Electrochemical reduction of p-chloronitrobenzene (p-CNB) at silver cathode in dimethylformamide

Abstract

Chloronitrobenzenes (CNBs) are a group of extensively used raw materials in the production of many important chemicals, which also belong to among the common pollutants in the environment. Electrochemical reduction is recognized as an important and promising strategy widely applied in the environment and synthesis fields. However, few studies on the possible electrochemical reduction of CNBs have been reported before; in particular, the effects of proton donor on the reduction mechanisms have not been fully addressed. In this work, cyclic voltammetry and bulk electrolyses have been investigated for the electrochemical reduction of p-CNB at silver (Ag) cathode in DMF under different solvent conditions. Voltammetric reduction of p-CNB at Ag electrode shows two successive reduction peaks and three anodic waves in the return cycle, the first reduction peak of which is attributed to the sequential reduction of p-CNB to p-chlorophenylhydroxylamine (p-CPHA), while the second one is assigned to the further reduction to p-chloroaniline (p-CAN). No catalytic effect of Ag was observed for the reduction of p-CNB with respect to GC electrode, which was mainly due to the reversible nature of both reduction waves in the cyclic voltammetry. The presence and the type of proton donor (e.g., acetic acid and water) was found to have significant effects on the reduction of p-CNB. The general observation is that the first reduction peak develops at the expense of the second one with increasing concentrations of proton donors at Ag cathode. Bulk electrolysis of p-CNB shows that three principal reduction mechanisms of sequential reduction, condensation and radical anions’ coupling reactions were involved. The primary electrolysis products were 4,4′-dichloroazoxybenzene (DOB) and 4,4′-dichloroazobenzene (DAB), as well as a small amount of p-CAN, and the product selectivity was dependent on the solvent conditions. This work may provide an effective tool for the reduction of p-CNB and other CNBs by obtaining useful compounds (e.g., chlorinated azo-benzenes).