Question

Starting from aniline how will you prepare benzene diazonium chloride and chlorobenzene from the later? Explain giving chemical reactions involved.

Answer 1

Hint: Aniline undergoes substitution reaction with nitrous acid giving benzene diazonium salt which further undergoes Sandmeyer reaction giving the chlorobenzene product.

Complete answer :
Aniline is an aromatic compound. When one hydrogen is replaced by $N{H_2}$ on a benzene ring; aniline is formed. We know the property of aromatic compounds that they are not so reactive due to resonance. But the compounds can undergo substitution reactions.
The conversion of aniline to chlorobenzene on a whole is a substitution reaction only.
In the first step, aniline is diazotized with $NaN{O_2}$ and HCl to give benzene diazonium chloride. The reaction occurs as –

First, $NaN{O_2}$ reacts with HCl giving nitrous acid. This nitrous acid reacts with aniline giving benzene diazonium salt as –

The aniline undergoes reaction with nitrous acid giving benzene diazonium chloride, NaCl and water molecules.
The benzene diazonium chloride then undergoes reaction in presence of Cu and HCl giving chlorobenzene. This reaction is known as the Sandmeyer reaction. It can be presented as -

When benzene diazonium chloride undergoes reaction; Nitrogen gas is released as a byproduct which confirms the reaction.

Note :
We are normally asked to prepare aromatic compounds from aniline but not benzene because benzene is exceptionally stable and is less reactive. So, one can not easily perform any reaction with benzene.
For the preparation of benzene diazonium salt from aniline, we require cold conditions.