Change in reaction pathway in the reduction of 3,5-di-tert-butyl-1,2-benzoquinone with increasing concentrations of 2,2,2-trifluoroethanol

The electrochemical reduction of 3,5-di-tert-butyl-1,2-benzoquinone,1, has been studied in acetonitrile with added 2,2,2-trifluoroethanol, 2. At low concentrations of 2 the reaction proceeds by the following pathway: reduction of the quinone (Q) to its anion radical (Q(center dot-)) followed by complexation of the anion radical with 2 ( HA) and the further reduction of the hydrogen-bonded complex (Q(center dot-)( HA)) to form HQ(-) and A(-). The latter reaction is a concerted proton and electron-transfer reaction (CPET). At higher concentrations of 2, the pathway changes. The first steps remain the same, but now Q(center dot-)(HA) is reduced to HQ(-) via a disproportionation reaction with Q(center dot-) along with proton transfer from HA to Q(center dot-) to form HQ(center dot) which is reduced to HQ-. The only mechanism that could be found which would account for all of the data involves proton transfer to Q(center dot-) occurring within a higher complex, Q(center dot-)(HA)(3).