One-versus two-electron reaction pathways in the electrocatalytic reduction of benzyl bromide at silver cathodes

The electrocatalytic reduction of benzyl bromide at a silver cathode has been investigated in acetonitrile in the absence and presence of acids, using cyclic voltammetry (CV) and controlled-potential electrolysis (CPE). CV gives rise to two reduction waves, which represent the dissociative 1e- reduction of PhCH2Br to PhCH2 and Br-, followed by a further reduction of the benzyl radical to PhCH2- at more negative potentials. The charge stoichiometry (1e- vs 2e-/molecule) and product distribution depend on the applied potential and reaction medium. In the absence of added acids, the reduction of PhCH2Br at potentials of the first wave is a 1e- process mainly yielding bibenzyl, whereas toluene becomes the principal product at potentials beyond the second wave. The addition of acids strongly modifies the dependence of selectivity on the applied potential. The presence of a strong acid changes the mechanism of the process, which now becomes a 2e- reduction to toluene, even at potentials corresponding to the first reduction wave.