TA from 2

Article pubs.acs.org/est

Quantum Chemical Study on Formation of PCDT/TA from 2‑Chlorothiophenol Precursor Tajwar Dar,† Mohammednoor Altarawneh,*,†,‡ and Bogdan Z. Dlugogorski† †

Priority Research Centre for Energy, Faculty of Engineering & Built Environment, The University of Newcastle, Callaghan NSW 2308, Australia ‡ Chemical Engineering Department, Al-Hussein Bin Talal University, Ma’an, Jordan S Supporting Information *

ABSTRACT: 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 2chlorothiophenol (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.

1. INTRODUCTION Chlorinated thiophenols (CTP) are widely used as intermediates in various chemical industries, such as in manufacturing of dyes, insecticides, inks, pharmaceuticals and polyvinyl chloride (PVC).1,2 Owing to the presence of sulfur and chlorine, CTP exhibit human and environmental toxicity.3,4 In particular, pentachlorothiophenol, an important additive in the vulcanization process of rubber in tire industry, acts as a potent precursor for the formation of sulfur analogs of the notorious dioxins compounds (PCDD/F), namely, polychlorinated dibenzothiophenes (PCDT) and polychlorinated thianthrenes (PCTA).5−7 Consensus of opinion in literature indicates that CTP operate as precursors for the formation of PCDT/TA.8−10 Figure 1 depicts the generic structures for PCDT/TA. Benz et

evident from the fact that the toxic equivalency factor (TEF) for the 2378-TCTA congener appears 10 times lower than for its oxygenated counterpart.7 Isomers of PCDT/TA have been identified in various environmental samples such as soil and sediment,12−14 pulp bleaching,15 incineration of municipal waste,16,17 wastes from petroleum refineries,18 petroleum spills19 and in tissues of crab.20 High correlation between concentrations of PCDT/TA and PCDD/F in the environmental samples suggests that, both group form via similar operating mechanisms during homogeneous gas-phase reactions and heterogeneous pathways involving the catalytically assisted coupling of precursors.21,22 These observations have prompted us to propose similar operating reactions for PCDT/TA formation from CTP and to pursue the elucidation of their formation based on an analogy to the mechanism of formation of PCDD/F. As a first step to confirm this suggestion, we consider gas-phase formation pathways. The principal reactions in synthesis of PCDD/F in gas-phase from chlorophenols include dimerization of precursors (chlorophenoxy radicals and chlorophenol molecules), intermolecular cyclization of intermediates to produce PCDD/F, as well as related chlorination and dechlorination reactions.23,24 To this end, the purpose of this study comprises the development of mechanistic pathways for gas-phase

Figure 1. Generic structure of polychlorinated thianthrene (PCTA) and polychlorinated dibenzothiophene (PCDT).

al. identified PCDT/TA as byproducts from the synthesis of chlorothiophenol. There are in total 135 congeners of PCDT and 75 congeners of PCTA. PCDT/TA exhibit less toxicity than dioxins. For instance, replacement of oxygen by sulfur decreases the potency of PCDT/TA to cause endocrine disruption in comparison to PCDD/F.11 Less toxicity is also © 2013 American Chemical Society

Received: Revised: Accepted: Published: 11040

March 6, 2013 July 19, 2013 August 21, 2013 August 21, 2013 dx.doi.org/10.1021/es4009823 | Environ. Sci. Technol. 2013, 47, 11040−11047

Environmental Science & Technology

Article

Figure 2. Calculated electronic densities for 2-chlorothiophenoxy and 2-chlorophenoxy.

