Spectroscopic determination of acid dissociation constants of some pyridyl-substituted 2-aminothiazole derivatives

The acid dissociation constants, K-a, of nine biologically active 2-amino-4-(x-pyridyl)- thiazole, 2-methylimino-3-methyl- 4-(x-pyridyl)- 2,3-dihydrothiazole, and 2-methylamino-4-(x-pyridyl)- 2,3-dihydrothiazole were determined using UV-vis spectroscopic technique. The acidity constants for the first protonation, pK(a1), of parent molecules and their fixed model molecules are found to be associated with the protonation of aza or imino nitrogen atom. The acidity constants for the second proton uptake, pK(a2), are found to correspond to aza, imino, or amino nitrogen atom protonation. The contribution of the imino tautomeric form to tautomeric equilibria was found to be considerably important.