Voltammetric investigation of the dissociative electron transfer to polychloromethanes at catalytic and non-catalytic electrodes

The electrochemical behavior of CCl₄, CHCl₃ and CH₂Cl₂ has been investigated by cyclic voltammetry at glassy carbon and silver electrodes in DMF + 0.1 M Et₄NClO₄ in the absence and presence of a good proton donor. At both electrodes, each compound exhibits a series of reduction peaks which represent sequential hydrodechlorination steps up to methane. The nature of the electrode material and the proton availability of the medium affect drastically the voltammetric pattern of the compounds. Silver exhibits extraordinary electrocatalytic properties toward the reduction process, with positive shifts of the peak potentials of about 0.57-0.95 V as compared to glassy carbon. Reduction of any polychloromethane, CH_nCl_(4−n) (n = 0-2), yields the carbanion CH_nCl_(3−n)⁻ which partitions into two reaction channels: (i) protonation and (ii) Cl⁻ elimination to give a carbene :CH_nCl_(2−n). If a strong proton donor is added into the solution, sequential hydrodechlorination becomes the principal reaction route at both electrodes. When, instead, purposely added acid is not present in solution, both reaction pathways ought to be considered. In these conditions, when possible, self-protonation reactions play an important role in the overall reduction process.