inertia. It should be noted that internal rotations about C−C linked aromatic moieties possess significant barriers to rotation as a consequence of the overlapping of the two S/SH groups and thus they are excluded from our thermochemical and kinetic analyses. Despite continuing advancements in formulating new DFT functionals with improved predictive capabilities, the B3LYP functional retains its relative accuracy in general applications to organic systems. The pioneering work by the Bozzelli group35 demonstrated that B3LYP predicts thermochemical parameters for polychlorinated dibenzo-p-dioxns and furans (PCDD/F) in a satisfactory agreement with corresponding experimental measurements. For instance, the calculated estimate of Δf Ho298 by the B3LYP functional for the dibenzo-p-dioxin (−12.38 kcal/mol) agrees with the experimental value (−11.97 kcal/mol). In the same study, theoretical and experimental values of Δf Ho298 concur for chlorinated congeners of benzene. Along the same line of inquiry, our previously calculated36 Δf Ho298 for dibenzofuran (14.7 kcal/mol) accords with the corresponding experimental measurement of 14.8 kcal/mol. The B3LYP functional has been shown to provide accurate thermochemical parameters for PCTA and PCDT.37,38

formation of PCDT/TA from CTP. A second objective involves deducing rate constants of key elementary reactions participating in the formation of PCDT/TA from the 2monochlorothiophenol (2-CTP) molecule.

2. COMPUTATIONAL DETAILS Structural optimization and energy calculations were performed at the B3LYP/GTLarge//B3LYP/6-311+G(d,p)25−27 theoretical level using the Gaussian09 program.28 The unrestricted B3LYP functional served to optimize all open shell singlet structures, with a stability test carried out to ensure correctness of the wave function. Spin contaminations (⟨S2⟩) for all singlet and doublet species attained final expected values of 0.00 and ∼0.75, correspondingly. Calculations of intrinsic reaction coordinates (IRC) confirmed the structures of transition states. Figure S1 of the Supporting Information plots the energy profiles as function of reaction coordinates for selected transition states. Evaluation of the reaction rate constants involved the application of the conventional transition state theory as implemented in the ChemRate code.29 Eckart functional afforded corrections to values of rate constants for possible contributions from quantum tunneling effects.30 Application of the Chemissian software facilitated the constructions of contours of electron densities of two structures.31 Table S1 of Supporting Information provides the thermochemical parameters including standard enthalpies of formation (Δf Ho298), standard entropies (So298) and heat capacities (Cpo (T)) at various temperatures. Evaluation of Δf Ho298 involves combination of calculated enthalpies of reactions (ΔrHo298) and our previously computed Δf Ho298 values for 2-chlorothiophenol (19.9 kcal/mol) and 2-chlorothiophenoxy (56.3 kcal/mol).32 The ChemRate code facilitated the estimation of the thermodynamic parameters of So298 and Cpo (T). The methodology of calculations of thermochemical data is well explained in the literature.33 Energy profiles for the internal rotations about the S−C bond in various intermediates required performing relaxed scans on the corresponding dihedral angles at an interval of 30°. Calculations with MultiWell suite of programs34 yielded moments of inertia for these rotors. Figure S2 of the Supporting Information depicts the energy profiles for the S−C rotors. The Supporting Information also provides associated values for moments of

3. RESULTS AND DISCUSSION 3.1. 2-Chlorothiophenol versus 2-Chlorophenol. From a kinetic point view, the most notable difference between 2chlorophenol (2-CP) and 2-chlorothiophenol (2-CTP) arises from the noticeable discrepancy in bond dissociation energies of O−H (87 kcal/mol) in 2-CP and S−H (77 kcal/mol) in 2CTP.32 This in turn implies the unimolecular decomposition of 2-CTP to start at a lower onset temperature in reference to that of 2-CP.39 Despite a noticeable difference in the endothermicities of the H−O/S bonds, abstraction of the thiol’s H by the O/H radical pool proceeds with rate constants comparable to those corresponding to hydroxyl’s H in 2-chlorophenol.40 The difference between the two compounds could also be rationalized within the formalism of the electron density distribution. As the OH constitutes a stronger electron donating group than SH, one expects that, the π-conjugated system in the 2-CTP molecule displays less nucleophilic character in reference to that of the 2-CP molecule.32 As a consequence, the self-coupling of two 2-CP molecules appears more accessible than the corresponding process for the 2-CTP 11041

dx.doi.org/10.1021/es4009823 | Environ. Sci. Technol. 2013, 47, 11040−11047


Article

Figure 3. Stable structures from the dimerization of two 2-chlorothiophenoxy radicals (Scheme A), self-condensation of two 2-CTP molecules (Scheme B) and coupling of a 2-CTP molecule and a 2-chlorothiophenoxy radical (Scheme C). Values in bold are reaction enthalpies and values in italic are activation enthalpies in reference to the reactants in each reaction. All values are in kcal/mol at 298.15 K.41

