Cation-radicals: oxidation of propan-2-ol by paraquat dichloride, photocatalysed by benzophenone and other ketones

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Abstract
Photoreduction of paraquat salts by aqueous propan-2-ol is very efficient with light of wavelength 366 nm in the presence of typical triplet sensitisers such as benzophenone, decafluorobenzophenone, and xanthone. Quantum yields for production of paraquat cation-radical are very similar to those for photoreduction of the ketone sensitiser under identical conditions. In contrast to the direct irradiation of paraquat in aqueous alcohols, the yield of radical which may be obtained is not limited by a reversible electron transfer, and the reactions are further complicated by a highly efficient process in which all the cation-radical produced is subsequently destroyed (photobleaching phenomenon) by an apparent chain reaction initiated by hydrogen transfer from intermediate semipinacol radicals. The reactions illustrate further the electron transfer capabilities of semipinacol radicals and paraquat. Photoreduction of xanthone (but not benzophenone) by aqueous propan-2-ol is efficiently quenched by iodide ion.

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