The acid ionization constants of some thiazolidine compounds: Medium effects and thermodynamic studies

The pK(a) values of some thiazolidine compounds in various aqueous-organic solvent mixtures were determined pH-metrically at 25 degrees C and at an ionic strength of I = 0.02 mol dm(-3) (KNO3). The organic solvents used are methanol or ethanol (as amphiprotic solvent), DMF or DMSO (as dipolar aprotic solvent), and acetonitrile (as a low basic aprotic solvent). The pK(a) values obtained are discussed in terms of solvent characteristics. The pK(a) values of investigated compounds are increased on increasing the mole fraction of the amphiprotic or low basic aprotic organic solvent, whereas they are decreased in the presence of a dipolar aprotic solvent. Hydrogen bonding interactions and the medium basicity, in addition to the electrostatic effect, are the main factors influencing the ionization process in aqueous-amphiprotic or a low basic aprotic solvent mixture. The high stabilization of the conjugate base by dispersion forces and of the proton by its interaction with solvent as well as the solvent basicity play a vital role in determining the ionization equilibria of the investigated compounds in aqueous mixture of dipolar aproptic solvents. The effect of molecular structures of the compounds on pK(a) values is also discussed. Thermodynamic functions (Delta H, Delta G degrees, and Delta S degrees) of the ionization processes of all compounds in aqueous media are determined and discussed.