Theoretical study in the dimerisation of 2-chlrothiophenol/2-chlorothiophenoxy: Precursors to PCDT/TA
Despite wide use of chlorinated thiophenols as intermediates in organic synthesis, their role as persistent organic pollutant has not yet been fully addressed. Chlorothiophenols have found widespread applications in manufacturing of pharmaceutiacal, dyes, insecticides, printing inks, pesticides and polyvinyl chloride (PVC). Species formed from chlorinated thiophenols are toxic and hazardous to health, with their toxicity increasing with the degree of chlorination. For instance, pentachlorothiophenol, an additive in vulcanisation process of rubber in tyre industry, represents an important precursor for the formation of polychlorinated dibenzothiophenes (PCDT) and polychlorinated thianthrenes (PCTA), with their structures illustrated in Fig. 1. Consensus of opinion in the literature indicates that chlorothiophenols act as precursors for the formation of PCDT/TA. Benz et al. detected and identified sulfur analogues of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F, dioxins) during manufacturing of chlorothiophenol. As PCDT/TA constitute sulfur equivalents of PCDD/F, there are 135 isomers of PCDT and 75 isomers of PCTA. From a toxicological point of view, PCDT/TA have been identified to cause endocrine disruption. But, in general, these compounds are less toxic than PCDD/F with TEF of 2378-TeCTA and 2378-TeCDT as 0.01 and 0.001 respectively.
PCDT/TA appear to form in conjunction with PCDD/F. This has prompted us to suggest similar operating conditions for the formation of PCDT/TA, and to pursue the elucidation of their formation based on an analogy to the mechanism of formation of PCDD/F. The principal steps in formation of PCDD/F in gas phase from chlorophenols include dimerisation of precursors (chlorophenoxy radical and chlorophenol molecules), such as coupling of molecule/molecule, radical/radical or radical/molecule, as well as intermolecular cyclisation of preformed intermediates to produce PCDD/F, and related chlorination and dechlorination reactions.
The purpose of this study is therefore to develop initial pathways for the gas phase formation of PCDT/TA and evaluate the relevant thermochemical parameters, in particular, activation energies and reaction enthalpies. In addition, the contribution compares the steps in the formation of PCDT/TA and PCDD/F to pinpoint the effect of the SH functional group.
|Publication Type:||Journal Article|
|Notes:||Paper presented at the 32nd International Symposium on Halogenated Persistent organic Pollutants - dioxin, 26 - 31 August, Cairns, Qld, Australia|
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