Advanced Higher Unit 3- Organic Chemistry

Advanced Higher
Chemistry

Unit 3 - Organic Chemistry

REACTION MECHANISMS

KHS Chemistry Mar 2001 page 1 Reaction Mechanisms

Advanced Higher Unit 3- Organic Chemistry

Reaction Types
3.1.1 Equations can be written for the following reaction types and, given
equations, these reaction types can be identified.
substitution addition elimination condensation
hydrolysis oxidation reduction

Reaction Mechanisms
3.1.2 The following reaction mechanisms can be described in terms
of electron shifts.
( i) radical substitution of alkanes
( ii) electrophilic addition to alkenes
carbocation mechanism
cyclic ion intermediate mechanism
(iii) nucleophilic substitution
SN1 and SN2

Hydrocarbons & Haloalkanes

3.2.4 Alkanes undergo substitution reactions with chlorine and bromine
by a chain reaction mechanism.

3.2.5 The chain reaction includes the following steps

( i) initiation by homolytic fission to produce radicals
( ii) propagation
(iii) termination

3.2.7 Alkenes can be prepared in the laboratory by

( i) dehydration of alcohols using aluminium oxide,
concentrated sulphuric acid or orthophosphoric acid
(ii) base-induced elimination of hydrogen halides from
monohalogenoalkanes.

3.2.8 Alkenes undergo:

( i) catalytic addition with hydrogen to form alkanes
( ii) addition with halogens to form dihalogenoalkanes
(iii) addition with hydrogen halides according to Markownikoff’s
rule to form monohalogenoalkanes
( iv) acid-catalysed addition with water according to
Markownikoff’s rule to form alcohols.

3.2.9 The mechanisms of the above reactions involve

( i) for halogenation cyclic ion intermedate
( ii) for hydrohalogenation carbocation intermediate
(iii) for acid catalysed hydration carbocation intermediate

KHS Chemistry Mar 2001 page 2 Reaction Mechanisms

Advanced Higher Unit 3- Organic Chemistry
3.2.12 Monohalogenoalkanes undergo nucleophilic substitution reactions.

3.2.13 Monohalogenoalkanes undergo elimination reactions to form

alkenes.

3.2.14 Monohalogenoalkanes react with:

( i) alkalis to form alcohols
( ii) alcoholic alkoxides to form ethers
(iii) ethanolic cyanide to form nitriles which can be hydrolysed to
carboxylic acids(chain length increased by one carbon atom)
( iv) ammonia to form amines via alkyl ammonium salts.

Alcohols and ethers

3.2.17 Alcohols can be prepared from
( i) alkenes by hydration
(ii) halogenoalkanes by substitution.

3.2.19 Alcohols react with some reactive metals to form alkoxides .

3.2.20 Alcohols can be dehydrated to alkenes.

3.2.21 Alcohols undergo condensation reactions slowly with carboxylic

acids and more vigorously with acid chlorides to form esters.

3.2.27 Ethers can be prepared by the reaction of halogenoalkanes with

alkoxides.

Aldehydes, ketones & carboxylic acids

3.2.31 Aldehydes and ketones can be reduced to primary and secondary
alcohols, respectively, by reaction with lithium aluminium hydride
in ether.

3.2.32 Aldehydes and ketones undergo

( i) addition reactions in which the carbon atom in the polar
carbonyl group submits to nucleophilic attack.
(ii) condensation reactions with derivatives of ammonia (XNH2)
which proceed by nucleophilic addition of XNH2 followed by
elimination of a water molecule.

3.2.39 Carboxylic acids can be prepared by:

( i) oxidising primary alcohols and aldehydes
( ii) hydrolysing nitriles, esters or amides

3.2.40 Reactions of carboxylic acids include:

( i) formation of salts by reactions with metals, carbonates and alkalis
( ii) condensation reactions with alcohols to form esters
(iii) reaction with ammonia or amines and subsequent heating of the
ammonium salt to form amides
( iv) reduction with lithium aluminium hydride to form primary
alcohols.
KHS Chemistry Mar 2001 page 3 Reaction Mechanisms
Advanced Higher Unit 3- Organic Chemistry

Aromatics
3.2.49 Most reactions of benzene involve attack of an electrophile on the
cloud of delocalised electrons, that is electrophilic substitution.

3.2.50 Benzene resists addition reactions but undergoes electrophilic

substitution reactions. These include:
( i) chlorination and bromination to produce chlorobenzene and
bromobenzene
( ii) nitration to produce nitrobenzene
(iii) sulphonation to produce benzene sulphonic acid
( iv) alkylation to produce alkylbenzenes.

