Orientation in photochemical substitution of phenyl and methyl radicals in toluene: MO calculation

Abstract

The photochemical reaction of toluene with phenyl radicals generated from benzoyl peroxide affords methylbiphenyls, the order for the orientation of which is ortho/2 > para > meta/2; this result is the same as that for the thermal reaction. In contrast, the photochemical reaction with methyl radicals generated from acetyl peroxide affords xylenes in the order ortho/2 > meta/2 > para for 254 nm light, and ortho/2 > para > meta/2 for light > 290 nm. The orientation of the products obtained with light at 254 nm is different from that obtained in thermal reaction: ortho/2 > para > meta/2. The electron density of toluene and the energies of intermediate cyclohexadienyl radicals have been calculated by molecular orbital methods and likely reaction mechanisms are discussed on the basis of these calculations.