The electrochemical reduction of nitrobenzene and azoxybenzene in neutral and basic aqueous methanolic solutions at polycrystalline copper and nickel electrodes

The electrochemical reduction of nitrobenzene, azoxybenzene, azobenzene, phenylhydroxyl-amine and hydrazobenzene was studied at polycrystalline copper and nickel electrodes in neutral and basic aqueous methanolic solutions. Electronation of nitrobenzene, azo- and azoxybenzene was observed by voltammetry before the discharge potential of the medium whereas no wave obtained in the case of phenylhydroxylamine and hydrazobenzene. Under preparative electrolysis conditions, nitrobenzene gave a mixture of phenylhydroxylamine and aniline; the proportion of aniline increased when decreasing the potential even beyond the discharge potential of the medium and was larger at copper than at nickel electrodes. The reduction of phenylhydroxylamine to aniline under the same conditions did not go to completion. The reduction of azoxybenzene and azobenzene stopped at hydrazobenzene. An electrocatalytic hydrogenation mechanism is proposed for the reduction of phenylhydroxylamine to aniline. Aniline would be adsorbed on the surface and impede the catalytic process.