Mechanisms of transformation of polychlorinated diphenyl ethers into polychlorinated dibenzo-p-dioxins and dibenzofurans

This study presents a detailed mechanistic account of the formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) from polychlorinated diphenyl ethers (PCDEs). It applies the recently developed meta hybrid M06-2X functional and deploys the 2,2'-dichlorodiphenylether (2,2'-DCDE) molecule as a representative model compound for all PCDEs congeners. We find that, exceedingly high activation enthalpies prevent the direct formation of PCDFs from PCDEs via unimolecular elimination of HCl or Cl₂. Rather, loss of an ortho H/Cl atom initiates the transformation of PCDEs into PCDD/Fs. Subsequent formation of PCDFs takes place through ring-closure reactions with modest activation enthalpies, whereas the addition of a ground state oxygen molecule at an apparent ortho radical site of a PCDE congener commences a complex, yet very exothermic, mechanism leading to the formation of PCDDs. Splitting the ether linkage through H/Cl addition at the pivot carbon constitutes a major source for the formation of chlorophenoxy radicals and chlorobenzene molecules. Our kinetic and mechanistic analyses demonstrate that, the degree and pattern of chlorination of PCDEs display a negligible effect on the formation pathways of PCDD/Fs from PCDEs.