10

HALOALKANES AND
HALOARENES

elements, have great

Halogens, being among the
most electronegative

derivatives of organic compounds. hese

CHAPTER CHECKLST
tendency to form various in terms of day-to-day applications, .Classification, Nomenclat
derivatives are of great importance
They can De and Nature of Haloalkans
useful compounds, chemical research,
etc.
synthesis of other and Haloarenes
found in nature.
synthesised in laboratories and are also Methods of Preparation of
due to their resistance to
persist in the environment
hese compoundsbacteria. in as
well Haloalkanes and Haloarens
breakdown by soil They have great importance medicines
as in healthcare, e.g chlorine containing
antibiotic, chloramphenicol
fever.
.Properties of Haloalkan
and Haloarenes
is effective in treatment of typhoid the
produced by soil microorganisms hormone, thyroxine,
Even our body produces iodine containing Polyhalogen Compounds
disease called goitre. These halogenated
deficiency of which causes a haloarenes which are discussed
in this
haloalkanes and
compounds involve
chapter. the treatment of
Synthetic halogen compounds, e.g. chloroquine
inis used
used an anaesthetic in surgery.
In
malaria and halothane (CF, CHCIBr) is as
methods of preparation, physical and
this unit, we will study the important
chemical properties and uses of organohalogen compounds.

TOPIC 1
Classification, Nomenclature and
Nature of Haloalkanes and Haloarenes
The replacement of hydrogen atom(s) fromresults hydrocarbon,
a aliphatic or

B, in the formation of alkyl

aromatic, by halogen atom(s) (1.e. F, Cl, I)
halide (haloalkane) and aryl
halide (haloarene), respectively.
Haloalkanes contain halogen atom(5)
attached
to the sp-hybridised carbon
contain halogen atom(s) attached
atom(s) of an alkyl group, whereas haloarenes
sp-hybridised carbon
atom(s) of an aryl group.
to
 kanes and Haloarenes

AASSIFI
FICATION 355
and haloarenes may be
H a l b a l k a n e sa n o

classified as follows: H
CH CH3
Basis of
,
Onthe
Halogen Atoms
Number of CH-C-Br; H-C-Br CH-C-Br
H CH
Depending
an
on the number of
halogen atoms in Bromoethane CH3
rures, haloalkanes and 2-bromopropane 2-bromo-2-methylpropane
aloarenes may be mono,
their (Primary or 1°)
(Secondary or 2°) (Tertiary or 3°)
yhalogen (tetra, enta, etc.) di, tri
compounds. i) Allylic halides In these halides, the halogen is bonded
CH,X to the
sp°-hybridised carbon atom next to
CH,X carbon-carbon double bond which is also called
CH,X CHX carbon. Hence, these halides are allylic
or called allylic halides.
CHsX CHX X
CH,X
Monohaloalkane Dihaloalkane Trihaloalkane
3

X
CH,X,
X
X
3-haloprop-1-ene
(Allyl halide) 3-halocyclohex-1-ene
Monohaloarene Dihaloarene
Trihaloarene CH2=CH-CH2-Cl,
(where, X= F, Cl, Br, 1)
3-chloroprop-1-ene
3-chlorocyclohex-1-ene
9, On the Basis of Nature of ii) Benzylic halides In these halides, the halogen atom is
Carbon ofC-X Bond bonded to the
sp°-hybridised carbon atom next to an
Monohalo compounds may further be classified aromatic ring, i.e. to a
benzylic carbon.
to the
hybridisation of the carbon atom to which accordingis R
bonded, as discussed below. halogen CH2X
e.g R"
A. Compounds Containing sp°C-X Bond
These are classified into three (1°)
types:
() Alkyl halides or Haloalkanes (RX) In these halides, If R =CH3, R" =H (2°)
the halogen atom (X) is bonded to an alkyl group IfR=R" =CH, (3°)
They form a homologous series of (R).
represented the
by formula C,H2n +1X.
compounds CH3
They are further classified as primary (1), CH,CI C-C
2) or secondary
tertiary (3°) depending upon the nature of H
carbon atom to which the
halogen is attached. (1°) (2°)
H R R'
CH
R-C-X ; R"-C-X; R"-Ç-X -CI
H R" CH3
Primary (1°) Secondary (2°) Tertiary (3°)
(3)
 Allinone| CHEMISTRV
356
Disubstituted Haloalka.
B. Compounds Containing sp*c-X Bond 2. For
having the same type of h
alides.haloThe
gen adih
to one or
compounds, halogen is directly attached
dihaloalkanes
In these The o r alkylene dihal:
as
alkylidene
named
are same type ofhalogen ms are hurh,
the doubly bonded carbon atom ( = -X). compounds
having
classified as:
These are classified into the following two types: ) Geminal halides Here halogen atoms are present
Is
G) Vinylic halides Inthese halides, the halogen atom carbon a t o m .
of one of the the same

bonded to the sp -hybridised carbon Here

(i) Vicinalhalides Here halogen atoms are
carbon atoms of a double bond, i.e. vinylic
carbon.
adjacent carbon atoms.
sent on he
X
eg system, genm-dihalides
X In common
name

halides,
whereas vic-dihalides named
Haloethene
alkylidene
dihalides. In
lUPAC system, thev
are named
amed
(Vinyl halide) 1-halocyclohex-1-ene alkylene
atom is dihaloalkanes.
i) Aryl halides In these halides, the halogen CI
bonded to the sp -hybridised carbon
atom of an
CH,CH CH-CH,
aromatic ring. CI C C
X e. gem-dihalide
vic-dihalide
e.g. X
Common
name
Ethylidene chloride Exthylene dichloride
H3C p-halotoluene
IUPAC name
1,1-dichloroethane
1,2-dichloroethane
Halobenzene
3. For Haloarenes
NOMENCLATURE prefixing halo betore the name ofa
They are named by
aromatic hydrocarbon.
Haloalkane Haloarenes are the common as well as 1UPAC namessof ami
1. For Monosubstituted
are derived by naming
derivatives, the prefixes 0, m-
The names of alkyl halides
common halides. For dihalogen
followed by the name of halide used in common system
but
in luPAC system, th
the alkyl group
n, is0-, sec, are used respectively.
the prefixes numerals 1,2; 1,3 and 1,4
(chloride, bromide, etc). Here, halides are
tert, etc., are used.
In IUPAC system, alkyl Br
i.e. haloalkanes. The
named as halosubstituted hydrocarbons, of
names are written by prefixing
the word 'halo' to the name Br
continuous carbon chain
alkane corresponding to the longest
having the halogen atom.

e.g CH,CH,CH,I; (b)

n-propyl iodide
(a)
Common name
IUPAC name (1-iodopropane) Br

CH-CH-CH-Br;

CH3 Br Br
iso-butyl bromide
Common name
IUPAC name (1-bromo-2-methylpropane)
of above tigures are ghu
Common and 1UPAC names
CH3
below:
IUPAC name
Common name
CH-C-CH,C
Bromobenzene
(a) Bromobenzene

CH (b) m-dibromobenzene
1,3-dibromobenzene

neo-pentyl chloride
Common name (C) sym-tribromobenzene 1,3,5-tribromobenzene
IUPAC name (1-chloro-2, 2-dimethylpropane)
fabalkanesand Haloarenes
357
C
ommon
o m
and JUPAC names of some halides
Common name i) The expanded structure is:
Sructure UPAC name
n-propyl fluoride
-fluoropropane CH
neo-pentyl bromide0romo-2,2-dimethy!
sec-butyl chloride
propane CH,-CH=¢-a
HOHCH(C)CH,
tert-butyl bromide
2-chlorobutane
bbC-B
2-bromo-2-methyl
propane
CH,
h =CH-CHBr Allyl bromide

