organic compounds containing the hydrazo group —NH—NH— bound to the two hydrocarbon radicals RNH—NHR.
The aromatic hydrazo compounds Ar—NH—NH—Ar, colorless crystalline substances that have very weak basic properties and are insoluble in water and soluble in alcohol, ether, and benzene, are of practical importance. On exposure to strong reducing agents the aromatic hydrazo compounds for amines: AR—NH—NH—Ar + 2H→ 2ArNH2; hydrazo compounds can be oxidized to azo compounds: Ar—NH—NH— Ar → ArN = NAr. On exposure to mineral acids the aromatic hydrazo compounds isomerize into diaminodiphenyls (benzidine rearrangement). Aromatic hydrazo compounds are produced by reducing nitro compounds in an alkaline medium (with zinc dust or electrolytically). The simplest aromatic hydrazo compound is hydrazobenzene, C6H5NH—NHC6H5, discovered by N. N. Zinin in 1845. Aromatic hydrazo compounds are produced in large quantities as intermediate products in the production of benzidine and its derivatives (tolidine, dianisidine, and others), which are important starting materials for the production of azo dyes.