Unit 12 Diazonium Salts

UNIT 12
DIAZONIUM SALTS

Structure
12.1 Introduction
Formation of Aryl Fluorides
Expected Learning Outcomes
With Potassium Iodide
12.2 Preparation from Aromatic
Arylation
Amines
Conversion to Azo Dyes:
12.3 Reactions of Diazonium Salts
Coupling Reactions
Conversion to Phenols
12.4 Summary
Conversion to Benzene
12.5 Terminal Questions
The Sandmeyer Reaction
12.6 Answers

12.1 INTRODUCTION
In Unit 11, you have studied about the chemistry of amines. The reaction of
primary amines and nitrous acid gives diazonium salts. The diazonium salts
obtained from primary aliphatic amines are unstable but those formed by the
primary aromatic amines are stable. The diazonium salts are important
category of compounds which can be used to synthesise a large variety of
organic compounds.

In this unit, we will first discuss the methods of preparation of diazonium salts.
Then, the reactions of diazonium salts will be explained. These reaction are
useful in the synthesis of a variety of organic compounds. These compounds
have a variety of uses and especially the azo dyes obtained by the coupling
reactions of diazonium salts. Here, you will study about interesting examples
of azo dyes with which some of you might be already familiar.

Expected Learning Outcomes

After studying this unit, you should be able to:
 discuss the methods of preparation of diazonium salts;
 describe various reactions exhibited by the diazonium salts;
 explain the synthetic uses of diazonium salts;
 give some examples of azo dyes; and
 outline the synthesis of some important azo dyes.
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Block 3 Carboxylic Acids, their Derivatives and Amino Compounds

12.2 PREPARATION FROM AROMATIC AMINES

In Unit 11, you have already studied that the products of nitrosation of amines
depend on whether the amine is primary, secondary or tertiary and whether it
is aliphatic or aromatic.

The nitrosation reactions are interesting reactions of amines wherein the

nitrosonium ion acts as an electrophile. As you have studied earlier in
sub-Sec.11.5.5 of Unit 11, the nitrosonium ion is obtained from the nitrous acid
as given below:

H2O
NaNO 2 + aq.HCl HO N O + NaCl
273 K
nitrous acid

H
+ +
HO N O + H3O HO N O H2O + N O
+
nitrous conjugate acid nitrosonium
acid of nitrous acid ion

In strongly acidic solutions, nitrous acid gets protonated and then loses water
to give the nitrosonium ion.

Nitrous acid, HNO2, is a weak unstable acid. It is usually prepared in situ by

treating sodium nitrite with a strong mineral acid, usually hydrochloric acid at
273-278 K.

The diazonium salts can be prepared by the nitrosation reaction of amines

with nitrous acid.

Nitrosation of Primary Amines

Primary amines react with nitrous acid to yield diazonium salts via a number
of intermediate species. This reaction is called diazotisation.

Diazotisation of primary aliphatic amines yields alkyl diazonium salts. Alkyl

diazonium salts are unstable and decompose even at low temperatures to give
nitrogen and various other products via the intermediate carbocation. This was
explained in Unit 11, sub-Sec.11.5.5.

H2O
CH 3CH 2CH 2CH 2NH 2 + NaNO 2 + HCl
+  N2 +
[CH 3CH 2CH 2CH 2 N N:Cl ] CH 3CH 2CH 2CH 2

1-butanediazonium chloride (a carbocation)

(a diazonium salt)
(highly unstable)

This carbocation yields a complex mixture of various products about which you
studied in sub-Sec.11.5.5.

