The azo functional group is N=N, which consists of two nitrogen atoms double-bonded to each other.

The azo group is also called the diazonium group.

1. Prepare nitrous acid:

NaNO₂ + HCl ➔ HNO₂ + NaCl

1. Diazotise phenylamine:

C₆H₅NH₂ + HNO₂ + HCl ➔ C₆H₅N≡NCl + 2H₂O

1. Couple the diazonium salt with phenol:

C₆H₅N≡NCl + C₆H₅OH ➔ C₆H₅N=NC₆H₄OH + HCl

Nitrous acid is generated in situ. because the acid is unstable and will decompose easily.

 Nitrous acid decomposes into nitrogen dioxide, nnitrogen monoxide and water.

The reaction mixture is kept below 10°C because the diazonium ion is unstable and will decompose easily, giving off nitrogen gas at higher temperatures.

Azo compounds are commonly used as dyes.

An azo dye is stable because its delocalised π bonding system extends between the two benzene rings through the N=N azo group.

Different coloured azo dyes can be prepared by varying the aryl compound used in the coupling reaction with phenylamine.

For example, coupling with phenol produces an orange dye whereas coupling with N,N-dimethylphenylamine produces a yellow dye.

Phenol can be prepared from phenylamine by first reacting phenylamine with nitrous acid (HNO₂) to form a diazonium salt. Then, the diazonium salt is warmed with water, yielding phenol.

Phenylamine is prepared by reducing nitrobenzene. This is done by heating nitrobenzene under reflux with tin and concentrated hydrochloric acid to form the ammonium salt, phenylammonium chloride, which is then converted to phenylamine by a reaction with excess sodium hydroxide.

Phenylamine reacts with aqueous bromine at room temperature to produce a white precipitate of 2,4,6-tribromophenylamine.