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

Abstract

The UV-vis spectra of recently synthesized 5-benzoyl-1-(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-one, (1), and 5-benzoyl-1-(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-thione, (11) 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 pK(a) 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 pK(a1) = 5.121, pK(a2) = 7.929 and PKa3 = 11.130 for I and pK(a1) = 4.684, pK(a2) = 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. (C) 2009 Elsevier B.V. All rights reserved.