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 me thylene halides (bromides and iodides) in aqueous solution, are investigate d 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 th e electrode material, similar measurements have also been carried out emplo ying 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 (severa l hundred mV) in their reduction potentials (E,), Consequently, the reducti on of CH2Br2, CH2I2, and CH3I starts at potentials positive with respect to the potentials of zero charge (pzc) of Ag and Cu, producing significant am ounts of ethylene and ethane, respectively. Methane becomes the dominant re action product only at the potentials more negative than E,. The extent of interaction between the reaction intermediates and the electrode surface cl early depends not only on the potential but also on the electronic structur e of the electrode. This was confirmed by the results of experiments conduc ted 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 coll ected results demonstrate that the reactions undergone by the electrogenera ted CH2 and .CH3 species can be oriented toward formation of either ethylen e/ethane or methane by a suitable choice of the electrode material and of t he range of applied potentials. The use in this study of Ag and Cu electrod es allows generation of the CH2 and .CH3 radicals above the onset potential for H-2 evolution, so that their reactivity can be investigated virtually in the absence of competing reactions.