Effect of Metal Ions on the Electrochemical Reduction of Benzil in Non-Aqueous Solvents
Format of Original
Journal of Electroanalytical Chemistry and Interfacial Electrochemistry
The reduction of benzil has been studied in N,N-dimethylformamide (DMF), dimethyl-sulfoxide (DMSO) and acetonitrile (AN) in the presence of group IA and IIA metal ions. In DMF, the first voltammetric peak for the reversible, one-electron reduction of benzil obtained with tetra-n-butylammonium perchlorate as electrolyte, was observed to shift toward positive potentials as potassium, sodium or lithium perchlorate was added due to rapid, reversible ion pair formation between the metal cation and benzil radical anion. The ion pair formation constants were 2, 7.8 and 42 M−1 for K+, Na+, and Li+ respectively. In the absence of metal ions, benzil radical anion is reduced in an irreversible, one-electron step which occurs about 1 V negative of the first peak. When K+, Na+, or Li+ (<30 mM LiClO4) was added, two new peaks appeared between the original reduction peaks for benzil. The more positive of these disappeared at rapid scan rates and was attributed to the reduction of an ion triplet containing two cations per radical anion. The more negative peak was assigned to the reduction of the ion pair. Values of K2 k−21/2 (where K2 is the ion triplet formation constant and k−2 is the rate constant for ion triplet dissociation) are 380, 300 and 4100 M−1 s−1/2 for K+, Na+, and Li+ respectively. In the presence of barium, strontium, calcium and magnesium perchlorates, benzil was reduced in a single, two-electron process. For the first three metals, an anodic peak was observed in the cyclic voltammograms. Double potential step chronoamperometry showed that both the reduction and return oxidation were two-electron, diffusion controlled processes. The shapes of the voltammograms and their dependence on scan rate were best explained by a reaction scheme involving reversible reduction to the radical anion, rapid ion pair formation, and quasi-reversible reduction of the ion pair with the standard potential for ion pair reduction being slightly positive of that for the reduction of benzil to the radical anion. The reduction in the presence of magnesium perchlorate was a completely irreversible, two-electron process at all scan rates employed. The effect of metal ions on the reduction in DMSO was very similar to that observed in DMF but in AN the ion pairing interactions were much stronger. In the presence of sodium perchlorate in AN, benzil was reduced in two closely spaced, reversible, one-electron steps, first to the sodium—benzil radical anion ion pair, then to a species containing two sodium ions per benzil dianion.