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

Abstract

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.

Keywords: PCDD/Fs; PCDEs; Rate constants; Reaction mechanism; Trace pollutants.