KHS Chemistry Mar 2001 page 4 Reaction Mechanisms

Advanced Higher Unit 3- Organic Chemistry
ELECTROPHILIC ADDITION ELECTROPHILIC ADDITION
(CARBOCATION INTERMEDIATE) (CARBOCATION INTERMEDIATE)

H H H H
| | | |
H — C = C — H H — C = C — H
Though water is polar,
the presence of H+ ions
H makes this reaction easier
As the polar HBr
H δ+ molecule approaches, π O H+ (catalyst).

electrons from the C=C π electrons from the C=C

| bond come out to form a H bond come out to form
new bond with the a new bond with the H+
Br δ— Hydrogen atom. ion

The electrons of the H—Br bond H H

move onto the bromine to form a
bromide ion, Br—. | |
H — C — C — H
H H | ⊕
H
| | H :O
H — C — C — H
H
| ⊕
The carbocation is now attacked by
a lone pair on the oxygen atom
H :Br—
The carbocation is then attacked by H H
the lone pair of the bromide ion, a
neucleophile. | |
H — C — C — H
H H | |⊕
| | H O — H
H — C — C — H |
| | H
H Br
A hydrogen ion is reformed (catalyst) and an alkanol is
A monohaloalkane is prduced and overall the reaction can be produced. Overall the reaction can be represented by the
represented by the equation: equation:

C2H4 + HBr → C2H5Br C2H4 + H2O → C2H5OH

ADDITION ADDITION, HYDRATION

KHS Chemistry Mar 2001 page 5 Reaction Mechanisms
Advanced Higher Unit 3- Organic Chemistry

ELECTROPHILIC ADDITION FREE RADICAL

(CYCLIC ION INTERMEDIATE) SUBSTITUTION

INITIATION
H H UV radiation is
| | absorbed
‘half arrows’ are used to
H — C = C — H show the movements of
single electrons

Cl : Cl
As the non-polar Br2
Br δ+ molecule approaches, π Homolytic fission results in the
| electrons from the C=C
bond induce polarity and
formation of two chlorine free 2 Cl•
radicals
then come out to form a
Br δ— new bond with the nearer
Bromineatom. PROPAGATION
The electrons of the Br—Br bond
move onto the further bromine to
form a bromide ion, Br—.
Cl• H—CH3→ HCl + CH3•
:Br—
H
H — C — C — H CH3• Cl— Cl → HCl + Cl•
⊕
H
Br A series of reactions between a free radical and a molecule
keep the reaction going.
Instead of a carbocation being formed, the positive
charge is shared between the 2 carbon atoms and the Because each step makes another free readical, the reaction
bromine atom. The Bromide ion (nucleophile) will then is a chain reaction.
attack from the other side. A TRANS arrangement.

TERMINATION
Br H
| |
Cl• •
CH3→ CH3Cl
H — C — C — H
Any collisions betwee two free radicals will stop a chain.
| |
H Br
A dihaloalkane is prduced and overall the reaction can be A mixture of haloalkanes will be produced but the first
represented by the equation: stage can be represented by the equation:

C2H4 + Br2 → C2H4Br2 CH4 + Cl2 → CH3Cl + HCl

ADDITION SUBSTITUTION
KHS Chemistry Mar 2001 page 6 Reaction Mechanisms
Advanced Higher Unit 3- Organic Chemistry
NUCLEOPHILIC SUBSTITUTION NUCLEOPHILIC SUBSTITUTION
(SN1) (SN2)

R' R'
—
R — C δ+ — Br δ— Nu C δ+ — Br δ—
The nucleophile
R'' The polar C—Hal bond breaks approaches the R R''
heterolytically. electon deficient carbon

A carbocation is
produced which can R' At this stage the molecule is
trigonal planar and
The intermediate is a
trigonal bipyramid R'
be attacked by any
nucleophilic group |
⊕
nucleophilic attack from ei-
ther side is equally likely.
shape
|
C Nu C Br
R R'' R R''
Nu — Nu : As the nucleophiles' electron pair moves in, the electons of
the C—Br bond move onto the bromine

If R , R' and R'' are all different (assymetric carbon), then Even if R , R' and R'' are all different (assymetric carbon),
a mixture of optical isomers will be produced. only one possible isomer can be produced.

Nucleophiles include: Nucleophiles include:

: :

: :

HO — → alcohols HO — → alcohols
(NaOH(aq) or other aqueous solutions) (NaOH(aq) or other aqueous solutions)
: :

: :

RO — → ethers RO — → ethers
(Alkoxide ions, from Na/alcohols) (Alkoxide ions, from Na/alcohols)

H3N: → amines H3N: → amines

(Ammonia) (Ammonia)

CN: — → nitriles CN: — → nitriles

(Alcoholic cyanides) (Alcoholic cyanides)

A variety of products can be made by this reaction but over- A variety of products can be made by this reaction but over-
all the reaction can be represented by the equation: all the reaction can be represented by the equation:

