparations, 195
Dienes, reactions, 196-7, 207
isomerism, 243 Ethane-l,2-diol,
manufacture, 315, 318, 338 £1 reactions, 124 manufacture, 152, 168
Diesel oil, 309 £2 reactions, 124 physical properties, 152
Diethylamine, Electrophilic reactions, 82 preparation, 83
physical properties, 251 Elimination reactions, 123-4 reactions, 153,162
preparation, 252-3 Empirical formula, 35 uses, 154
reactions, 253-6 determination of, 35-6 Ethanedioyl chloride, 196
Diethyl ether, Enantiomers, 241-2 Ethanenitrile, 225, 228
manufacture, 167 Enantiomorphs, 242 Ethanoic acid,
physical properties, 166 Energy, dimerisation, 190
preparations, 167 bond, 4, 54-5 dissociation constant, 65, 192
reactions, 167-8 delocalisation, 101 manufacture, 191, 316
uses, 168 hydrogen-bond, 145 physical properties, 65, 189-90

384
 preparations, 190 Ethylene dichloride—see Gasoline, 304-5
reactions, 191-3, 206-7
1,2-Dichloro-ethane Gas-turbine fuels, 309
uses, 194, 315 Ethyl ethanoate, Gene, 288
Ethanoic anhydride, physical properties, 211 Genetic engineering, 345
manufacture, 220 preparation, 211-12, 226-7 Geometrical isomerism—see Isomerism
physical properties, 219 reactions, 212-14 Gluconic acid, 291
preparations, 218, 220 uses, 214 Glucosazone, 292, 298
reactions, 220, 228, 266, 299 Ethylene glycol—see Ethane-1,2-diol Glucose,
uses, 220, 297 Ethylene oxide—see Epoxyethane physical properties, 291
Ethanol, Ethyl ether—see Diethyl ether preparations, 290
absolute, 146-7 Ethylidene dibromide—see reactions, 291-2, 298
manufacture, 83, 146, 316 1,1 -Dibromoethane uses, 292-3
physical properties, 144-5 Ethylidene dichloride—see Glucose-6-phosphate, 292
preparations, 145-6 1,1-Dichloroethane Glycerides, 214
reactions, 146-51,160-2 Ethyl iodide—see lodoethane Glycerine—see Propane-1,2,3-triol
uses, 152, 316 Ethyl methyl ether, 166 Glycerol—see Propane-1,2,3-triol
Ethanoylation, 216 Ethyl nitrite, 123 Glycine,
Ethanoyl bromide, 216 Ethyl 3-oxobutanoate, 215, 236 physical properties, 258, 267
Ethanoyl chloride, Ethyl propanoate, 211 preparations, 258-9
physical properties, 216 Ethyne, reactions, 259-60, 267
preparations, 216 manufacture, 73, 94 Glycogen, 289, 293
reactions, 216-19, 227-8 physical properties, 94 Glycol—see Ethane-1,2-diol
uses, 219 preparations, 96 Glycollic acid—see Hydroxyethanoic
Ethanoyl iodide, 216 reactions, 95, 97-8 acid
Ethene, structure, 6-7, 58 Glyoxylic acid—see Oxoethanoic acid
manufacture, 80, 315-6 uses, 96 Glyptal resins, 335
physical properties, 79 Extraction, 16-17 Greases, 311
preparations, 89, 95 E; Z nomenclature, 237 Griess, 260
reactions, 80-5, 89 Grignard, 133
Fats, 201,227
structure, 5-6, 56-7 Grignard reagents,
Fehling’s solution, 181, 375
uses, 85, 316-17 preparations, 125, 133, 203
Fehling’s test, 181,185,186, 193-4, 291,
Ethene ozonide, 84 reactions, 133-4, 203-4
294, 298
Ethenone, 220 Guanine, 286-8
Fermentation, 296
Ethenyl ethanoate, Gun cotton, 155
Fillers, 327
manufacture, 331
Flame-ionisation detector, 24
polymerisation, 331 Halides—see Alkyl halides. Aromatic
