Catalytic hydrogenation of o-nitrochlorobenzene to 3,3′-dichlorobenzidine

2,2′-Dichlorohydroazobenzene was prepared by selective hydrogenation of o-nitrochlorobenzene with hydrogen in the presence of 0.8% and 5% Pd/C catalyst. o-Chloroaniline was a minor product in the catalytic hydrogenation of o-nitrochlorobenzene. The effects of base, Pd/C catalyst, and co-catalyst were discussed on catalytic hydrogenation. 2,2′-Dichlorohydroazobenzene, as an intermediate, was rearranged to 3,3′-dichlorobenzidine after reacting with HCl. It was shown that selectivity of catalytic hydrogenation of o-nitrochlorobenzene is affected strongly by concentration of base, Pd/C catalyst, and co-catalyst. ¹H and ¹³C NMR spectroscopy confirmed the chemical structures of 2,2′-dichlorohydrazobenzene and 3,3′-dichlorobenzidine.