Similarly, diazotisation of primary aromatic amines leads to aryl diazonium

96 salts or arenediazonium salts. Arenediazonium salts are more stable than
Unit 12 Diazonium Salts

alkyldiazonium salts and can be stored at temperatures between 273 K and

278 K. Arenediazonium ions undergo a variety of reactions and are versatile
intermediates in the synthesis of a variety of aromatic compounds about which
you will study in detail in the next section. The formation of an arenediazonium
salt can be represented as shown below:

+ 
Ar NH 2 + 2 NaNO 2 + 2 HX Ar N N:X + NaX + 2 H 2O

primary arenediazonium salt

aryl amine (stable below 278 K)

SAQ 1
Why are aryl diazonium salts more stable than alkyl diazonium salts?

12.3 REACTIONS OF DIAZONIUM SALTS

You have studied the formation of diazonium salts in the last section. It has
been pointed out that arenediazonium salts are stable at temperatures below
278 K and can be used in the synthesis of aromatic compounds. You will now
study various reactions undergone by arenediazonium salts.

12.3.1 Conversion to Phenols–Reaction with Water

The most general method for the preparation of phenols involves the heating
of the diazonium salt in aqueous acid.

+ 
NH2 N2 HSO4 OH

1. H2SO4, NaNO2 H2SO4, H2O

1. 2.
2. H2O 373 K
Br Br Br

3-bromobenzennamine 3-bromobenzenol
(m-bromoaniline) 3-bromophenol
(m-bromophenol)
(78%)

12.3.2 Conversion to Benzene-Replacement of

Diazonium Group by Hydrogen
When a diazonium salt is treated with hypophosphorous acid, H3PO2, the
diazonium group is replaced by hydrogen. Reactions of this type are called
reductive deaminations.

This reaction is useful when we introduce an amino group into an aromatic

ring to influence the orientation of a subsequent reaction. Later the amino
group can beremoved by converting it into the diazonium salt and then treating
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Block 3 Carboxylic Acids, their Derivatives and Amino Compounds

the diazonium salt with H3PO2. For example, direct bromination of benzene
leads to 1,2,3- and 1,2,4-tribromobenzenes.
Br Br Br
Br
Br2 Fe/FeBr3
+
FeBr3

benzene bromobenzene 1,2-dibromobenzene Br

1,4-dibromobenzene

Br2/FeBr3 Br
Br
Br
Br
+

Br
1,2,3-tribromobenzene Br
1,2,4-tribromobenzene

Thus, 1,3,5-tribromobenzene which cannot be prepared by direct bromination

of benzene can be obtained by the reaction of the diazonium salt of
tribromoaniline with H3PO2.
NH2
N 2 Cl 
Br Br
Br Br Br Br
NaNO 2,HCl H3PO2
H2 O

Br
Br Br
2,4,6-tribromoaniline diazonium salt 1,3,5-tribromoaniline

SAQ 2
How will you convert 4-methylbenzenamine to toluene?

12.3.3 The Sandmeyer Reaction

The reactions of diazonium salts involving cuprous salts are called
Sandmeyer reactions. Arenediazonium salts react with cuprous chloride,
cuprous bromide and cuprous cyanide to give products in which the diazonium
group has been replaced by Cl, Br and CN groups, respectively.
+ 
NH2 N2 Cl Cl

NaNO2, aq. HCl CuCl,HCl + N2

298 K 273-298 K

CH3 CH3 CH3

p-chloroaniline p-chlorotoluene
(70-79%)

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Unit 12 Diazonium Salts
+ 
NH2 N2 Br Br
Cl Cl Cl
NaNO2, aq. HBr CuBr
+ N2
283 K HBr

o-chloroaniline (89-95%)
o-bromochlorobenzene

NH 2 N2+Cl CN
Cl Cl Cl
HCl/NaNO2 CuCN
+ N2
263-273 K

o-chlorobenzenamine o-chlorobenzonitrile
(65% overall)

Reactions similar to the Sandmeyer reactions may be accomplished by the

use of copper powder as a catalyst for decomposing the diazonium salt. This
method is particularly useful in cases where the corresponding cuprous salt
cannot be prepared. This variation is called the Gattermann reaction.

