FUNDAMENTAL AND APPLIED TOXICOLOGY 5, 546-554 (1985) Characterization of the Photolysis of 2,4,5,2',4',5'-Hexabromobiphenyl1

(2,4,5-HBB) was irradiated with ultraviolet light in hexane with stirring, and photolysis was monitored by gas chromatography (GC). 2,4,5-HBB decomposed at an average rate of 0.66 ±0.02 Mmol/min and the reaction appeared zero order from 0.159 to 1.59 mM 2,4,5-HBB. Several polybrominated biphenyl (PBB) congeners were identified as photoproducts of 2,4,5-HBB. 2,4,5,2',5'-Pentabromobiphenyl (-PBB), formed by para debromination, accumulated at a higher rate than did 2,4,5,3',4'-PBB, formed by ortho debromination. 2,4,5,2',4'-PBB was formed by meta debromination. 3,4,3',4'-Tetrabromobiphenyl (-TBB) was found as a secondary photoproduct, formed by ortho debromination of 2,4,5,3',4'-PBB. 2,5,2',5'-TBB and 2,4,2',5'-TBB were formed by debromination of 2,4,5,2',5'-PBB para and meta, respectively. 2,5,3',4'-TBB could be formed by either ortho debromination of 2,4,5,2',5'-PBB or para debromination of 2,4,5,3',4'-PBB. Rates of degradation and accumulation of the penta- and tetrabrominated biphenyls were also studied. The ultraviolet spectra of the 2,4,5-HBB photolysis mixture, as well as the purified components, were studied and are also reported. © 1985 Society of Toxicology. The toxicity of polyhalogenated biphenyls appears to be dependent upon the number and position(s) of the halogens (Poland and Knutson, 1982) and therefore a nontoxic polybrominated biphenyl (PBB) congener, such as 2,4,5,2',4',5'-hexabromobiphenyl (2,4, 5-HBB), can in theory yield toxic congeners through photolytic debromination. Poly-brominated biphenyls and polychlorinated biphenyls (PCB) have been shown to be susceptible to photodegradation. The photo-products observed were consistent with a reductive dehalogenation pathway where preferential cleavage of the ortho halogen occurred (Ruzo el al, 1976; Ruzo et al