Acidities of Benzyltetrahydrothiophenium Halides in Water. A Simple Method of Estimation

摘要

The pKa values of benzyltetrahydrothiophenium halides 1a-f in water have been estimated by measuring the absorbances of the solution in aqueous hydroxide ion solution. Assuming that the ratios of the activity coefficients remains close to unity, the absorbance of the solution can be expressed as A/[SH]o=(ュSH+ュ S-K[OH-])/(1+K[OH-]), where A, [SH]o, K, ュSH, and ュS- are the absorbance of solution, the initial concentration of 1a-f, the equilibrium constant, and the extinction coefficients for SH and S-, respectively. The ュS- and K values that best fit with this equation were calculated by a nonlinear regression analysis with a large number of absorbance data determined at different [OH-] and [SH]o. The pKa values of the SH were then calculated with the relationship Ka=-log K+14. The validity of this method has been demonstrated by the excellent agreements between the experimental and literature pKa values of three organic acids. The pKa values of 1a-f estimated by this method are in the range of 12.5-15.3 and correlate well with the Hammett equation. The large negative deviation for the pKa values of 1e and 1f from the Hammett plot has been attributed to the extra hydrogen bonding between the phenyl group and water molecules attracted by the hydrophilic substituents.