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 a (not) + zoe (to live).