| Résumé | There are conflicting reports on the origin of the effect of Y substituents on the S-H bond dissociation enthalpies (BDEs) in 4-Y-substituted thiophenols, 4-YC₆H₄S-H. The differences in S-H BDEs, [4-YC₆H₄S-H] - [C₆H₅S-H], are known as the total (de)stabilization enthalpies, TSEs, where TSE = RSE - MSE, i.e., the radical (de)stabilization enthalpy minus the molecule (de)stabilization enthalpy. The effects of 4-Y substituents on the S-H BDEs in thiophenols and on the S-C BDEs in phenyl thioethers are expected to be almost identical. Some S-C TSEs were therefore derived from the rates of homolyses of a few 4-Y-substituted phenyl benzyl sulfides, 4-YC₆H₄S-CH₂C₆H₅, in the hydrogen donor solvent 9,10-dihydroanthracene. These TSEs were found to be -3.6 ± 0.5 (Y = NH₂), -1.8 ± 0.5 (CH₃O), 0 (H), and 0.7 ± 0.5 (CN) kcal mol⁻¹. The MSEs of 4-YC₆H₄SCH₂C₆H₅ have also been derived from the results of combustion calorimetry, Calvet-drop calorimetry, and computational chemistry (B3LYP/6-311+G(d,p)). The MSEs of these thioethers were -0.6 ± 1.1 (NH₂), -0.4 ± 1.1 (CH₂O), 0 (H), -0.3 ± 1.3 (CN), and -0.8 ± 1.5 (COCH₂) kcal mol⁻¹. Although all the enthalpic data are rather small, it is concluded that the TSEs in 4-YC₆H₄SH are largely governed by the RSEs, a somewhat surprising conclusion in view of the experimental fact that the unpaired electron in C₆H₅S⋅ is mainly localized on the S. The TSEs, RSEs, and MSEs have also been computed for a much larger series of 4-YC₆H₄SH and 4-YC₆H₄SCH₃ compounds by using a B3P86 methology and have further confirmed that the S-H/S-CH₃ TSEs are dominated by the RSEs. Good linear correlations were obtained for TSE = ρ⁺σp⁺(Y), with ρ⁺ (kcal mol⁻¹) = 3.5 (S-H) and 3.9 (S-CH₃). It is also concluded that the SH substituent is a rather strong electron donor with a σp⁺(SH) of -0.60, and that the literature value of -0.03 is in error. In addition, the SH rotational barriers in 4-YC₆H₄SH have been computed and it has been found that for strong electron donating (ED) Ys, such as NH₂, the lowest energy conformer has the S-H bond oriented perpendicular to the aromatic ring plane. In this orientation the SH becomes an electron withdrawing (EW) group. Thus, although the OH group in phenols is always in-plane and ED irrespective of the nature of the 4-Y substituent, in thiophenols the SH switches from being an ED group with EW and weak ED 4-Ys, to being an EW group for strong ED 4-Ys. |
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