Basicity of the thionamide group of α-aminothionamides. Attenuation of electrostatic effects by solvation

Abstract

The first dissociation constants pK1of four diprotonated α-aminothionamides, determined spectrophotometrically, are between −8.1 to −8.8; the second dissociation constants pK2, of three of these compounds, determined titrimetrically, are between 7.3 and 7.7. Both pK1and pK2 values are close to those calculated from current electrostatic theory for equilibria in solutions of zero ionic strength, in spite of the fact that data for pK1values were obtained in solutions of high ionic strength. Results indicate a much larger effect of the charged ammonium group on the basicity of the thionamide group than on the basicity of the amide group. A possible explanation based on the greater hydration of the protonated amide group is advanced.