Figure 4. Transition structures for self-dimerization of 2-chlorothiophenoxy radicals, self-condensation of 2-chlorothiophenol and combination of 2chlorothiophenol with 2-chlorothiophenoxy. Interatomic distances are in Å.

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bimolecular reaction with H/OH radicals display significant exothermicity. The TA molecule could be sourced from the Pre D3 intermediate via ring closure accompanied by an intramolecular elimination of Cl atom. This reaction takes place through the transition structure TS8 with an activation enthalpy amounting to 19.3 kcal/mol. Another plausible route for the formation of a TA molecule arises from an intramolecular elimination of an HCl molecule from the Pre D1 intermediate as characterized by the transition state TS10. Due to the considerable enthalpy of activation of TS10 (57.8 kcal/mol), the bimolecular abstraction of the thiol’s H by radicals most likely controls the fate of the Pre D1 intermediate. As shown in Figure 5, abstractions by H and SH radicals, leading from Pre D1 to Pre D3, are highly exothermic (−26.9 and −37.9 kcal/mol, respectively) and require trivial barriers (0.4 and −0.2 kcal/mol, respectively). In view of the very comparable activation enthalpies of transition structures TS9 and TS8, that is, 18.2 kcal/mol versus 19.3 kcal/ mol, ring closure of Pre D3 toward an ortho C/Cl atom competes with ring closure toward an ortho C/H atom. The formation of the D6 moiety via TS9 entails endothermicity of 12.6 kcal/mol. The out-of-plane H atom in the D6 structure could be readily abstracted by species of the O/H radical pool. These bimolecular reactions lead to the production of a 1MCTA molecule in highly exoergic reactions. Pathways in Figure 5 suggest comparable yields for the formation of TA and 1-MCTA molecules from the D5, Pre D3 and Pre D1 intermediates. Figure 6 illustrates the routes for the formation of 1-MCTA and 1,6-DCTA molecules from the potent precursor D4. A

molecules; especially at ortho and para positions. Along the same line of inquiry, the 2-chlorophenoxy radical exists as a strongly oxygen centered moiety, whereas the 2-chlorothiophenoxy emerges as a rather moderately sulfur centered radical. This can be explained with the contours of electron densities depicted in Figure 2 for 2-chlorothiophenoxy and 2chlorophenoxy radicals. These contours indicate less radical character on S atom in comparison to that of the O atom of a phenoxy radical. 3.2. Formation of Direct Intermediates of PCDT/TA. In a recent study, we investigated energies for the formation of direct intermediates for PCDT/TA from radical/radical, molecule/molecule and radical/molecule coupling, where molecule and radical signify the 2-chlorothiophenol molecule and the 2-chlorothiophenoxy radical, respectively.41 Schemes A, B and C in Figure 3 summarize our findings for the formation of direct intermediates that produce PCDT/TA from radical/ radical, molecule/molecule and radical/molecule coupling, respectively, along with their estimated reaction and activation energies. In Scheme A in Figure 3, structures D4 and D5 serve as intermediates or prestructures for the formation of PCTA whereas structures D1, D2 and D3 function as intermediates for the formation of PCDT. Formation of D4 and D5 occurs without encountering a reaction barrier while the formation of D1−D3 structures necessitates overcoming modest reaction barriers. As shown in Scheme B, sizable enthalpic barriers hinder the self-couplings of two 2-CTP molecules. On the basis of energy values of Scheme C, the most plausible channel operating in the molecule/radical coupling entails the formation of the Pre D1 structure. Figure 4 shows the geometries of the transition structures for Schemes A, B and C. Note that in this paper, the predioxin structures (Pre D1−Pre D3) denote neutral and radical species of chlorinated thiophenoxythiophenols, whereas, within the context of PCDD, predioxins signify chlorinated phenoxyphenols. 3.3. Formation of PCTA. Figure 5 depicts channels for the formation of three thianthrene congeners. These congeners comprise thianthrene (TA) and 1-monochlorothianthrene (1MCTA). It follows from Figure 5 that, all pathways leading to the formation of thianthrene involve sulfur−carbon coupling as the initial step, subsequent abstraction of Cl or H, ring closure and intra annular elimination of Cl or H. Routes that involve

