Transformation of phenolic compounds upon UVA irradiation of anthraquinone-2-sulfonate.

This paper studies the transformation of phenol and of 3,5-dichlorophenol (3,5-DCP) upon UVA irradiation of anthraquinone-2-sulfonate (AQ2S). Light-excited AQ2S is able to oxidise phenol, 3,5-DCP, and AQ2S. Transformation reactions do not proceed at a significant extent in the absence of molecular oxygen, in which case recombination reactions of initially formed oxidised and reduced radical species (and/or radical ions) would yield back the initial substrates. AQ2S hydroxyderivatives are the main transformation intermediates, while the phenoxyl radicals arising upon oxidation of phenol and of 3,5-DCP react with the substrates to yield dihydroxybiphenyls and phenoxyphenols. Very small amounts of catechol and of 3,5-dichlorocatechol were observed, indicating a possible minor role of the hydroxyl radicals in the reactivity of the system. Interesting results from an environmental point of view are the formation of 2-hydroxydibenzofuran from phenol and of various tetrachlorinated dihydroxybiphenyls and phenoxyphenols from 3,5-DCP, suggesting that quinone photochemistry can be an important pathway for the formation of hazardous secondary pollutants in the environment.