Question

Give the diazotisation reaction of aniline. Also give the chemical reaction involved in the preparation of red azo dye and light yellow azo dye.

Answer 1

The nitrosation of primary aromatic amines with nitrous acid (generated in situ from sodium nitrite and a strong acid, such as hydrochloric acid, sulfuric acid, or $HB{F}_4$) leads to diazonium salts.

Diazonium salts are important intermediates for the preparation of halides (Sandmeyer Reaction, Schiemann Reaction), and azo compounds.

Red Azo dye:

The naphthalen-2-ol ($\beta$-naphthol) is dissolved in sodium hydroxide solution and is cooled and mixed with the diazonium chloride solution.

An intense orange-red precipitate is formed.

Yellow azo dye:

The aniline is mixed with the diazonium chloride solution. An yellow colored precipitate is formed.