WAVELENGTH EFFECTS IN THE PHOTOLYSIS OF KETONES - INTRAMOLECULAR HYDROGEN ABSTRACTION OF O-METHYL-DIBENZYL KETONES VS ALPHA-CLEAVAGE IN MICELLAR SOLUTION

The chemical yields of indanols, the products of intramolecular delta- hydrogen abstraction produced in the photolysis of o-methyldibenzyl ke tone (o-MeDBK) and o, o'-dimethyldibenzyl ketone (o, o'-Me2DBK) in SDS micelles are sensitive to the wavelength of excitation: the yields of indanols are about two times smaller for excitation at 254 nm than at ca. 320 nm. At the same time the magnitude of the secondary cage effe ct, arising from coupling of geminate benzyl radical pairs is increase d slightly for excitation at 254 nm relative to 300 nm. In contrast to the results for micellar solution, in hexane no wavelength effect on the yields of indanols is observed. A kinetic analysis of the reaction scheme shows that the decrease in the yield of indanol in micelles fo r shorter wavelength excitation is caused mainly by a decrease in the recombination probability P(r) of the primary phenacyl/benzyl radical pair resulting from alpha-cleavage. This decrease in probability for r ecombination of the primary geminate pair causes an increase in the fo rmation of the secondary geminate benzyl/benzyl pair and a higher rela tive yield of geminate cage recombination. Intramolecular hydrogen abs traction as well as further cyclization of the biradical to indanol ar e not wavelength dependent processes. A Salem correlation diagram for hydrogen abstraction provides theoretical support for the proposed mec hanism.