Article pubs.acs.org/jced
Thermochemical Parameters and pKa Values for Chlorinated Congeners of Thiophenol Mohammednoor Altarawneh,*,† Tajwar Dar, and Bogdan Z. Dlugogorski Priority Research Centre for Energy, Faculty of Engineering & Built Environment, The University of Newcastle, Callaghan NSW 2308, Australia S Supporting Information *
ABSTRACT: 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 pKa values based on gas phase deprotonation free energies and calculated solvation energies for chlorinated thiophenol molecules and chlorinated thiophenolate anions. Calculated pKa values are in good agreement with limited available experimental measurements.
1. INTRODUCTION Over the last few decades, numerous efforts have been undertaken to investigate emission sources,1 transformation pathways,2 and thermodynamic data3 of chlorinated phenols. In contrast, corresponding data pertinent to chlorinated thiophenols have been very limited. Chlorothiophenols have been widely employed as an intermediate in the manufacturing of dyes, insecticides, and printing inks.4 In particular, parachlorothiophenol has gain commercial importance as being a key ingredient in the synthesis of pesticides, rubber, and polyvinylchloride (PVC).5 In addition, the use of dichlorothiophenols has been mounted in pharmaceutical industry due of their inhibitory effect on human cytochrome.6 Thiophenol itself is reported to be the major aromatic sulfur-carrier in coal.7 Chlorothiophenols are believed to be connected with serious health and environmental hazards. For instance, Shi et al.8 investigated acute toxicities of chlorinated thiophenols in a water environment. They explained that the order of toxicity increases with the degree of chlorination. While chlorinated thiophenols are toxic on their own right, they also act as potent precursors for the formation of sulfur analogues of the notorious dioxins compounds (PCDD/Fs),9 namely, polychlorinated dibenzothiophenes (PCDTs) and polychlorinated thianthrenes (PCTAs).10,11 Benz et al.12 identified PCDT/TAs during the course of synthesis of pentachlorothiophenol from hexachlorobenzene. Furthermore, reaction pathways suggested by Benz et al. to account for the formation of PCDT/TAs © 2012 American Chemical Society
involve analogous steps to the mechanism of formation of PCDD/Fs. Due to their structural resemblance on PCCD/Fs, PCDT/ TAs display ecotoxicological and toxicological properties similar to those exhibited by their dioxins counterparts.13 They have been detected in various environmental matrices such as soil14 and surface water15 in additions to samples taken from sediment,16 incineration of municipal waste,17 pulp bleaching,18 and metal reclamation.19 PCDT/TAs could also arise in uncontrolled burning of materials containing appreciable amounts of sulfur and chlorine compounds as in the case of automobile tires.20 Accruing reliable thermochemical parameters could serve as a prominent stride toward developing insight into the occurrence, environmental fate, and behavior of chlorinated thiophenols in thermal systems, as well as their potential role in the homogeneous formation of PCDTs and PCTAs. For example, such parameters will be instrumental to determine whether or not the formation and the distribution of chlorinated congeners are controlled by their relative thermodynamic stability. To this end, this study reports theoretically derived thermochemical and structural parameters for chlorinated congeners of thiophenol. This contribution Received: March 11, 2012 Accepted: May 7, 2012 Published: May 11, 2012 1834
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Figure 1. Optimized structures at the B3LYP/CBSB7 level. Distances are in angstroms, and angles are in degrees.
calculations, followed by CCSD(T), MP4SDQ, and MP2 single-point calculations and then a CBS extrapolation and a correction term for spin contamination. Isodesmic work reactions are utilized to obtain standard enthalpies of formation, that is, ΔfH°298. Calculations of entropies and heat capacities are performed using the ChemRate code.25 In these calculations, vibrational frequencies corresponding to the internal rotations of the SH group in chlorinated congeners of thiophenols are treated as hindered
complements our ongoing interest in deriving reliable thermochemistry data for benzene-substituted compounds.21,22
2. METHODOLOGY All structural optimizations and energy calculations are performed using the Gaussian 03 suite of programs.23 Calculations are carried out using the composite chemistry model of CBS-QB3 theory.24 CBS-QB3 is a five-step method that starts with B3LYP/CBSB7 geometry and frequency 1835
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Figure 2. Potential energy profiles for the internal rotations of the H around the C−S bond in chlorinated congeners of thiophenol (R).
