Protolytic properties of 2-acylthioacetamides

Concentration acidity of 2-acylthioacetamides dissolved in DMSO-H2O was measured by the method of pH-metric titration. The data obtained were extrapolated for the determination of pK a values of 2-acylthioacetamides in water and DMSO. We found that in aqueous solutions the acidity of thioacetarylamide (pK a = 5.75–6.01) was almost independent of the nature of substituents in their N-phenyl rings, whereas in DMSO solution the nature of substituents significantly affects their ionization constants (pK a = 7.88–11.70). The graphs were displayed and the acidity of N-aryl-3-oxobutanethioamides dependence on the Hammett constants of meta- and para-substituents in the phenyl rings derived. Water is shown to be a leveling, and DMSO a differentiating solvent for 2-acylthioacetamides.