Solvent Effect in the Electrochemical Reduction of p‐Quinones in Several Aprotic Solvents

The potential separation between the first and second electroreduction steps of 13 para quinone derivatives has been determined in six aprotic solvents. It was confirmed that negligible solvent effect appears in the first electron transfer reaction forming anion radicals but the effect becomes appreciable in the second electron transfer. Actually, when either or determined in one solvent M are plotted against those in a different solvent N, a straight line is obtained. With , the line slope is independent of the solvent combination, while with , the slope varies depending on the combination. The former fact indicates negligible solvent effect on while is affected by the nature of solvents. It was suggested that the slope of line connecting values in two solvents will provide a new solvent parameter. Detailed examination suggested a linear correlation between solvation energy (including the effect of ion‐pairing) and electron affinity of individual substrate. Relative order of solvent effect was determined to be , which agrees with the order so far known. The free energy associated with the transfer of 1,4‐BQ dianion from DMF to PC was estimated to be in the order of −10 kcal/mol while the corresponding value from DMF to DMSO was less than −1 kcal/mol.