3.2. Internal Rotation, Heat Capacities, and Standard Entropies. Energy potential profiles for the rotation of thiols' H atoms around the C−S bond are depicted in Figure 2 (relaxed scan). All profiles are 2-fold rotors with peaks corresponding to the perpendicular position of H atom with respect to the planar phenyl moiety. The calculated barrier for unsubstituted thiophenol amounts to 0.69 kcal·mol−1, in satisfactory agreement with the reported experimental value of 0.76 kcal·mol−1.31 Barriers for rotations based on the height of genuine transition structures of rotations are presented in Table 1 for selected isomers and compared to corresponding values obtained from relaxed and fixed scans. As given in Table 1, the barrier-based relaxed scans and transition structures give
rotors. The methodology of applying this treatment as well as its influence on deriving accurate thermochemical parameters is well-documented in the literature.26 In a nutshell, this approach requires scanning corresponding dihedral angles at an interval of 30°, determining the barriers of rotation, moments of inertia of the rotors, and the rotational symmetry number. Reduced moments of inertia for internal rotations have been calculated using the MultiWell suite of programs27 that utilizes a model of two unsymmetrical moieties that rotate with respect to each other. Values of solvation energies, that is, ΔG°solv, are calculated based on the polarizable continuum method (PCM)28 as implemented in the G03 code. PCM yields reasonable estimates of ΔG°solv.29 This is basically achieved by adapting a continuum surface charge formalism that enhances smoothness in external perturbing solvent media while maintaining robustness in regard to atomic composition of the solute.
Table 1. Barriers for the Rotation of Thiols H Atoms around the C−S Bond in Selected Congeners Based on Barriers of Transition Structures (TS) of Rotations, Relaxed Scan, and Fixed Scana
3. RESULTS AND DISCUSSION 3.1. Optimized Structures. Figure 1 shows optimized geometries and important atomic distances for chlorinated thiophenols. The variation in the degree and pattern of chlorination induces minor differences in structural parameters. Generally, geometries of chlorinated thiophenols are very close to the corresponding values of unsubstituted thiophenols. The calculated distances of S−H in chlorinated thiophenols (1.340 Å to 1.343 Å) are very close to the corresponding experimental values in alkyl thiols such as 2-propanethiol (1.345 Å) and ethanethiol (1.322 Å).30
barrier of TS
barrier from relaxed scan
barrier from fixed scan
0.69 2.60 2.74
0.69 2.49 2.54
1.12 2.97 3.20
2.84
2.65
3.32
4.17
4.12
5.41
4.28
4.27
5.66
thiophenol 2-chlorothiophenol 2,3dichlorothiophenol 2,3,5trichlorothiophenol 2,3,4,6trichlorothiophenol pentachlorophenol a
1836
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Table 2. S°298 in cal·mol−1·K−1 and C°p(T) in cal·mol−1·K−1 Cp°(T) thiophenol 2-chlorothiophenol 3-chlorothiophenol 4-chlorothiophenol 2,3-dichlorothiophenol 2,4-dichlorothiophenol 2,5-dichlorothiophenol 2,6-dichlorothiophenol 3,4-dichlorothiophenol 3,5-dichlorothiophenol 2,3,4-trichlorothiophenol 2,3,5-trichlorothiophenol 2,3,6-trichlorothiophenol 2,4,5-trichlorothiophenol 2,4,6-trichlorothiophenol 3,4,5-trichlorothiophenol 2,3,4,5-tetrachlorothiophenol 2,3,4,6-tetrachlorothiophenol 2,3,5,6-tetrachlorothiophenol pentachlorothiophenol a
S°298
300 K
500 K
800 K
1000 K
1500 K
82.37 80.48a 88.21 89.61 89.79 94.97 95.52 95.24 93.29 96.67 96.87 101.82 101.78 99.30 101.93 99.67 103.13 108.40 105.95 105.83 112.34
25.95 25.07b 30.73 29.98 29.68 34.51 34.54 34.57 34.26 33.58 33.97 38.22 38.37 38.01 38.26 38.13 37.43 41.99 41.75 41.73 45.41
39.38 39.07b 43.46 42.77 42.62 46.75 46.66 46.77 47.02 45.92 46.16 49.88 50.09 50.31 49.86 50.27 49.21 53.16 53.49 53.50 56.71
51.31 51.59b 54.19 53.80 53.73 56.65 56.57 56.65 57.04 56.18 56.29 59.00 59.14 59.55 58.97 59.41 58.63 61.46 61.92 61.93 64.39
56.20 56.79b 58.48 58.20 58.16 60.49 60.42 60.48 60.81 60.15 60.22 62.41 62.51 62.88 62.38 62.74 62.15 64.42 64.82 64.82 66.84
63.30 64.64 64.50 64.48 65.85 65.81 65.84 66.04 65.69 65.71 67.03 67.07 67.30 67.01 67.20 66.90 68.25 68.48 68.48 69.72
Experimental value from ref 32. bExperimental value from ref 33.
