PHOTOLYSIS OF A SERIES OF ALPHA-BROMINATED ORTHO-XYLENES IN APOLAR SOLVENTS

The alpha-brominated ortho-xylenes have been subjected to 254-nm irrad iation in deaerated benzene, isooctane and benzene-cyclohexene. The pr oduct analysis revealed that homolysis of the C-Br bond is followed by a series of hydrogen abstraction and radical recombination reactions resulting in xylenes more and less brominated than the starting compou nd. The less brominated products are formed with higher quantum yield when cyclohexene is present, due to hydrogen abstraction by the o-benz yl radical formed initially, together with cyclohexene dimers. Additio nally, the formation of 2-bromo-2,4,4-trimethylpentane is observed whe n isooctane is the solvent. The quantum yields observed for the photol ysis of 1 and 2 are higher in benzene than in isooctane, suggesting se nsitization by benzene. A biradical intermediate of the type o-quinodi methane was expected in the case of (a) photolysis of the o-benzyl rad ical formed (biphotonic process) or (b) intramolecular hydrogen abstra ction. However, the addition of cyclohexene failed to produce the expe cted Diels-Alder adduct; The synthesis of the novel alpha,alpha-dibrom o-o-xylene 3 is reported.