The effect of substituents (CH3 and NH2) in positions 4 and 6 on the acid-base properties of 2(1H)-pyrimidinethiones

The acid-base properties of 4,6-dimethyl- and 4,6-diamino-2(1H)-pyrimidinethiones (1 and2) were investigated by potentiometric titration. Dissosiation of1 and protonation of its anion occur in two steps, the first of which affords the deprotonated dimer HA₂)^–. The relative stability of the dimer depends on the nature of the solvent. The effect of kinetic factors on the acid-base behavior of1 was elucidated by comparing the results of potentiometric titrations carried out under various kinetic conditions. It was concluded that the difference between the acid-base properties of1 and2 results from the electron-donating effect of the amino groups.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2145–2148, December, 1994.