Mechanisms for the formation of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) from chlorinated toluenes

In this contribution, we study a new mechanism of the formation of PCDD/F polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) from chlorinated toluenes, using quantum chemical calculations. Pathways involving 2-chlorotoluene incur high activation barriers which diminish its direct role in the formation of PCDD/F. Alternatively, the 2-chlorobenzyl and 2-methylphenoxy radicals, which are important intermediates in the early stages of the 2-chlorotoluene oxidation, yield PCDD/F through a less energy demanding pathway. The proposed mechanisms resemble the well-established pathways for the oxidative coupling of chlorinated phenols. It is predicted that the presence of an ortho methyl-bearing site facilitates initial coupling over that of an ortho chlorine-bearing site. Furthermore, chlorinated toluenes may initially be oxidized to phenoxy and phenolic intermediates to form PCDD/F. Results presented herein should be instrumental for gaining a better understanding of general formation routes of PCDD/F.