Rate and Driving Force for Protonation of Aryl Radical Anions in Ethanol

Aryl radical anions created in liquid alcohols decay on the microsecond time scale by transfer of protons from the solvent.1,2 This paper reports a 4.5 decade range of rate constants for proton transfer from a single weak acid, ethanol, to a series of unsubstituted aryl radical anions, Ar-•. The rate constants correlate with free energy change, ΔG°, despite wide variations in the two factors that contribute to ΔG°:  (a) the reduction potentials of the aryls and (b) the Ar−H• bond strengths in the product radicals. For aryl radical anions containing CH2OH substituents, such as 2,2‘-biphenyldimethanol•- which is protonated with a rate constant of 3 × 109 s-1, the faster rates do not fit well in the free energy correlation, suggesting a change in mechanism.