Fluorobenzene, 130, 261
uses, 331, 337 halides. Dihalides and individual
Fluorocarbons, 131-2
Ether—see Diethyl ether compounds
Fluoroethanoic acid,
Ethers, Halogen carrier, 104
dissociation constant, 64
nomenclature, 166 Halogens,
Fluoromethane, 132
physical properties, 166 qualitative analysis of, 46-7
Fluorotrichloromethane, 132
preparations, 122, 167 quantitative analysis of, 35
Fluothane, 132
reactions, 167-8 Heat of hydrogenation, 100
Formaldehyde—see Methanal
uses, 168 Heisenberg’s Uncertainty Principle, 50
Formalin, 171, 183
Ethyl acetoacetate—see Ethyl Helium, 304
Formamide—see Methanamide
3-oxobutanoate a-Helix, 283, 284
Formic acid—see Methanoic acid
Ethyl alcohol—see Ethanol Heptane, 69, 304
Formula weight,
Ethylamine, Heterolysis, 76
determination of, 36-7
physical properties, 252 Hexachlorocyclohexane, 106
Fragmentation pattern, 38
preparation, 252-3 Hexachloroethane, 76
Free radicals, 66, 75
reactions, 253-6 Hexamethylenediamine—see
Freons, 132
Ethylbenzene, 1,6-Diaminohexane
Friedel-Crafts reaction, 105, 173
manufacture, 318, 329 Hexane,
Fructose,
preparation, 105 preparation, 291 physical properties, 69
reactions, 109-10, 329 reactions, 299 reactions, 77-8
uses, 329 Fructose-1,6-diphosphate, 292 Hexanedinitrile, 320
Ethyl benzoate, Fructose-6-phosphate, 292 Hexanedioic acid,
saponification of, 203 Fumaric acid—see trans-Butenedioic
manufacture, 320
Ethyl bromide—see Bromoethane acid physical properties, 195
Ethyl chloride—see Chloroethane reactions, 336
Functional group, 5
Ethylene—see Ethene uses, 336
Ethylene bromohydrin—see Galactose, 294 Hexanedioyl chloride, 342
2-Bromoethanol Gammexane—see Hexan-l-ol, 144
Ethylene chlorohydrin—see Hexachlorocyclohexane Hex-l-ene, 79
2-Chloroethanol Gas oil, Hexoses, 289, 290-3
Ethylene dibromide—see cracking, 309 Hofmann reaction, 223, 263-4
1,2-Dibromoethane manufacture, 304-5 Homologous series, 5, 69

385
 Homolysis, 76 lodoethanoic acid, Le Bel, 241
Hund’s rule, 51 dissociation constant, 64 Lipids, 281
Hybridisation, 56 Iodoform—see Tri-iodomethane Liquefied petroleum gas, 304, 316
Hydrocarbons—see Alkanes, Alkenes, Iodoform reaction, 150, 162, 181,185 Lithium aluminium hydride—see
Alkynes, Aromatic hydrocarbons. lodomethane, Lithium tetrahydridoaluminate
Cycloalkanes and individual physical properties, 117 Lithium tetrahydridoaluminate,
compounds reaction, 105 reduction of acid halides, 219
Hydrocracking, 309 Ion-exchange resins, 21 reduction of acids, 193, 204-5
Hydrodealkylation, 101, 318 Iron{III) chloride test, reduction of aldehydes, 177
Hydroforming, 307 for acids, 206 reduction of 2-chlorobenzoic acid,
Hydrogen, for enols, 158-9,162 204-5
manufacture, 321-2 Isobutane—see 2-Methylpropane reduction of esters, 214
qualitative analysis of, 46 Isobutene—see 2-Methylpropene reduction of ketones, 177
quantitative analysis of, 33-5 Isobutyl alcohol—see reduction of nitro compounds, 275
uses, 323 2-Methylpropan-l-ol LPG, 304,316
Hydrogenation, Isobutylene—see 2-Methylpropene Lubricating oil, 304, 311
