Photochemical reaction of diacyl peroxides with aromatic compounds. Effects of temperature and wavelength on the orientation of radical substitution

The orientation in aromatic substitution of methyl and phenyl radicals generated in the decomposition of acyl peroxide under u. v. irradiation has been studied. Methyl radical, generated from acetyl peroxide, reacts with toluene to give xylenes, where their isomer distribution depends on both wavelength and temperature and is in the order of ortho > meta > para with 254nm light at 30℃, but the meta fraction increases with decreasing temperature leading to meta > ortho > para below ―50℃. On the other hand, the orientation with > 290nm light is the same as that for thermal dark reaction, and is almost independent of the temperature. The analogous photochemical methylation of anisole with acetyl peroxide and phenylation of toluene with benzoyl peroxide show little dependence on temperature and wavelength. Conceivable mechanisms for these reactions are discussed.