Thermodynamics of the Ionization of Some Thiols in Aqueous Solution.

Abstract

Dissocn. consts. of a no. of thiols were detd. spectroscopically using the strong absorption of the thiolate ion in the 250 mm region where the absorption of the undissocd. thiol group is very low. The corresponding heats of ionization were detd. calorimetrically. For the ionization reactions in H2O at 25 Deg the following values were obtained (pKa values cor. to zero ionic strength; DHi values detd. at the indicated ionic strengths, I): EtSH 10.61, 6.42, 0.015; iso-PrSH 10.86, 5.38, 0.010; tert-BuSH 11.22, 5.30, 0.01-0.1; HOCH2CH2SH 9.72, 6.21, 0.015; AcNHCH2CH2SH 9.92, 6.26, 0.015; HSCH2CO2- 10.68, 6.28, 0.05; HSCH2CH2CO2- 10.84, 6.10, 0.05; HSCH2CH2NH3+ 8.23, 7.43, 0.01; o-HSC6H4CO2- 8.88, 5.72, 0.1; p-HSC6H4CO2-5.80, --, --; and AcSH 3.62, 0.56, 0.015, for pKa, DHi kcal./mole, I, resp. Current ideas concerning energy changes for chem. processes in soln. and in particular aq. ionization processes are reviewed. Free energies, heats, and entropies for thiol ionization are compared with corresponding values for analogous carboxylic acids and phenols. The free energies of ionization of the thiols are .apprx.7 kcal./mole higher than for the corresponding carboxylic acids, this difference resulting from an essentially const. difference in the enthalpy term while entropy changes were generally similar. As with carboxylic acids, differences in the strengths of the thiols are, therefore, mainly due to entropy differences, but thiolacetic acid is a marked exception to this rule. Strongly polar groups exert only a small influence on the ionization enthalpy of the thiols; the more important consideration in relation to acid strength is their marked effect on the ionization entropy. In common with carboxylic acids with the exception of thiolacetic acid, a linear relation exists between pKa (or DGi) and DSi, again indicating that acid strength differences arise mainly from the entropy term, with DHi essentially const. [on SciFinder (R)]