Diazonium Salt

DIAZONIUM SALT:

When primary aromatic amine is treated with nitrous acid in a cool solution, product is unstable compound, known as diazonium salt.

This reaction is known as diazotisation. Diazonium salts have the structure

Mechanism of diazotisation:

$\begin{align} & HO-NO+{{H}^{+}}{{H}_{2}}{{O}^{+}}-NO{{H}_{2}}O+N{{O}^{+}} \\ &\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,Nitrosonium \\ &\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,ion\end{align}$

Illustration 1. Why ice cold condition have to be maintained in the diazotisation reaction of aniline?

Solution. Because benzene diazonium chloride is unstable and decomposes to give phenol above 278 K.

Reactions of diazonium salts:

These salts give substitution reactions and coupling reactions as follows:

Substitution reactions:

COUPLING REACTIONS

Coupling reactions are electrophilic substitution reactions. Some examples are as follows :

General reaction

Illustration 2. Give a test to distinguish between aniline and diethyl amine.

Solution: Aniline with benzene diazonium salt form a yellow dye but diethyl amine does not.