Electrochemical reduction of benzyl halides at a silver electrode

Abstract

The electrochemical reduction of benzyl halides PhCH 2X (X = Cl, Br and I) has been investigated at Ag and glassy carbon (GC) electrodes in CH 3CN + 0.1 M Et 4NClO 4. At both electrodes reduction of PhCH 2X involves irreversible electron transfer concerted with breaking of the carbon–halogen bond. All three halides exhibit a single 2e − reduction peak at GC, whereas up to three peaks can be observed at the Ag electrode. Silver exhibits remarkable catalytic properties for the reduction process, which is positively shifted by 0.45–0.72 V with respect to GC. The mechanism of reduction of the organic halides at Ag involves adsorption of both the starting reagents and their reduction products. Adsorption of PhCH 2Cl and PhCH 2Br is weak and slow, whereas PhCH 2I is more rapidly and strongly adsorbed, so that two distinct peaks can be observed for the reduction of the dissolved and adsorbed molecules. Controlled-potential electrolyses at Ag have shown that the process may be directed to the production of bibenzyl or toluene, depending on the applied potential.