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Unit

10
Objectives
After studying this Unit, you will be
able to
• name haloalkanes and haloarenes
according to the IUPAC system of
nomenclature from their given
structures;
• describe the reactions involved in
the preparation of haloalkanes and
haloarenes and understand
various reactions that they
undergo;
• correlate the structures of
haloalkanes and haloarenes with
various types of reactions;
• use stereochemistry as a tool for
understanding the reaction
mechanism;
• appreciate the applications of
organo-metallic compounds;
• highlight the environmental effects
of polyhalogen compounds
...


The replacement of hydrogen atom(s) in a
hydrocarbon, aliphatic or aromatic, by halogen
atom(s) results in the formation of alkyl halide
(haloalkane) and aryl halide (haloarene), respectively
...
Many
halogen containing organic compounds occur in
nature and some of these are clinically useful
...
They are used
as solvents for relatively non-polar compounds and
as starting materials for the synthesis of wide range
of organic compounds
...

Our body produces iodine containing hormone,
thyroxine, the deficiency of which causes a disease
called goiter
...

chloroquine is used for the treatment of malaria;
halothane is used as an anaesthetic during surgery
...

In this Unit, you will study the important methods
of preparation, physical and chemical properties and
uses of organohalogen compounds
...
1 Classification

Haloalkanes and haloarenes may be classified as follows:

10
...
1 On the
Basis of
Number of
Halogen
Atoms

These may be classified as mono, di, or polyhalogen (tri-,tetra-, etc
...
For example,

Monohalocompounds may further be classified according to the
hybridisation of the carbon atom to which the halogen is bonded, as
discussed below
...
1
...

They form a homologous series represented by CnH2n+1 X
...


(b) Allylic halides
These are the compounds in which the halogen atom is bonded to an
sp3-hybridised carbon atom next to carbon-carbon double bond (C=C)
i
...
to an allylic carbon
...


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10
...
3 Compounds This class includes:
Containing
(a) Vinylic halides
sp2 C—X
These are the compounds in which the halogen atom is bonded to
Bond
an sp2-hybridised carbon atom of a carbon-carbon double bond
(C = C)
...


10
...
The common names of alkyl halides are
derived by naming the alkyl group followed by the halide
...
Haloarenes are the common as well as IUPAC names of
aryl halides
...


The dihaloalkanes having the same type of halogen atoms are named
as alkylidene or alkylene dihalides
...
In common name
system, gem-dihalides are named as alkylidene halides and vic-dihalides
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are named as alkylene dihalides
...


Some common examples of halocompounds are mentioned in Table 10
...

Table 10
...
1 Draw the structures of all the eight structural isomers that have the
molecular formula C5H 11Br
...


Solution CH3CH2CH2CH2CH2Br

1-Bromopentane (1 )

CH3CH2CH2CH(Br)CH3

2-Bromopentane(2o )

CH3CH2CH(Br)CH2CH3
(CH3)2CHCH2CH2Br

3-Bromopentane (2o )
1-Bromo-3-methylbutane (1o )

o

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(CH3)2CHCHBrCH3

2-Bromo-3-methylbutane(2o )

(CH3)2CBrCH2CH3

2-Bromo-2-methylbutane (3o )
o

CH3CH2CH(CH3)CH2Br 1-Bromo-2-methylbutane(1 )
(CH3)3CCH2Br
1-Bromo-2,2-dimethylpropane (1 o)

Example 10
...
1 Write structures of the following compounds:
(i) 2-Chloro-3-methylpentane
(ii) 1-Chloro-4-ethylcyclohexane
(iii) 4-tert
...
butyl-2-methylbenzene
...
3 Nature of
C-X Bond

Since halogen atoms are more electronegative than carbon, the carbonhalogen bond of alkyl halide is polarised; the carbon atom bears a
partial positive charge whereas the halogen atom bears a partial negative
charge
...
Consequently the carbon-halogen bond length also increases
from C—F to C—I
...
2
...
2: Carbon-Halogen (C—X) Bond Lengths, Bond
Enthalpies and Dipole Moments
Bond

Bond length/pm

C-X Bond enthalpies/ kJmol-1

Dipole moment/Debye

CH3–F

139

452

1
...
860

CH3–Br

193

293

1
...
636

10
...
4
...

