Voltammetric behaviour of benzil at a glassy carbon electrode

Abstract

The voltammetric behaviour of benzil in DMF and in aqueous solutions on a glassy carbon electrode (GCE) was investigated and the results were compared with the reported voltammetric behaviour of benzil on a mercury electrode. Benzil gives two reversible 1e reduction peaks on a glassy carbon electrode in DMF. The apparent charge transfer rate constant for the first 1e step is found to be 2.819 × 10–3 cm s–1 which is much slower than reported on a mercury electrode. In DMF media, a surface prewave corresponding to the reduction of the species resulting from the slow time-dependent surface protonation of benzil is observed. A small anodic wave in the positive potential region corresponding to the oxidation of a small concentration of the isomeric semidione intermediate is also observed. In the presence of o-cresol, further reduction of the anion radical occurs in between the two cathodic peaks owing to simultaneous protonation and reduction. Benzoic acid protonates benzil itself, which results in a direct 2e reduction in the prewave region. In the presence of both o-cresol and benzoic acid, the benzoin formed from 2e reduction gives rise to a further reduction peak at a more negative potential. The benzil anion radical–Li+ ion-pair also undergoes easier reduction than the free anion radical species.In aqueous media an irreversible 2e reduction peak corresponding to the reduction of benzil to benzoin is observed. In neutral and alkaline media further reduction of benzoin to hydrobenzoin is also observed. In alkaline conditions at high sweep rates, oxidation waves corresponding to the Z and E forms of stilbenediolate are observed.