Reductive Cleavage of the Carbon−Halogen Bond in Simple Methyl and Methylene Halides. Reactions of the Methyl Radical and Carbene at the Polarized Electrode/Aqueous Solution Interface

Reactivities of methyl radical and carbene species, electrogenerated on the surfaces of group IB metals via reductive cleavage of simple methyl and methylene halides (bromides and iodides) in aqueous solution, are investigated using cyclic voltammetry associated with the products' identification by gas chromatography/mass spectrometry. To appreciate specific changes in the electrochemical behavior of the above halides, related to the nature of the electrode material, similar measurements have also been carried out employing an inert glassy carbon electrode. The replacement of glassy carbon by the group IB metals Ag and Cu was found to lead to a sensible shift (several hundred mV) in their reduction potentials (E p). Consequently, the reduction of CH2Br2, CH2I2, and CH3I starts at potentials positive with respect to the potentials of zero charge (pzc) of Ag and Cu, producing significant amounts of ethylene and ethane, respectively. Methane becomes the dominant reaction product only at the potentials more negative than E p. The extent of interaction between the reaction intermediates and the electrode surface clearly depends not only on the potential but also on the electronic structure of the electrode. This was confirmed by the results of experiments conducted using the Au and glassy carbon electrodes, having pzc values much more positive than Ag and Cu, at which methane was the only detected product of the CH3X and CH2X2 reduction, over the entire range of potentials. The collected results demonstrate that the reactions undergone by the electrogenerated CH2 and •CH3 species can be oriented toward formation of either ethylene/ethane or methane by a suitable choice of the electrode material and of the range of applied potentials. The use in this study of Ag and Cu electrodes allows generation of the CH2 and •CH3 radicals above the onset potential for H2 evolution, so that their reactivity can be investigated virtually in the absence of competing reactions.