The hydroxyl group of an alcohol is replaced by halogen on reaction
with concentrated halogen acids, phosphorus halides or thionyl chloride
...
Hence the reaction gives pure alkyl halides
...
The preparation of alkyl chloride is carried out either
by passing dry hydrogen chloride gas through a solution of alcohol or
by heating a solution of alcohol in concentrated aqueous acid
...
With tertiary alcohols, the reaction is
conducted by simply shaking with concentrated HCl at room
temperature
...
Good yields of R—I may be obtained by heating alcohols
with sodium or potassium iodide in 95% orthophosphoric acid
...
The
above methods are not applicable for the preparation of aryl halides
because the carbon-oxygen bond in phenols has a partial double bond
character and is difficult to break being stronger than a single bond
(Unit 11, Class XI)
...
4
...
Consequently, the yield of any one
compound is low (Unit 13, Class XI)
...


Example 10
...

Replacement of these hydrogen atoms will give the following

Solution

(CH3)2CHCH2CH2Cl

(CH3)2CHCH(Cl)CH3

(CH3)2C(Cl)CH2CH3

CH3CH(CH2Cl)CH2CH3

(b) By electrophilic substitution
Aryl chlorides and bromides can be easily prepared by electrophilic
substitution of arenes with chlorine and bromine respectively in the
presence of Lewis acid catalysts like iron or iron(III) chloride
...
Reactions with iodine are reversible
in nature and require the presence of an oxidising agent (HNO3,
HIO4) to oxidise the HI formed during iodination
...

(c) Sandmeyer’s reaction
When a primary aromatic amine, dissolved or suspended in cold
aqueous mineral acid, is treated with sodium nitrite, a diazonium
salt is formed (Unit 13, Class XII)
...


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Replacement of the diazonium group by iodine does not require the
presence of cuprous halide and is done simply by shaking the diazonium
salt with potassium iodide
...


Propene yields two products, however only one predominates as
per Markovnikov’s rule
...
The addition results in the synthesis
of vic-dibromides, which are colourless (Unit 13, Class XI)
...
4 Write the products of the following reactions:

Solution

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10
...
3 Halogen
Exchange

Alkyl iodides are often prepared by the reaction of alkyl chlorides/
bromides with NaI in dry acetone
...


NaCl or NaBr thus formed is precipitated in dry acetone
...

The synthesis of alkyl fluorides is best accomplished by heating an
alkyl chloride/bromide in the presence of a metallic fluoride such as
AgF, Hg2F2, CoF2 or SbF3
...


Intext Questions
10
...
3 Write structures of different dihalogen derivatives of propane
...
4 Among the isomeric alkanes of molecular formula C5H 12, identify the one that
on photochemical chlorination yields
(i) A single monochloride
...

(iii) Four isomeric monochlorides
...
5 Draw the structures of major monohalo products in each of the following
reactions:

10
...
However, bromides and iodides
develop colour when exposed to light
...


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Melting and boiling points
Methyl chloride, methyl bromide, ethyl chloride and some
chlorofluoromethanes are gases at room temperature
...
As we have already learnt, molecules of organic
halogen compounds are generally polar
...
That is why the boiling points
of chlorides, bromides and iodides are considerably higher than those
of the hydrocarbons of comparable molecular mass
...
The pattern of variation of boiling points of different
halides is depicted in Fig
...
1
...
This
is because with the increase in size and mass of halogen atom, the
magnitude of van der Waal forces increases
...
10
...
For example, 2-bromo-2-methylpropane
has the lowest boiling point among the three isomers
...

However, the para-isomers are high melting as compared to their orthoand meta-isomers
...

