Coupling and uses in synthesis

Coupling reactions keep both nitrogens. The azo products join two aromatic rings through –N=N–, forming an extended conjugated system; they are often coloured and are used as dyes.

Benzenediazonium chloride couples with phenol at phenol's para position, in alkaline (OH⁻) solution, to give p-hydroxyazobenzene, an orange dye.

With aniline, in acid (H⁺), the product is p-aminoazobenzene, a yellow dye. Coupling is an electrophilic substitution on the phenol or amine ring.

Diazonium salts can introduce –F, –Cl, –Br, –I, –CN, –OH and –NO₂ onto a ring. Aryl fluorides and iodides cannot be made by direct halogenation, and –CN cannot replace Cl in chlorobenzene by substitution, but all come easily from the diazonium salt.

So diazonium chemistry reaches substituted aromatics that direct substitution on benzene or its derivatives cannot give, such as 1,3,5-tribromobenzene from aniline (brominate, diazotise, then replace by H).

Tertiary amines such as trimethylamine are used as insect attractants.