a very similar barrier of rotation. It is concluded herein that relaxed scans yield barrier heights that match corresponding values obtained from the barriers of transition structures of rotations and that fixed scans give rather higher values. Barriers to rotation appear to be the highest for the 2,6substituted congeners. The occurrence of local minima at 180° with very similar energies to the global minima at 0° indicates that there is no noticeable energy difference between syn and anti conformers of chlorothiophenols. Vibrational frequencies corresponding to the rotation of H atoms around the C−S bonds are treated as hindered rotors in calculations of standard entropies and heat capacities. Values of standard entropies and heat capacities at selected temperatures are given in Table 2. A value of S°298 for unsusbtituted thiophenol is calculated to be 82.37 cal·mol−1·K−1, that is, in a relative agreement with the corresponding experimental value of 80.48 cal·mol−1·K−1.32 Comparisons of calculated and experimental values of entropies at elevated temperatures are given in Table S1 of the Supporting Information. Calculated values of Cp°(T) are also close to the corresponding experimental33 values for thiophenol. The contributions of treating internal rotation as hindered rotors to values of S°298, Cp°(300), and Cp°(1000) are given in Table 3. Values in Table 3 correspond to the difference between S°298 and Cp°(T) calculated with and without the inclusion of treatment for internal rotations. As given in Table 3, the contribution of this treatment is more profound for S°298 where it ranges from 0.72 kcal·mol−1 (2-chlorothiophenol) to 3.28 cal·mol−1·K−1 (pentachlorophenol). In general, the contribution of anharmonicity to thermodynamic functions increases with temperature. 3.3. Standard Enthalpies of Formation of Chlorothiophenols and Their Derived Chlorothiophenoxy Radicals. To provide a benchmark of the accuracy of CBS-QB3 in calculated standard enthalpies of formation (ΔfH°298) for chlorinated congeners of thiophenol, we first calculated ΔfH°298
Table 3. Contribution of Treatment of Internal Rotors as Hindered Rotors to the Values of S°298, Cp°(300), and Cp°(1000)a thiophenol 2-chlorothiophenol 3-chlorothiophenol 4-chlorothiophenol 2,3-dichlorothiophenol 2,4-dichlorothiophenol 2,5-dichlorothiophenol 2,6-dichlorothiophenol 3,4-dichlorothiophenol 3,5-dichlorothiophenol 2,3,4-trichlorothiophenol 2,3,5-trichlorothiophenol 2,3,6-trichlorothiophenol 2,4,5-trichlorothiophenol 2,4,6-trichlorothiophenol 3,4,5-trichlorothiophenol 2,3,4,5-tetrachlorothiophenol 2,3,4,6-tetrachlorothiophenol 2,3,5,6-tetrachlorothiophenol pentachlorothiophenol
S°298
Cp°(300)
Cp°(1000)
1.12 0.72 1.2 1.17 2.13 1.9 2.18 3.47 1.25 1.01 2.09 2.42 2.9 2.38 2.85 1.39 2.51 3.16 3.14 3.28
0.69 0.33 0.36 0.8 0.33 0.29 0.32 0.02 0.64 0.38 0.29 0.33 0.24 0.26 0.29 0.57 0.3 0.24 0.25 0.23
0.92 0.66 0.94 0.98 0.64 0.71 0.65 0.32 0.98 0.91 0.69 0.61 0.22 0.72 0.36 0.95 0.67 0.25 0.25 0.23
All values are in cal·mol−1·K−1. Values correspond to the difference between S°298 and Cp°(T) values calculated with and without the inclusion of treatment for internal rotations. a
value for thiophenol itself using the atomization method. Based on calculated CBS-QB3 enthalpy values for C (−37.783017 Ha), H (−0.497457 Ha), and S (−397.6550 Ha) and the corresponding experimental values of gaseous atoms of C (171.3 kcal·mol−1), H (52.1 kcal·mol−1), and S (65.66 kcal·mol−1),34 the ΔfH°298 value for thiophenol is calculated 1837
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to be 26.8 kcal·mol−1. This value matches excellently the corresponding experimental value of 26.8 kcal·mol−1.34 Standard enthalpies of formation (ΔfH°298) for chlorinated congeners of thiophenol are calculated based on the isodesmic reaction R1:
Reaction R1 utilizes the experimental enthalpies of benzene (19.8 ± 0.2 kcal·mol−1), thiophenol (26.8 ± 0.2 kcal·mol−1), and chlorobenzene (13.01 kcal·mol−1 ± 0.2).34 Table 4 gives Table 4. ΔfH°298 (kcal·mol−1) for Chlorinated Congeners of Thiophenol thiophenol 2-chlorothiophenol 3-chlorothiophenol 4-chlorothiophenol 2,3-dichlorothiophenol 2,4-dichlorothiophenol 2,5-dichlorothiophenol 2,6-dichlorothiophenol 3,4-dichlorothiophenol 3,5-dichlorothiophenol 2,3,4-trichlorothiophenol 2,3,5-trichlorothiophenol 2,3,6-trichlorothiophenol 2,4,5-trichlorothiophenol 2,4,6-trichlorothiophenol 3,4,5-trichlorothiophenol 2,3,4,5-tetrachlorothiophenol 2,3,4,6-tetrachlorothiophenol 2,3,5,6-tetrachlorothiophenol pentachlorothiophenol
ΔR1H°298
ΔfH°298
−0.1 2.6 0.4 2.2 0.8 0.7 0.6 2.6 1.2 5.0 3.6 3.4 3.5 2.2 5.3 8.1 6.4 6.4 11.3
26.8 19.9 22.6 20.4 15.4 14.0 13.9 13.8 15.8 14.4 11.4 10.0 9.8 9.9 8.6 11.7 7.7 6.0 6.0 4.1
Figure 3. Calculated atomic polar tensor (APT) charges for syn and anti conformers of 2-chlorothiophenol and 2-chlorophenol at the B3LYP/6-311+G(d,p) level of theory.
