Photolysis of benzil in 2-propanol and in cumene

Benzil has been irradiated in 2-propanol and in cumene with a mercury lamp filtered through Pyrex. The principal products are benzoin, 1,2,3,4-tetraphenyl-2,3-dihydroxybutane-1,4-dione (3), benzoin benzoate, benzaldehyde, benzoic acid, 1,2,3,4-tetraphenyl-2,3-dihydroxybutane-1,4-dione dibenzoate (5), and some unidentified high molecular weight compounds. The yield of 3 is strongly dependent on initial benzil concentration and temperature and on the solvent used. The quantum yield of benzil disappearance depends on initial benzil concentration to a lower limit which varies with solvent. At an initial benzil concentration of 0.083 moles/l, the quantum yield of benzil disappearance is 0.11 in 2-propanol and 0.018 in cyclohexane. Compound 3 does not appear to be formed by a coupling of the appropriate ketyl radicals, as in the benzophenone case but instead, a complex is formed between a benzil molecule and the ketyl radical. Compound 3 decomposes at temperatures as low as 25°. The principal path for the formation of benzaldehyde and benzoyl radicals is the dissociation of the ketyl radical. Excited benzil abstracts a hydrogen from 2-propanol at least ten times faster than it dissociates into two benzoyl radicals.