=HC Vinyl chloride 3-bromopropene

Chloroethene
Methylene chloride Dichloromethane In case, if a halogen (functional group) is attached to a
Chloroform benzene ring which in turn, is attached to an alkane or
CHU Bromoform
Trichloromethane alkene then the
Tribromomethane position of that halogen will be written
CHB
Carbon in bracket as
tetrachloride Tetrachloromethane 1-chloro-1-(4-iodophenyl)-3,3-dimethylbut-1-ene.
C 0-chlorotoluene 1-chioro-2-methyl
benzene
ISOMERISM
Or Haloalkanes exhibit the following two types of isomerism:
2-chlorotoluene
Chain Isomerism
CHC
Ifa haloalkane has four or more carbon atoms, it can exhibit
Benzyl chloride
Chlorophenylmethane chain isomerism by
making a branched or straight chain
compound. e.g. Buryl bromide has two possible chain
isomers as follows:
EKAMPLE 1| Write the IUPAC names of the following
compounds:
CH-CH-CH-CH;-Br,
1-bromobutane
) (Cl,),CCl
() (CHy )3CCH= C(CI)C,H,l-p
CH
NCERT
ol CH-CH-CH,Br
) We follow the following steps: 1-bromo-2-methylpropane
L Towrite the IUPAC name, we first
of the compound as follows:
expand the structure Positional Isomerism
A haloalkane
CI having atleast three carbon atoms show
position isomerism by having halogen atoms at different
carbon number, in a chain, in diferent isomers.
C-c-C
CI e.g. Propyl bromide exhibits two
follows: position isomers as
a-c-a Br

C CH CHCH,Br CH-H-CH
CIC-C 1-bromopropane 2-bromopropane
CI NATURE OF C-X BOND
. Now we do the
numbering of the carbon chain,
Specifically from the end where functional group 1s Electronegativity of halogen atom is greater than that of
nearer and/or carbon atom due to which the shared
more in number. In this case, pair of electron in
C-X bond lies closer to the halogen atom. As a
humbering from both the ends is equivalent. result, the
hen we write the IUPAC name, counting the halogen atom bears a partial negative charge whereas, the
Tunctional groups and their carbon atom bears a
partial positive charge.
positions.is
Thus, the name of the given structure
tnchloromethyl)-1,1,1,2,3,3,3,-heptachloropropane -
 358
e of
the size o
of the periodic table,
in a group
AS we go down is the smallest and
halogen atom increases, fluorine atom
Consequently, the
iodine atom is
the largest. C - F to
also increases from
carbon-halogen bond length
C-I.
electronegativity of the
Further, as we move from F to I, the C-X bond
decreases, therefore the polarity of the
halogen
decreases accordingly.
bond
Carbon-halogen (C-X) bond lengths,
enthalpies and dipole moments
C-X Bond
Bond between Bond Dipole
carbon and enthalpies/ moment/Debye
length/pm kJ mol
halogen
452 1.847
CH3-F 139
1.860
CH C l 178 351
293 1.830
CH3-Br 193

214 234 1.636

CH3

In haloarenes, sp hybridised carbon of benzene is bonded

in haloalkanes
to halogen and C-X bond is also polar as
due to higher electronegativity of halogen. Apart from this,
of electrons of halogen atom are involved in
lone pair
resonance with benzene ring. So, this C--X bond acquires
partial double bond character.
The C-X bond of haloarenes is less polar than C-X
bond of haloalkanes. This is supported by the fact that
dipole moment of chlorobenzene (u 1.69 D) is little lower
=

than thar of CH,Cl (H =1.83 D). The C-X bond of a

haloarene is usually shorter than haloalkane.
The reason for this occurrence is that haloarenes are more
electron rich, due to the presence of double bonds, than
haloalkanes and the benzene ring as a whole due to
resonance is strongly attracted towards electronegativve
halogen atoms.
 reactive and therefore,
OPIC2 Tertiary alcohols are very
conducted by simply shaking
their reactions are at room temperature
even

othods
Ie off p
thodso Preparation of With concentrated HCI
in the absence of zinc chloride.

alkanesand Haloarenes CH3

Room temperature

CH-C-OH + HCl (conc.)

PREPARATIO OF
CH3
HALOALKANES
tert-butyl alcohol CH3
Ikanes canbe prepared from wide range of organic
Haloalkar CH C-C +H20
Dunds through various nmethods. Some
cthodsof preparation. are discussed below: important
CH3
tert-butyl chloride
Preparation from Alcohols (2-chloro-2-methylpropane)
method for the preparation of (i) Bromoalkanes are prepared by heating an alcohol
Thisis the most convenient In with constant boiling of HBr (48%) in the
haloalkanes in the oratory. this method, the hydroxyl
of an alcohol is replaced by halogen on reaction with presence of conc. H,SO4, which acts as a catalyst.
concentra halogen acids, phosphorus halides or thionyl
Aloride. Different reagents can be used to get haloalkanes CH,CH,OH +HBr
Conc.
H,SO4
A
CH CHBr
Ethyl alcohol Ethyl bromide
fom alcohols
as described below:
+H20
a By the Action of Halogen Acids HBr can also be generated in situ by the action of
Haloalkanes are prepared by the treatment of alcohols with conc. HS04 on KBr or NaBr.
halogen acids as follows:
CH CHOH + KBr + H,SO4
ROH + HX RX +H,O Ethyl alcohol
Alcohol Halogen acid Haloalkane Water
The rate of reaction depends on the nature of alcohol as
CH,CH,Br +KHSO4 +H,O
Ethyl bromide
well as the halogen acid.
0 The reaction of primary (1°) and secondary (2°) Gii) Iodoalkanes are prepared by heating an alcohol
with constant boiling HI (57%).
alcohols with halogen acids require the presence of a
catalyst, ZnCl, which acts as a Lewis acid and helps in Reflux CH,CH,I+H,O
CH,CH,OH +HI
the cleavage of C-O bond. Chloroalkanes are Ethyl alcohol lodoethane
prepared by the action of hydrochloric acid to the
HI can also be generated in situ by the action of
primary and secondary alcohols in the presence of
anhydrous zinc chloride (Groove's process). 95% phosphoric acid on KI.

CHCH,OH +HCI()
Anhydrous ZnCla CH,CH,OH+KI+
Ethyl alcohol
H3PO4 CH,CH,I
Ethyl alcohol (1) Phosphoric lodoethane
acid
CHCHCI + HO +KHPO4 + H20
CH3 Ethyl chloride
This reaction gives good yields of alkyl iodides.
Anhydrous ZnCl, Note
CH-COH+ HCI(g) ) Secondary and tertiary bromides and iodides cannot be
H CH3 prepared from their respective alcohols in the presence of
conc. HS04 as they undergo dehydration to form alkenes
150-propyl alcohol (2°) (i) The order of reactivity of alcohols with a given haloacid is
CHC-Cl+H,O 3°>2 1°.
(i) The order of reactivity of halogen acids with alcohols is
H HI > HBr > HCI.
50-propyl chloride
 TRY
Allinone | CHEMISTRY Class
364
amounts
of these isomeric haloalkan
The relative halogen and
Action of Phosphorus Halides depend upon
the nature of the
and the number
2° and
b) By the atoms (1°, 3)
stitution,whicvarh iou
PCl3 of hydrogen
action of PCl, or
and type of substi
) Chloroalkanes are obtained by the substituted.
For the ease

on alcohols. being the sequence as

tollows
R-OH+PCI R-Cl+POC3 +HCI hydrogens
3°>2°>1°
CH,CH,OH +PCI CH,CH,Cl+ POCl +HCl (b) From Alkenes
Ethyl alcohol
Chloroethane Phosphoryl Halides
of Hydrogen
chloride
(i) Addition
the reaction
with hydrogen halide, an
alk
3CH,CHOH +PCl 3CH,CH,CI + H,PO3 By halide ne
Phosphorous converted to
the corresponding alkyl
Ethyl alcohol Chloroethane
acid
the
i) Bromoalkanes and iodoalkanes are prepared by
with the alcohols.
reactionof PBr3 and Plz, respectively the reaction
e.g C C +HX.

