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 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 pK(a3)=11.130 for I and pK(a1)=4.684, pK(a2)=7.245 and pK(a3)=10.630 for II. The preferred dissociation mechanisms were discussed based on UV-vis data and a mechanism was proposed for each compound.
Kiliç
H
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Department of Physical Chemistry, Faculty of Sciences and Letters, Marmara University, Ziverbey 34722, Goztepe, Istanbul, Turkey. hkilic@marmara.edu.tr
D000136:Acid-Base Equilibrium; D004581:Electronics; D000432:Methanol / Q000737:chemistry*; D015394:Molecular Structure; D011743:Pyrimidines / Q000737:chemistry*; D012664:Semicarbazones / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D013882:Thiosemicarbazones / Q000737:chemistry*
D011743:Pyrimidines; D012664:Semicarbazones; D013882:Thiosemicarbazones; D000432:Methanol