R—Hal + Nu → R—Nu + Hal— R—Hal + Nu → R—Nu + Hal—

SUBSTITUTION SUBSTITUTION
The first step is the rate determining step and, since it only The rate determining step involves both chemicalss so the
involves one substance, the reaction is first order reaction is second order

SN1 or SN2 ? Can depend on the polarity of the C—Hal bond. Can depend on the polarity of the solvent used. Can depend on the
size of the R , R' and R'' groups. No easy answer. Just need to be aware of the the two possibilities.
KHS Chemistry Mar 2001 page 7 Reaction Mechanisms
Advanced Higher Unit 3- Organic Chemistry
ALCOHOL ELIMINATION HALOALKANE ELIMINATION
(USING ACID) (ETHANOLIC KOH)

H H
H H
| |
| |
: :

HO — C — C — H
H — C — C — Br
| |
H |
H+ H H
H
A lone pair from the hydroxyyl oxygen forms a new bond HO:—

:
with the H+ ion. The alcohol ibecomes protonated. The OH— ion acts like a base by removing a
Hydrogen from the molecule as an H+ ion.

H H The electrons move from the C—H bond to

form a π bond between the 2 carbon atoms.
⊕
| |
HO — C — C — H The electrons move from the C—Br bond to
form a bromide ion which is eliminated.
| | |
A hydrogen atom and a bromine atom have been eliminated
H H H from the molecule to form an alkene. Overall the reaction can
The electrons of the C—O bond move onto the be represented by the equation:
oxygen and a water molecule is eliminated
C2H5Br → C 2H 4 + HBr
H H A carbocation is formed. ELIMINATION
| |
⊕C — C — H

| | Electrons move from the

C—H bond to form a π bond
H H between the 2 carbon atoms

A hydrogen ion is
H H
eliminated, replacing
the original H+ ion.
| |
H — C = C — H
An alkene is prduced and overall the reaction can be
represented by the equation:

C2H5OH → C2H4 + H2O

ELIMINATION
KHS Chemistry Mar 2001 page 8 Reaction Mechanisms
Advanced Higher Unit 3- Organic Chemistry
ELECTROPHILIC SUBSTITUTION
This is the main reaction of benzene rings. Though they have even more π electrons than the C = C bond
in alkenes, they resist addition because the loss of the delocalised ring is too destabilising.

ALKYLATION - USING ALKYL HALIDES/AlCl3 HALOGENATION - USING HALOGENS/AlCl3

The production of the electrophile requires the presence of The production of the electrophile is helped by the presence
AlCl3 . (The Al make use of its empty 4th orbital). of AlCl3 . The reaction would be very slow otherwise.

H An AlCl4— ion is
also formed
An AlCl4— ion is
also formed
H— C — Cl : AlCl3 Cl — Cl : AlCl3
H
⊕
CH3 ⊕
Cl π electrons move from the
π electrons move from the ring to form a new bond with
ring to form a new bond with the chlorine ion, Cl+.
the methyl ion.

H CH3 H Cl
The positive charge is shared The positive charge is shared
over the whole ring. ⊕ over the whole ring. ⊕
Electrons move from the C—H Electrons move from the C—H
bond, a H+ ion is eliminated. bond, a H+ ion is eliminated.

CH3 Cl
Overall, an alkyl group takes the place Overall, a halogen atom takes the place
of a hydrogen atom, SUBSTITUTION. of a hydrogen atom, SUBSTITUTION.
The H+ ion reacts with the AlCl4— to reform The H+ ion reacts with the AlCl4— to reform
AlCl3 and a molecule of HCl. AlCl3 and a molecule of HCl.

NITRATION - USING H2SO4/HNO3 SULPHONATION - USING H2SO4/SO3

+
The production of the electrophile , NO2 is a result of a The electrophile is a molecule of SO3. The 3 oxygen atoms
reaction between these two strong acids. are more electronegative; a large δ+ forms on the sulphur.

2 H2SO4 + HNO3 → 2 HSO4— + NO2+ + H3O+ Oδ−

δ+
S Oδ−ringπ electrons move from the
to form a new bond with
⊕
NO2 π electrons move from the Oδ−the sulphur trioxide, SO3.
ring to form a new bond with
Electrons also move in one of the S—O
the nitro ion, NO2+.
bonds.
O−
H NO2 H S O
The positive charge is shared The positive charge is shared O
over the whole ring. ⊕ over the whole ring. ⊕
Electrons move from the C—H Electrons move from the C—H
bond, a H+ ion is eliminated. bond, a H+ ion is eliminated.

OH
NO2 S O The H+ ion that is eliminated from
Overall, a nitro group takes the place of O the benzene ring attaches itself to the
a hydrogen atom, SUBSTITUTION. oxygen ion.
Overall, a HSO3 group takes the place
of a hydrogen atom, SUBSTITUTION.

KHS Chemistry Mar 2001 page 9 Reaction Mechanisms