The reaction of joining two aromatic rings through the --N=N—bond is known as coupling reaction.

Arenediazonium salts (Ar-N₂⁺Cl^-) such as benzene diazonium salts (C₆H₅-N₂⁺Cl^-) react with phenol or aromatic amine (C₆H₅-NH₂) to form coloured Azo compounds.

The coupling reaction is an example of electrophilic substitution reaction in which the diazonium cation with the positive charge on the terminal nitrogen acts as the electrophile while the electron rich compounds such as phenols and amines act as the nucleophiles.

Note: Electrophilic substitution reaction is a chemical reaction in which an electrophile (electron loving) displaces a group in a compound. For example: