Oxidation potentials of alpha-hydroxyalkyl radicals in acetonitrile obtained by photomodulated voltammetry
2001 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 123, no 50, 12590-12595 p.Article in journal (Refereed) Published
Oxidation potentials, E-1/2(ox) of alpha -hydroxyalkyl radicals of the type (RRCOH)-R-1-C-2-O-. (denoted (KH)-H-1 .) have been obtained in acetonitrile by photomodulated voltammetry. The values of E-1/2(ox) increase as the R-1 and R-2 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 + (KH.)-H-1 --> KH. + K-1, it is found that as the difference in the standard potential of the ketone K, E-K(o), and the oxidation potential of (KH)-H-1 . (,) E-1/2(ox) 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 an-les. Finally, acid dissociation constants for (KH.)-H-1 are provided by means of a thermochemical cycle.
Place, publisher, year, edition, pages
2001. Vol. 123, no 50, 12590-12595 p.
reduction, ketones, constants, electron, exchange, ketyl
IdentifiersURN: urn:nbn:se:kth:diva-21177DOI: 10.1021/ja011217bISI: 000172745300018OAI: oai:DiVA.org:kth-21177DiVA: diva2:339874
QC 201005252010-08-102010-08-10Bibliographically approved