The replacement of hydrogen atom(s) in an aliphatic

or aromatic hydrocarbon by halogen atom(s) results

in the formation of alkyl halide (haloalkane) and aryl

halide (haloarene), respectively. Haloalkanes contain

halogen atom(s) attached to the sp

hybridised carbon

atom of an alkyl group whereas haloarenes contain

halogen atom(s) attached to sp

hybridised carbon

atom(s) of an aryl group. Many halogen containing

organic compounds occur in nature and some of

these are clinically useful. These classes of compounds

find wide applications in industry as well as in day-

to-day life. They are used as solvents for relatively

non-polar compounds and as starting materials for

the synthesis of wide range of organic compounds.

Chlorine containing antibiotic, chloramphenicol,

produced by microorganisms is very effective for the

treatment of typhoid fever. Our body produces iodine

containing hormone, thyroxine, the deficiency of which

causes a disease called goiter. Synthetic halogen

compounds, viz. chloroquine is used for the treatment

of malaria; halothane is used as an anaesthetic

during surgery. Certain fully fluorinated compounds

are being considered as potential blood substitutes

in surgery.

In this Unit, you will study the important methods

of preparation, physical and chemical properties and

uses of organohalogen compounds.

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.

Objectives

Unit

Haloalkanes andHaloalkanes and

Haloalkanes and

Haloar

enesenes

enes

Haloalkanes andHaloalkanes and

Haloalkanes and

HaloarHaloar

Haloar

enesenes

enes

Halogenated compounds persist in the environment due to their

resistance to breakdown by soil bacteria.

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Haloalkanes and haloarenes may be classified as follows:

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

compounds depending on whether they contain one, two or more halogen

atoms in their structures. For example,

Monohalocompounds may further be classified according to the

hybridisation of the carbon atom to which the halogen is bonded, as

discussed below.

This class includes

(a) Alkyl halides or haloalkanes (R—X)

In alkyl halides, the halogen atom is bonded to an alkyl group (R).

They form a homologous series represented by C

2n+1

X. They are

further classified as primary, secondary or tertiary according to the

nature of carbon to which halogen is attached. If halogen is attached to

a primary carbon atom in an alkyl halide, the alkyl halide is called

primary alkyl halide or 1° alkyl halide. Similarly, if halogen is attached

to secondary or tertiary carbon atom, the alkyl halide is called

secondary alkyl halide (2°) and tertiary (3°) alkyl halide, respectively.

(b) Allylic halides

These are the compounds in which the halogen atom is bonded to an

-hybridised carbon atom adjacent to carbon-carbon double bond

(C=C) i.e. to an allylic carbon.

These are the compounds in which the halogen atom is bonded to an

-hybridised carbon atom attached to an aromatic ring.

10.110.1

10.1

ClassificationClassification

Classification

10.1.1 On the

Basis of

Number of

Halogen

Atoms

10.1.2 Compounds

Containing

C—X

Bond (X= F,

Cl, Br, I)

Allylic carbon

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Haloalkanes and Haloarenes

This class includes:

(a) Vinylic halides

These are the compounds in which the halogen atom is bonded to

a sp

-hybridised carbon atom of a carbon-carbon double bond

(C = C).

(b) Aryl halides

These are the compounds in which the halogen atom is directly

bonded to the sp

-hybridised carbon atom of an aromatic ring.

Having learnt the classification of halogenated compounds, let us now learn

how these are named. The common names of alkyl halides are derived by

naming the alkyl group followed by the name of halide. In the IUPAC system

of nomenclature, alkyl halides are named as halosubstituted hydrocarbons.

For mono halogen substituted derivatives of benzene, common and IUPAC

names are the same. For dihalogen derivatives, the prefixes o-, m-, p- are

used in common system but in IUPAC system, as you have learnt in Class

XI, Unit 12, the numerals 1,2; 1,3 and 1,4 are used.

10.1.3 Compounds

Containing

C—X

Bond

The dihaloalkanes having the same type of halogen atoms are named

as alkylidene or alkylene dihalides. The dihalo-compounds having both

the halogen atoms are further classified as geminal halides or gem-dihalides

when both the halogen atoms are present on the same carbon atom of the

10.2 Nomenclature10.2 Nomenclature

10.2 Nomenclature

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chain and vicinal halides or vic-dihalides when halogen atoms are present

on adjacent carbon atoms. In common name system, gem-dihalides are

named as alkylidene halides and vic-dihalides are named as alkylene

dihalides. In IUPAC system, they are named as dihaloalkanes.

Structure Common name IUPAC name

CH(Cl)CH

sec-Butyl chloride 2-Chlorobutane

(CH

)

CCH

Br neo-Pentyl bromide 1-Bromo-2,2-dimethylpropane

(CH

)

CBr tert-Butyl bromide 2-Bromo-2-methylpropane

= CHCl Vinyl chloride Chloroethene

= CHCH

Br Allyl bromide 3-Bromopropene

Methylene chloride Dichloromethane

CHCl

Chloroform Trichloromethane

CHBr

Bromoform Tribromomethane

CCl

Carbon tetrachloride Tetrachloromethane

F n-Propyl fluoride 1-Fluoropropane

o-Chlorotoluene 1-Chloro-2-methylbenzene

2-Chlorotoluene

Benzyl chloride Chlorophenylmethane

Table 10.1: Common and IUPAC Names of some Halides

Example 10.1Example 10.1

Example 10.1

SolutionSolution

Solution

Draw the structures of all the eight structural isomers that have the

molecular formula C

Br. Name each isomer according to IUPAC system

and classify them as primary, secondary or tertiary bromide.

Br 1-Bromopentane (1

)

CH(Br)CH

2-Bromopentane(2

)

CH(Br)CH

3-Bromopentane (2

)

(CH

)

CHCH

Br 1-Bromo-3-methylbutane (1

)

Some common examples of halocompounds are mentioned in Table 10.1.

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Haloalkanes and Haloarenes

Intext QuestionIntext Question

Intext Question

10.1 Write structures of the following compounds:

(i) 2-Chloro-3-methylpentane

(ii) 1-Chloro-4-ethylcyclohexane

(iii) 4-tert. Butyl-3-iodoheptane

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

(v) 1-Bromo-4-sec. butyl-2-methylbenzene.

Halogen atoms are more electronegative than carbon, therefore,

carbon-halogen bond of alkyl halide is polarised; the carbon atom bears

a partial positive charge whereas the halogen atom bears a partial

negative charge.