Figure 6. Formation of 1-monochlorothianthrene (1-MCTA) and 1,6dichlorothianthrene (1,6-DCTA) from dimers of radical/molecule and radical/radical introduced in Figure 3. Values in bold are reaction enthalpies and values in italic are activation enthalpies in reference to the reactants in each reaction. All values are in kcal/mol at 298.15 K.

closed-shell pathway for the formation of 1-MCTA advances in two steps. The first forms the Pre D2 intermediate, in a hydrogen atom transfer to the thiophenolic sulfur via an enthalpic barrier of 34.8 kcal/mol. The second incorporates ring closure and concurrent expulsion of an HCl molecule through a reaction enthalpy of 57.7 kcal/mol, as necessitated by the transition structure TS14. Abstraction of an H atom from the S−H bridge in D4 produces the Pre D4 intermediate. Based on the calculated reaction enthalpies of TS18 (11.6 kcal/mol) and TS17 (18.5 kcal/mol), cyclization toward an ortho C/H atom dominates

Figure 5. Formation of thianthrene (TA) and 1-monochlorothianthrene (1-MCTA) from dimers of radical/molecule and radical/radical illustrated in Figure 3. Values in bold are reaction enthalpies and values in italic are activation enthalpies in reference to the reactants in each reaction. All values are in kcal/mol at 298.15 K. 11043



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Radical/radical coupling of two carbon-centered 2-chlorothiophenoxy mesomers produces the keto−keto intermediate of D1 (Figure 3). Abstraction of an H atom from the C−H bridge yields the D9 intermediate. Tautomerization of D9 into D10 occurs via the transfer of the remaining H atom on the C− C bridge to the S atom. This process reveals an exothermicity of 24 kcal/mol and a trivial reaction barrier of 2.8 kcal/mol (TS19). The ring closure step starting from the D10 moiety necessitates a barrier of 11.6 kcal/mol (TS20). This step results in the formation of 4,6-dichloro-4a-mercaptodibenzothiophene radical (4a-SH-4,6-DCDT). In the final reaction, the out-ofplane SH moiety departs 4a-SH-4,6-DCDT to produce a 4,6DCDT molecule. Loss of the SH group proceeds without encountering a genuine enthalpic barrier, as the located transition structure for this reaction resides below its reactant. Figure 8 shows geometries of all transition structures involved in the formation pathways of PCDT and PCTA included in Figures 5−7. Pathway B of Figure 7 shows a closed-shell mechanism for the formation of a 4,6-DCDT molecule in two different routes. The first leads to the formation of keto−enol structure D12 via a single H atom transfer. This process takes place via a trivial activation barrier of 3.4 kcal/mol, as characterized by the transition structure TS24. A barrier for a concentric H transfer, resulting in the formation of the enol−enol structure of D11, resides 1.0 kcal/mol below the D1 intermediate. Cyclization of the D12 intermediate into the 4,6-dichloro-1a-hydro-4amercaptodibenzothiophene (1a-H-4a-SH-4,6-DCDT) structure demands a sizable barrier of 50.2 kcal/mol through the transition structure TS25. TS26 and TS22 highlight the formation of 4,6-DCDT by elimination of an H2S molecule from 1a-H-4a-SH-4,6-DCDT and D11 intermediates, requiring enthalpies of 26.6 and 60.4 kcal/mol, respectively. In view of the difference in the properties of OH/O and SH/ S functional groups, formation of PCDT and PCDF exhibits distinct enthalpic profiles, notably in terms of enthalpies of activation. Generally, pathways for the formation of PCDT require lower activation enthalpies in reference to the corresponding steps involved in the formation of PCDF. For instance, activation enthalpies for the H shift along the reaction (D9 → D10) correspond to 2.8 and 48.1 kcal/mol for sulfur and oxygen systems, respectively.42 3.5. Calculations of Rate Constants. Table 1 lists the modified Arrhenius parameters for reaction rate constants for pathways involved in the formation of PCDT/TA. To the best of our knowledge, literature offers no experimental measurements of rate constants for the PCDT/TA formation. In order to check the reliability of calculated rate constants involved in the formation of the sulfur analogs of PCDD/F, our calculated pre-exponential A factors are compared with the corresponding literature values43 pertinent to the formation of PCDD/F. We find that our A factors accord with the corresponding values quoted for the oxygenated systems for the synthesis of PCDD/ F. For example, the calculations yield the value of A for the ring closure reaction (Pre D3→ TA + Cl) of 3.52 × 1011 s−1 at 800 K, whereas A of the analogous reaction for the formation of dibenzo-p-dioxin corresponds to 3.58 × 1011 s−1. The calculated value of the modified A factor for the reaction (PreD3 → D6) amounts to 7.86 × 1011 s−1 at 800 K, in good agreement with the corresponding value for the analogous oxygenated reaction of 4.26 × 1011 s−1. The underlying overall mechanism for the yield of PCDT/TA is not only governed by formation reactions considered herein, but also by decomposition (i.e., oxidation