of alkenes, 80 Isobutyric acid—see 2-Methylpropanoic Lucas test, 148
of alkynes, 95 acid Lycra, 335
of benzene, 100, 106, 320 Isocyanobenzene, 256 Lysine, 258
of phenol, 159 Isocyano-compounds, 226, 256
Hydrogen bonding, Isomerisation,
in acid amides, 221 of alkanes, 306-7 Maleic acid—see cis-Butenedioic acid
in alcohols, 144-5 Isomerism, 8-9, 235-47 Maleic anhydride—see Butenedioic
in carboxylic acids, 190 chain, 235, 246 anhydride
in diols, 152 cis-trans, 58, 236-7, 246, 247 Malonic acid—see Propanedioic acid
in ethanoic acid, 190 functional group, 235-6, 246 Malt, 296
in ethanol, 144-5 geometrical, 58, 236-7, 246, 247 Maltase, 296
in nitrophenols, 160 optical, 238-46, 246-7 Maltose, 294-5
Hydrogen cyanide, position, 235, 246 Markownikoff’s rule, 86
manufacture, 317 stereo, 235, 236-46, 246-7 Mass spectrometry, 36-7, 38-9
uses, 174-5, 198, 331 structural, 235-6, 246 Melamine, 333
Hydrogen peroxide, Iso-octane—see 2,2,4-Trimethylpentane Melaware, 333
decomposition, 298 Isoprene—see 2-Methylbuta-1,3-diene Melting-point method, 27
Hydrogen sulphide, 304, 323 Isopropyl alcohol—see Propan-2-ol Mesomerism, 61
4-Hydroxyazobenzene, 262, 267 Isopropyl bromide—see 2-Bromopropane Metaldehyde—see Ethanal tetramer
3- Hydroxybutanal, 179 Isopropyl chloride—see 2-Chloropropane Methanal,
2-Hydroxy-1,2-diphenylethanone, 183 manufacture, 174, 322
Hydroxyethanoic acid, 198 Jet fuels, 309 physical properties, 171-2
Hydroxylamine, preparations, 172-3
manufacture, 275 reactions, 174-82,184-6
reactions, 178-9, 291 Katharometer, 24-5, 35 uses, 183, 333
4- Hydroxy-4-methylpentan-2-one, 179 Keratin, 284
Methanamide, 221
2-Hydroxypropanoic acid, Kerosine,
Methane,
biological importance, 293 cracking, 309
chlorination, 73-6
isomerism, 199, 238, 242 manufacture, 304 occurrence, 72
physical properties, 199 uses, 308-9
physical properties, 69
preparation of (-)enantiomer, 242 Keten—see Ethenone
reactions, 72-6
reactions, 199 Ketones,
structure, 1-2
identification, 186
uses, 314-5
Imines, 178 nomenclature, 171
Methanenitrile—see Hydrogen cyanide
Inductive effect, 62 physical properties, 171-2
Methanoic acid,
Infrared spectroscopy, 39-41, 145 preparations, 105, 172-3,183-4, 200-1
dissociation constant, 65, 192
Initiator, 330 reactions, 174-83, 184-6
manufacture, 190
Internal compensation, 243 uses, 183
physical properties, 65, 189-90, 192
Inversion of sucrose, 294 Kjeldhal’s method, 35
preparations, 190
Invertase, 294 Knock, 304-5
reactions, 193-4
Invert sugar, 294 Kolbe, 1
Methanol,
Iodine, Kolbe’s reaction, 200
manufacture, 73, 146, 321-2
qualitative analysis of, 46-7 Krebs’ cycle, 293
NMR, 42
quantitative analysis of, 35 physical properties, 144-5
lodobenzene, Lactic acid—see 2-Hydroxypropanoic preparations, 145-6
physical properties, 130 acid reactions, 147-50,161-2
preparations, 104, 261, 267 Lactides, 198 uses, 151-2, 345
reactions, 130-1 Lactose, 294-5 Methylacetylene—see Propyne
1-Iodobutane, 136 Lassaigne test, 46-7 Methyl alcohol—see Methanol
lodoethane, Lauroyl peroxide—see Di(dodecanoyl) Methylamine,