As we go down the group in the periodic table, the size of halogen

atom increases. Fluorine atom is the smallest and iodine atom is the

largest. Consequently the carbon-halogen bond length also increases

from C—F to C—I. Some typical bond lengths, bond enthalpies and

dipole moments are given in Table 10.2.

Alkyl halides are best prepared from alcohols, which are easily accessible.

10.310.3

10.3

Nature ofNature of

Nature of

C-X BondC-X Bond

C-X Bond

(CH

)

CHCHBrCH

2-Bromo-3-methylbutane(2

)

(CH

)

CBrCH

2-Bromo-2-methylbutane (3

)

CH(CH

)CH

Br 1-Bromo-2-methylbutane(1

)

(CH

)

CCH

Br 1-Bromo-2,2-dimethylpropane (1

)

Write IUPAC names of the following:

(i) 4-Bromopent-2-ene (ii) 3-Bromo-2-methylbut-1-ene

(iii) 4-Bromo-3-methylpent-2-ene (iv) 1-Bromo-2-methylbut-2-ene

(v) 1-Bromobut-2-ene (vi) 3-Bromo-2-methylpropene

Example 10.2Example 10.2

Example 10.2

SolutionSolution

Solution

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10.4.2 From

Hydrocarbons

Bond Bond length/pm C-X Bond enthalpies/ kJmol

-1

Dipole moment/Debye

–F 139 452 1.847

– Cl 178 351 1.860

–Br 193 293 1.830

–I 214 234 1.636

Table 10.2: Carbon-Halogen (C—X) Bond Lengths, Bond

Enthalpies and Dipole Moments

The preparation of alkyl chloride is carried out either by passing

dry hydrogen chloride gas through a solution of alcohol or by heating

a mixture of alcohol and concentrated aqueous halogen 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).

(I) From alkanes by free radical halogenation

Free radical chlorination or bromination of alkanes gives a complex

mixture of isomeric mono- and polyhaloalkanes, which is difficult to

The hydroxyl group of an alcohol is replaced by halogen on reaction with

concentrated halogen acids, phosphorus halides or thionyl chloride.

Thionyl chloride is preferred because in this reaction alkyl halide is formed

along with gases SO

and HCl. The two gaseous products are escapable,

hence, the reaction gives pure alkyl halides. The reactions of primary and

secondary alcohols with HCl require the presence of a catalyst, ZnCl

With tertiary alcohols, the reaction is conducted by simply shaking the

alcohol with concentrated HCl at room temperature. Constant boiling

with HBr (48%) is used for preparing alkyl bromide. Good yields of

R—I may be obtained by heating alcohols with sodium or potassium

iodide in 95% orthophosphoric acid. The order of reactivity of alcohols

with a given haloacid is 3°>2°>1°. Phosphorus tribromide and triiodide

are usually generated in situ (produced in the reaction mixture) by the

reaction of red phosphorus with bromine and iodine respectively.

10.410.4

10.4

Methods ofMethods of

Methods of

PreparationPreparation

Preparation

of Haloalkanesof Haloalkanes

of Haloalkanes

10.4.1 From Alcohols

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Haloalkanes and Haloarenes

separate as pure compounds. Consequently, the yield of any single

compound is low (Unit 13, Class XI).

Identify all the possible monochloro structural isomers expected to be

formed on free radical monochlorination of (CH

)

CHCH

In the given molecule, there are four different types of hydrogen atoms.

Replacement of these hydrogen atoms will give the following

(CH

)

CHCH

Cl (CH

)

CHCH(Cl)CH

(CH

)

C(Cl)CH

CH(CH

Cl)CH

Example 10.3Example 10.3

Example 10.3

SolutionSolution

Solution

(II) From alkenes

(i) Addition of hydrogen halides: An alkene is converted to

corresponding alkyl halide by reaction with hydrogen chloride,

hydrogen bromide or hydrogen iodide.

Propene yields two products, however only one predominates as

per Markovnikov’s rule. (Unit 13, Class XI)

(ii) Addition of halogens: In the laboratory, addition of bromine in

CCl

to an alkene resulting in discharge of reddish brown colour

of bromine constitutes an important method for the detection of

double bond in a molecule. The addition results in the synthesis

of vic-dibromides, which are colourless (Unit 13, Class XI).

Alkyl iodides are often prepared by the reaction of alkyl chlorides/

bromides with NaI in dry acetone. This reaction is known as Finkelstein

reaction.

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

the forward reaction according to Le Chatelier’s Principle.

The synthesis of alkyl fluorides is best accomplished by heating an

alkyl chloride/bromide in the presence of a metallic fluoride such as

10.4.3 Halogen

Exchange

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296Chemistry

AgF, Hg

, CoF

or SbF

. The reaction is termed as Swarts reaction.

(i) From hydrocarbons 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.

The ortho and para isomers can be easily separated due to large

difference in their melting points. Reactions with iodine are reversible

in nature and require the presence of an oxidising agent (HNO

HIO

) to oxidise the HI formed during iodination. Fluoro compounds

are not prepared by this method due to high reactivity of fluorine.

(ii) From amines by 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). Mixing the solution of freshly

prepared diazonium salt with cuprous chloride or cuprous

bromide results in the replacement of the diazonium group

by –Cl or –Br.

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.

10.510.5

10.5

Preparation ofPreparation of

Preparation of

HaloarenesHaloarenes

Haloarenes

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Haloalkanes and Haloarenes

Alkyl halides are colourless when pure. However, bromides and iodides

develop colour when exposed to light. Many volatile halogen compounds

have sweet smell.

10.2 Why is sulphuric acid not used during the reaction of alcohols with KI?

10.3 Write structures of different dihalogen derivatives of propane.

10.4 Among the isomeric alkanes of molecular formula C

, identify the one that

on photochemical chlorination yields

(i) A single monochloride.

(ii) Three isomeric monochlorides.

(iii) Four isomeric monochlorides.

10.5 Draw the structures of major monohalo products in each of the following

reactions:

Intext QuestionsIntext Questions

Intext Questions

Write the products of the following reactions:

Example 10.4Example 10.4

Example 10.4

SolutionSolution

Solution

10.610.6

10.6

PhysicalPhysical

Physical

PropertiesProperties

Properties

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Melting and boiling points

Methyl chloride, methyl bromide, ethyl chloride and some

chlorofluoromethanes are gases at room temperature. Higher members

are liquids or solids. As we have already learnt, molecules of organic

halogen compounds are generally polar. Due to greater polarity as well

as higher molecular mass as compared to the parent hydrocarbon, the

intermolecular forces of attraction (dipole-dipole and van der Waals)

are stronger in the halogen derivatives. That is why the boiling points

of chlorides, bromides and iodides are considerably higher than those

of the hydrocarbons of comparable molecular mass.

