Azo compounds

Azo compounds are compounds bearing the functional group R−N=N−R′, in which R and R′ can be either aryl or alkyl. IUPAC defines azo compounds as: "Derivatives of diazene (diimide), HN=NH, wherein both hydrogens are substituted by hydrocarbyl groups, e.g. PhN=NPh azobenzene or diphenyldiazene." The more stable derivatives contain two aryl groups. The N=N group is called an azo group. The name azo comes from azote, the French name for nitrogen that is derived from the Greek ἀ- (a-, "not") + ζωή (zōē, life).

Many textile and leather articles are dyed with azo dyes and pigments.

Aryl azo compounds are usually stable, crystalline species. Azobenzene is the prototypical aromatic azo compound. It exists mainly as the trans isomer, but upon illumination, converts to the cis isomer.

Aromatic azo compounds can be synthesized by azo coupling, which entails an electrophilic substitution reaction where an aryl diazonium cation is attacked by another aryl ring, especially those substituted with electron-donating groups:

Since diazonium salts are often unstable near room temperature, the azo coupling reactions are typically conducted near ice temperatures. The oxidation of hydrazines (R−NH−NH−R′) also gives azo compounds. Azo dyes are also prepared by the condensation of nitroaromatics with anilines followed by reduction of the resulting azoxy intermediate:

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