in turn, makes phenolic H atoms stronger hydrogen-bond acceptors than thiol H atoms as is evident from APT values given in Figure 3. Accordingly, hydrogen bonding (i.e., difference in energy between syn and anti conformers) is stronger in chlorophenols than in chlorothiophenols. Substitution on an ortho site stabilizes the molecule via intramolecular hydrogen bonding. For instance, 2-chlorothiophenol and 2,4-dichlorothiophenol are more stable than 4chlorothiophenol and 3,4-dichlorothiophenol by (0.6 and 1.8) kcal·mol−1, respectively. An average estimate of enthalpic penalty pertinent to steric repulsion between two adjacent chlorine atoms could be obtained by considering enthalpic differences between 3,5-dichlorothiophenol and 3,4-dichlorothiophenol or between 2,3,5-trichlorothiophenol and 2,4,5trichlorothiophenol isomers. In both cases, chlorine steric repulsion incurs a value of 1.4 kcal·mol−1. To the best of our knowledge, there are no experimental ΔfH°298 values to compare with our calculated values. We suggest using calculated values of ΔfH°298 until reliable experimental measurements become available. Values of ΔfH°298 for chlorothiophenoxy radicals are calculated based on reaction R2:
values of ΔfH°298 of chlorinated thiophenols and enthalpies of reactions R1 (ΔR1H°298). Based on the uncertainty limits in the enthalpies of formation and the margin of accuracy of the adopted computational method, one could assign a value of ± 1.0 kcal·mol−1 as the uncertainty limit of the calculated ΔfH°298 values. ΔfH°298 values in Table 4 are calculated for syn conformers (thiolic hydrogen is intramolecularly bounded to chlorine at ortho positions). ΔfH°298 for the anti conformer of 2chlorothiophenol is found to differ from that of syn conformer by only 0.15 kcal·mol−1, that is, a value that is most plausible to reside within the computational accuracy of our adopted methodology. In case of chlorophenols, the difference in thermodynamic stability between syn and anti conformers of 2chlorophenol was found to reach 3.5 kcal·mol−1.35 This noticeable difference between 2-chlorothiophenol and 2chlorophenol could be rationalized based on the charge distribution within their syn and anti conformers. Atomic polar tensor (APT) charges are calculated for syn and anti conformers of 2-chlorothiophenol and 2-chlorophenol and depicted in Figure 3. As expected, more charge density is located on oxygen atoms in comparison with sulfur atoms. This,
