Photochemistry of ketones in solution LVIII: Mechanism of photoreduction of benzophenone by benzhydrol

Abstract

New studies of the mechanism of photoreduction of benzophenone by benzhydrol are reported. They demonstrate conclusively that the primary photochemical act involves hydrogen abstraction from benzhydrol by the n,π* uncomplexed triplet state of benzophenone to give a triplet radical pair which does not couple to a measurable extent within the solvent cage. Escape of the radicals from the cage is followed by a series of hydrogen transfer reactions with ground state ketone, leading to scrambling of label when either the ketone or the hydrol is labeled initially. The competition between coupling and disproportionation of Ph 2ĊOH and (CH 3) 2ĊOH is contrasted and rationalized, as is the virtual absence of in-cage reaction in the benzophenone—benzhydrol system and the moderately efficient in-cage reactions in the benzophenone—isopropyl alcohol system. Thus, textbook descriptions of this prototype photoreduction should reflect the fact that the initially produced pair of radicals do not couple directly, but undergo a series of hydrogen transfers prior to coupling outside the initial solvent cage.