Electron Spin Resonance Study of Short-Lived Free Radicals in Photoreduction of Benzophenone

Abstract By observing electron spin resonance spectra during photolysis at room temperature, kinetic behaviors of short-lived free radicals are studied in photoreduction of benzophenone in alcohols, especially in ethanol, with and without sodium methoxide. In neutral solution, diphenylhydroxymethyl radicals and hydroxyethyl radicals are observed simultaneously. This is a firm evidence, obtained from ESR, of hydrogen abstraction of excited benzophenone from ethanol. Results indicate that (1) free radicals are the most efficiently generated by the light of ∼350 nm, (2) hydroxyethyl radicals transform into diphenylhydroxymethyl radicals if the concentration of benzophenone is high, and (3) the latter radicals disappear following the second order reaction. In the presence of sodium methoxide, diphenylhydroxymethyl radicals transform into benzophenone ketyl anions. Rate constants are determined, being 3×107 and ∼104 mole−1·1·sec−1 for the combination reaction between diphenylhydroxymethyl radicals and the proton transfer reaction from diphenylhydroxymethyl radical to methoxide ion, respectively.