Oxidation of Phenols by Triplet Aromatic Ketones in Aqueous Solution

Abstract

Aromatic ketones efficiently mediate the photo-oxidative degradation of phenols in aerated aqueous solution, a process likely to be relevant in sunlit natural waters. Absolute bimolecular rate constants for the quenching of three model ketone triplets by nine phenols bearing various substituents, from electron-donating alkyl and alkoxy groups to the electron-withdrawing cyano group, were measured by nanosecond laser flash photolysis. Triplet benzophenone (BP) is quenched at nearly diffusion-controlled rates (2.6−5.6 × 109 M-1 s-1). Triplet state quenching of 3‘-methoxyacetophenone (3‘-MAP) and 2-acetonaphthone (2-AN) by the same set of phenols occurs more selectively, with rate constants spanning a range of 1 and more than 2 orders of magnitude, respectively. Quenching rate constants obey a Rehm−Weller relationship to the free energy of electron transfer from the phenol to the ketone triplet. By comparison of the quenching constants with overall photo-oxidation rates obtained by stationary irradiation in ...