The efficient electrochemical reduction of nitrobenzene and azoxybenzene to aniline in neutral and basic aqueous methanolic solutions at devarda copper and raney nickel electrodes: electrocatalytic hydrogenolysis of NO and NN bonds

The preparative electrochemical reduction of nitrobenzene, phenylhydroxylamine, azoxybenzene, azobenzene, and hydrazobenzene was studied under controlled potential conditions at Devarda copper and Raney nickel electrodes in neutral (E = −0.6 V vs sce) and basic (E = −1.0 V vs sce) aqueous methanolic solutions. The reduction of NB at Devarda copper electrodes was carried out also at constant current (∼ 2.2 A dm−2) in basic aqueous methanol. The lifetime of the Devarda copper electrode was about 21 days under these conditions. In all cases, aniline was obtained in high chemical (85–100%) and current (80–100%) yields, thus showing that hydrogenolysis of the NO bond of phenylhydroxylamine and of the NN bond of hydrazobenzene is an efficient electrochemical process at these electrodes. An electrocatalytic hydrogenation (ECH) mechanism is most probably operating in this hydrogenolysis process. For the reduction of nitrobenzene to phenylhydroxylamine and of azoxybenzene to hydrazobenzene, an ECH mechanism and/or an electronation—protonation (EP) mechanism can be involved.