ESTIMATION OF REDUCTION AND STANDARD POTENTIALS OF ACYL RADICALS

Acyl radicals are formed in the electron transfer reaction between electrogenerated anion radicals of aromatic or heteroaromatic compounds and acid chlorides or anhydrides. A method based on the steady-state properties of the rotating disk electrode allows the determination of a dimensionless parameter q, which represents the competition between the reactions of coupling and reduction that the acyl radical can undergo with the aromatic anion radicals. Both reactions are fast compared with the decarbonylation of the acyl radical. From the measurement of q it is possible to extract the reduction potential E(1/2)(q), the standard potential E(R)(o). and the self-exchange reorganization energy lambda(R).(0) of the radicals investigated. The standard potentials of aliphatic acyl radicals are between -1.68 and -1.75 V vs. SCE. The corresponding values for the aromatic radicals are much more positive (between -1.02 and -1.16 V vs. SCE). In all cases the self-exchange reorganization energy lambda(R).(0) was found to be 30 kcal mol(-1).