over cyclization involving an ortho C/Cl atom. The reactions result in the formation of D8 and D7 intermediates with modest endothermicity. The ejection of the out-of-plane H and Cl atoms from the D7 and D8 moieties affords the 1-MCTA and 1,6-DCTA molecules accompanied by enthalpy changes of −1.4 and 26.7 kcal/mol, respectively. Generally, the results show that, pathways for the formation of PCTA require lower activation barriers than analogous reactions operating in the formation of PCDD. For example, activation enthalpy for the initial tautomerization channel (D4 → Pre D2) declines by 21.3 kcal/mol from the barrier requisite for the corresponding enolization reaction in the oxygenated system. Furthermore, cyclization reactions of (Pre D4 → D7/ D8) require activation enthalpies comparable to those of reactions of chlorinated DD/F. Notably, the intra-annular elimination of HCl in both systems entails very comparable activation enthalpies, that is, 57.7 kcal/mol versus 56.1 kcal/ mol.42 The difference in the enthalpic profiles for the sulfur (CDT/ TA) and oxygen (CDD/F) systems reflects the variance in properties between the 2-chlorothiophenol/2-chlorothiophenoxy and 2-chlorophenol/2-chlorophenoxy discussed above. An elucidation of lower activation enthalpies for the sulfur system in reference to the oxygen system also stems from the fact that a longer C−S bond induces less electron overlapping between the S atom and the π-conjugated system of the phenyl rings. This, in turn, facilitates the occurrence of prominent reactions such as cyclization and H atom transfer from the C− C/S linkages into the thiol’s S atom. 3.4. Formation of PCDT. Figure 7 illustrates potential energy surfaces to elucidate mechanisms for the formation of

Figure 7. Formation of 4,6-DCDT from 3,3′-dichloro-1,1′-bi(cyclohexa-3,5-diene)-2,2′-dithione (D1). Values in bold are reaction enthalpies and values in italic are activation enthalpies in reference to the reactants in each reaction. All values are in kcal/mol at 298.15 K.

