Mechanism of the electrochemical reduction of 3,5-di-tert-butyl-1,2-benzoquinone. Evidence for a concerted electron and proton transfer reaction involving a hydrogen-bonded complex as reactant

The electrochemical reduction of 3,5-di-tert-butyl-1,2-benzoquinone (Q) has been investigated in acetonitrile with glassy carbon electrodes in the abs ence and presence of the hydrogen-bond and proton donating additives, water and 2,2,2-trifluoroethanol (TFE). Under nominally anhydrous conditions, th e first step of the reduction is the reversible formation of the persistent radical anion, Q(-.). However, in long-term experiments such as controlled potential electrolysis, the radical anion disappears slowly with the rate being enhanced by the addition of water. This reaction was shown to be the water-promoted disproportionation of the radical anion giving neutral quino ne (Q), the protonated dianion (HQ(-)), and hydroxide. This reaction is too slow to affect the voltammetric experiments. Variation of the standard pot ential for the first step with the addition of water was explained by the f ormation of hydrogen-bonded complexes between Q(-.) and water. The 1:1 comp lex, (Q(-.))(H2O), is proposed to be the reactant in the second step of the reduction and it is suggested that the reaction is a concerted electron an d proton-transfer reaction in which insertion of the electron into the comp lex is concerted with transfer of a proton from water to the developing qui none dianion. The principal support of this suggestion is the observation o f very small electron-transfer coefficients, cc, for the second process in the presence of both water (alpha = 0.20) and TFE (alpha = 0.14-0.18). The voltammograms were adequately accounted for by digital simulation using thi s mechanism. Finally, the extremely small size of the second reduction peak at low water concentrations has been explained by the rapid reaction of hy droxide, formed in the second step, with quinone that continues to arrive a t the electrode. This reaction scheme adequately accounts for the steady-st ate voltammograms at a microelectrode. The product of the reaction of quino ne and hydroxide is suggested to be a gem-diolate.