physical properties, 117 peroxide dissociation constant, 65, 254

386
 physical properties, 65, 251, 252, 254 Molecular sieves, 315 Nitrophenylamines,
preparation, 252-3,263-4 Monochloroacetic acid—see separation of, 30
reactions, 253-6, 265-6 Chloroethanoic acid JV-(Nitrophenyl)ethanamides, 267
Methylammonium chloride, 255, 263-4 Monomer, 327 1- Nitropropane, 274-5, 315
Methylbenzene, Monosaccharides, 289,290-3, 298-9 2- Nitropropane, 274-5, 315
manufacture, 106, 307, 318 Mutarotation, 290 Nitrosamines, 255
physical properties, 106-7 NMR,41-5
preparation, 105 carbon, 45
reactions, 107-9,113, 276, 277-8, Naphtha, hydrogen, 41-5
318 cracking, 315-16 Nobel, 155
uses, 276,318,319, 334 manufacture, 304-5 Nobel Prizes, 133, 328
Methyl benzoate, 211, 268 uses, 305-8 Node, 51
Methyl bromide—see Bromomethane Naphthalen-2-ol, 262 Nomenclature,
2-Methylbuta-1,3-diene, 315, 318 Natta, 328 of acid amides, 221
2- Methylbutane, 71, 339 Natta process, 328 of acid anhydrides, 219
3- Methylbutanoic acid, 189 Natural gas, 302-4 of acid chlorides, 10, 216
Methyl t-butyl ether, 308 dry, 304 of acid halides, 216
Methyl chloride—see Chloromethane wet, 304 of acid nitriles, 10, 225
Methyl cyanide—see Ethanenitrile Neoprene rubber, 339 of acids, dicarboxylic, 195
Methylene chloride—see Nicol prisms, 239 of acids, monocarboxylic, 10, 189
Dichloromethane Nicotine, 242 of alcohols, 10,143-4
Methylene dichloride—see Nitration, of aldehydes, 10,171
Dichloromethane of benzene, 102-3, 275 of alkanes, 9,10, 69
Methylene group, 5, 69 of chlorobenzene, 110-11, 277-8 of alkenes, 9,10, 79
Methyl ethanoate, 211 of methylbenzene, 107-8, 276, 277-8 of alkyl halides, 10, 116
Methylethylamine, 251 of methyl benzoate, 276-7 of alkynes, 9, 10, 94
(l-Methylethyl)benzene, 105, 156 of nitrobenzene, 110, 276 of amines, 10, 251-2
Methyl ethyl ketone—see Butan-2-one of phenol, 111-12, 159, 275, 277-8 ofaromatic amines, 252
Methyl group (radical), 5, 66 of phenylamine, 111-12, 257 of aromatic halides, 129
Methyl iodide—see lodomethane Nitrile rubber, 338 of carboxylic acids, 10,189, 195
Methyl methanoate, 211 Nitriles—see Acid nitriles of esters, 211
Methyl 2-methylpropenoate, Nitroalkanes, of ethers, 166
preparation, 331, 341 manufacture, 275, 315 of ketones, 10, 171
uses, 331-2, 341 nomenclature, 274 of nitroalkanes, 274
Methyl-3-nitrobenzene, 274 physical properties, 274 of nitro compounds, 274
Methylnitrobenzenes, 107 preparations, 102-3, 274 of phenols, 155
Methyl 3-nitrobenzoate, reactions, 275-6, 278 Nonane, 69
preparation, 276-7 uses, 276 Nuclear magnetic resonance
Methylphenols, 155 Nitrobenzene, spectroscopy, 41-5
Methyl phenyl ether, 166 manufacture, 275, 276 Nucleic acids, 285-8
2-Methylpropane, physical properties, 274 Nucleophiles, 120
manufacture, 306-7, 309 preparation, 275 Nucleophilic reagents, 120,175-7
uses, 307 reactions, 275-6, 276, 278 Nucleotides, 286
Methyl propanoate, 211 Nitro compounds—see Nitroalkanes and Nylon, 336-7, 341-2
2-Methylpropanoic acid, 189 Nitro compounds, aromatic
