Electrocatalytic dechlorination of volatile organic compounds at copper cathode. Part II: Polychloroethanes

[Display omitted] ▸ Cu shows catalytic activity for the reduction of polychloroethanes, PCAs. ▸ The H+ availability and the PCA structure influence the catalytic activity of Cu. ▸ Added or electro-generated bases prompt a dehydrodehalogenation pathway. ▸ Hydrodehalogenation is favored in the presence of a good proton donor. ▸ Cu is a hopeful cathode for selective activation of R-X bond in industrial processes. Copper is considered as a catalytic electrodic material for the reduction of organic halides for possible application in environmental remediation. In Part I, we have demonstrated that Cu is a good electrodic material in the reduction of polychloromethanes (PCMs). In this second part, we extended the study to geminal polychloroethanes (PCAs) with the aim of understanding whether the catalytic activity and the reduction mechanism observed for PCMs are maintained or significantly affected by molecular structure. To this end, we considered the electroreduction of 1,1,1-trichloroethane (TCA) and 1,1-dichloroethane (DCA), which are the simplest molecules belonging to the homologous series of chloroform and dichloromethane, respectively, at a Cu electrode in DMF under controlled proton availability. Voltammetric investigations point out that PCAs can be sequentially reduced at both GC and Cu. Copper shows modest catalytic effects for TCA and DCA; with respect to GC, Ep at Cu anodically shifts by 210mV and 76 for TCA and DCA, respectively. In contrast to the reduction of PCMs, Cu exhibits a good electrocatalytic activity only in the presence of acetic acid, HAc, indicating a strong influence of the structure of the polychlorinated molecule. Controlled-potential electrolyses have shown that the reduction mechanism and therefore the intermediates and final products of the reduction process are profoundly affected by the presence of H2O or HAc. In analogy to what was previously observed for PCMs, sequential hydrodehalogenation leading to ethane as the final product becomes the principal reaction pathway in the presence of HAc. In the presence of H2O both hydrodehalogenation and dehydrodehalogenation mechanisms are possible. In the latter case the mechanism involves α,β-elimination of H+ and Cl− and leads to the formation of chlorinated olefins and acetylene.