Question

In the diazotisation of aniline with sodium nitrite and hydrochloric acid, an excess of hydrochloric acid is used primarily to :

• A. Suppress the concentration of free aniline available for coupling
• B. Suppress hydrolysis of phenol
• C. Insure a stoichiometric amount of nitrous acid
• D. Neutralize the base liberated

Answer 1

The correct option is A Suppress the concentration of free aniline available for coupling

Mixture of sodium nitrite and hydrochloric acid is used for production of nitrosonium ion which convert aniline into diazonium ion.
If some amount of aniline is left in the reaction mixture then it will react with diazonium salt to form azo coupling product as follow:

Therefore excess of $HCl$ is taken so that it quench free aniline for further coupling by converting them into anilinium salt.