+  NaNO2, Cu
Ar&N2X Ar&NO 2

+ SO2, H2O
Ar&N2X
 Ar&SO 2H
Cu sulphinic acid

+  KNCO, Cu
Ar&N2X Ar&N=C=O
aryl isocyanate

12.3.4 Formation of Aryl Fluorides

The diazotisation of an amine with sodium nitrite and fluoboric acid, HBF4,
yields diazonium fluoborate salt as a precipitate which is isolated and
decomposed thermally to yield aryl fluorides. This reaction is also known as
Schiemann reaction.
+ +
NH2 N2 BF4 F

NaNO2

aq. HBF4, 273 K + N2 + BF3
CH3 CH3 CH3

3-methylbenzenamine 3-methylbenzenediazonium 3-methylfluo-

(m-toluidine) fluoborate orobenzene
(m-methylbenzenediazonium (m-fluorotoluene)
fluoborate)
(76-84%)

12.3.5 With Potassium Iodide

Diazonium salts react with potassium iodide to yield aryl iodides. The
diazonium salt is prepared in the usual way and a solution of potassium iodide
is then added to the reaction mixture which is heated to yield the aryl iodide. 99
Block 3 Carboxylic Acids, their Derivatives and Amino Compounds
NH2 I
Br Br
1. NaNO2, HCl, H2O, 273 K
2. Kl

2-bromobenzenamine 2-bromoiodobenzene
(o-bromoaniline) (o-bromoiodobenzene)
(72-83%)

SAQ 3
Give reagents used in the following reactions of benzenediazonium salt to give
the products shown below:

i) Nitrobenzene

ii) Phenol

iii) Chlorobenzene
iv) Iodobenzene

12.3.6 Arylation
Basic solutions of diazonium salts react with aromatic compounds in cold to
yield biaryl compounds in which the diazonium group has been replaced by an
aromatic ring. This is illustrated by the following example.
+ _
NH2 N2 Cl

1. NaNO2 2. NaOH
CH 3
aq. HCl benzene, 278 K

CH3 CH3 4-methylbiphenyl

(21%)
4-methylbenzenamine a diazonium salt

This reaction is called the Gomberg-Bachmann reaction.

12.3.7 Conversion to Azo Dyes: Coupling Reactions

Arenediazonium salts are weak electrophiles and attack aromatic ring of highly
activated compounds such as amines and phenols to yield azo compounds.

This electrophilic aromatic substitution reaction is called diazo coupling and

is shown below:
Coupling takes place
preferably at the para
273 K
position, if it is free. If + 
N2 Cl  + OH N N OH
it is not, then, the NaOH, H2O
coupling takes place
at the ortho position.
benezenediazonium benzenol 4-hydroxyazobenzene
chloride (phenol) (orange)

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Unit 12 Diazonium Salts

+ 
N2 Cl  + N(CH 3)2 N N N(CH 3)2

benezenediazonium N,N-dimethylbenzenenamine 4-(N,N-dimethyl)aminoazobenzene

chloride (butter yellow)

The azo compounds thus obtained are highly coloured and many of them are
used as colouring agents and are called azo dyes. Butter yellow was once
used as a food colouring agent.

Azo dyes are also used as indicators and for textile dying. Some examples are
given below:

N(CH 3)2
 +
N N SO 3 Na
OH

N
Orange II
N
(an industrial dye
for testiles)
 +
SO 3 Na  +
SO 3 Na
 +
SO 3 Na
methyl orange
(an indicator) N N N N
NH2 NH2
(red at pH = 3.1
yellow at pH = 4.4) Congo red

(blue-violet at pH = 3
red at pH = 5)

After studying the reactions of diazonium salts, answer the following SAQ.

SAQ 4
Write the starting materials required for the preparation of azo compounds
methyl orange and Congo red.

Let us now summarise what we have learnt in this unit.