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Density
Bromo, iodo and polychloro derivatives of hydrocarbons are heavier than
water
...
3)
...
3: Density of Some Haloalkanes
Compound

Density (g/mL)

Compound

Density (g/mL)

n–C 3H 7Cl

0
...
336

n–C 3H 7Br

1
...
489

n-C3H 7I

1
...
595

Solubility
The haloalkanes are only very slightly soluble in water
...
Less energy is released when new
attractions are set up between the haloalkane and the water molecules
as these are not as strong as the original hydrogen bonds in water
...
However, haloalkanes
tend to dissolve in organic solvents because the new intermolecular
attractions between haloalkanes and solvent molecules have much the
same strength as the ones being broken in the separate haloalkane and
solvent molecules
...
6 Arrange each set of compounds in order of increasing boiling points
...

(ii) 1-Chloropropane, Isopropyl chloride, 1-Chlorobutane
...
6 Chemical
Reactions

10
...
1 Reactions of Haloalkanes
The reactions of haloalkanes may be divided into the following categories:
(i) Nucleophilic substitution
(ii) Elimination reactions
(iii) Reaction with metals
...
A substitution reaction takes place and halogen atom,
called leaving group departs as halide ion
...


It is one of the most useful classes of organic reactions of alkyl
halides in which halogen is bonded to sp3 hybridised carbon
...
4
...
4: Nucleophilic Substitution of Alkyl Halides (R–X)

Reagent

Nucleophile
(Nu–)

Substitution
product R–Nu

Class of main
product

NaOH (KOH)

HO–

ROH

Alcohol

H2O

H2 O

ROH

Alcohol

NaOR′

R ′O –

ROR′

Ether

NaI

I–

R—I

Alkyl iodide

NH3

NH3

RNH2

Primary amine

R′NH2

R′NH2

RNHR′

Sec
...
amine

RCN

Nitrile
(cyanide)

KCN
AgCN

Ag-CN:

RNC
(isocyanide)

Isonitrile

KNO2

O=N—O

R—O—N=O

Alkyl nitrite

AgNO2

Ag—Ö—N= O

R—NO2

Nitroalkane

R′COOR

Ester

–

R′COOAg

R′COO

LiAlH4

H

RH

Hydrocarbon

R′ – M+

R′–

RR′

Alkane

Groups like cyanides and nitrites possess two nucleophilic centres
and are called ambident nucleophiles
...
e
...
Similarly nitrite ion also represents an ambident
nucleophile with two different points of linkage [–O— N =O]
...

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Haloalkanes react with KCN to form alkyl cyanides as main product
while AgCN forms isocyanides as the chief product
...


Example 10
...

Although both carbon and nitrogen atoms are in a position to donate
electron pairs, the attack takes place mainly through carbon atom and
not through nitrogen atom since C—C bond is more stable than C—N
bond
...


Solution

Mechanism: This reaction has been found to proceed by two different
mechanims which are described below:
(a) Substitution nucleophilic bimolecular (SN 2)
The reaction between CH3Cl and hydroxide ion to yield methanol and
chloride ion follows a second order kinetics, i
...
, the rate depends
upon the concentration of both the reactants
...
3
...


This can be represented diagrammatically as shown in Fig
...
2
...
10
...


It depicts a bimolecular nucleophilic displacement (SN2) reaction;
the incoming nucleophile interacts with alkyl halide causing the carbonhalide bond to break while forming a new carbon-OH bond
...
As the reaction progresses and the bond between the
nucleophile and the carbon atom starts forming, the bond between
carbon atom and leaving group weakens
...
This process is called as
inversion of configuration
...
Sc
...


group and such structures are unstable and cannot be isolated
...

Since this reaction requires the approach of the nucleophile to the
carbon bearing the leaving group, the presence of bulky substituents
on or near the carbon atom have a dramatic inhibiting effect
...
Tertiary halides are
the least reactive because bulky groups hinder the approaching
nucleophiles
...


Fig
...
3: Steric effects in S N2 reaction
...
The reaction between tert-butyl
bromide and hydroxide ion yields tert-butyl alcohol and follows
the first order kinetics, i
...
, the rate of reaction depends upon the
concentration of only one reactant, which is tert- butyl bromide
...
In step I, the polarised C—Br bond undergoes
slow cleavage to produce a carbocation and a bromide ion
...