2-Methylpropan-l-ol, 144 Nitro compounds, aromatic, Octadecanoic acid, 201
2-Methylpropan-2-ol, 144 manufacture, 275, 276 Octane, 69
2-Methylpropene, nomenclature, 274 Octane number, 305
physical properties, 79 physical properties, 274 Oil,
uses, 307 reactions, 275 distillation, 304
Methyl-2,4,6-trimtrobenzene, 276 uses, 275-6,276, 278 formation, 301-2
Molasses, 293 Nitroethane, 274, 278, 315 Olefins—see Alkenes
Molecular formula, 4 Nitrogen, Optical isomerism—see Isomerism
determination of, 37 qualitative analysis of, 46-7 Orange II, 263
Molecular ion, 36, 38 quantitative analysis of, 33-5 Orbitals—see Atomic orbitals and
Molecular orbitals, 53 Nitroglycerin, 154-5 Molecular orbitals
acid amides, 221-2 Nitromethane, 274, 315 Orion, 337
benzene, 59-60 Nitronium ion, 103 Osazones, 292, 298
carbon-carbon double bond, 56-7 Nitroparaffins—see Nitroalkanes Oxalic acid—see Ethanedioic acid
carbon-carbon single bond, 56 Nitrophenols, Oxalyl chloride—see Ethanedioyl chloride
carbon-carbon triple bond, 58 hydrogen bonding, 160 Oximes,
carboxylic acids, 193-4 physical properties, 155,160 isomerism, 237
chloroethene, 128-9 preparations, 159, 278 preparation, 178, 321
ethene, 56-7 separation, 31 Oxoethanoic acid, 198
ethyne, 58 2- Nitrophenylamine, 30, 257 0X0 process, 322
phenoxide ion, 157-8 3- Nitrophenylamine, 30 2-Oxopropanoic acid, 199
phenylamine, 254-5 4- Nitrophenylamine, 30, 252, 257 Ozonolysis, 84

387
 Paraffin hydrocarbons—see Alkanes reactions, 330,341 physical properties, 69
Paraffin oil, uses, 329-30, 338 reactions, 76
cracking of, 311-12 Phenylethyne, 94,97-8 uses, 315
Paraffin wax—see Wax Phenylhydrazine, Propanedioic acid,
Paraformaldehyde—see Polymethanal preparation, 263 physical properties, 195
Paraldehyde—see Ethanal trimer reactions, 178, 292, 298 preparation, 195-6
Parent ion, 36 Phenylhydrazones, 178 reactions, 197
Pasteur, 244 Phenyl isocyanate, Propane-1,2-diol, 152
Penicillium glaucum, 242, 245 determination of polypeptide structure, Propane-1,3-diol, 152
Pentachloroethane, 76 285 Propanenitrile, 225
Pentane, Phenyl isocyanide, 256 , ,
Propane-l,2,3-triol, 154,162 201, 227
physical properties, 69, 71 Phenylmagnesium bromide, 334, 335
reactions, 77-8 preparation, 203 Propane-1,2,3-triyl trinitrate,
Pentanoic acid, 189 reactions, 203-4 154-5
Pentan-l-ol, 144 Phenylmethanol, 144,160-2 Propanoic acid,
Pentan-3-one, 171,184-5 (Phenylmethyl)amine, 252, 254, 255, dissociation constant, 65, 192
Pent-l-ene, 79 265-6 physical properties, 189
Pentoses, 289 Photochemical reactions, 73 Propan-l-ol, 144
Pent-l-yne, 94 Phthalic acid—see Benzene-1,2- Propan-2-ol,
,
Peptide link, 229 281, 298 dicarboxylic acid ’* manufacture, 147
Peptides, 281 Phthalic anhydride—see Benzene-1,2- physical properties, 144-5
Peroxide effect, 86-7 dicarboxylic anhydride preparations, 145-6
Peroxobenzoic acid, 83 Phthalimide—see Benzene-1,2- reactions, 147-51,162
Peroxoethanoic acid, 337 dicarboximide uses, 152
Perspex, 332, 341 Picric acid, Propanone,
