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 × 10 9 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 air-saturated aqueous solution, phenols bearing electron-donating substituents were found to be depleted with quantum yields generally exceeding 0.5, whereas parent phenol and, presumably, acceptor-substituded phenols are transformed at only ∼0.1 efficiency. The present quenching data were used to interpret the efficiency of photosensitized oxidation of phenols by dissolved natural organic matter (DNOM), an important sunlight absorber present in surface waters. The effective reduction potential of reactive excited triplet states in DNOM was estimated to be at least 1.36 V vs NHE.