COMPETITIVE PROCESSES IN RETARDATION BY MERCAPTANS OF PHOTO-REDUCTION BY ALCOHOLS

Reactions of mercaptans and disulfides have been studied in photoreduction of benzophenone by 2-propanol and by benzhydrol, and of acetophenone by α-methylbenzyl alcohol, and by the deuterated alcohols. In photoreduction of benzophenone by 2-propanol, the rate is halved by 0.001 M 2-mercaptomesitylene, and a decrease in rate is greater in the deuterio than in the protio system. Retardation is due in part to light absorption and quenching of triplet, and largely to hydrogen-transfer reactions from mercaptan to 2-hydroxy-2-propyl radical and from benzophenone ketyl to thiyl radical. Measured formation of 2-propanol-2-d is consistent with calculation of the relative importance of hydrogen transfer. This retardation depends on competition between transfer of hydrogen from mercaptan to the alcohol-derived radical and from this radical to ground-state ketone. The ratio of rate constants for these two reactions of the 2-hydroxy-2-propyl radical is ~150. Photoreduction of acetophenone by α-methylbenzyl alcohol is retarded by diphenyl disulfide, and carbon-bound deuterium is introduced into the carbinol. The acetophenone ketyl radical is both reduced by mercaptan and oxidized by thiyl. This retardation is less efficient in the deuterio system and depends on competition between reduction of ketyl by mercaptan and coupling of ketyl radicals. In benzophenone-benzhydrol the rate is halved by 0.01 M 2-mercaptomesitylene and carbon-bound deuterium is not introduced into the carbinol. This retardation is due predominantly to quenching of triplet by mercaptan and disulfide, in small part to masking, and is the same in the protio and deuterio systems. Photoreduction of benzophenone by 2-mercaptomesitylene, initial quantum yield ~0.1, leads to benzpinacol and disulfide. Benzhydrol is not found, confirming that transfer of hydrogen from mercaptan to benzophenone ketyl is too slow to compete with ketyl coupling. Mesityl disulfide is photoreduced by benzhydrol, φ~ 0.06; such reduction is increased in the presence of benzophenone, presumably by reduction of disulfide and/or thiyl radical by benzophenone ketyl. © 1979, American Chemical Society. All rights reserved.