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Step I is the slowest and reversible
...
Since
the rate of reaction depends upon the slowest step, the rate of reaction depends only on the
concentration of alkyl halide and not on the concentration of hydroxide ion
...
In case of alkyl halides, 30 alkyl halides undergo SN 1 reaction
very fast because of the high stability of 3 0 carbocations
...
The carbocation thus formed gets stabilised through resonance (Unit 12, Class XI) as
shown below:
+

+

H2C C CH2
H

H2C

C
H

CH2

For a given alkyl group, the reactivity of the halide, R-X, follows the same order in both the
mechanisms R–I> R–Br>R–Cl>>R–F
...


Example 10
...

Predict the order of reactivity of the following compounds in S N1 and
SN 2 reactions:
(i) The four isomeric bromobutanes
(ii) C6H5CH2Br, C 6H5CH(C6H5)Br, C6H5CH(CH3)Br, C6H 5C(CH3)(C6H5)Br

Example 10
...
Therefore,
(CH3)2CHCH 2Br is more reactive than CH 3CH 2CH 2CH 2Br in S N1 reactions
...
Hence the above order is followed in SN1
...

(ii) C6H 5C(CH3)(C6H 5)Br > C6H5CH(C6H5)Br > C6H5CH(CH3)Br > C6H5CH2Br (SN1)
C6H 5C(CH3)(C6H 5)Br < C6H5CH(C6H5)Br < C6H5CH(CH3)Br < C6H5CH2Br (SN2)
Of the two secondary bromides, the carbocation intermediate obtained
from C6H 5CH(C6H5)Br is more stable than obtained from C6H5CH(CH3)Br
because it is stabilised by two phenyl groups due to resonance
...
A
phenyl group is bulkier than a methyl group
...


William Nicol (17681851) developed the first
prism that produced
plane polarised light
...

In order to understand this concept, we need to learn some basic
stereochemical principles and notations (optical activity, chirality,
retention, inversion, racemisation, etc
...
Such compounds are called optically active
compounds
...
If the
compound rotates the plane polarised light to the right, i
...
,
clockwise direction, it is called dextrorotatory (Greek for right
rotating) or the d-form and is indicated by placing a positive (+)
sign before the degree of rotation
...
Such (+) and (–) isomers of a compound
are called optical isomers and the phenomenon is termed as
optical isomerism
...
He demonstrated that
aqueous solutions of both types of crystals showed optical
rotation, equal in magnitude (for solution of equal concentration)
but opposite in direction
...

Dutch scientist, J
...
Le Bel in
Jacobus Hendricus
the same year (1874), independently argued that the spatial
Van’t Hoff (1852-1911)
arrangement of four groups (valencies) around a central carbon
received the first Nobel
is tetrahedral and if all the substituents attached to that carbon
Prize in Chemistry in
1901 for his work on
are different, such a carbon is called asymmetric carbon or
solutions
...
The resulting molecule would lack symmetry and
is referred to as asymmetric molecule
...

The symmetry and asymmetry are also observed in many day to day
objects: a sphere, a cube, a cone, are all identical to their mirror images
and can be superimposed
...
For example, your left and
right hand look similar but if you put your
left hand on your right hand, they do not
coincide
...
While
the objects, which are, superimposable on
their mirror images are called achiral
...
There are other aids, however, that
can assist us in recognising chiral molecules
...
Let us consider
two simple molecules propan-2-ol and
Fig 10
...


As you can see very clearly, propan-2-ol does not contain an
asymmetric carbon, as all the four groups attached to the tetrahedral
carbon are not different
...

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Butan-2-ol has four different groups attached to
the tetrahedral carbon and as expected is chiral
...

The stereoisomers related to each other as nonFig
...
5: A chiral molecule
superimposable mirror images are called enantiomers
and its mirror image
(Fig
...
5)
...
They only differ with respect
to the rotation of plane polarised light
...

However, the sign of optical rotation is not necessarily related to
the absolute configuration of the molecule
...
Such a mixture is
known as racemic mixture or racemic modification
...
The process of conversion of enantiomer into a racemic
mixture is known as racemisation
...
8 Identify chiral and achiral molecules in each of the following pair of
compounds
...
1)
...
It is also the configurational
correlation when a chemical species XCabc is converted into the
chemical species YCabc having the same relative configuration
...
Such a reaction is said
to proceed with retention of the configuration
...


