395 Amines

Hence, the lone pair of electrons on nitrogen is less available for

donation to benzene ring by resonance. Therefore, activating

effect of –NHCOCH

group is less than that of amino group.

(b) Nitration: Direct nitration of aniline yields tarry oxidation

products in addition to the nitro derivatives. Moreover, in the

strongly acidic medium, aniline is protonated to form the

anilinium ion which is meta directing. That is why besides the

ortho and para derivatives, significant amount of meta derivative

is also formed.

However, by protecting the –NH

group by acetylation reaction

with acetic anhydride, the nitration reaction can be controlled

and the p-nitro derivative can be obtained as the major product.

to form anilinium hydrogensulphate which on heating with

sulphuric acid at 453-473K produces p-aminobenzene sulphonic

acid, commonly known as sulphanilic acid, as the major product.

Aniline does not undergo Friedel-Crafts reaction (alkylation and

acetylation) due to salt formation with aluminium chloride, the

Lewis acid, which is used as a catalyst. Due to this, nitrogen of

aniline acquires positive charge and hence acts as a strong

deactivating group for further reaction.

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396Chemistry

Intext QuestionsIntext Questions

Intext Questions

13.4 Arrange the following in increasing order of their basic strength:

(i) C

, C

, NH

, C

and (C

)

(ii) C

, (C

)

NH, (C

)

N, C

(iii) CH

, (CH

)

NH, (CH

)

N, C

, C

13.5 Complete the following acid-base reactions and name the products:

(i) CH

+ HCl → (ii) (C

)

N + HCl →

13.6 Write reactions of the final alkylation product of aniline with excess of methyl

iodide in the presence of sodium carbonate solution.

13.7 Write chemical reaction of aniline with benzoyl chloride and write the name of

the product obtained.

13.8 Write structures of different isomers corresponding to the molecular formula,

N. Write IUPAC names of the isomers which will liberate nitrogen gas on

treatment with nitrous acid.

II. DIAZONIUM SALTS

The diazonium salts have the general formula

–

R N X

where R stands

for an aryl group and

–

ion may be Cl

–

Br,

–

HSO

−

, etc. They are

named by suffixing diazonium to the name of the parent hydrocarbon

from which they are formed, followed by the name of anion such as

chloride, hydrogensulphate, etc. The

group is called diazonium

group. For example,

–

6 5

C H N C l

is named as benzenediazonium

chloride and C

HSO

–

is known as benzenediazonium

hydrogensulphate.

Primary aliphatic amines form highly unstable alkyldiazonium salts

(refer to Section 13.6). Primary aromatic amines form arenediazonium

salts which are stable for a short time in solution at low temperatures

(273-278 K). The stability of arenediazonium ion is explained on the

basis of resonance.

Benzenediazonium chloride is prepared by the reaction of aniline with

nitrous acid at 273-278K. Nitrous acid is produced in the reaction

mixture by the reaction of sodium nitrite with hydrochloric acid. The

conversion of primary aromatic amines into diazonium salts is known

as diazotisation. Due to its instability, the diazonium salt is not

generally stored and is used immediately after its preparation.

−

+ + → + +

273 2 78K

6 5 2

–

6 5 22

C H NaNO 2H C H NaCl Cl ClH N 2HN

13.713.7

13.7

Method ofMethod of

Method of

PreparationPreparation

Preparation

of Diazoniunof Diazoniun

of Diazoniun

SaltsSalts

Salts

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397 Amines

Benzenediazonium chloride is a colourless crystalline solid. It is readily

soluble in water and is stable in cold but reacts with water when

warmed. It decomposes easily in the dry state. Benzenediazonium

fluoroborate is water insoluble and stable at room temperature.

The reactions of diazonium salts can be broadly divided into two

categories, namely (A) reactions involving displacement of nitrogen and

(B) reactions involving retention of diazo group.

A. Reactions involving displacement of nitrogen

Diazonium group being a very good leaving group, is substituted

by other groups such as Cl

–

, Br

–

and OH

–

which displace

nitrogen from the aromatic ring. The nitrogen formed escapes from

the reaction mixture as a gas.

1. Replacement by halide or cyanide ion: The Cl

–

, Br

–

and CN

–

nucleophiles can easily be introduced in the benzene ring in the

presence of Cu(I) ion. This reaction is called Sandmeyer reaction.

