Acidity constants of some tetrazole compounds in various aqueous-organic solvent media

The acidity constants of some tetrazole compounds were determined pH-metrically at 25 degreesC and at the constant ionic strength I = 0.1 mol dm(-3) (KNO3) in pure water as well as in various aqueous mixtures having variable mole fractions (0.05-0.3) of organic solvents. The organic solvents used are methanol, ethanol, N,N-dimethylformamide, dimethyl sulfoxide, and acetone. Generally, the pK(a) values increase with an increase in the content of the organic solvent. It was concluded that the major effects responsible for this behavior are, in addition to the electrostatic effect, the stabilization of the conjugate base by hydrogen-bonding interactions with solvent molecules and solvent basicity. Effect of molecular structure of tetrazoles on the pK(a) values is discussed. Moreover, the thermodynamic functions (DeltaH, DeltaG degrees, and DeltaS degrees) associated with the ionization process of investigated compounds in a pure aqueous medium are derived and their values are discussed.