(iv) Inversion, retention and racemisation: There are three outcomes
for a reaction at an asymmetric carbon atom
...

If (B) is the only compound obtained, the process is called inversion
of configuration
...

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Now let us have a fresh look at SN1 and SN2 mechanisms by
taking examples of optically active alkyl halides
...
This is because the nucleophile attaches itself on the
side opposite to the one where the halogen atom is present
...


Thus, SN2 reactions of optically active halides are accompanied by
inversion of configuration
...
Can you think of the reason why it
happens? Actually the carbocation formed in the slow step being sp2
hybridised is planar (achiral)
...
This may be illustrated
by hydrolysis of optically active 2-bromobutane, which results in the
formation of (±)-butan-2-ol
...
Elimination reactions
When a haloalkane with β-hydrogen atom is heated with alcoholic
solution of potassium hydroxide, there is elimination of hydrogen
atom from β-carbon and a halogen atom from the α-carbon atom
...
Since β-hydrogen
atom is involved in elimination, it is often called β -elimination
...
These form part of a pattern first
observed by Russian chemist, Alexander Zaitsev (also pronounced as
Saytzeff) who in 1875 formulated a rule which can be summarised as
“in dehydrohalogenation reactions, the preferred product is that
alkene which has the greater number of alkyl groups attached to the
doubly bonded carbon atoms
...


Elimination versus substitution
A chemical reaction is the result of competition; it is a race that is won by the fastest
runner
...
An
alkyl halide with β-hydrogen atoms when reacted with a base or a nucleophile has two
competing routes: substitution (SN1 and SN2) and elimination
...
Thus, a bulkier nucleophile will prefer to act as a base and abstracts
a proton rather than approach a tetravalent carbon atom (steric reasons) and vice versa
...


3
...
Such
compounds are known as organo-metallic compounds
...
These reagents are obtained by the reaction of
haloalkanes with magnesium metal in dry ether
...
When he eventually switched to chemistry, it was
not to the mathematical province of physical chemistry but to organic
chemistry
...
Grignard reagents were first reported in 1900 and Grignard
used this work for his doctoral thesis in 1901
...


In the Grignard reagent, the carbon-magnesium bond is covalent but
highly polar, with carbon pulling electrons from electropositive
magnesium; the magnesium halogen bond is essentially ionic
...
Even water, alcohols, amines are sufficiently
acidic to convert them to corresponding hydrocarbons
...
On the other hand, this could be considered as one of the
methods for converting halides to hydrocarbons
...

This reaction is known as Wurtz reaction
...

10
...
2Reactions of
Haloarenes

1
...


C—Cl bond acquires a partial double bond character due to
resonance
...

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(ii) Difference in hybridisation of carbon atom in C—X bond: In
haloalkane, the carbon atom attached to halogen is sp3
hybridised while in case of haloarene, the carbon atom attached
to halogen is sp2-hybridised
...
Thus, C—Cl bond length in haloalkane is
177pm while in haloarene is 169 pm
...

(iii) Instability of phenyl cation: In case of haloarenes, the phenyl
cation formed as a result of self-ionisation will not be stabilised
by resonance and therefore, SN 1 mechanism is ruled out
...

Replacement by hydroxyl group
Chlorobenzene can be converted into phenol by heating in aqueous
sodium hydroxide solution at a temperature of 623K and a pressure
of 300 atmospheres
...


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The effect is pronounced when (-NO 2) group is introduced at orthoand para- positions
...

Mechanism of the reaction is as depicted:

Can you think why does NO2 group show its effect only at ortho - and para- positions
and not at meta- position?
As shown, the presence of nitro group at ortho- and para-positions withdraws the
electron density from the benzene ring and thus facilitates the attack of the nucleophile
on haloarene
...
The negative
charge appeared at ortho - and para- positions with respect to the halogen substituent is
stabilised by –NO2 group while in case of meta -nitrobenzene, none of the resonating
structures bear the negative charge on carbon atom bearing the –NO2 group
...

