The Photolysis of 4-Hydroxybenzil and the Effect of Triethylamine on the Photolysis of Benzil and 4-Hydroxybenzil

Abstract

Benzil is photoreduced in neat triethylamine and mixtures of triethylamine and 2-propanol. In neat 2-propanol the principal products are; benzil pinacol, 20%; benzoin benzoate, 19%; benzoin, 18%; and benzoic acid, 12%. When the amine concentration is as low as 0.2 mM the benzil pinacol formation is almost completely suppressed. At amine concentrations higher than 0.36 mM the principal product is benzoin, 80%. At intermediate amine concentrations benzoin benzoate is important. The rate of benzil disappearance reaches a maximum and then declines as either the benzil concentration in triethylamine is increased or the amine concentration in 2-propanol is increased.4-Hydroxybenzil is not photoreduced by 2-propanol. It is reduced by triethylamine. The quantum yield of disappearance is 0.04 at a concentration of 47.8 mM and in solutions containing at least 0.36 M of amine. 4-Hydroxybenzoin is the principal product but at low starting 4-hydroxybenzil concentrations 4,4′-dihydroxybenzil is important. A rate maximum is also observed in this case.4,4′-Dihydroxybenzil is not photoreduced by 2-propanol. It does undergo photoreaction in neat triethylamine but no products were isolated.Triethylamine also appears to suppress biacetyl pinacol formation in the photoreaction of biacetyl with 2-propanol.