Electrochemical reduction of (1R,2r,3S,4R,5r,6S)-hexachlorocyclohexane (Lindane) at silver cathodes in organic and aqueous–organic media

In the present work, the electrocatalytic ability of a silver cathode to dechlorinate Lindane (1R,2r,3S,4R,5r,6S-hexachlorocyclohexane) in acetonitrile (ACN), dimethylformamide (DMF), ethanol (EtOH), and 50:50 organic–water mixtures (by volume), each containing 0.050 M tetra-n-butylammonium tetrafluoroborate (TBABF4) as supporting electrolyte, has been investigated with the aid of cyclic voltammetry and controlled-potential (bulk) electrolysis. For each solvent system, a cyclic voltammogram for direct reduction of Lindane exhibits a single cathodic peak at a silver electrode with a peak potential (vs. a saturated calomel electrode, SCE) of −1.40 V (ACN), −1.43 V (DMF), −1.50 V (EtOH), −0.89 V (50:50 ACN–H2O), −0.94 V (50:50 DMF–H2O), and −0.90 V (50:50 EtOH–H2O). Bulk electrolytic reduction of Lindane at a silver gauze cathode, held at a potential that is 150 mV more negative than each of the preceding peak potentials, affords predominantly benzene (60–100% yield) in a six-electron process; however, in a pure ACN or EtOH medium, chlorobenzene is found in significant yield after an electrolysis. When bulk reduction of Lindane is carried out in pure ACN at a potential 350 mV more negative than the peak potential, no chlorobenzene is detected. Pentachlorocyclohexene, tetrachlorocyclohexene, three isomers of trichlorobenzene, and three isomers of dichlorobenzene have been identified as intermediates in the reduction of Lindane in ACN. Reduction of Lindane involves a combination of one- and two-electron cleavage of carbon–chlorine bonds. Due to its ability to serve as an effective proton donor, water (when added to ACN, DMF, and EtOH) promotes the complete conversion of Lindane to benzene.

► Cyclic voltammograms for reduction of Lindane at silver exhibit one irreversible peak.
► Reduction of Lindane at silver leads exclusively to benzene in aqueous–organic media.
► Formation of benzene via reduction of Lindane at silver is a six-electron process.
► Chlorobenzene cannot be reduced at silver in organic or aqueous–organic media.