Study of spectral characteristics, kinetics, and equilibria of radicals derived from hydroxy benzophenones

Abstract

Studies on radical equilibria and the spectral evaluation have been carried out for three monohydroxy-substituted benzophenones (HOBP's) in aqueous solution. The transient ketyl or anion radicals were generated via hydrated electron (e_aq^-) reaction or dimethyl ketyl radical reaction in a pulse radiolysis experiment. The reactivity of both the undissociated (HOBP) as well as dissociated (^-OBP) forms toward these reducing agents were studied. e_aq^- was found to react with the HOBP's and ^-OBP's with diffusion-controlled rates which are of the order 3 × 10¹⁰ dm³ mol^-1 s^-1. In the case of the para derivative, two acid-base equilibria were identified (pK_a 9 and 11) in the pH range 6-13, one corresponding to the protonation-deprotonation at the OH site of the ketyl radical and the other for the protonation-deprotonation at the carbonyl site. For the ortho derivative, the intramolecularly hydrogen bonded structure of the radical anion has been found to be stable even in strongly alkaline solution (pH 13). Three different radical forms for the meta derivative have been identified at different pH conditions. The spectral evaluation of these radical species at different pH conditions was carried out. Reactions of dimethyl ketyl radicals with HOBP's and ^-OBP's were found to be different at different pH conditions. At acidic pH, it forms an adduct with the HOBP's, whereas at alkaline pH, the reaction with the ortho and meta derivatives occurs by H atom transfer. However, no reaction is seen with the para derivative in alkaline solution. Detailed spectral and kinetic data on the formation and decay of the various transient intermediates have been obtained.