The photosensitized decomposition of benzoyl peroxide in benzene

Abstract

The mechanism of the benzophenone and 2-acetonaphthone sensitized decomposition of benzoyl peroxide in benzene has been investigated. When benzophenone is the sensitizer, the yield of acid resulting from peroxide decomposition increases as the initial peroxide concentration decreases. We interpret this to mean that benzophenone ketyl radicals are formed by hydrogen abstraction from solvent benzene at low peroxide concentrations, and the ketyl radicals then induce the decomposition of peroxide. When 2-acetonaphthone is the sensitizer, the acid yield is unaffected by peroxide concentration, but does depend upon the 2-acetonaphthone concentration and the light intensity. At the same light intensity, the acid yield with 2-acetonaphthone as sensitizer is less than half the yield when benzophenone is the sensitizer in the region of peroxide concentrations where the latter sensitizer should not form ketyl radicals. This can be rationalized by assuming (i) induced decomposition of benzoyl peroxide by benzophenone, but not 2-acetonaphthone, triplets; (ii) vertical and nonvertical energy transfer, with multibond cleavage occurring preferentially from the non-spectroscopic peroxide triplet; or (iii) quenching of sensitizer triplets by benzoyloxy radicals. These may not be mutually exclusive, and additional data are needed to clarify these points.