Reaction R2 maintains the same number of radical species via inclusion of two sulfur-centered radicals on both sides of the reaction. Reaction R2 utilizes experimental enthalpies of formation of H 2 S (−4.9 kcal·mol −1 ) and HS· (32.4 kcal·mol−1)34 and the calculated ΔfH°298 for chlorothiophenols given in Table 4. Values of ΔfH°298 for chlorothiophenoxy radicals are given in Table 5. The calculated ΔfH°298 for 1838
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Table 5. ΔfH°298 (kcal·mol−1) for Chlorinated Thiophenoxy Radicals and Bond Dissociation Enthalpies (BDH) for S−H Bonds in Chlorinated Thiophenols
thiophenoxy 2-chlorothiophenoxy 3-chlorothiophenoxy 4-chlorothiophenoxy 2,3-dichlorothiophenoxy 2,4-dichlorothiophenoxy 2,5-dichlorothiophenoxy 2,6-dichlorothiophenoxy 3,4-dichlorothiophenoxy 3,5-dichlorothiophenoxy 2,3,4trichlorothiophenoxy 2,3,5trichlorothiophenoxy 2,3,6trichlorothiophenoxy 2,4,5trichlorothiophenoxy 2,4,6trichlorothiophenoxy 3,4,5trichlorothiophenoxy 2,3,4,5tetrachlorothiophenoxy 2,3,4,6tetrachlorothiophenoxy 2,3,5,6tetrachlorothiophenoxy pentachlorothiophenoxy
ΔR2H°298
BDH(S−H) in the parent molecule
ΔfH°298
−2.9 −3.3 −6.2 −11.1 −3.5 −3.1 −3.1 −3.9 −10.4 −2.6 −3.8
77.4 77.0 74.1 69.2 76.8 77.2 77.2 76.4 69.9 77.7 77.1
63.6 56.3 56.2 49.0 51.6 50.7 50.5 49.5 45.1 51.4 46.0
−3.4
76.9
46.3
−4.2
76.1
45.3
−3.0
77.4
46.6
−3.8
76.5
44.5
−9.7
70.6
41.8
−3.3
77.0
44.1
−6.6
73.7
39.2
−3.9
76.4
41.8
−3.0
74.3
37.8
Table 6. Standard Gibbs Free Energies in the Gas Phase ΔfG°298, Gibbs Free Energies for Solvation (ΔsolvG), and Standard and Relative Gibbs Free Energies in Aqueous Phase ΔfGaq298 for Chlorinated Congeners of Thiophenola thiophenol 2-chlorothiophenol 3-chlorothiophenol 4-chlorothiophenol 2,3-dichlorothiophenol 2,4-dichlorothiophenol 2,5-dichlorothiophenol 2,6-dichlorothiophenol 3,4-dichlorothiophenol 3,5-dichlorothiophenol 2,3,4-trichlorothiophenol 2,3,5-trichlorothiophenol 2,3,6-trichlorothiophenol 2,4,5-trichlorothiophenol 2,4,6-trichlorothiophenol 3,4,5-trichlorothiophenol 2,3,4,5-tetrachlorothiophenol 2,3,4,6-tetrachlorothiophenol 2,3,5,6-tetrachlorothiophenol pentachlorothiophenol
ΔG°solv
ΔfG°298
ΔfGaq298
−2.3 −0.7 −2.0 −2.0 0.2 0.0 −0.3 −0.2 −1.3 −1.2 1.1 1.3 0.8 1.2 1.3 −0.1 2.5 2.5 2.5 3.8
35.1 29.7 32.1 29.8 26.6 25.2 25.0 25.4 26.4 25.3 23.8 22.4 22.9 22.2 21.6 23.7 21.4 20.5 20.5 20.0
32.9 29.0 30.0 27.8 26.7 25.1 24.7 25.2 25.1 23.8 24.9 23.7 23.7 23.5 22.9 23.6 23.9 23.0 23.1 23.8
All values are in kcal·mol−1. The most stable congener in each homologue group is highlighted in bold font.