The attractions get stronger as the molecules get bigger in size and

have more electrons. The pattern of variation of boiling points of different

halides is depicted in Fig. 10.1. For the same alkyl group, the boiling

points of alkyl halides decrease in the order: RI> RBr> RCl> RF. This

is because with the increase in size and mass of halogen atom, the

magnitude of van der Waal forces increases.

The boiling points of isomeric haloalkanes decrease with increase

in branching (Unit 12, Class XI). For example, 2-bromo-2-

methylpropane has the lowest boiling point among the three isomers.

Boiling points of isomeric dihalobenzenes are very nearly the same.

However, the para-isomers are high melting as compared to their ortho-

and meta-isomers. It is due to symmetry of para-isomers that fits in

crystal lattice better as compared to ortho- and meta-isomers.

Fig. 10.1: Comparison of boiling points of some alkyl halides

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Haloalkanes and Haloarenes

Solubility

The haloalkanes are very slightly soluble in water. In order to dissolve

haloalkane in water, energy is required to overcome the attractions between

the haloalkane molecules and break the hydrogen bonds between water

molecules. 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. As a result, the solubility of haloalkanes

in water is low. 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.

Table 10.3: Density of Some Haloalkanes

Density

Bromo, iodo and polychloro derivatives of hydrocarbons are heavier than

water. The density increases with increase in number of carbon atoms,

halogen atoms and atomic mass of the halogen atoms (Table 10.3).

The reactions of haloalkanes may be divided into the following categories:

1. Nucleophilic substitution

2. Elimination reactions

3. Reaction with metals.

(1)Nucleophilic substitution reactions

You have learnt in Class XI that nucleophiles are electron rich species.

Therefore, they attack at that part of the substrate molecule which

is electron deficient. The reaction in which a nucleophile replaces

10.6 Arrange each set of compounds in order of increasing boiling points.

(i) Bromomethane, Bromoform, Chloromethane, Dibromomethane.

(ii) 1-Chloropropane, Isopropyl chloride, 1-Chlorobutane.

Intext QuestionIntext Question

Intext Question

10.710.7

10.7

ChemicalChemical

Chemical

ReactionsReactions

Reactions

Compound Density (g/mL) Compound Density (g/mL)

n–C

Cl 0.89 CH

1.336

n–C

Br 1.335 CHCl

1.489

n-C

I 1.747 CCl

1.595

10.7.1 Reactions of

Haloalkanes

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already existing nucleophile in a molecule is called nucleophilic

substitution reaction. Haloalkanes are substrate in these reactions.

In this type of reaction, a nucleophile reacts with haloalkane (the

substrate) having a partial positive charge on the carbon atom bonded

to halogen. A substitution reaction takes place and halogen atom,

called leaving group departs as halide ion. Since the substitution

reaction is initiated by a nucleophile, it is called nucleophilic

substitution reaction.

It is one of the most useful classes of organic reactions of alkyl

halides in which halogen is bonded to sp

hybridised carbon. The

products formed by the reaction of haloalkanes with some common

nucleophiles are given in Table 10.4.

Groups like cyanides and nitrites possess two nucleophilic centres

and are called ambident nucleophiles. Actually cyanide group is a

hybrid of two contributing structures and therefore can act as a

nucleophile in two different ways [

C≡N ↔ :C=N

], i.e., linking through

Table 10.4: Nucleophilic Substitution of Alkyl Halides (R–X)

Reagent Nucleophile Substitution Class of main

(Nu

–

) product R–Nu product

NaOH (KOH) HO

–

ROH Alcohol

O H

O ROH Alcohol

NaOR′ R′O

–

ROR′ Ether

NaI I

–

R—I Alkyl iodide

RNH

Primary amine

R′NH

RNHR′ Sec. amine

R′R′′NH R′R′′NH RNR′R′′ Tert. amine

KCN RCN Nitrile

(cyanide)

AgCN Ag-CN: RNC Isonitrile

(isocyanide)

KNO

O=N—O R—O—N=O Alkyl nitrite

AgNO

Ag—Ö—N=O R—NO

Nitroalkane

R′COOAg R′COO

–

R′COOR Ester

LiAlH

H RH Hydrocarbon

R′

–

R′

–

RR′ Alkane

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Haloalkanes and Haloarenes

carbon atom resulting in alkyl cyanides and through nitrogen atom

leading to isocyanides. Similarly nitrite ion also represents an ambident

nucleophile with two different points of linkage [

–

O—

i i

=O]. The linkage

through oxygen results in alkyl nitrites while through nitrogen atom, it

leads to nitroalkanes.

Mechanism: This reaction has been found to proceed by two different

mechanims which are described below:

(a) Substitution nucleophilic bimolecular (S

The reaction between CH

Cl and hydroxide ion to yield methanol and

chloride ion follows a second order kinetics, i.e., the rate depends

upon the concentration of both the reactants.

Haloalkanes react with KCN to form alkyl cyanides as main product

while AgCN forms isocyanides as the chief product. Explain.

KCN is predominantly ionic and provides cyanide ions in solution.

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. However, AgCN is mainly covalent in nature and nitrogen is free

to donate electron pair forming isocyanide as the main product.

Example 10.5Example 10.5

Example 10.5

SolutionSolution

Solution

The above reaction can be represented diagrammatically as shown in

Fig. 10.2.

It depicts a bimolecular nucleophilic substitution (S

2) reaction;

the incoming nucleophile interacts with alkyl halide causing the

carbon-halide bond to break and a new bond is formed between

carbon and attacking nucleophile. Here it is C-O bond formed between

C and -OH. These two processes take place simultaneously in a

Fig. 10.2: Red ball represents the incoming hydroxide ion and green ball represents

the outgoing halide ion

In the year 1937,

Edward Davies Hughes

and Sir Christopher

Ingold proposed a

mechanism for an S

reaction.

You have already learnt in Section 12.3.2 of Class XI, the solid wedge represents the bond

coming out of the paper, dashed line going down the paper and a straight line representing

bond in the plane of the paper.

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single step and no intermediate is formed. As the reaction progresses

and the bond between the incoming nucleophile and the carbon

atom starts forming, the bond between carbon atom and leaving

group weakens. As this happens, the three carbon-hydrogen bonds

of the substrate start moving away from the attacking nucleophile. In

transition state all the three C-H bonds are in the same plane and the

attacking and leaving nucleophiles are partially attached to the

carbon. As the attacking nucleophile approaches closer to the carbon,

C-H bonds still keep on moving in the same direction till the attacking

nucleophile attaches to carbon and leaving group leaves the carbon.

