Electrochemical Reduction of Some o-Quinone Anion Radicals: Why Is the Current Intensity so Small?

Reduction of quinones in aprotic media proceeds in two steps: initial reduction to the anion radical, followed by reduction of the anion radical to the dianion. In the cases of 3,5-di-tert-butyl-1,2-benzoquinone, 1, and 3,6-di-tert-butyl-1,2-benzoquinone, 2, the second reduction peak seen in a cyclic voltammogram is much smaller than the first reduction peak. Similar effects are seen with other quinones, but none is so pronounced as with 1 and 2. Various attempts to explain this anomalous behavior have been presented in the past, and in the present work, new results have been obtained that rule out the previous explanations. It is shown that a series of addition reactions of the quinone dianion to the anion radical can adequately account for the observed behavior. This conclusion is supported by the ability to simulate the experimental voltammograms both in the absence of added water with no added sodium perchlorate and also with added sodium perchlorate up to a 1:1 ratio of sodium to quinone.