PHOTOCHEMISTRY OF POLYHALOARENES .12. THE PHOTOCHEMISTRY OF PENTACHLOROBENZENE IN MICELLAR MEDIA

The dependence of the reciprocal of the quantum yield for the photohydrodechlorination of pentachlorobenzene (1) in aqueous 0.100 M hexadecyltrimethylammonium bromide (CTAB) solution upon the reciprocal of the microconcentration of 1 and upon the reciprocal of the probability for excited state 1 reacting with ground-state 1 provides a linear correlation at high microconcentrations of 1. The regiochemistry of the photohydrodechlorination process in CTAB favors formation of 1,2,4,5-tetrachlorobenzene to a significantly smaller extent than is observed in the analogous process in acetonitrile solution in the presence of triethylamine. The bromotetrachlorobenzene byproduct is formed in the photolysis in the following average composition: 1-bromo-2,3,4,5-tetrachloro- (5): 2-bromo-1,3,4,5-tetrachloro- (6): 3-bromo-1,2,4,5-tetrachlorobenzene (7) = 9.7:66.7:23.3. In a trapping experiment carried out during an irradiation of 1 in CH3CN/H2O (8:2) in the presence of excess KBr at 254 nm, bromo-tetrachlorobenzenes (5:6:7) were formed in a ratio of 11.3:66.8:21.9. These experiments are rationalized by proposing that product in these micellar photohydrodechlorination reactions is formed by fission of triplet-state 1 and a competing process which involves conversion of triplet-state 1 to triplet excimer which then undergoes fragmentation.