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2
...
Halogen atom besides being slightly deactivating is o, pdirecting; therefore, further substitution occurs at ortho- and parapositions with respect to the halogen atom
...
Further, the halogen atom
because of its –I effect has some tendency to withdraw electrons from
the benzene ring
...

(i) Halogenation

(ii) Nitration

(iii) Sulphonation

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(iv) Friedel-Crafts reaction

Example 10
...
Why?

Solution Chlorine withdraws electrons through inductive effect and releases
electrons through resonance
...


Through resonance, halogen tends to stabilise the carbocation and
the effect is more pronounced at ortho- and para- positions
...
The resonance effect
tends to oppose the inductive effect for the attack at ortho- and parapositions and hence makes the deactivation less for ortho- and paraattack
...

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3
...


Fittig reaction
Aryl halides also give analogous compounds when treated with sodium
in dry ether, in which two aryl groups are joined together
...


Intext Questions
10
...


10
...
9 Identify A, B, C, D, E, R and R1 in the following:

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10
...
Many of these compounds are
useful in industry and agriculture
...


10
...
1 Dichloromethane
(Methylene
chloride)

Dichloromethane is widely used as a solvent as a paint remover, as a
propellant in aerosols, and as a process solvent in the manufacture of
drugs
...

Methylene chloride harms the human central nervous system
...
Higher levels of methylene chloride in air cause
dizziness, nausea, tingling and numbness in the fingers and toes
...
Direct contact with the eyes can
burn the cornea
...
7
...
The major use of chloroform today is in
methane
(Chloroform) the production of the freon refrigerant R-22
...
As might be expected from its use as
an anaesthetic, inhaling chloroform vapours depresses the central
nervous system
...
Chronic chloroform exposure may cause damage
to the liver (where chloroform is metabolised to phosgene) and to the
kidneys, and some people develop sores when the skin is immersed in
chloroform
...
It is therefore stored in closed dark coloured bottles
completely filled so that air is kept out
...
7
...
Due
to its objectionable smell, it has been replaced by other formulations
containing iodine
...
7
...
It is also used as
feedstock in the synthesis of chlorofluorocarbons and other chemicals,
pharmaceutical manufacturing, and general solvent use
...
There is some evidence that exposure to carbon
tetrachloride causes liver cancer in humans
...
In severe cases, these effects can lead
rapidly to stupor, coma, unconsciousness or death
...
The chemical may irritate
the eyes on contact
...
Depletion of the
ozone layer is believed to increase human exposure to ultraviolet rays,
leading to increased skin cancer, eye diseases and disorders, and
possible disruption of the immune system
...
7
...
They are extremely stable, unreactive,
non-toxic, non-corrosive and easily liquefiable gases
...
It is manufactured
from tetrachloromethane by Swarts reaction
...
By
1974, total freon production in the world was about 2 billion pounds
annually
...
In stratosphere, freon is able to initiate radical chain
reactions that can upset the natural ozone balance (Unit 14, Class XI)
...
7
...
Paul Muller was awarded the Nobel Prize in Medicine
and Physiology in 1948 for this discovery
...
However, problems related to extensive use of DDT
began to appear in the late 1940s
...
The chemical stability of DDT and its fat solubility
compounded the problem
...
If
ingestion continues at a steady rate, DDT builds up within the animal
over time
...


309 Haloalkanes and Haloarenes

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Summary
Alkyl/ Aryl halides may be classified as mono, di, or polyhalogen (tri-, tetra-, etc
...
Since halogen atoms are more electronegative than carbon, the carbonhalogen bond of alkyl halide is polarised; the carbon atom bears a partial positive
charge, and the halogen atom bears a partial negative charge
...
Aryl halides are prepared by
electrophilic substitution to arenes
...

The boiling points of organohalogen compounds are comparatively higher than the
corresponding hydrocarbons because of strong dipole-dipole and van der Waals forces
of attraction
...

The polarity of carbon-halogen bond of alkyl halides is responsible for their
nucleophilic substitution, elimination and their reaction with metal atoms to form
organometallic compounds
...
Chirality has a profound role in
understanding the reaction mechanisms of SN1 and SN2 reactions
...