12.4 SUMMARY
In this unit, you have studied that

 Amines undergo nitrosation reaction with nitrous acid which gives

various products depending upon whether the amine is primary,
secondary or tertiary and is aliphatic or aromatic.

 The diazonium salts can be prepared by the reaction of primary amines

with nitrous acid.
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Block 3 Carboxylic Acids, their Derivatives and Amino Compounds

 Diazonium salts can be used to synthesise a variety of aromatic

compounds and azo dyes.

 The reactions of diazonium salts can be summarised as follows:

4-chloro-1-methylbenzene
Cl
OH Br

CH3
CH3 CuCl, 363 K CuBr, CH3
HCl HBr
4-methylbenzenenol H2O 4-bromo-1-methylbenzene
373 K
+ -
NH2 N2 HSO4 I

H2SO4, NaNO2, H2O KI

273-278 K (no catalyst necessary)

CH3 CH3 CH3

CuCN 4-iodo-1-methylbenzene
4-methylbenzenamine 4-methylbenzenediazonium
(para-toluidine) hydrogen sulphate CN
(not isolated)
NaNO2

hypophophorous
acid, H3PO2 NO 2
CH3
4-cyano-1-methylbenzene

CH3
4-nitro-1-methylbenzene

CH3

12.5 TERMINAL QUESTIONS

1. Write the product of nitrosation of the following compound:
NH2

2. Why is it recommended to keep to temperature low while carrying out the

diazotisation reaction?

3. How will you carry out the following conversions?

a) 4-bromoaniline to 4-bromophenol

b) 3- chlorobenzenamine to 3- chlorofluorobenzene

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Unit 12 Diazonium Salts

4. Identify A, B, C and D in the following reactions:

i) NH2
NO2
HCl/NaNO2 CuCN
A B + N2
263-273 K

ii) NH2

1. NaNO2 2. NaOH
C D
aq. HCl toluene, 278 K

CH3
4-methylbenzenamine

5. How will you synthesise 2-bromoiodobenzene using a diazotisation

reaction?

12.6 ANSWERS
Self-Assessment Questions
1. Resonance structures are possible for aryl diazonium ion in which the
positive charge is delocalised in the aromatic ring; this increases its
stability as compared to the alkyl diazonium ion in which such resonance
stabilisation is not possible

2. By first converting it to diazonium salt by diazotisation reaction and then

treating the diazonium salt with hypophosphorous acid.

3. i) Copper powder, NaNO2

ii) Water

iii) CuCl/HCl

iv) KI

4. For Methyl orange:

NH2 N(CH 3)2

and

 +
SO 3 Na

 +
For Congo red SO 3 Na

H2N NH2 and 2 equivalents of

NH2
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Block 3 Carboxylic Acids, their Derivatives and Amino Compounds

Terminal Questions
+ 
1. N2 Cl

2. Because at higher temperatures, the water present will react with the
diazonium salt and lead to formation of phenols.
+ -
3. a) NH2 N2 HSO4 OH

1. H2SO4, NaNO2 H2SO4, H2O

1. 2.
2. H2O 373 K

Br Br Br
4-bromobenzenamine 4-bromobenzenol
(p-bromoaniline) 4-bromophenol
(p-bromoaniline)

+ +
b) NH2 N2 BF 4 F

NaNO2

aq. HBF4, 273 K + N2 + BF3
Cl Cl Cl

3-chlorobenzenamine 3-chlorobenzenediazonium 3-chlorofluorobenzene

fluoborate
(m-chlorobenzenediazonium
fluoborate)

A is B is
4. i)
+ &
N2 Cl CN

NO 2 NO 2
and

ii) C is D is
+ 
N2 Cl
CH3 N N CH3

CH3

5. Starting with 2-bromobenzenamine and converting it to the diazonium

salt. This is followed by the reaction of the diazonium salt obtained with
KI.

NH2 I

Br 1.NaNO2,HCl,H2O,273 K Br

2.KI

2&bromoaniline 2&bromoiodobenzene

104