As a result configuration is inverted, the configuration (See box) of

carbon atom under attack inverts in much the same way as an

umbrella is turned inside out when caught in a strong wind. This

process is called as inversion of configuration. In the transition

state, the carbon atom is simultaneously bonded to incoming

nucleophile and the outgoing leaving group. Such structures are

unstable and cannot be isolated. Thus, in the transition state, carbon

is simultaneously bonded to five atoms.

Hughes worked under

Ingold and earned a

D.Sc. degree from the

University of London.

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. Of the

simple alkyl halides, methyl halides react most rapidly in S

2 reactions

because there are only three small hydrogen atoms. Tertiary halides

are the least reactive because bulky groups hinder the approaching

Configuration

Spacial arrangement of functional groups around carbon is called its configuration.

See the structures (A) and (B) given below carefully.

These are the two structures of the same compound. They differ in spacial arrangement

of functional groups attached to carbon. Structure (A) is mirror image of Structure (B).

We say configuration of carbon in structure (A) is mirror image of the configuration of

carbon in structure (B).

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Haloalkanes and Haloarenes

nucleophiles. Thus the order of reactivity followed is:

Primary halide > Secondary halide > Tertiary halide.

(b) Substitution nucleophilic unimolecular (S

1 reactions are generally carried out in polar protic solvents

(like water, alcohol, acetic acid, etc.). The reaction between tert-

butyl bromide and hydroxide ion yields tert-butyl alcohol and

follows the first order kinetics, i.e., the rate of reaction depends

upon the concentration of only one reactant, which is tert- butyl

bromide.

It occurs in two steps. In step I, the polarised C—Br bond undergoes

slow cleavage to produce a carbocation and a bromide ion. The

carbocation thus formed is then attacked by nucleophile in step II

to complete the substitution reaction.

Fig.10.3: Steric effects in S

2 reaction. The relative rate of S

2 reaction is given in parenthesis

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Step I is the slowest and reversible. It involves the C–Br bond breaking for which the

energy is obtained through solvation of halide ion with the proton of protic solvent. 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. Further, greater

the stability of carbocation, greater will be its ease of formation from alkyl halide and faster

will be the rate of reaction. In case of alkyl halides, 3

alkyl halides undergo S

1 reaction

very fast because of the high stability of 3

carbocations. We can sum up the order of reactivity

of alkyl halides towards S

1 and S

2 reactions as follows:

For the same reasons, allylic and benzylic halides show high reactivity towards the S

reaction. The carbocation thus formed gets stabilised through resonance (Unit 12, Class XI) as

shown below:

In the following pairs of halogen compounds, which would undergo

2 reaction faster?

Example 10.6Example 10.6

Example 10.6

SolutionSolution

Solution

It is primary halide and therefore undergoes S

reaction faster.

As iodine is a better leaving group because of its

large size, it will be released at a faster rate in the

presence of incoming nucleophile.

Example 10.7Example 10.7

Example 10.7

Predict the order of reactivity of the following compounds in S

1 and

2 reactions:

(i) The four isomeric bromobutanes

(ii) C

Br, C

CH(C

)Br, C

CH(CH

)Br, C

C(CH

)(C

)Br

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.

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Haloalkanes and Haloarenes

In order to understand the stereochemical aspects of substitution

reactions, we need to learn some basic stereochemical principles

and notations (optical activity, chirality, retention, inversion,

racemisation, etc.).

(i) Optical activity: Plane of plane polarised light produced by

passing ordinary light through Nicol prism is rotated when it

is passed through the solutions of certain compounds. Such

compounds are called optically active compounds. The angle

by which the plane polarised light is rotated is measured by an

instrument called polarimeter. If the compound rotates the plane

of plane polarised light to the right, i.e., 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. If the light is rotated towards left (anticlockwise

direction), the compound is said to be laevo-rotatory or the

l-form and a negative (–) sign is placed before the degree of

rotation. Such (+) and (–) isomers of a compound are called

optical isomers and the phenomenon is termed as optical

isomerism.

(ii) Molecular asymmetry, chirality and enantiomers: The

observation of Louis Pasteur (1848) that crystals of certain

compounds exist in the form of mirror images laid the

foundation of modern stereochemistry. 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. He believed that this difference in

optical activity was associated with the three dimensional

arrangements of atoms in the molecules (

configurations) of

(i) CH

Br < (CH

)

CHCH

Br < CH

CH(Br)CH

< (CH

)

CBr (S

Br > (CH

)

CHCH

Br > CH

CH(Br)CH

> (CH

)

CBr (S

Of the two primary bromides, the carbocation intermediate derived from

(CH

)

CHCH

Br is more stable than derived from CH

Br because

of greater electron donating inductive effect of (CH

)

CH- group. Therefore,

(CH

)

CHCH

Br is more reactive than CH

Br in S

1 reactions.

CH(Br)CH

is a secondary bromide and (CH

)

CBr is a tertiary

bromide. Hence the above order is followed in S

1. The reactivity in S

reactions follows the reverse order as the steric hinderance around the

electrophilic carbon increases in that order.

(ii) C

C(CH

)(C

)Br > C

CH(C

)Br > C

CH(CH

)Br > C

Br (S

C(CH

)(C

)Br < C

CH(C

)Br < C

CH(CH

)Br < C

Br (S

Of the two secondary bromides, the carbocation intermediate obtained

from C

CH(C

)Br is more stable than obtained from C

CH(CH

)Br

because it is stabilised by two phenyl groups due to resonance. Therefore,

the former bromide is more reactive than the latter in S

1 reactions. A

phenyl group is bulkier than a methyl group. Therefore, C

CH(C

)Br

is less reactive than C

CH(CH

)Br in S

2 reactions.

SolutionSolution

Solution

William Nicol (1768-

1851) developed the first

prism that produced

plane polarised light.

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306Chemistry

two types of crystals. Dutch scientist, J. Van’t Hoff and French

scientist, C. Le Bel in the same year (1874), independently

argued that the spatial arrangement of four groups (valencies)

around a central carbon is tetrahedral and if all the substituents

attached to that carbon are different, the mirror image of the

molecule is not superimposed (overlapped) on the molecule;

such a carbon is called asymmetric carbon or stereocentre.

The resulting molecule would lack symmetry and is referred to

as asymmetric molecule. The asymmetry of the molecule along

with non superimposability of mirror images is responsible for

the optical activity in such organic compounds.

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.

