Electroreduction of hexachlorobenzene in protic solvent at Hg cathodes

The electrochemical reduction of hexachlorobenzene (HCB) to lower chlorinated benzenes has been studied in methanol solution. Differential pulse voltammetry at a sessile mercury drop cathode showed four reduction waves at E = -1.13, -1.33, -1.57 and -1.84 V vs the standard hydrogen electrode, corresponding to sequential reduction to penta-, tetra-, tri-and dichlorobenzenes, respectively. The formation of these products was followed with time using a recirculating flow-through cell; the isomer distributions of the lower chlorinated benzenes were as predicted from a mechanism involving chloride elimination from radical anion intermediates. A maximum current efficiency of 60% was achieved in single-pass experiments in which the potential was controlled near the peak of the first reduction peak (-1.2 V) and the initial concentration of HCB was 1 mM.