Alternatively, chlorine or bromine can also be introduced in the

benzene ring by treating the diazonium salt solution with corresponding

halogen acid in the presence of copper powder. This is referred as

Gatterman reaction.

The yield in Sandmeyer reaction is found to be better than

Gattermann reaction.

2. Replacement by iodide ion: Iodine is not easily introduced into

the benzene ring directly, but, when the diazonium salt solution

is treated with potassium iodide, iodobenzene is formed.

3. Replacement by fluoride ion: When arenediazonium chloride is

treated with fluoroboric acid, arene diazonium fluoroborate is

precipitated which on heating decomposes to yield aryl fluoride.

4. Replacement by H: Certain mild reducing agents like

hypophosphorous acid (phosphinic acid) or ethanol reduce

diazonium salts to arenes and themselves get oxidised to

phosphorous acid and ethanal, respectively.

13.813.8

13.8

PhysicalPhysical

Physical

PropertiesProperties

Properties

13.913.9

13.9

ChemicalChemical

Chemical

ReactionsReactions

Reactions

2 2

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398Chemistry

5. Replacement by hydroxyl group: If the temperature of the

diazonium salt solution is allowed to rise upto 283 K, the salt

gets hydrolysed to phenol.

6. Replacement by –NO

group: When diazonium fluoroborate is

heated with aqueous sodium nitrite solution in the presence of

copper, the diazonium group is replaced by –NO

group.

B. Reactions involving retention of diazo group

coupling reactions

The azo products obtained have an extended conjugate system having

both the aromatic rings joined through the –N=N– bond. These compounds

are often coloured and are used as dyes. Benzene diazonium chloride

reacts with phenol in which the phenol molecule at its para position is

coupled with the diazonium salt to form p-hydroxyazobenzene. This

type of reaction is known as coupling reaction. Similarly the reaction of

diazonium salt with aniline yields p-aminoazobenzene. This is an example

of electrophilic substitution reaction.

From the above reactions, it is clear that the diazonium salts are very

good intermediates for the introduction of –F, –Cl, –Br, –I, –CN, –OH,

–NO

groups into the aromatic ring.

Aryl fluorides and iodides cannot be prepared by direct halogenation.

The cyano group cannot be introduced by nucleophilic substitution of

chlorine in chlorobenzene but cyanobenzene can be easily obtained

from diazonium salt.

Thus, the replacement of diazo group by other groups is helpful in

13.1013.10

13.10

ImportanceImportance

Importance

ofof

DiazoniumDiazonium

Diazonium

Salts inSalts in

Salts in

SynthesisSynthesis

Synthesis

of Aromaticof Aromatic

of Aromatic

CompoundsCompounds

Compounds

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399 Amines

preparing those substituted aromatic compounds which cannot be

prepared by direct substitution in benzene or substituted benzene.

13.9 Convert

(i) 3-Methylaniline into 3-nitrotoluene.

(ii) Aniline into 1,3,5 - tribromobenzene.

Intext QuestionIntext Question

Intext Question

How will you convert 4-nitrotoluene to 2-bromobenzoic acid ?

Example 13.5Example 13.5

Example 13.5

SolutionSolution

Solution

Amines can be considered as derivatives of ammonia obtained by replacement of

hydrogen atoms with alkyl or aryl groups. Replacement of one hydrogen atom of

ammonia gives rise to structure of the type R-NH

, known as primary amine.

Secondary amines are characterised by the structure R

NH or R-NHR

′′

′ and tertiary

amines by R

N, RNR

′′

′R

′′′′

′′ or R

′.′.

′. Secondary and tertiary amines are known as

simple amines if the alkyl or aryl groups are the same and mixed amines if the

groups are different. Like ammonia, all the three types of amines have one unshared

electron pair on nitrogen atom due to which they behave as Lewis bases.

Amines are usually formed from nitro compounds, halides, amides, imides, etc.

They exhibit hydrogen bonding which influence their physical properties. In

alkylamines, a combination of electron releasing, steric and H-bonding factors

influence the stability of the substituted ammonium cations in protic polar solvents

and thus affect the basic nature of amines. Alkyl amines are found to be stronger

bases than ammonia. In aromatic amines, electron releasing and withdrawing groups,

respectively increase and decrease their basic character. Aniline is a weaker base

SummarySummary

Summary

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