Petrol, 304-8, 316 dissociation constant, 158 manufacture, 156-7, 174, 317
Petroleum, 301-2, 304-11 preparation, 160 physical properties, 171-2
distillation, 304 Plasticisers, 327 preparations, 172-3,183
formation, 301-2 Plastics, 327-35, 341-2 reactions, 174-82,184-6
stabilisation, 304 thermosetting, 332-5, 342 uses, 183, 331
uses, 304-11 thermosoftening, 327-32, 341 Propanone cyanohydrin, 331
Phenol, Platforming, 307 Propanone hydrogensulphite, 176
manufacture, 156-7 Polarimeter, 240 Propanone oxime, 178
physical properties, 155-6 Polarised light, 238-40 Propanoyl chloride, 216
preparation, 156, 261 Polaroids, 239, 342 Propenal, 154
reactions, 157-60,160-2, 262, Polyacrylic esters, 331-2,341 Propene,
278 Poly(chloroethene), 330-1 manufacture, 315-16
structure, 157-8 Polyesters, 331-2, 337 physical properties, 79
uses, 160,319,333 Poly(ethene), 328 reactions, 85-7
Phenols, 155-60, 261, 262, 335 Poly(ethenyl ethanoate), 331 uses, 88, 154, 317
Phenoxide ion, 157-8 Polymerisation, 308, 327-41, Propenenitrile, 317, 337, 338
Phenylalanine, 282 341-3 Propenenitrile fibres, 337
Phenylamine, addition, 327,341 Propenoic acid, 317
dissociation constant, 254 CO-, 337, 338 Propyl alcohol—see Propan-l-ol
manufacture, 253 condensation, 327, 341-3 Propyl bromide—see 1-Bromopropane
physical properties, 252 Polymers, Propyl chloride—see 1-Chloropropane
preparations, 253, 264-5 atactic, 328 Propylene—see Propene
reactions, 227-8, 253-7, 265-6 isotactic, 328 Propylene oxide—see Epoxypropane
Phenylammonium chloride, 264 -,
natural, 281-9, 295-7, 297 8 299 Propyl hydrogensulphate, 77
Phenyl benzoate, synthetic, 327-41, 341-3 Propyne, 94
preparation, 161, 217 Polymethanal, 182,185 Protection, 257
Phenyl cyanide—see Benzonitrile Polyoxymethylene, 132, 332 -,
Proteins, 281-5, 297 8 345
A-Phenylethanamide, Polypeptides, 281 denaturation, 284, 297
preparations, 255, 266 Poly (phenylethene), 329-30, 341 fibrous, 284
Phenyl ethanoate, Poly(propene), 328-9 globular, 284
preparation, 157, 212 Polysaccharides, 289, 295-7, 299 a-helix, 283, 284
Phenylethanoic acid, Poly(tetrafluoroethene), 332 structure, 283-4
dissociation constant, 65, 192 Polythene, 328 synthetic, 345
Phenylethanone, Polyurethanes, 334-5, 342 Pruteen, 345
physical properties, 171 Position isomerism—see Isomerism PTFE, 132, 332
preparations, 105, 173 Potassium benzene-1,2-dicarboximide, 259 Purines, 286
reactions, 184-6 Promotion energy, 54 Purity,
Phenylethanone Propadienes, 243 criteria of, 27-8
2,4-dinitrophenylhydrazone, 178 Propanal, 171 PVC, 330-1
Phenylethene, Propanamide, 221 Pyrimidines, 286
manufacture, 329 Propane, Pyroxylin, 297
physical properties, 79 manufacture, 76, 315 Pyruvic acid—see 2-Oxopropanoic acid

388
 Quaternary ammonium salts, 252-3 Sodium dodecyl sulphate, Sulphur tetrafluoride, 132
Quinine, 245 preparation, 205 Synthesis gas,
Sodium ethanedioate—see Disodium composition, 321
Racemic compound, 244 ethanedioate manufacture, 146, 321, 324
Racemic form, 241 Sodium ethanoate, uses, 146, 321-2, 324