a
Calculating Gibbs free energies of formation in the aqueous phase (ΔfGaq298) requires estimating values of Gibbs free energies of solvation (ΔG°solv). As described in Section 2, the PCM model is used to calculate values of ΔG°solv. The PCM model yields values of ΔG°solv in accord with experimental measurements.29 Table 6 shows estimated values of ΔG°solv. As inferred from the values of ΔG°solv, the tendency for solvation of chlorinated thiophenols gradually decreases as the degree of chlorination increases. An analogous trend has been observed for chlorinated congeners of benzoic acid.38 It has been explained that, as chlorination increases, water molecules become less accessible by the chlorinated moiety. Calculated values of ΔfGaq298 are given in Table 5 based on the equation:
thiophenoxy (63.6 kcal·mol−1) is higher than the available experimental value36 by 3.0 kcal·mol−1; however, it is in agreement with recent theoretical predictions at G3B3 (64.1 kcal·mol−1) and CBS-QB3 (63.2 kcal·mol−1).37 By considering ΔfH°298 for an H atom as 52.1 kcal·mol−1, bond dissociation enthalpies (BDH; the endothermicity of breaking a bond) of S−H in chlorinated thiophenols can be estimated based on calculated values of ΔfH°298 for chlorinated thiophenol and thiophenoxy. Table 5 assemblies the estimated BDH values. Our calculated BDH value in the unsubstituted thiophenoxy amounts to 77.4 kcal·mol−1. This value agrees well with the lowest available experimental measurement of 77.5 kcal·mol−1.36 Overall, changes in pattern and degree of chlorination induce a rather modest variation in BDH values, and they reside between (69.2 and 77.4) kcal·mol−1. 3.4. Standard Gibbs Free Energies of Formation. Gaseous standard Gibbs free energies of formation (ΔfG°298) are calculated based on:
Δf Gaq 298 = Δf Go 298 + ΔGo solv
3.5. pKa Values. Accruing accurate values of acid dissociation constants (pKa) is of a prominent practical interest in biological as well as environmental applications. The theoretical derivation of pKa values is usually carried out via considering a thermodynamic cycle that deploys calculated gasphase deprotonation free energies, calculated solvation energies, and experimental values for the proton solvation and gas phase energies. A detailed description of the methodology is given in many recent studies.39,40 The thermodynamic cycle used herein is given in Scheme 1. All calculations in this section are carried out at the B3LYP/ 6-311+G(d,p)41 level of theory. Gas-phase deprotonation free energies for reaction R3 (ΔR3G(g)) are given in Table 7. It is a worthwhile mentioning that values in second column in Table 7 correspond to ΔG°solv for thiophenolate anions and not for the gaseous thiophenoxy radicals. Calculated ΔR3G(g) values include a correction term to signify an aqueous phase standard state (1 atm → 1 mol·L−1; 298.15 K). Calculated ΔR3G(g) for unsusbtituted thiophenol using the CBS-QB3 differs by only
Δf Go 298 = Δf H o 298 − TS o 298(molecule) + T ∑ S o 298(elements at their standard state)
Values are given in Table 6. The most stable congener in each homologue group is highlighted in bold font. Thermodynamic stability based on ΔfGaq298 matches the corresponding ordering obtained from enthalpic values shown in Table 2. To enable the derivation of H, S, and G values at elevated temperatures, NASA polynomials for all congeners are given in the Supporting Information (SI). 1839
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experimental measurement of ΔG°solv (H+) of −265.9 kcal·mol−1.42 pKa values are shown in Table 7. As given in Table 7, our calculated pKa values for thiophenol and 4chlorophenol are in a good agreement with limited corresponding experimental measurements.43−45 It can be deduced from values in Table 7 that the acidity of chlorinated thiophenols increases with the degree of chlorination and that the change in pattern of chlorination influences the degree of acidity. Calculated pKa values should be useful to understand the fate and transformation of chlorinated thiophenols in aqueous media.
