Photodecomposition of alkyl benzoates and S-alkyl thiobenzoates. Possibility of a barton-type transition state

Abstract

U.v. irradiation of alkyl (C₃–C₅) benzoates under nitrogen gives mainly benzoic acid (ca. 40%), via a Norrish type II process, along with benzaldehyde (< 4%) and other products. In the case of pentyl benzoate, a small amount of propiophenone (1·6%) was obtained. In contrast, S-pentyl thiobenzoate gives mainly benzaldehyde (27%)(via a Norrish type I process), dipentyl disulphide (23%), and some other minor products, viz, biphenyl (1·4%), pentyl phenyl sulphide (3·2%), pentylbenzene (trace), and propiophenone (trace), but no thiobenzoic acid. Electron impact-induced fragmentation of these alkyl benzoates and of S-alkyl thiobenzoates correlates with their photochemical behaviour; 50–60% benzoic acid cation radical was obtained from alkyl benzoates, but only ca. 5% thiobenzoic acid cation radical from S-pentyl thiobenzoate.