Racemic mixture, 243 reactions, 200, 206
Sodium ethoxide, Tartaric acid—see 2,3-
Racemisation, 245-6
Rayon, 297 preparation, 147 Dihydroxybutanedioic acid
Recrystallisation, 17-18
,
reactions, 161 215 Tautomerism, 236, 246
Rectified spirit, 147 Sodium hydrogenethanedioate, 196 Teflon, 132, 332
Refinery gas, 309 Sodium methanoate, Terephthalic acid—see Benzene-1,4-
Reflux, 12 reactions, 200, 206 dicarboxylic acid
Sodium phenoxide, 156 Terylene, 337
Reforming, 101, 307-8
Sodium potassium 2,3- Tetrabromoethane, 95
catalytic, 307-8
of heptane, 307 dihydroxybutanedioate, 375 Tetrachloroethanes, 76,95
Sodium tetrahydridoborate, Tetrachloromethane, 117,128
of hexane, 101
reducing properties, 177 Tetraethyllead,
Refractive index, 239
Spin-spin coupling, 44 manufacture, 125
Residual crude, 304, 309
Stabilisation energy, 101 uses, 308
Resolution, 244-5
Starch, Tetrafluoroethene,
Resonance, 61
fermentation, 296-7 manufacture, 132
Resonance hybrid, 61
. hydrolysis, 290, 296, 299 polymerisation, 332
Restricted rotation, 58, 237, 247
reactions, 290, 296, 299 Tetrahydrofuran, 168
Retention time, 21, 26
structure, 295-6 Tetramethylsilane, 43
Ribonucleic acid—see RNA
Stationary phase, 24 Thymine, 286-7
RNA, 286-8
Stearic acid, 201 TMS, 43
messenger, 288
Stereoisomerism—see Isomerism TNT, 276
transfer, 287
Stick diagram, 38 Toluene—see Methylbenzene
Rosenmund reaction, 219
Structural formula, Transition state, 119
R,S nomenclature, 241
determination of, 37-46 Triacontane, 69
Rubber,
Structural isomerism—see Isomerism, Trialkylaluminiums, 316, 328, 339
natural, 338
Structure, Tribromomethane, 181
synthetic, 338-9
of benzene, 59-60, 99-101 2.4.6- Tribromophenol, 112, 159,162
determination of, 37-46 2.4.6- Tribromophenylamine, 256, 266
Saccharic acid, 291 of dichloromethane, 2 Tricarbon dioxide, 197
Salts, of ethene, 6, 57-8 Trichloroethanoic acid, 64
of carboxylic acids, 199-201, 206-7 of ethyne, 6, 58 Trichloroethane, 76
Sandmeyer reaction, 261, 267 of methane, 1-2 Trichloroethene, 129
Saponification, of proteins, 283-4 T richloromethane,
of esters, 203, 213, 227 of polypeptides, 285 physical properties, 117, 126
of ethyl benzoate, 203 Styrene—see Phenylethene preparations, 73,126, 180
of a fat, 227 Substitution reactions, 73 reactions, 126-8
Saturated compounds, 5 of aldehydes, 180 (Trichloromethyl)benzene, 109, 130
Schiff’s base, 178 of alkanes, 73-5 2.4.6- Trichlorophenol, 159
Schiff’s reagent, 181,375 of alkyl halides, 118-23 Triethanolamine—see
Schiff’s test, 181 of aromatic compounds, 102-12 Tris-(2-hydroxyethyl)amine
Schotten-Baumann reaction, of benzene, 102-6 Triethylaluminium, 316, 339
of phenol, 161 of carboxylic acids, 193 Triethylamine, 251
of phenylamine, 227 of chlorobenzene, 110-11 Trifluoroethanoic acid, 132
Sebacic acid—see Decanedioic acid of ethylbenzene, 109-10 Tri-iodomethane,
Sebacoyl chloride—see Decanedioyl chloride of ketones, 180 physical properties, 117,156
Silica gel, 22, 24, 30 of methylbenzene, 107-8 ,
preparation, 150,162 181,185
Silicones, 339-41, 342-3 of nitrobenzene, 110-11, 275-6, Trimethylamine, 251
