ACIDITIES OF BENZYLTETRAHYDROTHIOPHENIUM HALIDES IN WATER - A SIMPLE METHOD OF ESTIMATION

The pK(a) values of benzyltetrahydrothiophenium halides 1a-f in water have been estimated by measuring the absorbances of the solution in aq ueous hydroxide ion solution. Assuming that the ratios of the activity coefficients remains close to unity, the absorbance of the solution c an be expressed as SH](o)=(epsilon(SH)+epsilon(S)-K[OH-])/(1+K[OH-]), where A, [SH](o), K, epsilon(SH), and epsilon(S-) are the absorbance o f the solution, the initial concentration of 1a-f, the equilibrium con stant, and the extinction coefficients for SH and S-, respectively. Th e epsilon(S-) and K values that best fit with this equation were calcu lated by a nonlinear regression analysis with a large number of absorb ance data determined at different [OH-] and [SH](o). The pK(a) values of the SH were then calculated with the relationship K-a = -log K+14. The validity of this method has been demonstrated by the excellent agr eements between the experimental and literature pK(a) values of three organic acids. The pK(a) values of 1a-f estimated by this method are i n the range of 12.5-15.3 and correlate well with the Hammett equation. The large negative deviation for the pK(a) values of 1e and 1f from t he Hammett plot has been attributed to the extra hydrogen bonding betw een the phenyl group and water molecules attracted by the hydrophilic substituents.