ESTIMATION OF REDUCTION AND STANDARD POTENTIALS OF ACYL RADICALS

Acyl radicals are formed in the electron transfer reaction between ele ctrogenerated anion radicals of aromatic or heteroaromatic compounds a nd acid chlorides or anhydrides. A method based on the steady-state pr operties of the rotating disk electrode allows the determination of a dimensionless parameter q, which represents the competition between th e reactions of coupling and reduction that the acyl radical can underg o with the aromatic anion radicals. Both reactions are fast compared w ith the decarbonylation of the acyl radical. From the measurement of q it is possible to extract the reduction potential E(1/2)(q), the stan dard potential E(R)(o). and the self-exchange reorganization energy la mbda(R).(0) of the radicals investigated. The standard potentials of a liphatic acyl radicals are between -1.68 and -1.75 V vs. SCE. The corr esponding values for the aromatic radicals are much more positive (bet ween -1.02 and -1.16 V vs. SCE). In all cases the self-exchange reorga nization energy lambda(R).(0) was found to be 30 kcal mol(-1).