Silver dicarbide, 95 276 2.4.6- T rinitrophenol,
Silver mirror test, 181,185, 193, 206, 291, ,
of phenol, 111-12, 159, 162 278 dissociation constant, 158
294, 295, 298 of phenylamine, 111-12, 256-7, 266 preparation, 160
SnI reaction, 121 Succinic acid—see Butanedioic acid Trinitrotoluene—see Methyl-2,4,6-
Sn2 reaction, 120 ,
Sucrose, 293-4, 299 324 trinitrobenzene
Soaps, 201 Sugar—see Sucrose Trioxan, 182
Sodium alkylbenzenesulphonate, Sugar charcoal, 294 Tripeptide, 281
preparation, 205-6 Sugars, 289-95 Triphenylamine, 252
Sodium ammonium 2,3- Sulphonation, 105-6 Triplet coding, 288
dihydroxybutanedioate, Sulphur, Tris-(2-hydroxyethyl)amine, 205
isomerism, 244 manufacture, 323 Tyrosine, 282
Sodium benzenesulphonate, 156 qualitative determination of, 46-7
Sodium borohydride—see Sodium Sulphuric acid, Undecane, 69
tetrahydridoborate manufacture, 323 Unimolecular reaction, 121

389
Unsaturated halides, 128-9 Viscose rayon, 297 X-rays, 285
Unsaturated compounds, 5 Vulcanisation, 338 Xylenes—see Dimethylbenzenes
Uracil, 286-7
Urea—see Carbamide Wacker process, 173 Yeast, 296
Urease, 298 Wax,
cracking, 311 Zeolites, 315
Valine, 282 manufacture, 304, 311 Ziegler, 328
van’t Hoff, 241 uses, 311 Ziegler catalyst, 328, 339
Veronal, 224 Wohler, 1 Ziegler process, 328
Vinyl chloride—see Chloroethene Wurtz reaction, 125 Zwitterions, 258
Now in its fourth edition this well established book is designed for 'A'
level students and for many college courses. It will also be useful for
the early part of some university courses.
This new edition maintains the structure of earlier editions. The
preparations and properties of organic compounds are set out in terms
of functional groups and against a background of the principles of
bonding, energetics and reaction mechanisms.
However, organic chemistry is a constantly evolving subject and the new
edition reflects this, in particular, in introducing
• the newer techniques for the separation and purification of
organic compounds
• the newer techniques for the determination of molecular
structures
• the latest industrial processes which themselves reflect the
response of industry to rising costs of basic feedstocks (oil) and
energy, and the use of alternative feedstocks throughout the
world (e.g. natural gas in the U.K., biomass in developing
countries). These changes have been dramatic and form the basis
of two chapters
• a chapter on possible developments in the future, with its
emphasis on biotechnology and newer catalysts.
The new edition also contains revised experiments and information on
films, videotapes and further reading. The latest examination questions
are included at the end of each chapter.
The nomenclature used is based on that recommended by the
Association for Science Education and the examination boards.
Companion texts in this series:
Modern Physical Chemistry G.F. Liptrot, J.J. Thompson and G.R. Walker
Modern Inorganic Chemistry G.F. Liptrot

Bell & Hyman Limited ISBN 0 7135 1363 2