Pl3 are usually generated in byrespectively.

and siu
PBt3
and iodine,
Alkene
H X
of red phosphorus with bromine Haloalkane
R-OH RedP/X, R-X (where, X =Br, ) halogen acide
The decreasing order of reactivity ofthe
Red P/Br2 HF
CH-OH CH-Br HI>HBr>HCl >
of addition to symmetrical alkenes, only oe
(c) By the Action of Thionyl Chloride In case
formed.
alcohols with addition product is
Chloroalkanes are prepared by refluxing the to unsymmetrical alkenes
in the presence of pyridine. But in case of addition
SOCl, Markovnikov's rule is followed, e.g propene yields owo
CH,CHOH+ SOCl CH,CH,C+ SO1+ HCIT products but only one predominates according
to

Ethyl alcohol Thionyl Chloroechane Markovnikov's rule.

chloride
of this CH,CH-CH, +HI CH,CH,CHI
This method is usually preferred since both the products the Propene 1-iodopropane
Hence,
reaction (SO2 and HCl) are escapable gases. (Minor)
reaction gives pure alkyl halide. +CH-CH-CH
not applicable for the preparation
Note The above three methods are
the C-O bond in phenol has a partial double
of aryl halides because I
than a single
bond character and is difficult to break being stronger 2-iodopropane
bond. (Major)
Preparation from Hydrocarbons However, in the presence of a peroxide, the addition
of

in contray
halides be prepared from alkanes through HBr to an unsymmetrical alkene, proceeds
Alkyl can
addition of halogen the Markovnikov's rule. This effect is
known #
substitution and from alkenes through to
Kharasch effect or peroxide effect.
acids or through allylic substitution.

(a) By Free Radical Halogenation CH,CH-CH,+HBr CH,CH,CH,Br

in the presence of 1-bromopropane
Alkanes react with halogens (Cl2 and Brz) Propene
UV light to form haloalkanes.
The reaction proceeds through It is informally called as anti-Markovnikov's addituo
free radical mechanism and gives a complex mixture of
isomeric mono and polyhaloalkanes which are difficult to i) Addition of Halogens
separate as pure compounds. In the laboratory, addition of bromine (Br) 0
C,/UV light alkene in CCl4 results in discharge of reddish-bror
eg. CH,CH,CH,CHoror Heat colour of bromine constitutes an important nethod.
h
the detection of double bond in a molecule
CHCH,CH,CH,C+CH,CHCH,CH, addition results in the synthesis ofvic-dibromides
Cl are colourless.
 nes and Haloarenes
Hooat 365
n adang
B, Br orClhalkenes, the
to
addition occurs
nd forming vic-dihalides, at the CI

Hc-C CCla, BrCHCH,Br O).a FeCl +HCI

Alkene vic-dibromide Ch310-320K Chlorobenzene

Prepanaration by Halogen Exchange CI CI

ides are often
prepared by the reaction C
Alyl
s/bromides with Nal in dry acetone. This of alkyl
dhlorides/,
Finkelstein reaction. reaction is +Cl, , (
w
known a 4s (Excess)
R-X + Nal
-

R-I+NaX [X =Cl, Br] C

Acetone o-dichlorobenzene p-dichlorobenzene
CH,CHBr +Nal ACHCH,I+NaBr (minor) (major)
NaBr or NaCl, thus formed gets precipitated in dry
which facilitates the forward reaction according to
CH CH3 CH
CI
hatelier's principle. Fluoroalkanes are best
aCctone

prepared by
ating alkyl chloride/bromide in the presence of a metallic
O).C+Clh- FeCl3 (O
fuoride such as AgF,IHgF2, CoF2 or SbF3. This reaction
as Swarts reaction.
is known CI
CH,Br + AgF CHF+ AgBr o-chlorotoluene p-chlorotoluene
Methyl bromide Silver fluoride
Fluoromethane
0-
and p-isomers can be easily separated due to large
Preparation from Silver Salts of Acids difference in their melting points.
Reaction with iodine is reversible and require the
Generally, bromoalkanes are prepared by refluxing the silver
alts of acids with bromine in CCI4: This reaction is known presence of an oxidising agent (HNO3, HIO4) to
Borodine-Hunsdiecker reaction. The reaction can be oxidise the HI formed during iodination.
The reaction with fluorine is violent and
depicted as:
uncontrolable, hence fluoroarene be
cannot
prepared by direct fluorination of aromatic
CH,COO Ag +Br CCl
CH,Br +CO T+AgBr hydrocarbon.
Methyl
bromide
From Diazonium Sats

PREPARATION OF HALOARENES (Sandmeyer's Reaction)

The
The diazonium salt is prepared by
treating aniline dissolved
Haloarenes can be prepared by the following methods: in cold aqueous mineral acid with an
aqueous solution of
sodium nitrite at low temperature (0-5°C). Bromo and
By Electrophilic Substitution of chloroarenes can be prepared by treating a freshly
prepared
Aromatic Hydrocarbons diazonium salt solution with cuprous bromide or
cuprous
chloride.
Aryl chlorides and bromides can be easily prepared by
cectrophilic substitution of arenes with chlorine and NH2 N,X
NaNO +HX
Omine, respectively in dark, at ordinary temperature in the
273-278 K
Prsence of a Lewis acid catalysts such as iron or ferric halides
0r Benzene diazonium
aluminium halides (FeClz, FeBr3, AICI3). Lewis acid is
halide
dto generate the electrophile, i.e. CI and Br which
Teacts with
aryl halide. NX
CH
+X-
(Dark
CH CH C ,
Aryl halide
+NT
XX p-halotoluene
o-halotoluene (X= Cl, Br)
 366
Replacement of the diazonium group by iodine is donc
iodide.
simply by shaking the diazonium salt with potassium
N,X KI + N,T

The Sandmeyer's reaction has been modified by using

is
copper powder in place of cuprous halide. This reaction
called Gattermann reaction.
NCI CI

O] HC[O]+NT
Chlorobenzene
fluoroboric acid,
If the diazonium group is replaced using
the reaction is called Balz-Schiemann reaction.
NC NBE
|+HBF4 A|OJ+ BFs+ N,T
Fluoroboric Fluorobenzene
acid
ITOPIC 3
Properties of Haloalkanes and Haloarenes
PHYSICAL PROPERTIES OF atom, the magnitude of van
der
HALOALKANES AND increases. Waal
HALOARENES
Some of the
As the size of alkyl group
decreases, heil
decrease for the same halogen atom. botling pin
and haloarenesimportant physical properties of haloalkanes
are as follows:
CH,CH,CH,X > CH,CH,X >CH, X
) Physical state (where, X =F,C,
are
Alkyl halides are colourless when they
pure. However, bromides and
For isomeric alkyl halides, Brandt
boiling point decre
iodides the branching increases. This iis because with
colour when
exposed to light. Methyl chloride, develop
methyl in branching, the surface area of
alkyl halid n
bromide, ethyl chloride and some and hence, the magnitude of the van
are
gases at room chlorofluoromethanes der Wa
temperature.
solids. Many volatile Higher members are of attraction decreases.
torcer
liquids or
have sweet smell. halogen compounds CHCH CH,CH,Br>
i) Melting and boiling points Molecules of (bp=375 K)
halogen compounds are generally polar. Due to organic CH
polarity and higher molecular mass greater
parent hydrocarbon, the compared as
to the
intermolecular forces of
CH,CHCH-CH3 >CH3-C-CH
attraction
(dipole-dipole
molecules are
and van der Waals) between Br
(bp=364 K)
Br
stronger in the
halogen derivatives. bp 346 K)
That's why, the
and iodides are
boiling points of chlorides, bromides Generally, the boiling points of chloro,
of
higher than those of the
hydrocarbons iodo compounds increases the bromo a
comparable molecular mass. atom increases.
number of halogen
as

