Reactivities of some electrogenerated organic cation radicals in room-temperature ionic liquids:: Toward an alternative to volatile organic solvents?

Four different ionic liquids, based on 1-alkyl-3-methylimidazolium or quaternary ammonium cations, were used as reaction media for several typical electrochemical reactions with a strict control of the residual water concentration. The oxidation of organic molecules (anthracene, naphthalene, durene, 1,4-dithiafulvene, and veratrole) for which the cation radicals undergo first and second-order kinetics reactions were investigated in ionic liquids and compared with their behavior in acetonitrile. From the analysis of the voltammetric current responses, the reaction mechanism was established and the thermodynamic and kinetics parameters were extracted. The most interesting result is that the nature of investigated mechanisms is almost unchanged in ionic liquids as compared with conventional organic media. A decrease of the electron-transfer kinetics from the aromatic molecules to the electrode (around I order of magnitude) is observed for all of the studied molecules, indicating an higher solvent reorganization during the charge transfer. The kinetics of the chemical reactions triggered by the electron transfer are slightly affected by the use of ionic liquid. The only noticeable effect is a decrease of the bimolecular reactions rates partly due to a lowering of the limiting diffusion-controlled kinetics rate constant together with a specific solvation effect of reactants in these special media. These preliminary electrochemical experiments appear as encouraging results for the use of ionic liquids in a green electrochemistry.