Electrochemical Investigation of Benzil in Ionic Liquids

Abstract

The voltammetric behavior of benzil (1,2-ethanedione, 1,2-diphenyl) has been investigated by cyclic voltammetry in the ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4), 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (BDMIm BF4), and 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate BMPY TfO. At 100 mV/s, protonation of the radical anions of benzil by the EMIm cation is extensive and leads to complicated voltammetry; however, the protonation reaction is slow and is much less significant at 1000 mV/s. Use of BDMIm BF4, in which the acidic imidazolium 2-position is substituted, precludes protonation of these radical anions. It has been found that addition of BF3 etherate to solutions of benzil in EMImBF4 results in the complexation of benzil by BF3. Benzil has been found to undergo two successive reversible one-electron redox processes in the ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate (BMPY TfO). This behavior is observed due to the very low proton-donating ability of the BMPY cation. Upon addition of acetonitrile, however, the second redox process becomes irreversible as is observed in acetonitrile alone. The benzil dianion is evidently undergoing protonation by acetonitrile, and the resulting voltammogram is very similar to that in acetonitrile itself.