Mechanism of benzidine and semidine rearrangements. Part XXIV. Photochemical decomposition of hydrazoarenes

Ultraviolet irradiation of N-1-naphthyl-N′-phenylhydrazine (I), 1,1′-hydrazonaphthalene (II), 4-nitrohydrazobenzene (III), or 4-methoxyhydrazobenzene (IV), in n-hexane led to over 80% recovery of disproportionation products but only traces of those from benzidine-type rearrangements. The proportions of azo-compounds formed were greater than that expected for bimolecular redox disproportionation and were altered by changes in reaction temperature and substrate concentration.No azo-compounds were formed by cross-combination of the possible fission-fragments of (I), (III) or (IV), and isotopic analysis of products from deuteriated (I) and (II) confirmed that azo-compounds resulted from oxidation without fission of the N,N-bond. The rates of disappearance of isotopically normal (II) and hydrazobenzene and their corresponding [15N]-labelled analogues were the same.These results are consistent with a mechanism involving formation of a hydrazyl radical in the rate-limiting step.