As the molecular increases, the

mass
boiling points increases. The attractions getmelting andas eg.CH3Cl<CH,Ch
249 K
<CHCl, < CCl
the molecules stronger 313K 334K 350 K
get bigger in size and have more
electrons. Boiling points of isomeric dihalobenzenes
The pattern of variation of the same. are
neaty
halides is boiling points of different CI CI Cl
depicted in table given below:
bp ( C
400
Chlorides C
300 Bromides
mp/K 256
lodides bp/K 453
249
323
200 446 448
However, the p-isomers have
100 high melting point
compared to their o-and m-isomers. This is due to
symmetry of p-isomers that fits in the crystal latieur
better
CH3X CH,CH2X CH,CHCH X as
compared to 0- and m-isomers.
Comparison of boiling points of some alkyl halides ii) Density Fluoro and chloroalkanes han
water while bromo, iodo and polychloro lighter
are
For the same alkyl derivatives
group, the boiling
point of alkyl heavier than water.
halides decreases in the order R1> RBr> With the increase in the numb
RCl> RE, Catoms, halogen atoms and atomic mass of alogen
because with increase in size and of atom, density increases
mass
halogen
dafoalkanesaand Haloarenes 373
i )Solubilir For haloalkane
olve in water,
to

overcome the attractions

to
i+ y c - N u +X
energy s r e q u i r e d

che haloalkane molecules and break the

berwee
onds berween water molecules. Less
halides(R-X)
hydrogen
Nucleophilic substitution of alkyl
energyi s
ased when new attractions are set up R-X+Nu R-Nut X
haloalkane and the water molecules as these
berween Class of main
not
strong
as the original hydrogen bonds in Reagent Nucleophile (Nu )oroduct (R-Nu) produc
ae
As a result, solubility of haloalkanes
the solubilir
in Alcohol
warer.
water i s l o w .
NaOH(KOH) HO ROH
HO H20 ROH Alcohol
are soluble in organicsolvents because the new
NaOR Ether
nolecular attractions between haloalkanes and A'O ROR
Alkyl iodide
Nal
solvent moleecules
cu are of same strength as the ones RI
Primary arnine
exists NH3 NH3 NH2
being broken that separately between the alkyl sec-amine
molecules and:solvent molecules. R'NH2 R'NH2 RNHR
halide tert-aminne
R'R'NH R'R NH RNR R
Stability The stability
also decreases as the strength Nitrile (cyanide)
KCN C=N: RCN
ofC-X bond decreases (RF> RBr> RCl>
RI). AgCN Ag CN: RNC (isocyanide) Isonitrile

moment As the
electronegativity of the KNO2 O=N-0 R-O-N=O Alkyl nitrite
Dipole
i )Dip
from
decreases Cl to I, dipole moment also AgNO2 R NO2 Nitroalkane
halogen Ag-0-Ñ=O
whereas
es fluorides have lower dipole Ester
ment than chlorides because of very small size of R'COOAg R'COO R'COOR

LiAIH4 H RH Hydrocarbon
F which outweighs the effect of greater
RMt RR' Alkane
electronegativity.
Groups like cyanides and nitrites possess two nucleophilic
CHCl>CH3F >CH,Br>CH,I centres and are called ambident nucleophiles. Cyanide
group is a hybrid of two contributing structures and
CHEMICAL REACTIONS OF therefore, can act as nucleophile in two different ways
HALOALKANES :C=N:G:C=NI.
Due to the presence of a polar C X bond, haloalkanes
e highly reactive compounds. The reactions may be When it links through carbon atom, it results in formation of
dvided into following categories: alkyl cyanides, while that through nitrogen atom, it results in
formation of isocyanides.
Nucleophilic Substitution Reactions Similarly, in nitrite ion. There are two different points of
When a nucleophile (i.e. electron rich species) stronger linkage (O-N=O). The linkage through oxygen results
than the halide ion, reacts with haloalkane having a in alkyl nitrites while that through nitrogen atom, results in
partial positive charge on the carbon atom bonded to nitroalkanes.
alogen, a substitution reaction takes place and the Note Among alky! halides, iodide ion is the best leaving group and
leaving group departs as ion.
alogen atom called halide hence, iodoalkanes undergo nucleophilic substitution reactions at the
ince, the substitution reaction is initiated by a fastest rate in comparison to fluoroalkane, as fluoride ion is the poorest
nuclecophile, hence it is called nucleophilic substitution leaving group. Better the leaving group, more facile is the nucleophilic
reaction. substitution reaction.
 Allinone| CHEMISTRY Css
374 follow:
as
halides
(R-X) are
cophilic
substitution
reactions of alkyl Nucleophiles

Reactions
(HO)
+aq. KOH ROH+ KX;
Alcohol
(R'O)
Ni'OR ROR'+ NaX;
Ether
(H,0)
+H,0 ROH
Alcohol
-C=N)
+KCN R - C N + KX;
Alkyl cyanide
(Ag-C=N)

+AgCN R-NC+ AgX;

R-X Alkyl isocyanide
Alkyl R - 0 - N = 0 + KX; (0=N-0)
halides +KNO;
Alkyl nitrite
(Ag-0-Ñ=O)

+AgNO2 >R-NO0, +AgX;

Nitroalkane

(R' COO)
O-A, R ' - C - 0 - R + AgX;
Ester

+LiAlH4 R-H
Alkane

+NH3 R-NHHai (NH3)

1 amine

+Na CCH R-C=CH + NaX ; C=CH)

Higher alkyne
R"M* R-R'+ MX; (R)
Alkane

EXAMPLE |1| Haloalkanes react with KCN to form alkyl Mechanism

cyanides while AgCN forms isocyanides as the chief product. The nucleophilic substitution reactions proceed hu .
different mechanisms:
Why? following two

Sol. K"CN" is predominantly ionic and provides cyanide ions I. Bimolecular Nucleophilic Substitution(s,
in solution. Although, both carbon and nitrogen atoms are
in position to donate electron pairs, the attack takes place
When two molecules take part in determining the rae
mainly through carbon atom and not through nitrogen
the reaction, it is called bimolecular nucleophite
atom since, C-C bond is more stable than C - N bond. substitution (Sy 2). Here, the rate depends upon the
However, Ag-CNis mainly covalent in nature and only concentration of both the reactants.
electron pair of nitrogen is available for bond formation.
e.g. The reaction berween CH3Cl and hydroxide ion
As a result, alkyl isocyanides are the chief products.
yield methanol and chloride ion follow a second order
EXAMPLE |2| Haloalkanes react with KNO2 to form kinetics.

alkyl nitrites while AgN02 forms nitroalkanes as the chief OH+ A 8-

product. why? HO-
Sol. KNO2 (or O=N--0K) is predominantly ionic and H H H
one of the oxygen atom have a negative charge.
Nucleophile H Transition state