A number of polyhalogen compounds e
...
, dichloromethane, chloroform, iodoform,
carbon tetrachloride, freon and DDT have many industrial applications
...


Exercises
10
...
2

(xii) o-Br-C 6H 4CH(CH 3)CH2 CH3

Give the IUPAC names of the following compounds:
(i) CH3 CH(Cl)CH(Br)CH3
(iv) (CCl3 )3CCl

10
...

(i) 2-Chloro-3-methylpentane
(iii) 1-Chloro-4-ethylcyclohexane
(v) 2-Bromobutane

(ii) p-Bromochlorobenzene
(iv) 2-(2-Chlorophenyl)-1-iodooctane
(vi) 4-tert-Butyl-3-iodoheptane

(vii) 1-Bromo-4-sec-butyl-2-methylbenzene

(viii) 1,4-Dibromobut-2-ene

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10
...
5 A hydrocarbon C 5H 10 does not react with chlorine in dark but gives a single
monochloro compound C5H 9Cl in bright sunlight
...

10
...

10
...


10
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10 Predict all the alkenes that would be formed by dehydrohalogenation of the
following halides with sodium ethoxide in ethanol and identify the major alkene:
(i) 1-Bromo-1-methylcyclohexane

(ii) 2-Chloro-2-methylbutane

(iii) 2,2,3-Trimethyl-3-bromopentane
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11 How will you bring about the following conversions?
(i) Ethanol to but-1-yne (ii) Ethane to bromoethene (iii) Propene to
1-nitropropane (iv) Toluene to benzyl alcohol (v) Propene to propyne
(vi) Ethanol to ethyl fluoride (vii) Bromomethane to propanone (viii) But-1-ene
to but-2-ene (ix) 1-Chlorobutane to n-octane (x) Benzene to biphenyl
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12 Explain why
(i) the dipole moment of chlorobenzene is lower than that of cyclohexyl chloride?
(ii) alkyl halides, though polar, are immiscible with water?
(iii) Grignard reagents should be prepared under anhydrous conditions?
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15 Write the mechanism of the following reaction:
nBuBr

+

KCN

nBuCN

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p
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Discuss
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19 How the following conversions can be carried out?
(i)
(ii)
(iii)
(iv)
(v)

Propene to propan-1-ol
Ethanol to but-1-yne
1-Bromopropane to 2-bromopropane
Toluene to benzyl alcohol
Benzene to 4-bromonitrobenzene

(vi)
(vii)
(viii)
(ix)
(x)

Benzyl alcohol to 2-phenylethanoic acid
Ethanol to propanenitrile
Aniline to chlorobenzene
2-Chlorobutane to 3, 4-dimethylhexane
2-Methyl-1-propene to 2-chloro-2-methylpropane

(xi)
(xii)
(xiii)
(xiv)
(xv)

Ethyl chloride to propanoic acid
But-1-ene to n-butyliodide
2-Chloropropane to 1-propanol
Isopropyl alcohol to iodoform
Chlorobenzene to p-nitrophenol

(xvi)
(xvii)
(xviii)
(xix)

2-Bromopropane to 1-bromopropane
Chloroethane to butane
Benzene to diphenyl
tert-Butyl bromide to isobutyl bromide

(xx) Aniline to phenylisocyanide
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Explain
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21 Primary alkyl halide C 4H 9 Br (a) reacted with alcoholic KOH to give compound (b)
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When
(a) is reacted with sodium metal it gives compound (d), C8 H 18 which is different
from the compound formed when n-butyl bromide is reacted with sodium
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1

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The replacement of equivalent hydrogens will
give the same product
...
Thus, four isomeric products are
possible
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5

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7

(i) Chloromethane, Bromomethane, Dibromomethane, Bromoform
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(ii) Isopropylchloride, 1-Chloropropane, 1-Chlorobutane
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p
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(i) CH3 CH2 CH2CH2Br
Being primary halide, there won’t be any steric
hindrance
...

(iii)

The presence of methyl group closer to the halide
group will increase the steric hindrance and
decrease the rate
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8

(i)

Tertiary halide reacts faster than secondary halide
because of the greater stability of tert-carbocation
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10

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