A mechanistic study of the reduction of quinones by ascorbic acid

Abstract

Rates of the redox reaction between ascorbic acid, 1, and p-benzoquinone, 2, which forms dehydroascorbic acid and hydroquinone, are highly pH-dependent, increasing by a factor of around 60-fold between pH 2 and 4.5. This is evidently due to the increased reactivity of ascorbate relative to the undissociated acid. The reaction shows a maximum primary kinetic isotope effect, k h /k d = 7.8 and volumes of activation which are appreciably negative in the acidic range (-12 to -16 cm 3 mol -1 ) but around only -4 at higher pH while the volume of reaction is -12 cm 3 mol -1 . Protic solvents are mandatory, water and methanol being the only practical solvents in which the reaction occurs at all. Rates in water–methanol mixtures show a non-linear correlation with Y-values, m s ≈ 0.27, suggesting charge dispersion in the transition state. Rates of reduction of several other quinones show similar behaviour. Hydrogen atom transfer following a one-electron transfer is proposed as the rate-determining step.