Nucleophilic attack through this negatively charged Reactant H

oxygen atom on the alkyl halides mainly gives alkyl
nitrites. In contrast, AgNO, is a covalent compound and HO- "
+C
both oxygen and nitrogen atoms carry lone pair of H
electrons. Since, nitrogen is less electronegative than H
oxygen, therefore, lone pair of electrons of nitrogen is Product
more easily available for bond formation. As a result,
This can be represented diagrammatically as shownbao
nitroalkanes are the chief products.
 ac aand
alkanes Haloarenes 375

Primary halide
Methyl halide
(1)

HH
Nu: X HC X
Nu-
represents t incoming hydroxide ion and
epresentS the

the outgoing
our halide ion black dot H
Greyents
nechanism, the Methyl Ethyl, 1°
nism, the incoming nucleophile approaches
hSy2
halide (CH3 X) lecule and starts
u h ea t
halide
which the carbon-h
interacting
(C-X) bond starts Secondary halide Tertiary halide
(2) (3)
rnd a new carbon-nucleophile (C-OH) bond
Aang These trwo processes take place simultaneously HH
forming.

no intermediate is formed. HH
Single
step
Nu H C
iha Occurs through the formation of a transition H
Nu X
such ransition state, the C-atom is
simultaneously HC
s t a t c

to incoming nucleophile and also to the outgoing H H H

bonded
kaving.group, i.e. carbon is bonded to five atoms. Thus, H H
tate is unstable. and results in the formation of the HH ter-buryl, 3°
iso-propyl, 2
product.
Steric effects in S2 reaction
H H
Nu+ Nu X Nu +X II. Unimolecular Nucleophilic
HH Substitution (SE 1)
Transition state
When only one molecule is involved in determining the rate
It Sy2 reactions, the attack of nucleophile (ie. Nu") occurs of the reaction, it is called unimolecular nucleophilic
fom the backside and the leaving group leaves from the front substitution. Such type of reactions are generally carried
out
in polar protic solvents such as water, alcohol, acetic acid,
sde As this happens, the configuration of C-atom under attack
nvert in the same way as an umbrella is turned inside out when etc., because they stabilise the carbocation by solvation.
aupht in astrong wind, while the leavinggroup is pushed away, Mechanism
this process is called inversion of configuration or Walden
SNl reaction occurs via two steps:
nversion and is a characteristic feature of Sy 2 reaction.
In step 1, formation of carbocation takes place due to the
Since, S 2 requires the backside approach of nucleophile
heterolytic cleavage o f C - X bond. This step is slow
to the carbon bearing the leaving group, i.e. X-atom, the
and reversible (rate determining step). This step involves
presence of bulky alkyl groups blocks the approach of the only one reactant, i.e. alkyl halide, therefore, rate of reaction
nucleophile to carbon due to steric hindrance. That's why
the simple alkyl halides, methyl halides and primary alky depends only on the concentration of alkyl halide and not
on the concentration of nucleophile.
halides always react predominantly by SN 2 mechanism.
In step II, nucleophile attack the carbocation formed in step
Teriary halides are least reactive because of the presence of I and the substitution reaction completes.
bully groups which hinders the approaching nucleophiles.
e.g. The reaction between tert-buryl bromide and hydroxide
Non-polar solvents favour SN 2 mechanism. ion yield tert-butyl alcohol.
hus, the order of reactivitry of alkyl halides towards Sn2
Teactions is as follows: (CH3),C-Br +OH (CH3)3 COH+Br
 Allinone CHEMISTRY G
376
need to be learne
and notations are hich are
Step I Formation of carbocation intermediate follows:
CH3 Light and ptical Activity
Optical
Plane Polarised
consists of
beam of ordinary light
Step I
C + Bf
(CH),C-Br (Slow)
HC CH
A
waves vibrating in all plane in space. Wher
a nicol prism,.it
dlecromag
becomes planethis le
passed through
Step II Attack of nucleophile on carbocation formed
light, which vibrates in one
only plane. pla
CH3 Passed through
+OH (CH,),COH
(Fast)
nicol
nicol prim
prism |
H,C CH Plane polarised lighr
Ordinary light
.Greater the stability ofcarbocation, greater will be its ease Polarisation of ordinary light
of formation from alkyl halide and faster will be the rate certain organic com
When the solutions of
of reaction. That's why, 3° alkyl halides undergo SN
reaction very fast because of high stability of 3 placed in the path of plane polarised light, theyr pounds a
path of the light either to left or right, such roate
carbocations.
Therefore, the order of reactivity of alkyl halides toward SN called optically active
substances. substancee
reactions is as follows: A substance which rotates the plane polarisedliohr
t to
right is called dextrorotatory d-form and is
or
Methyl halide< Primary halide < Secondary halide< Teriary indica
by placing a positive (4) sign before the degree of ro
halide We can sum up the order of reactivity of alkyl halide
towards SN l and Sn2 reactions as follows: The substance which rotates the plane polaised
towards the left is called laevorotatory or Lforma
S l reactivity increases
indicated by placing a negative (-) sign before the de
of rotation.
Tertriary halide, Seconday halide, Primary halide, CH3 X
Such d-and -forms of compound are called optic
SN 2 reactivity increases
isomers and the phenomenon 1s called opic
Allylic and benzylic halides show high reactivity towards the isomerism. The angle through which the plane polato
Syl reaction because carbocation thus formed gets stabilised light is rotated can be measured by an instrument alk
through resonance.
polarimeter.
H,CçH, H,ç>CH Molecular Asymmetry, Chirality and
H H Enantiomers
The four valencies of carbon atom are directed towards te
CH CH corners of regular tetrahedron and if al the atoms a
groups attached to a carbon atom are different, sia
carbon atom is called asymmetric carbon atom a
CH2 CH2 CH2 stereocentre. The resulting molecule would la
symmetry and the molecule is called asymmex
molecule. The asymmetry of the molecule is response
for theoptical activity in such organic compouns
For a given alkyl group, the reactivity of the halide,
All
R-X follows the same order in both the mechanisms, molecules or objects have mirror images. An objeaa
molecule which is not superimposable on
Sl and SN2. It5
image is called chiral and this property is calledchinly
R-I>R-Br>R-Cl>R-F C.g. human hand.
Stereochemical Aspects of Nucleophilic If we hold our left hand in front of a mirror, the
Substitution Reactions looks like the e

SN2 reaction proceeds with the complete stereochemical

right hand. If we try to superimpu
and right hands,
inversion while Syl reaction proceeds with racemisation. To
by putting the one on the otherrone,
observe that the hands hena

understand this concept, some basic stereochemical principles the hands are chiral.
cannot be superimposed
ynloalkanes and ldloarenes
377
Mirror Mirror
Racemic Mixtures and Racemisation
A
mixture containing two enantiomers in equal
Oportions will have zero optical rotation, as the rotation
C to one isomer will be cancelled by the rotation due to
ne other isomer. Such a mixture is called racemic mixture.
s represented as dl or t forms and will be optically
Chiral objects
Non-chiral objects nactive. The process of converting d- or L-form of an
(Superimposable
n - s u p e r i m p O s a b l e

Tor image)
mirror image) Optically active compound into racemic form (dl) is called
S o m ec o m m o mon examples of chiral and achiral racemisation.
objects
which are Retention
ed achiral assuperimposable their mirror
T h eo b y e c t s
on
the When the relative spatial arrangement of bonds
inage
a sr ec a l l e d

glass and the sphere in the asymmetric centre in a chiral molecule remains the same
to an

Let us take two simple molecule

ahove
igure, re.
and thei mirror images. propan-2-ol betore and after the
reaction, the reaction is said to occur
tan-2-0

contain asymmetric carbon Propan-2-ol does

atom.
with retention of
configuration,
e.g.
not
Therefore, it is
a c h i t a l . CH CH
Mirror H CH OH CH2 CI
After rotating B
H