PCDT from 2-CTP. An open-shell pathway, depicted in the top part of Figure 7, comprises five key steps, namely, selfdimerization of two 2-chlorothiophenoxy at the ortho C/H positions (shown in Figure 3), H abstraction from C−C bond by an H or OH radical, tautomerization, ring closure and elimination of an SH group. 11044



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Figure 8. Transition structures for the intermediates and products involved in the formation of PCDT and PCTA. Interatomic distances are in Å.

reactions) and chlorination/dechlorination reactions. The later operate mainly at elevated temperatures, that is, T ≥ 900 K. Overall reaction pathways for the formation of PCDT/TA from 2-CTP, involving computation of activation and reaction enthalpies, have been investigated using the density functional theory. The pathways produce four congeners of DT/TA, including TA, 1-MCTA, 1, 6-DCTA and 4,6-DCDT. In view of

differences in electron densities and bond dissociation of SH and OH in 2-CTP and 2-CP respectively, formation of PCDT/ TA exhibits lower enthalpic profiles than analogous reactions operating in oxygenated PCDD/F systems. The reaction steps requiring the largest barriers in the formation of PCTA include intra-annular elimination of Cl and HCl, and double enolization of bis keto dimer. The kinetic model incorporates the rate 11045



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Table 1. Arrhenius Parameters for Elementary Reactions Involved in the Formation of PCDT/TA from 2-CTP Fitted over the Temperature Range of 300−1200 K forward rate parameters reactions PreD3→TA+Cl PreD3→D6 PreD1+H→PreD3+H2 PreD1+SH→PreD3+H2S PreD1→TA+HCl D4+H→ PreD4+H2 D4+SH→ PreD4+H2S D4→PreD2 PreD2→1-MCTA+HCl PreD4→D8 PreD4→D7 D9→D10 D10→4a-SH-4,6-DCDT 4a-SH-4,6-DCDT→4,6-DCDT+SH D1→D12 D1→D11 D12→1a-H-4a-SH-4,6-DCDT 1a-H-4a-SH-4,6-DCDT→4,6-DCDT+H2S D11→4,6-DCDT+H2S

n

Ea/R (1/K)

× × × × × × × × × × × × × × × × × × ×

0.24 0.23 1.50 2.89 0.25 1.33 3.19 −0.12 0.15 0.16 0.27 2.00 0.26 0.01 0.56 0.72 0.60 0.72 0.53

9950 9400 255 −367 29000 12000 18900 18900 29500 6700 10300 11000 6060 172 1800 −98.0 25300 13500 30500

4.73 1.15 2.39 1.00 1.10 1.01 1.26 1.74 1.58 1.52 6.530 2.07 1.00 6.89 7.83 3.10 4.00 5.22 6.10

1010 1011 10−15 10−20 1011 10−15 10−22 1013 1011 1011 1010 1006 1011 1012 1009 1009 1011 1011 1010

ASSOCIATED CONTENT

S Supporting Information *

Cartesian coordinates, vibrational frequencies, intrinsic reaction coordinate and energy profiles for internal rotations are provided for selected structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (+61) 2 4985-4286; e-mail: Mohammednoor. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study has been supported by the Australian Research Council and a grant of computing time from the National Computational Infrastructure (NCI), Australia (Project ID: De3). T.D. thanks the University of Newcastle, Australia for a postgraduate research scholarship.

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A (1/s or cm3/molecule·s) 6.53 1.35 4.78 1.28 1.01 6.23 2.75 3.40 7.32 8.49 9.870 3.27 1.75 4.20 5.76 7.43 2.01 9.03 1.68

× × × × × × × × × × × × × × × × × × ×

10−18 1011 10−19 10−23 10−22 10−18 10−24 1014 10−21 1012 1010 1007 1013 10−21 1011 1010 1012 10−24 10−24

n

Ea/R (1/K)

1.79 1.85 2.35 3.39 3.23 1.54 3.44 −0.45 2.43 0.02 0.27 1.60 −0.05 2.54 0.49 0.20 0.29 3.10 3.33

7620 2570 13400 5359 30850 40250 40700 33800 325400 1500 7600 13260 5760 6900 20440 29000 22790 61960 65550

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parameters derived from the conventional transition state theory, with the model’s A factors in accordance with the corresponding values quoted in literature for the homogeneous formation of PCDD/F.

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reverse rate parameters

A (1/s or cm3/molecule·s)

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