Reactions of 2-chlorophenol and 2-bromophenol: Mechanisms fo formation of mixed halogenated dioxins and furans (PXDD/Fs)

Polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) and polybrominated dibenzo-p-dioxins and furans (PBDD/Fs) form invariably as unwanted byproducts, in the presence of trace sources of halogens, in thermal systems including smouldering, municipal waste incineration, fires as well as combustion stoves and burn barrels. PCDD/Fs and PBDD/Fs arise both in homogenous and heterogeneous processes, the former involving precursor species reacting in the gas phase, and the latter comprising the so-called precursor and de novo catalytic pathways. Gaseous chlorophenols (CPs) and boromphenols (BPs) act as potent precursors. The co-existence of brominated flame retardants (BFRs) and polyvinyl chlorides (PVC) in house fires and in thermal treatment and recycling of electrical and electronic wastes establishes a unique environment for the formation of mixed chlorinated and brominated dibenzo-p-dioxins and furans (PXDD/Fs). The toxicity of PXDD/Fs strongly depends on position and number of chlorine and bromine substituents. Overall, there are 4600 distinct PXDD/Fs congeners, with around one thousand of them exhibiting toxic nature as a consequence of their halogenation at positions 2, 3, 7 and 8 on the aromatic rings. While great deal of research on PCDD/Fs and PBDD/Fs has revealed in detail their formation mechanisms, the reaction pathways controlling emission of PXDD/Fs have received less attention. Dellinger’s group reported that, oxidation and pyrolysis of mixtures containing 2-chlorophenol (2-CP) and 2-bromophenol (2-BP) leads to the production of several congeners of PXDDs, in addition to the appearance of PCDD/Fs and PBDD/Fs. Based on demonstrated mechanisms of the formation of PCDD/Fs and PBDD/Fs, the principal steps overriding the formation of PXDDs from chlorinated and brominated phenols involve condensation reactions, intermolecular rearrangements of intermediates as well as halogenation and hydrogenation reactions. Nevertheless, a comprehensive understanding of the mechanism governing the formation of PXDDs necessities addressing all plausible chemical reactions on a precise atomic basis. To this end, this theoretical contribution investigates gas-phase bimolecular reactions involving 2-CP and 2-BP molecules (and their derived phenoxy radicals), as an initial corridor for the formation of PXDD/Fs. Our aim is twofold, firstly to survey energetic requirements for the formation of initial pre-PXDDs intermediates and secondly to assess the selectivity of prominent reaction steps depending on the presence of H, Cl or Br at the ortho site on the phenol molecule. Recently, we have reported the thermochemical and kinetic parameters, for reactions pathways operating in the formation of PCDD/Fs and PBDD/Fs from correspondingly chlorinated and brominated diphenyl ethers, to remain largely insensitive to degree and pattern of halogenation at meta-, and para-substituted sites. Thus, the results presented herein for 2-CP and 2-BP should also be applicable to more completely halogenated congeners of phenols