H -OH HO-C
CH H,Cy 180 CH2 + HCI i c a t
C-OH CH2
HCH HCH H,C H CH CH
A B
C 2-methyl-1-butanol (+)1-chloro-2-methylbutane
In the above
The structure C 1s superimposable on A. example, no bond at the asymmetric carbon is
broken, and the product will have the same general
L.ran-2-ol molecule contains asymmetric carbon atom and contiguration of groups around the stereocentre as that of
sexpected is chiral reactants. Therefore, the reaction
of
proceeds with retention
t has non-superimposable mirror images as shown in the configuration inspite of change of optical rotation from
igure:
to (+).
Mirror Inversion
CH3 CH3 If the relative spatial arrangement of bonds at an
asymmetric carbon atom becomes opposite as compared to
reactant after the reaction, then the reaction is
CHCH3H,CH,C H inversion.
called

OH OH Examples of Inversion, Retention

and Racemisation
Some common examples of chiral molecules are
During the substitution of a group X by Y in the reactions
ehlorobutane, (CH,CH(C)CH,CH3) given below, three possible products may be formed.
13-tihydroxy-propanal (OHC-CHOH-CH,OH), CaHs
romochloroiodomethane(BrCICHI), CHs
H C
oromopropanoic acid (HC-CHBr-COOH), l H H
Ihe optical isomers which are non-superimposable mirror
images of each other are called enantiomers and the H,C CH CH3
(B) A)
phenomenon is called enantiomerism.
(A+B)
ne enantiomers have identical physical and chemical
Poperties but differs with respect to the rotation of plane If A is the only product, the process is called retention of
aised light. If one of the enantiomer is dextrorotatory configuration because A has same contiguration as that of
then other will reactant. If B is the only product, the
be laevorotatory. process is called
inversion of configuration because B has the
configuration
378 Allinone| AEMISTRY

opposite to that of reactant. If an equimolar mixture 1.e.

Elimination Reactions
50:50 mixture of Aand B is formed, the is called Dehydrohalogenation (Formation of Alkeves
process
racemisation and the product is optically inactive. When a haloalkane with B-hydrogen aton
Now, stereochemical aspects of atom s heated
nucleophilic substitution alc. KOH solution, then alkene is formed. I n t
reactions can easily be understood.
hydrogen is eliminated
from ß-carbon and th this
hg reaci,
Stereochemical Aspects ofSy 2 Reaction from d-carbon atom. As a result, an alkene i
alkene ishalformed,
ogm si
In Sn2 mechanism, the nucleophile attacks the side
product. Due to the involvement
of
eliminari
atom, the process is often called as p-eliminat
on
opposite to one where halogen atom is present. Theretore, B-hydrogen
SN2 reaction is always accompanied by inversion of
configuration, that is referred to as Walden inversion.
B H
e.g. When ( 2-halobutane is allowed to react with
potassium hydroxide, (+) butan-2-ol is formed with the
-2 C=C+B-H+X
-OH group occupying the B Base; X=
had occupied.
position opposite to halogen Leaving groupj
e.g. H-CH,-CH2Br + KOH (alc.)-Hea
H H
CH CH2 +KBr+H,
C-X 44 KOH If there is possibility for the formation of more h
HO-C +X

CH alkene due to the availability of more than one Bhu

atoms, usually one alkene is formed as the major
CaH
2-halobutane
CH
+) Butan-2-ol
This occurs according to the Saytzeff rule, which
summarised as "in dehydrohalogenation reactions, the prctom
Thus, SN2 reaction of optically active alkyl halides is product is that alkene which has the greater number of i
always accompanied by the inversion of configuration. groups attached to the doubly bonded carbon atoms"
Stereochemical Aspects of Sy 1 Reaction e.g. 2-bromopentane gives pent-2-ene as the major produz
In Syl reactions, if
the alkyl halide is optically active, the HC-CH-CH=CH-CH O
product obtained is a racemic mixture. The intermediate alc
Pent-2-ene (81%)
carbocation formed in slowest step being
2 hybridised is (Major product) Br
planar (achiral) species .
Therefore, the attack of the nucleophile OH can occur HC-CH,-CH-CH-CH,
from both the faces with equal ease of 2-bromopentane
of rwo enantiomers. Thus,
forming a mixture
Syl reaction of optically active H
alkyl halides are accompanied by racemisation.
H,C-CH-CH-CH=CH
CH3 Pent-1-ene (19%)
CH3
X
Elimination versus Substitution
When alkyl halide with
CH,CH3 reacts
nucleophile, it may
undd
CH,CH Planar carbocation (Achiral) either substitution via Snl and SN2 or elimination teau
It depends upon the
CH
Rear Front following factors:
HO
attack
CH3atack OH
CH3 The of alkyl halide Primary alkyl halide
nature
HO-C (inversion) (Retention) -OH prefer Sn2 reaction, i.e. nucleoph subsirunon
"H H reaction via Sn2 mechanism. Secondary alky! halide
CH,CH H
CH,CH Planar carbocation
CH CH can
undergo substitution or elimination dc
(+) Butan-2-ol
Butan-2-o upon the
(50%) 59%) strength of the nucleophik
Racemic mixture
Tertiary alkyl halide will undergo substiru
elimination
or the
depending upon the stabilicy of carboc
more substituted alkene.
Aloalkanes
and and Haloarenes 379
and size of the nucleophi or base A with any source of proton
highly reactive and
The
strength
hese are react
cleophile will carry out the elimination more amines,
are water
hydrocarbons. Even alcohols,
hIkie bstitution as it preters to act as a base and Ogive corresponding
Sufficiently acidic to convert them
to
roton rather than
approaching
abstracts a proton

a
reravalent carbon tom (due to steric reasons) and hydrocarbons.
Stronger nucleophil like C2H,0 will
- v e r s a .
RMgX +H,0 RH +Mg(OH)X
avoid even traces
ination whereas, nucleophiles like
the eliminar
out Due to its high reactivity, it is necessary
to

hring of moisture from a Grignard reagent.

out the substitution.
will bring of the
OH On the other hand, this could be considered as one
i)The
nditions of the reaction such as temperature,
conditio.

pressure, e t c
methods for converting halides to hydrocarbons.

(i) Action with Sodium (Wurtz Reaction)

with sodium in
he reactions in which alkyl halides react
CH3 the presence of dry ether to form hydrocarbons containing
the halide,
double the number of carbon atoms present in
HyC-C-0
are known as Wurtz reaction.

CH3 Elimination 2RX +2Na-Dry etnes 2R +2NaX

VS

CHEMICAL REACTIONS
C
OF HALOARENES
Some important chemical reactions of haloarenes are
CHCH-0
Nucleophilic Substitution Reactions
Substitution Nucleophilic substitution reactions in aryl halides occur
only under drastic conditions. This is because haloarenes
Reaction with Metals are chemically less reactive towards nucleophilic
substitution reactions (than haloalkanes) due to the
Most organic chlorides, bromides and iodides react with following reasons:
carain metals to give compounds containing
arbon-metal bonds. Such compounds are known as ) Resonance effect In haloarenes like chlorobenzene, the
lone pair of electrons on halogen atom are in
anganometallic compounds. conjugation with T-electrons of the ring and hence,
) Action with Magnesium these are delocalised on the benzene ring as shown
(Formation of Grignard Reagents) below:
Viator Grignard in 1900 discovered an important class of
ganometalic compounds, ie. alkyl magnesium halide, Ck :C

-Mgk which is commonly called Grignard reagents.

rignard reagents are

obtained by the reaction of
1a0alkanes with magnesium metal in presence of dry ether.

