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

• Published in: Journal of the American Chemical Society 2001-12, Vol.123 (50), p.12590-12595
• Main Authors: Lund, Torben, Wayner, Danial D. M, Jonsson, Mats, Larsen, Allan G, Daasbjerg, Kim
• Format: Article
• Language: English
• Subjects: constants; electron; exchange; ketones; ketyl; reduction
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/ja011217b

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.