Scheme 1
4. CONCLUSIONS Thermochemical parameters, comprising standard enthalpies of formation, standard entropies, heat capacities, and standard Gibbs free energies of formation, have been evaluated for the complete series of chlorinated thiophenol congeners. Chlorinated isomers with an ortho-chlorine substitution tend to be more stable than other isomers based on the H or G thermodynamic scale. Enthalpic penalties associated with steric repulsion of neighboring chlorine atoms are found to amount to 1.4 kcal·mol−1. The solvation of chlorinated thiophenols in water is predicted to be either slightly exoergic (for lower chlorinated congeners) or slightly endoergic (for higher chlorinated congeners). Based on estimated pKa values, it is concluded that the acidity of chlorinated thiophenols increases gradually with the degree of chlorination.
0.4 kcal·mol−1 from the corresponding value evaluated at the B3LYP/6-311+d(d,p). This demonstrates the accuracy of B3LYP/6-311+G(d,p) in obtaining correct gaseous phase energies. As the calculation of pKa comprises the most recent
■
Table 7. Calculated pKa Values of Chlorinated Congeners of Thiophenola
thiophenol
2-chlorothiophenol 3-chlorothiophenol 4-chlorothiophenol 2,3-dichlorothiophenol 2,4-dichlorothiophenol 2,5-dichlorothiophenol 2,6-dichlorothiophenol 3,4-dichlorothiophenol 3,5-dichlorothiophenol 2,3,4trichlorothiophenol 2,3,5trichlorothiophenol 2,3,6trichlorothiophenol 2,4,5trichlorothiophenol 2,4,6trichlorothiophenol 3,4,5trichlorothiophenol 2,3,4,5tetrachlorothiophenol 2,3,4,6tetrachlorothiophenol 2,3,5,6tetrachlorothiophenol pentachlorothiophenol a
ASSOCIATED CONTENT
S Supporting Information *
ΔR3G(g)°
ΔG*solv for thiophenolate anions
calculated pKa
exptl pKa
Cartesian coordinates for optimized structures and NASA polynomials for all congeners. This material is available free of charge via the Internet at http://pubs.acs.org.
332.8
−58.5
7.84
8.2043 7.8144 6.5245
■
Corresponding Author
7.5044
Present Address
329.0 326.5 327.4 324.5 323.9 322.8 324.9 322.3 320.5 320.2
−54.99 −53.79 −54.12 −50.97 −50.41 −49.83 −51.16 −49.36 −48.2 −46.62
6.44 6.50 6.95 5.48 5.52 5.35 5.90 6.15 5.58 4.79
318.7
−45.47
4.44
320.3
−47.11
4.76
319.0
−45.73
4.50
320.0
−46.31
4.78
317.8
−45.3
4.91
315.8
−42.63
3.52
316.8
−42.63
4.20
316.1
−42.75
3.57
313.8
−40.54
2.58
AUTHOR INFORMATION
*Phone: (+61) 2 4985-4286. E-mail: Mohammednoor.
[email protected]. †
Also at Chemical Engineering Department, Al-Hussein Bin Talal University, Ma'an, Jordan. Funding
This study has been supported by 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. Notes
The authors declare no competing financial interest.
■
REFERENCES
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−1
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