CH,CH,Br +Mg y cner CHCH MgBr

Grignard reagent
the Grignard reagent, the carbon-magnesium bond is
alent but highly polar, with carbon pulling electrons
electropositive magnesium, the magnesium halogen
bond is
essentially ionic. V

R-MgX
 | Allinone| CHEMISTRYCass 12h
380
electron withdrawine
withdrawing8 Rroun.
The presence of
of electron
roups such
C-Cl bond acquires partial
a
and p-positions ofhaloa
As a result of resonance,
-NO-CNato- 0arenes wih
the bond cleavage in activates the hen
double bond character. Thus, is respect to halogen, greatly nIene ine
difficult than haloalkane (where carbon towards substitution. Further, o
nucleophilicsubstitution.
is
wiegreater
haloarene towards nucleophilic
bond) and thus, they
number of such groups o-and p-positions wi
at o-and
attached to
() Difference
halogenby a pure single substitution reaction.
in hybridisation of carbon atom in C-X
number ofsuch at
to halogen, more reactive
with
reactive is the haloarene.OH
haloarene.
respe
are reactive towards nucleophilic
less to halogen, more

bond In haloalkanes, the

carbon atoms attached to :C:
haloarenes, it is p*
halogenis sp-hybridised while in (i) NaOH, 443 K

hybridised. (i) H
X
p-hybrid
H sp-hybrid carbon

carbon R-C X NO2 NO

1-chloro-4-nitrobenzene p-nittophenol

is :Ci: OH
The sp hybridised C-atom with greater scharacter
electron pair of the
more electronegative. It can hold the NO2 g NaOH, 368 K NO
than the sp° C-atom with
hybridised
bond more tightly Ci)H
C-X bond in haloarenes
less s-character. As a result, the
haloalkanes (177 pm). Since, it
(169 pm) is shorter than in
bond than a longer NO NO
is difficult to break a shorter (C-X) 2, 4-dinitrochlorobenzene
2,4-dinitrophenol
reactive than haloalkanes
and therefore, haloarenes are less
reaction.
towards nucleophilic substitution
cation In haloarenes, the phenyl OH
(in) Unstability of phenyl
result of self-ionisation will not
be :CI
cation formed as a
mechanism cannot occur. O,N NO O,N NO
stabilised by resonance, hence Sl Warm
electron rich attacking HO
(iv) Repulsion between the electron
arenes Because of
nucleophiles and electron rich will not approach
rich arenes, electron rich nucleophile NO NO
for the attack due to repulsion. 2, 4,6-trinitrochlorobenzene 2,4, 6-trinitrophenol
closely (Picric acid
Replacement by Hydroxyl Group
NaOH solution at a atm-position to the chlorine has no
Chlorobenzene when heated in aq. The-NO2 group
is
temperature of 623
K and a pressure of 300 atm, phenol effect on reactivity.
formed.
Mechanism
ONa OH
:CI: The presence of groups at o- and p-positions
NO2 facilitates
withdraws electrons from benzene ring and
The carbanion
K,300atm Dil. HCI the attack of nucleophile on haloarenes.
+NaOH-523
-HCI NaC formed is stabilised through resonance as depicted
Phenol below:
Sodium phenoxide
Chlorobenzene
 A n e s
and Haloarenes 381
N Oa tp a r a - p o s i t i o n

COH C OH OH
OH .
COH CIOH COH
Slow step
Fast step
=

N
& O
nchloronitro benzene

III IV Resonance hybrid p-nitrophenol

NOg atortho-position

aOH0 COH0 ClOH

OH

o-chloronitro benzene
Slow step
Co o | O
N
OH
COH
COH,O
X o
=o Resonance hybrid
Fast step

o-nitrophenol
VIII

i) NOg at meta-position
C OH COH dOH
OH
0 Slowstep 0|

m-chloronitrobenzene IX X
OH
COH
-C
Fast step

Resonance hybrid m-nitrophenol

oteIn the above structures showing NO2 at meta position, there is no such structure in which the negative charge is present on carbon atom
eaing the-N02 group. Theretore, the presence of nitro group at meta postion does not stabilise the negative charge. Hence, no effect on
VIy 1S observed by the presence of-NO2 group at meta position.

Electrophilic Substitution Reactions

Taloarenes undergo the usual electrophilic reactions of the benzene ring such as halogenation, nitration, sulphonation and
Fiedel-Crafts reactions. In these reactions, stronger electrophile replaces weaker electrophile.The electrophilic substitution
ctions in haloarenes occur slowly and require more drastic oonditions. This is due to the ortho and para-directing
lence of halogen atom attached to a benzene ring which can be understood by the following resonating structures of
halobenzene
 382 Allinone CHEMISTRY
Cl Cl

Conc. H,SO4 SOJH

A

V
Chlorobenzene 2-chlorobenzene
sulphonic acid
Thus, due to resonance, the electron density increases more minor)

4s-cuhlolohoniSOHrobecnmn
at
-and p-positions than at m-positions. Hence,
electrophilic reactions occurat o- and
p-position. The
halogen atom has the tendency to withdraw electrons due
(iv) Friedel-Crafts Reactions
majon)
to which electron
density on benzene ring decreases
-effect) and the ring gets deactivated. Hence,
This reaction is carried out by treating haloaren.
electrophilic substitution in haloarene occurs at a slower chloride or acyl chloride in the presence
rate.
AICl3 acting as a catalyst. anhydr
The main types of
common
electrophilic substitution reactions of There are two Friedel-Crafts
reactiong
haloarenes are depicted as below:
(i) Alkylation
(i) Halogenation Cl
This reaction takes
place by reacting haloarenes with
halogens in the presence of ferric salt. + CHCl Anhyd. Ala,
CI
CI CI Chlorobenzene

+Cl Anhyd. FeCly,

C
C
CH3
Chlorobenzene
1,2-dichlorobenzene
Cl (minor)
1,4-dichlorobenzene 1-chloro-2-methylbenzene
(major) (minor) CH
1-chloro-4-methylbenzene
(ii) Nitration (major)
This reaction takes place by heating haloarenes with
(ii) Acylation
conc.HNO3 in the presence of conc.H2 SO4
CI
Cl C Cl
NO2 +H,C- -C AahydA
Conc. HNO
Conc. H,SO Chlorobenzene
Chlorobenzene 1-chloro-2-nitrobenzene O CI
(minor
NO2
1-chloro-4-nitrobenzene CH3 +
(major)

(ii) Sulphonation 2-chloroacetophenone

This reaction takes place by heating haloarenes with
(minor) CH
4-chloroacetophenone
conc.H, SO4. (major)
oalkanesand Haloarenes 383

CI ether
Cl+2Na+ Cl+
Anhyd AlC,
Ch
* * * * * * * *

+
(CH,CO),0 CH
+2NaCl
orDenzcNe 2-chloroacetophenone
(Minor)
Cl Diphenyl

reagent 15s
(u1) Reaction with magnesium Grignard and iodides
bromides
+ CH,COOH
tormed. Like alkyl halides, arylin form
also react with magnesium dry ether to
Grignard reagent.
O=C-CHH
4-chloroacetophenone Dry ether -MgBr
Br + Mg
(Major)
Phenyl magnesium
Bromobenzene
bromide
eaction with Metals
Wurtz-Fittig reaction When aryl halideis heated with and iodoarenes react
Bromo
(iv) Reaction with lithium ether to form
Ikyl halide in the presence of sodium in dry ether, with lithium metal in the presence of dry
their corresponding organometallic compounds.
halogenatom is eplaced by alkyl group and alkylarene
is formed. This reaction is called Wurtu-Fittig reaction.
R Br+Li Dry ether -LiBLiBr
Phenyl lithium
Dry Bromobenzene
+2Na + R-X-ether +2NaX bromide
Alkyl halide iodobenzene is
(v) Reaction with copper powder When
Alkylarene is
Aryl halide
heated with copper powder in a sealed tube, diphenyl
*******
formed. This reaction is called Ullmann reaction.
Cl+2Na+ C-CH,ether
-- ********* Heat

Chlorobenzene I+2Cu 2Cu +

-------------
1
--*
-
lodobenzene
-CH+2NaCI Iodobenzene
Toluene

with sodium in
+Cul
i) Fitig reaction When haloarenes react
the presence of dry ether, two aryl groups are joined Diphenyl
is called
together and diphenyl is formed. This reaction
Fitig reaction. Reduction
On reduction of haloarenes, hydrocarbons are formed.

