Azo coupling

An azo coupling is an organic reaction between a diazonium compound and another aromatic compound that produces an azo compound. In this electrophilic aromatic substitution reaction, the aryldiazonium cation is the electrophile and the activated arene is a nucleophile. In most cases, including the examples below, the diazonium compound is also aromatic.

The treatment of aniline with nitrous acid, produces a diazonium salt, in a reaction called diazotization. Diazonium salts are important synthetic intermediates that can undergo coupling reactions to form azo dyes and electrophilic substitution reactions to introduce functional groups.

The product will absorb longer wavelengths of light (specifically they absorb in the visible region) than the reactants because of increased conjugation. Consequently, aromatic azo compounds tend to be brightly colored due to the extended conjugated systems. Many are used as dyes (see azo dye). Important azo dyes include methyl red and pigment red 170.

Azo coupling is also used to produce prontosil and other sulfa drugs.

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