Presentation Date: Feb 14, 2026
AGSA Abstract
The mechanism for step-wise debromination of three (3) polybrominated biphenyls (PBBs) namely, 2,2',3,3',4,4'- hexabromobiphenyl (PBB-128), 2,2',3,3',4,5- hexabromobiphenyl (PBB-129), and 2,2',3,3',4,5'- hexabromobiphenyl (PBB-130) was studied theoretically using density functional theory (DFT) calculations. The DFT model employed in the study utilized the dispersion-corrected ωB97XD functional with the cc-pVTZ basis set. The debromination was assisted with zero-valent magnesium (Mg0) and zero-valent zinc (Zn0) as catalyst. For each hexabromobiphenyl, the various potential debromination routes were investigated, and the most stable pathway based on relative debromination energy was identified. Kinetics and thermodynamics studies as well as potential energy profile (PEP) assessments were considered. Step-wise mono-debromination of the PBBs occurred in a two-step process, with the first step being the formation of a partially charged covalent intermediate involving phenyl-metal-bromine interaction (rate-determining step), and the second step being the protonation of the intermediate by methanol molecule. The relative energy of the reactant and intermediate complexes were strongly connected with the opposing trends of the activation energies for both stages. Along the debromination pathway, total activation energy increases with the sequential elimination of one bromine. It was discovered that the preferred order for stepwise debromination was removal of Br at the position 3 or 3' followed by position 2 or 2' and then position 4 or 4'. The thermodynamic data showed that the reaction is an exothermic process given the negative enthalpy change while the negative free energy change indicated spontaneity. Mg0 serves as a better catalyst for the debromination reaction than Zn0. The ease of debromination of the studied species in a relative term could be summarized as PBB-128 > PBB-130 > PBB-129.