Dry CI
+2Naether N-A - .+HCI
2H
NaOH

+2NaX Chlorobenzene Benzene

Diphenyl
 olC 4/
OPIC 4| Polyhalogen Compounds of light to an
ompounds having more than one halogen oxidised by air in the presence
called
t is slowly chloride also
inOn to as po
halogen compounds. carbonyl
xtremely poisonous gas,
reterred
bottles.
usually coloured
are useful in industry and It is therefore, stored in dark
hese compounds
phosgene.
ame important polyhalogen compounds 2HC
2CHCl +0 Sunlight 2COClh
+
Kribed as tollows:

Phosgene
CHLOROMETHANE

Preservation of Chloroform
ylene hloride, CHCl2) the oxidation of chloroform,
it is preserved
in the
sweet smelling latile liquid, having low o prevent
olourless,
s Cnr of 313 K and specitic gravity of 1.37. It is following ways: bottles to protect it
coloured
olingpount

0) Chloroform is stored in dark

industrially by the direct chlorination of oxidation.
prepared

presence of iffused sunlight sunlight to avoid their

from
in the with chloroform upto
ethane
() These botles are complerely filled
to keep air out.
CH+2Cl2 CH,Cl, +2HCI the brim and are properly stoppered
to
in) 1% ethanol is added so, as to
convert phosgene gas
non-toxic.
non-volatile and
Uses
diethyl carbonate which is
S Used
COClh +2C,HOH>
as a solvent and
also as a paint remover. (C2Hs)2C03 +2HCI
in aerosols.
Phosgene
propellant Diethyl carbonate
i) as a
m) as a metal cleaning and finishing solvent.
and
(iv) as refrigerantsolvent
dewaxing agent. Uses
in the manufacture of drugs. It is used
() as a process
(i) as a solvent for fats, alkaloids, iodine and other substances.
Harmful Eftects (Physiological Effects) (ii) as a laboratory reagent.
of
Some of the important harmful effects methylene
follows:
ii) in the preparation of chloropicrin, chloretone, etc.

choride are as
iv) in medicines.
) It harms the human central system.
nervous
(v) in the production of freon refrigerant, R-22.
() Exposure to low levels of methylene chloride in air anaesthetic but now it has been replaced by other
vi) as an
can slightly impaired hearing and vision.
lead to
anaesthetics such as ether.
l) Exposure to high levels of methylene chloride can
Cause dizziness, nausea, tingling and numbness in
Harmful Effects (Physiological Effects)
the fingers and toes.
Some of its harmful effects are:
(w) Direct contact with skin causes intense burning and
() Chloroform affect the central nervous system. Inhaling it,
mild redness of the skin.
900 ppm (in air) for a short time, causes dizziness, fatigue
) Direct contact with eyes can burn the cornea.
and headache.
() Chronic exposure to chloroform damages liver and kidney
TRICHLOROMETHANE due to the formation of phosgene gas inside the body.
(Chloroform, CHCI3) (i) Skin develop sore if immersed directly into the
Chloroform is a colourless, oily liquid with a peculiar chloroform.
Sckly smell and a burning taste.
 Allinone CHEMISTRY
STRY Ca
396
FREONS
TRIIODOMETHANE compounds (CFC) ofr
Chlorofluorocarbon
methane nd
(lodoform, CHI 3) known as freons. They are
are collectively
non-corrosive:and sremely
easily
CCl,F)iqisuefiate
l
It is yellow coloured crystalline solid with characteristic unreactive,
non-toxic,

(dichlorodifluoromet
unpleasant odour. Freon-12

most
common
freons in industrial use. It isis m
It manufacn one
reaction.
Uses tetrachloromethane by Swarts

It is used SbCI
) as an antiseptic due
+50612
to the liberation of free iodine.
+4CO2 +2H,Oo
3CCl4 +2SbF3
2SbCl+3Ca,
FreonA2
4CHI3 +50-heat
Uses
Bur due to its unpleasant smell, it has been replaced by refrigerants in
other formulations containing iodine.
() These are used
as
refrigerato
conditioning
(ü) in the manufacture of pharmaceuticals.
i) These are used propellants aerosols ands.
as tor
spray out deodorants, cleansers, having reams,
TETRACHLOROMETHANE sprays and insecticides.

(ii) In stratosphere, freon is able to initiate chain reacio

(Carbon Tetrachloride, CCI4)
that can result in depletion ot ozone layer. Since s
It is colourless oily liquid wich sickly smel. Its boiling point has been found to be one of the factors responghi
is 350 K. It is formed from chloroform as follows:
the depletion of ozone layer, they are being replagi
CHCl3 +Clh CC4 +HCI other harmless compounds in many countries.

Harmful Effects (Environmental Eflects

Uses
Most freons make its way into the atmosphere, where
It is used
undergoes photochemical decomposition and initi
() in large quantities, in the manufacture of refrigerants
radical chain reactions for depletion of the ozone lar=
and propellants for aerosol cans.
Therefore, the use of freons as propellants and refrigeran
i) as a feed stock in the synthesis of chlorofluoro-carbons has been drastically discouraged.
(freons) and other chemicals.
(i) as a cleaning fluid, degreasing agent and spot remover. p.p-DICHLORODIPHENYLTR
(iv) as fire extinguisher. CHLOROETHANE (DDT)
Harmful Effects (Physiological Effects) DDT, the first chlorinated organic insecticide, wa
liver in humans.
originally prepared in 1873. Paul Muller was awarded the
) Exposure to CCI4 causes cancer Nobel prize in medicine and physiology in 1948 for thi=
(i) The most common effects are dizziness, light discovery.
headedness, nausea and vomiting which can cause C
cells. In severe cases, these
permanent damage to nerve
etfects can lead to coma, unconsciousness or even

death. CI
() Exposure of
CCl4 vapours can make heartbeat Cl
irregular or even can stop it. When brought in contact H
with eyes, it may cause irritation in eyes. (DDT)

(iv) It is also harmful to environment. When it is released

It is a white powder, insoluble in water but soluble mo
in air, it rises in the atmosphere and depletes the ozone

layer. Depletion of ozone layer increases the human

Uses
exposure to UV rays which lead skin cancer, eye ) DDT is a cheap but powerful insecticide.
diseases and disorders and possible disruption of the (i) It is popularly very cffective against anopnhe
immune system. mosquitoes (causes malaria) and lices which carry y
Haloalkanes and Haloarenes

Harmful Effects
(Environmental and Physiological Effects)
DDT is non-biodegradable and extremely stable compound
which acts as water pollutant and kills aquatic animals thus
imbalancing the water ecosystem. When higher animals
(including humans) feed on dead fishes killed by DDT, it
enters in their food chains. DDT gets stored in fat tissues of
animal and increases in amount over the time. Researches
have shown that such deposition in long run
may cause
cancer and other harmful diseases. Hence, DDT has been
banned in many countries but it is still in use, due to its
magical effects of controlling disease, in most of the
magical
developing countries.