Electrochemical characterization of ionic liquids

The need in organic synthesis to replace hazardous organic solvents with green solvents led to the development of ionic liquids (ILs) as alternative solvents. ILs are molten salts (melting points less than 100°C) and are typically comprised of a poorly coordinated bulky cation and smaller anion. We have examined ILs for their solvent-electrolyte properties using electrochemical techniques including cyclic voltammetry (CV) and chronoamperometry. The ILs studied were trialkylmethylammonium methylcarbonate salts synthesized using green synthetic methods whereby quaternization of tertiary amines was achieved with dimethylcarbonate under high pressure and temperature conditions instead of a methyl halide. The purity of ILs was examined using cyclic voltammetry and the results of low levels of amine impurity were further confirmed from mass spectrometry. Further investigations demonstrated that oxidation of ferrocene by CV was irreversible due to reaction of the ferrocenium cation with the methylcarbonate anion. Ferrocene reversibility was achieved when the methylcarbonate anion was replaced with tetrafluoroborate anion. Our investigations also focused on the proton donating capability of water in the ILs using p-benzoquinone as our proton probe. The electrochemical behavior of p-benzoquinone is different for aprotic and protic solvent system which acts as the basis for its proton probe activity. The p-benzoquinone in aprotic solvent system produces two reversible electron transfer reduction waves in cyclic voltammetry (formation of a radical anion for the first wave and a dianion for the second wave). In aqueous solvents, the reduction of p-benzoquinone is a single reversible two electron, two proton process (formation of p-hydroquinone). Instead of two separate reduction waves, one two-electron reduction wave is observed. As water is added to nonaqueous solvent, the two single electron reduction waves coalesce into one two-electron reduction wave.