This contribution investigates the thermochemical and kinetic parameters pertinent to the homogeneous gas-phase formation of two groups of pollutants, polychlorinated dibenzothiophenes (PCDT) and polychlorinated thianthrenes (PCTA) from their 2-chlorothiophenol (2-CTP) precursor. We compare the enthalpic profiles of the formation mechanism of PCDT/TA with the corresponding reactions involved in the gas-phase synthesis of PCDD/F (polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans, also known as dioxins). Overall, the presence of sulfur atoms greatly reduces the activation enthalpies of the rate determining steps in reference to the oxygenated system of PCDD/F. The rate constants of all elementary reactions are calculated using the transition state theory (TST) over a wide temperature range of 300-1200 K. We performed kinetic calculations for the formation of chlorinated dibenzothiophenes and chlorinated thianthrenes that could be applied to predict yields of these pollutants from 2-CTP up to ∼1200 K, that is, prior to the emergence of dechlorination and oxidation reactions. The results presented herein provide a greatly improved understanding of the gas-phase formation of the sulfur analogs of the notorious dioxins compounds.
Polychlorinated dibenzothiophene (PCDT) and polychlorinated thianthrene (PCTA) are sulfur analogues of dioxins, such as polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F). In this work, we present a detailed mechanistic and kinetic analysis of PCDT and PCTA formation from the combustion of 2,4,5-trichlorothiophenol. It is shown that the formation of these persistent organic pollutants is more favourable, both kinetically and thermodynamically, than their analogous dioxin counterparts. This is rationalised in terms of the different influences of the S-H and O-H moieties in the 2,4,5-trichlorothiophenol and 2,4,5-trichlorophenol precursors. Kinetic parameters also indicate that the yield of PCDT should exceed that of PCDD. Finally, we demonstrate here that the degree and pattern of chlorination on the 2,4,5-trichlorothiophenol precursor leads to subtle thermodynamic and kinetic changes to the PCDT/PCTA formation mechanisms. Graphical abstract Formation mechanisms of persistant organic pollutants, PCDT and PCTA, from 2,4,5-trichlorothiophenol combustion, has been investigated using density functional theory.
Thermochemical parameters of the complete series of congeners of chlorinated thiophenols are derived based on the accurate chemistry model of CBS-QB3. The effect of the change in pattern and degree of chlorination has been thoroughly investigated. Optimized geometries of chlorinated thiophenol molecules exhibit to a large extent very similar geometrical features. Standard enthalpies of formation of chlorinated thiophenol and thiophenoxy radicals are calculated using isodesmic work reactions. Thermodynamic scales of H and G enable the highlighting of the most stable isomer in each homologue group. Standard entropies and heat capacities are calculated with the treatment of internal rotations of the S−H group as hindered rotors. It is found that there is a rather minor effect of changes in pattern and degree of chlorination on the calculated bond dissociation enthalpies (BDH) of the S−H bond in chlorinated thiophenols. Values of solvation energies designate that the interaction of chlorinated congeners of thiophenols with water molecules decreases with the degree of chlorination; however, no apparent dependency can be deduced with regard to the pattern of chlorination. A thermodynamic cycle was constructed to estimate pK a values based on gas phase deprotonation free energies and calculated solvation energies for chlorinated thiophenol molecules and chlorinated thiophenolate anions. Calculated pK a values are in good agreement with limited available experimental measurements.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.