Electrochemical reduction of quinones in ethaline chosen as an example of deep eutectic solvent

The electrochemical reduction of a series of substituted benzoquinone have been examined in ethaline chosen as an example of ionic deep eutectic solvent. Experiments show the importance of hydrogen-bonding interactions between the quinones or its intermediates and the solvent. The effects are notably visible on the values of reduction potentials that are much more positive in ethaline than in a molecular solvent like acetonitrile and by the small difference between the first and second reduction potentials. The amplitude of the stabilization increases with the donor character of the substituent. Concerning the second reduction, the peak currents are considerably smaller than those of the first reduction and almost disappear at high scan rates (above 50 V s-1). This behavior could be explained considering a chemical step prior to the electron transfer that becomes the limiting step (CE mechanism). As a remarkable feature, the electron transfer kinetics remain fast despite the hydrogen-bonding interactions (ks = 0.12-0.14 cm s-1).