However, many objects are non superimposable on

their mirror images. For example, your left and right

hand look similar but if you put your left hand on

your right hand by moving them in the same plane,

they do not coincide. The objects which are non-

superimposable on their mirror image (like a pair

of hands) are said to be

chiral and this property is

known as chirality. Chiral molecules are optically

active, while the objects, which are, superimposable

on their mirror images are called achiral. These

molecules are optically inactive.

The above test of molecular chirality can be

applied to organic molecules by constructing

models and its mirror images or by drawing three

dimensional structures and attempting to

superimpose them in our minds. There are other

aids, however, that can assist us in recognising

chiral molecules. One such aid is the presence of

As you can see very clearly, propan-2-ol (A) does not contain an asymmetric

carbon, as all the four groups attached to the tetrahedral carbon are not

different. We rotate the mirror image (B) of the molecule by 180° (structure

C) and try to overlap the structure (C) with the structure (A), these structures

completely overlap. Thus propan-2-ol is an achiral molecule.

Jacobus Hendricus

Van’t Hoff (1852-1911)

received the first Nobel

Prize in Chemistry in

1901 for his work on

solutions.

Fig 10.4: Some common examples of chiral and

achiral objects

a single asymmetric carbon atom. Let us consider

two simple molecules propan-2-ol (Fig.10.5) and butan-2-ol

(Fig.10.6) and their mirror images.

Fig 10.5: B is mirror image of A; B is rotated by 180

and C is

obtained; C is superimposable on A.

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Haloalkanes and Haloarenes

Butan-2-ol has four different groups attached to

the tetrahedral carbon and as expected is chiral. Some common

examples of chiral molecules such as

2-chlorobutane, 2, 3-dihyroxypropanal, (OHC–CHOH–CH

OH),

bromochloro-iodomethane (BrClCHI), 2-bromopropanoic acid

C–CHBr–COOH), etc.

Fig. 10.7: A chiral molecule

and its mirror image

The stereoisomers related to each other as non-

superimposable mirror images are called enantiomers

(Fig. 10.7). A and B in Fig. 10.5 and D and E in Fig. 10.6 are

enantiomers.

Enantiomers possess identical physical properties namely,

melting point, boiling point, refractive index, etc. They only differ

with respect to the rotation of plane polarised light. If one of the

enantiomer is dextro rotatory, the other will be laevo rotatory.

A mixture containing two enantiomers in equal proportions

will have zero optical rotation, as the rotation due to one isomer

will be cancelled by the rotation due to the other isomer. Such

a mixture is known as racemic mixture or racemic

modification. A racemic mixture is represented by prefixing dl

or (±) before the name, for example (±) butan-2-ol. The process

of conversion of enantiomer into a racemic mixture is known as

racemisation.

Example 10.8Example 10.8

Example 10.8

Identify chiral and achiral molecules in each of the following pair of

compounds. (Wedge and Dash representations according to Class XI,

Fig. 12.1).

Fig 10.6: E is mirror image of D; E is rotated by 180

to get F and F is

non superimposable on its mirror image D.

However, the sign of optical rotation is not necessarily related to

the absolute (actual) configuration of the molecule.

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(iii) Retention: Retention of configuration is the preservation of the spatial

arrangement of bonds to an asymmetric centre during a chemical

reaction or transformation.

In general, if during a reaction, no bond to the stereocentre is broken,

the product will have the same general configuration of groups

around the stereocentre as that of reactant. Such a reaction is said

to proceed with retention of the configuration. Consider as an

example, the reaction that takes place when (–)-2-methylbutan-1-ol

is heated with concentrated hydrochloric acid.

SolutionSolution

Solution

It is important to note that configuration at a symmetric centre in

the reactant and product is same but the sign of optical rotation

has changed in the product. This is so because two different

compounds with same configuration at asymmetric centre may have

different optical rotation. One may be dextrorotatory (plus sign of

optical rotation) while other may be laevorotatory (negative sign of

optical rotation).

(iv) Inversion, retention and racemisation: There are three outcomes

for a reaction at an asymmetric carbon atom, when a bond directly

linked to an asymmetric carbon atom is broken. Consider the

replacement of a group X by Y in the following reaction;

If (A) is the only compound obtained, the process is called retention

of configuration. Note that configuration has been rotated in A.

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

of configuration. Configuration has been inverted in B.

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Haloalkanes and Haloarenes

If a 50:50 mixture of A and B is obtained then the process is called

racemisation and the product is optically inactive, as one isomer will

rotate the plane polarised light in the direction opposite to another.

Now let us have a fresh look at S

1 and S

2 mechanisms by

taking examples of optically active alkyl halides.

In case of optically active alkyl halides, the product formed as a

result of S

2 mechanism has the inverted configuration as compared

to the reactant. This is because the nucleophile attaches itself on the

side opposite to the one where the halogen atom is present. When

(–)-2-bromooctane is allowed to react with sodium hydroxide,

(+)-octan-2-ol is formed with the –OH group occupying the position

opposite to what bromide had occupied.

Thus, S

2 reactions of optically active halides are accompanied by

inversion of configuration.

In case of optically active alkyl halides, S

1 reactions are

accompanied by racemisation. Can you think of the reason why it

happens? Actually the carbocation formed in the slow step being sp

hybridised is planar (achiral). The attack of the nucleophile may be

accomplished from either side of the plane of carbocation resulting in

a mixture of products, one having the same configuration (the –OH

attaching on the same position as halide ion) and the other having

opposite configuration (the –OH attaching on the side opposite to halide

ion). This may be illustrated by hydrolysis of optically active

2-bromobutane, which results in the formation of (±)-butan-2-ol.

2. 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.

Location of

αα

α and

ββ

β carbon in a

molecule

Carbon on which

halogen atom is

directly attached is

called α-carbon and

the carbon atom

adjacent to this

carbon is called

β-carbon.

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310Chemistry

As a result, an alkene is formed as a product. Since β-hydrogen

atom is involved in elimination, it is often called

ββ

β-elimination.

If there is possibility of formation of more than one alkene due to

the availability of more than one β-hydrogen atoms, usually one alkene

is formed as the major product. 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.” Thus, 2-bromopentane gives

pent-2-ene as the major product.

3. Reaction with metals

Most organic chlorides, bromides and iodides react with certain

metals to give compounds containing carbon-metal bonds. Such

compounds are known as organo-metallic compounds. An

important class of organo-metallic compounds discovered by Victor

Grignard in 1900 is alkyl magnesium halide, RMgX, referred as

Grignard Reagents. These reagents are obtained by the reaction of

haloalkanes with magnesium metal in dry ether.

