Oxidation Potentials of α-Hydroxyalkyl Radicals in Acetonitrile Obtained by Photomodulated Voltammetry

Abstract

Oxidation potentials, , of α-hydroxyalkyl radicals of the type R¹R²C^•OH (denoted K¹H^•) have been obtained in acetonitrile by photomodulated voltammetry. The values of increase as the R¹ and R² groups are changed from alkyl to aryl and, in particular, strong electron-withdrawing functionalities such as CN and CF₃. Using rate data available in the literature for the pinacol photoexchange reaction K + K¹H^• → KH^• + K¹, it is found that as the difference in the standard potential of the ketone K, E and the oxidation potential of K¹H^•, , increases there is a modest increase in the exchange rate constant, k_ex. This indicates that even if some charge transfer may occur between the hydroxyalkyl radical and the ketone in the transition state, it is certainly not to the extent of a complete electron transfer. If the exchange reaction is treated as a simple hydrogen atom transfer process within the Marcus model, the intrinsic barrier is found to be 8−13 kcal mol^-1 due to the changes occurring in bonds, hybridizations, and bond angles. Finally, acid dissociation constants for K¹H^• are provided by means of a thermochemical cycle.