Photolytic transformation of polybrominated biphenyls leading to the structures of unknown hexa- to nonabromo-congeners

Polybrominated biphenyls (PBBs) have been used as flame-retardants mainly in the 1970s. Nowadays, they are found as ubiquitous contaminants in environmental samples. 2,2′,4,4′,5,5′-hexabromobiphenyl (PBB 153) is one of the persistent organic pollutants whose global ban is currently under discussion. Like the polychlorinated biphenyls (PCBs), 209 PBB congeners are existing in theory. However, only ∼40 PBBs have been identified to date. In this work, we therefore used UV light, a sun simulator and natural sunlight for the photochemical debromination of PBB 209. All techniques led to the reductive debromination of PBB 209 albeit at different speed. Shifts of bromine substituents were not observed. Normal phase and reversed phase high performance liquid chromatography (HPLC) was used for the isolation of 12 reaction products some of which could be identified by 1H NMR (PBB 202, 201, 197, 208, and 207). The other isolates (PBB 179, 178, 176, 199, 197, 196, and 194) were identified by realization of photolytic transformation studies with all isolates followed by comparison and evaluation of the obtained product spectra. In this way, we were able to establish (relative) retention times of the three nonabromobiphenyls, 9 of the 12 octabromobiphenyls, 14 of the 24 heptabromobiphenyl, and 16 of the 42 hexabromobiphenyls. Data on 24 PBBs are presented for the first time. Evaluation of the samples showed that Br was alternately removed from both phenyl rings and that positions with two vicinal Br substituents were most affected. Likewise, ortho-substituted PBBs were enriched.