A chemical reaction is the result of competition; it is a race that is won by the fastest

runner. A collection of molecules tend to do, by and large, what is easiest for them. An

alkyl halide with β-hydrogen atoms when reacted with a base or a nucleophile has two

competing routes: substitution (S

1 and S

2) and elimination. Which route will be taken

up depends upon the nature of alkyl halide, strength and size of base/nucleophile and

reaction conditions. 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.

Similarly, a primary alkyl halide will prefer a S

2 reaction, a secondary halide- S

2 or

elimination depending upon the strength of base/nucleophile and a tertiary halide- S

1 or

elimination depending upon the stability of carbocation or the more substituted alkene.

Elimination versus substitutionElimination versus substitution

Elimination versus substitution

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Haloalkanes and Haloarenes

Victor Grignard had a strange start in academic life for a chemist - he

took a maths degree. When he eventually switched to chemistry, it was

not to the mathematical province of physical chemistry but to organic

chemistry. While attempting to find an efficient catalyst for the process

of methylation, he noted that Zn in diethyl ether had been used for this

purpose and wondered whether the Mg/ether combination might be

successful. Grignard reagents were first reported in 1900 and Grignard

used this work for his doctoral thesis in 1901. In 1910, Grignard obtained

a professorship at the University of Nancy and in 1912, he was awarded

the Nobel prize for Chemistry which he shared with Paul Sabatier who

had made advances in nickel catalysed hydrogenation.

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.

Grignard reagents are highly reactive and react with any source of

proton to give hydrocarbons. Even water, alcohols, amines are sufficiently

acidic to convert them to corresponding hydrocarbons.

It is therefore necessary to avoid even traces of moisture from a Grignard

reagent. That is why reaction is carried out in dry ether. On the other

hand, this could be considered as one of the methods for converting

halides to hydrocarbons.

Wurtz reaction

Alkyl halides react with sodium in dry ether to give hydrocarbons

containing double the number of carbon atoms present in the halide.

This reaction is known as Wurtz reaction (Unit 13, Class XI).

1. Nucleophilic substitution

Aryl halides are extremely less reactive towards nucleophilic

substitution reactions due to the following reasons:

(i) Resonance effect : In haloarenes, the electron pairs on halogen

atom are in conjugation with π-electrons of the ring and the

following resonating structures are possible.

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

resonance. As a result, the bond cleavage in haloarene is difficult

than haloalkane and therefore, they are less reactive towards

nucleophilic substitution reaction.

10.7.2 Reactions of

Haloarenes

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312Chemistry

(ii) Difference in hybridisation of carbon atom in C—X bond: In

haloalkane, the carbon atom attached to halogen is sp

hybridised while in case of haloarene, the carbon atom attached

to halogen is sp

-hybridised.

The sp

hybridised carbon with a greater s-character is more

electronegative and can hold the electron pair of C—X bond

more tightly than sp

-hybridised carbon in haloalkane with

less s-chararcter. Thus, C—Cl bond length in haloalkane is

177pm while in haloarene is 169 pm. Since it is difficult to

break a shorter bond than a longer bond, therefore, haloarenes

are less reactive than haloalkanes towards nucleophilic

substitution reaction.

(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, S

1 mechanism is ruled out.

(iv) Because of the possible repulsion, it is less likely for the electron

rich nucleophile to approach electron rich arenes.

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.

The presence of an electron withdrawing group (-NO

) at ortho- and

para-positions increases the reactivity of haloarenes.

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Haloalkanes and Haloarenes

The effect is pronounced when (-NO

) group is introduced at ortho-

and para- positions. However, no effect on reactivity of haloarenes is

observed by the presence of electron withdrawing group at meta-position.

Mechanism of the reaction is as depicted:

Can you think why does NO

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 carbanion thus formed is stabilised through resonance. The negative

charge appeared at ortho- and para- positions with respect to the halogen substituent is

stabilised by –NO

group while in case of meta-nitrobenzene, none of the resonating

structures bear the negative charge on carbon atom bearing the –NO

group. Therefore,

the presence of nitro group at meta- position does not stabilise the negative charge and

no effect on reactivity is observed by the presence of –NO

group at meta-position.

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314Chemistry

2. Electrophilic substitution reactions

Haloarenes undergo the usual electrophilic reactions of the benzene

ring such as halogenation, nitration, sulphonation and Friedel-Crafts

reactions. Halogen atom besides being slightly deactivating is o, p-

directing; therefore, further substitution occurs at ortho- and para-

positions with respect to the halogen atom. The o, p-directing influence

of halogen atom can be easily understood if we consider the resonating

structures of halobenzene as shown:

Due to resonance, the electron density increases more at ortho- and

para-positions than at meta-positions. Further, the halogen atom

because of its –I effect has some tendency to withdraw electrons from

the benzene ring. As a result, the ring gets somewhat deactivated as

compared to benzene and hence the electrophilic substitution reactions

in haloarenes occur slowly and require more drastic conditions as

compared to those in benzene.

(i) Halogenation

(ii) Nitration

(iii) Sulphonation

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Haloalkanes and Haloarenes

(iv) Friedel-Crafts reaction

Although chlorine is an electron withdrawing group, yet it is ortho-,

para- directing in electrophilic aromatic substitution reactions. Why?

Chlorine withdraws electrons through inductive effect and releases

electrons through resonance. Through inductive effect, chlorine

destabilises the intermediate carbocation formed during the electrophilic

substitution.

Through resonance, halogen tends to stabilise the carbocation and

the effect is more pronounced at ortho- and para- positions. The

inductive effect is stronger than resonance and causes net electron

withdrawal and thus causes net deactivation. The resonance effect

tends to oppose the inductive effect for the attack at ortho- and para-

positions and hence makes the deactivation less for ortho- and para-

attack. Reactivity is thus controlled by the stronger inductive effect

and orientation is controlled by resonance effect.

Example 10.9Example 10.9

Example 10.9

SolutionSolution

Solution

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316Chemistry

Intext QuestionsIntext Questions

Intext Questions

3. Reaction with metals

Wurtz-Fittig reaction

A mixture of an alkyl halide and aryl halide gives an alkylarene when

treated with sodium in dry ether and is called Wurtz-Fittig reaction.

Fittig reaction

Aryl halides also give analogous compounds when treated with sodium

in dry ether, in which two aryl groups are joined together. It is called

Fittig reaction.

10.7 Which alkyl halide from the following pairs would you expect to react more

rapidly by an S

2 mechanism? Explain your answer.

