Hydrogen-Donor-Controlled Polybrominated Dibenzofuran (PBDF) Formation from Polybrominated Diphenyl Ether (PBDE) Photolysis in Solutions: Competition Mechanisms of Radical-Based Cyclization and Hydrogen Abstraction Reactions

Polybrominated dibenzofurans (PBDFs)
are characteristic
dioxin-like
products of polybrominated diphenyl ether (PBDE) photolysis. In this
study, competition mechanisms of radical-based cyclization and hydrogen
abstraction reactions are proposed in PBDF formation. Commonly, the ortho C–Br bond dissociation during photolysis generates
aryl radicals, which undergo intramolecular cyclization to form PBDFs
or hydrogen abstraction with hydrogen donors (such as organic solvents
and water) to form lower brominated PBDEs. By using 2,4,4′-tribromodiphenyl
ether (BDE-28) as the model reactant, the experimental PBDF formation
ratios in various solutions are explained quantitatively by the calculated
rate constants of cyclization and hydrogen abstraction reactions using
the density functional theory (DFT) method. The solvent effect of
pure and mixed solvents on PBDF formation is illustrated successfully.
The structure-related hydrogen donation ability for hydrogen abstraction
controls the bias of competition reactions and influences PBDF formation.
Water resulted to be the most significant generation of PBDFs. Fulvic
and humic acid display higher hydrogen donation ability than small-molecule
organics due to the partitioning effect in aqueous solution. Quantitative
structure–activity relationship (QSAR) models of the calculated
rate constants for 512 cyclization and 319 hydrogen abstraction reactions
using 189 PBDEs as the initial reactants in water are established,
revealing the high risk of PBDF formation in aqueous solution.