Diazonium salt formation and coupling reactions:

Amines react with nitrous acid to produce nitrogen gas and a variety of organic products.

If a reaction mixture of phenylamine and nitrous acid is kept below 5°C, diazonium salt is formed (the azo group is -N=N-). his reaction is known as a diazotisation reaction.

The azo group is unstable and decomposes readily to nitrogen. However due to the delocalisation of the azo groups bond electrons over the benzene ring this stabilises phenyldiazonium enough to enable it to form at low temps.

The phenyldiazonium acts as an electrophile and will attack other molecules such as phenol. This reaction is known as a coupling reaction.

The compound formed is a yellow azo dye.

Substitution reaction with Bromine:

Phenylamine is much more reactive than benzene due to the lone pair of electrons on the nitrogen atom being partly delocalised over the benzene ring. This enrichment of electron density makes phenylamine more active towards electrophiles such as bromine.