10.8 In the following pairs of halogen compounds, which compound undergoes faster

1 reaction?

10.9 Identify A, B, C, D, E, R and R

in the following:

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Haloalkanes and Haloarenes

Carbon compounds containing more than one halogen atom are usually

referred to as polyhalogen compounds. Many of these compounds are

useful in industry and agriculture. Some polyhalogen compounds are

described in this section.

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. It is also used as a metal cleaning and finishing solvent. Methylene

chloride harms the human central nervous system. Exposure to lower

levels of methylene chloride in air can lead to slightly impaired hearing

and vision. Higher levels of methylene chloride in air cause dizziness,

nausea, tingling and numbness in the fingers and toes. In humans, direct

skin contact with methylene chloride causes intense burning and mild

redness of the skin. Direct contact with the eyes can burn the cornea.

Chemically, chloroform is employed as a solvent for fats, alkaloids,

iodine and other substances. The major use of chloroform today is in

the production of the freon refrigerant R-22. It was once used as a

general anaesthetic in surgery but has been replaced by less toxic,

safer anaesthetics, such as ether. As might be expected from its use as

an anaesthetic, inhaling chloroform vapours depresses the central

nervous system. Breathing about 900 parts of chloroform per million

parts of air (900 parts per million) for a short time can cause dizziness,

fatigue, and headache. 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. Chloroform is slowly oxidised by air in the presence of

light to an extremely poisonous gas, carbonyl chloride, also known as

phosgene. It is therefore stored in closed dark coloured bottles

completely filled so that air is kept out.

It was used earlier as an antiseptic but the antiseptic properties are

due to the liberation of free iodine and not due to iodoform itself. Due

to its objectionable smell, it has been replaced by other formulations

containing iodine.

It is produced in large quantities for use in the manufacture of

refrigerants and propellants for aerosol cans. It is also used as

feedstock in the synthesis of chlorofluorocarbons and other chemicals,

pharmaceutical manufacturing, and general solvent use. Until the mid

1960s, it was also widely used as a cleaning fluid, both in industry,

as a degreasing agent, and in the home, as a spot remover and as fire

extinguisher. There is some evidence that exposure to carbon

tetrachloride causes liver cancer in humans. The most common effects

are dizziness, light headedness, nausea and vomiting, which can cause

permanent damage to nerve cells. In severe cases, these effects can lead

rapidly to stupor, coma, unconsciousness or death. Exposure to CCl

can make the heart beat irregularly or stop. The chemical may irritate

the eyes on contact. When carbon tetrachloride is released into the air,

it rises to the atmosphere and depletes the ozone layer. Depletion of the

10.810.8

10.8

PolyhalogenPolyhalogen

Polyhalogen

Compounds

CompoundsCompounds

Compounds

10.8.1 Dichloro-

methane

(Methylene

chloride)

10.8.3 Triiodo-

methane

(Iodoform)

10.8.4 Tetrachlo-

romethane

(Carbon

tetrachloride)

10.8.2 Trichloro-

methane

(Chloroform)

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318Chemistry

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.

The chlorofluorocarbon compounds of methane and ethane are collectively

known as freons. They are extremely stable, unreactive, non-toxic, non-

corrosive and easily liquefiable gases. Freon 12 (CCl

) is one of the

most common freons in industrial use. It is manufactured from

tetrachloromethane by Swarts reaction. These are usually produced

for aerosol propellants, refrigeration and air conditioning purposes. By

1974, total freon production in the world was about 2 billion pounds

annually. Most freon, even that used in refrigeration, eventually makes

its way into the atmosphere where it diffuses unchanged into the

stratosphere. In stratosphere, freon is able to initiate radical chain

reactions that can upset the natural ozone balance (Unit 14, Class XI).

DDT, the first chlorinated organic insecticides, was originally prepared

in 1873, but it was not until 1939 that Paul Muller of Geigy

Pharmaceuticals in Switzerland discovered the effectiveness of DDT as

an insecticide. Paul Muller was awarded the Nobel Prize in Medicine

and Physiology in 1948 for this discovery. The use of DDT increased

enormously on a worldwide basis after World War II, primarily because

of its effectiveness against the mosquito that spreads malaria and lice

that carry typhus. However, problems related to extensive use of DDT

began to appear in the late 1940s. Many species of insects developed

resistance to DDT, and it was also discovered to have a high toxicity

towards fish. The chemical stability of DDT and its fat solubility

compounded the problem. DDT is not metabolised very rapidly by

animals; instead, it is deposited and stored in the fatty tissues. If

ingestion continues at a steady rate, DDT builds up within the animal

over time. The use of DDT was banned in the United States in 1973,

although it is still in use in some other parts of the world.

10.8.5 Freons

10.8.6 p,p’-Dichlo-

rodiphenyl-

trichloro-

ethane(DDT)

SummarySummary

Summary

Alkyl/ Aryl halides may be classified as mono, di, or polyhalogen (tri-, tetra-, etc.)

compounds depending on whether they contain one, two or more halogen atoms in

their structures. Since halogen atoms are more electronegative than carbon, the carbon-

halogen bond of alkyl halide is polarised; the carbon atom bears a partial positive

charge, and the halogen atom bears a partial negative charge.

Alkyl halides are prepared by the free radical halogenation of alkanes, addition

of halogen acids to alkenes, replacement of –OH group of alcohols with halogens using

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Haloalkanes and Haloarenes

phosphorus halides, thionyl chloride or halogen acids. Aryl halides are prepared by

electrophilic substitution to arenes. Fluorides and iodides are best prepared by halogen

exchange method.

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. These are slightly soluble in water but completely soluble in organic

solvents.

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. Nucleophilic substitution reactions are categorised into

1 and S

2 on the basis of their kinetic properties. Chirality has a profound role in

understanding the reaction mechanisms of S

1 and S

2 reactions. S

2 reactions of

chiral alkyl halides are characterised by the inversion of configuration while S

1 reactions

are characterised by racemisation.

A number of polyhalogen compounds e.g., dichloromethane, chloroform, iodoform,

carbon tetrachloride, freon and DDT have many industrial applications. However,

some of these compounds cannot be easily decomposed and even cause depletion of

ozone layer and are proving environmental hazards.

