Electronic absorption study on acid–base equilibria for some pyrimidine derivatives containing semi- and thiosemicarbazone moiety

The UV–vis spectra of recently synthesized 5-benzoyl-1-(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-one, (I), and 5-benzoyl-1-(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-thione, (II) were studied in aqueous methanol (5%, v/v methanol). The nature of the electronic transitions and the roles of carbonyl oxygen of I and thiocarbonyl sulfur of II on the behavior of UV–vis spectra were discussed.Acid–base equilibria of the compounds against varying pH and pKa values related equilibria were determined at an ionic strength of 0.10 M by using the Henderson–Haselbalch equation. The mean acidity constants for the protonated forms of the compounds were determined as pKa1 = 5.121, pKa2 = 7.929 and pKa3 = 11.130 for I and pKa1 = 4.684, pKa2 = 7.245 and pKa3 = 10.630 for II. The preferred dissociation mechanisms were discussed based on UV–vis data and a mechanism was proposed for each compound.