10.1 Name the following halides according to IUPAC system and classify them as

alkyl, allyl, benzyl (primary, secondary, tertiary), vinyl or aryl halides:

(i) (CH

)

CHCH(Cl)CH

(ii) CH

CH(CH

)CH(C

)Cl

(iii) CH

C(CH

)

I (iv) (CH

)

CCH

CH(Br)C

(v) CH

CH(CH

)CH(Br)CH

(vi) CH

C(C

)

(vii) CH

C(Cl)(C

)CH

(viii) CH

CH=C(Cl)CH

CH(CH

)

(ix) CH

CH=CHC(Br)(CH

)

(x) p-ClC

CH(CH

)

(xi) m-ClCH

C(CH

)

(xii) o-Br-C

CH(CH

)CH

10.2 Give the IUPAC names of the following compounds:

(i) CH

CH(Cl)CH(Br)CH

(ii) CHF

CBrClF (iii) ClCH

C≡CCH

(iv) (CCl

)

CCl (v) CH

C(p-ClC

)

CH(Br)CH

(vi) (CH

)

CCH=CClC

I-p

10.3 Write the structures of the following organic halogen compounds.

(i) 2-Chloro-3-methylpentane (ii) p-Bromochlorobenzene

(iii) 1-Chloro-4-ethylcyclohexane (iv) 2-(2-Chlorophenyl)-1-iodooctane

(v) 2-Bromobutane (vi) 4-tert-Butyl-3-iodoheptane

(vii) 1-Bromo-4-sec-butyl-2-methylbenzene (viii) 1,4-Dibromobut-2-ene

10.4 Which one of the following has the highest dipole moment?

(i) CH

(ii) CHCl

(iii) CCl

10.5 A hydrocarbon C

does not react with chlorine in dark but gives a single

monochloro compound C

Cl in bright sunlight. Identify the hydrocarbon.

10.6 Write the isomers of the compound having formula C

Br.

10.7 Write the equations for the preparation of 1-iodobutane from

(i) 1-butanol (ii) 1-chlorobutane (iii) but-1-ene.

10.8 What are ambident nucleophiles? Explain with an example.

Exercises

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320Chemistry

10.9 Which compound in each of the following pairs will react faster in S

2 reaction

with

–

OH?

(i) CH

Br or CH

I (ii) (CH

)

CCl or CH

10.10 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.

10.11 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.

10.12 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?

10.13 Give the uses of freon 12, DDT, carbon tetrachloride and iodoform.

10.14 Write the structure of the major organic product in each of the following reactions:

(i) CH

Cl + NaI

(ii) (CH

)

CBr + KOH

(iii) CH

CH(Br)CH

+ NaOH

(iv) CH

Br + KCN

(v) C

ONa + C

(vi) CH

OH + SOCl

(vii) CH

CH = CH

+ HBr

(viii) CH

CH = C(CH

)

+ HBr

10.15 Write the mechanism of the following reaction:

nBuBr + KCN nBuCN

10.16 Arrange the compounds of each set in order of reactivity towards S

displacement:

(i) 2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane

(ii) 1-Bromo-3-methylbutane, 2-Bromo-2-methylbutane, 2-Bromo-3-methylbutane

(iii) 1-Bromobutane, 1-Bromo-2,2-dimethylpropane, 1-Bromo-2-methylbutane,

1-Bromo-3-methylbutane.

10.17 Out of C

Cl and C

CHClC

, which is more easily hydrolysed by aqueous

KOH.

10.18 p-Dichlorobenzene has higher m.p. than those of o- and m-isomers. Discuss.

10.19 How the following conversions can be carried out?

(i) Propene to propan-1-ol

(ii) Ethanol to but-1-yne

(iii) 1-Bromopropane to 2-bromopropane

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(iv) Toluene to benzyl alcohol

(v) Benzene to 4-bromonitrobenzene

(vi) Benzyl alcohol to 2-phenylethanoic acid

(vii) Ethanol to propanenitrile

(viii) Aniline to chlorobenzene

(ix) 2-Chlorobutane to 3, 4-dimethylhexane

(x) 2-Methyl-1-propene to 2-chloro-2-methylpropane

(xi) Ethyl chloride to propanoic acid

(xii) But-1-ene to n-butyliodide

(xiii) 2-Chloropropane to 1-propanol

(xiv) Isopropyl alcohol to iodoform

(xv) Chlorobenzene to p-nitrophenol

(xvi) 2-Bromopropane to 1-bromopropane

(xvii) Chloroethane to butane

(xviii) Benzene to diphenyl

(xix) tert-Butyl bromide to isobutyl bromide

(xx) Aniline to phenylisocyanide

10.20 The treatment of alkyl chlorides with aqueous KOH leads to the formation of

alcohols but in the presence of alcoholic KOH, alkenes are major products. Explain.

10.21 Primary alkyl halide C

Br (a) reacted with alcoholic KOH to give compound (b).

Compound (b) is reacted with HBr to give (c) which is an isomer of (a). When

(a) is reacted with sodium metal it gives compound (d), C

which is different

from the compound formed when n-butyl bromide is reacted with sodium.

Give the structural formula of (a) and write the equations for all the reactions.

10.22 What happens when

(i) n-butyl chloride is treated with alcoholic KOH,

(ii) bromobenzene is treated with Mg in the presence of dry ether,

(iii) chlorobenzene is subjected to hydrolysis,

(iv) ethyl chloride is treated with aqueous KOH,

(v) methyl bromide is treated with sodium in the presence of dry ether,

(vi) methyl chloride is treated with KCN?

Answers to Some Intext Questions

10.1

10.2 (i) H

cannot be used along with KI in the conversion of an alcohol to

an alkyl iodide as it converts KI to corresponding acid, HI which is then

oxidised by it to I

10.3 (i) ClCH

Cl (ii) ClCH

CHClCH

(iii) Cl

CHCH

(iv) CH

CCl

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10.4

10.5

10.6 (i) Chloromethane, Bromomethane, Dibromomethane, Bromoform.

Boiling point increases with increase in molecular mass.

(ii) Isopropylchloride, 1-Chloropropane, 1-Chlorobutane.

Isopropylchloride being branched has lower b.p. than 1-

Chloropropane.

10.7 (i) CH

Br Being primary halide, there won’t be any

steric hindrance.

(ii) Secondary halide reacts faster than tertiary

halide.

(iii) The presence of methyl group closer to the

halide group will increase the steric

hindrance and decrease the rate.

The equivalent hydrogens are grouped as a, b and

c. The replacement of equivalent hydrogens will

give the same product.

All the hydrogen atoms are equivalent and replacement

of any hydrogen will give the same product.

Similarly the equivalent hydrogens are grouped as

a, b, c and d. Thus, four isomeric products are

possible.

10.8 (i) Tertiary halide reacts faster than secondary halide

because of the greater stability of tert-carbocation.

(ii)

Because of greater stability of secondary carbocation than

primary.

10.9

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