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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., (Copyright (c) 2009 Elsevier B.V. All rights reserved.)

Tautomeric behaviors of the protonated forms of recently synthesized 1-Amino-5-benzoyl-4-phenyl-1H-pyrimidine-2-one (I), and 1-Amino-5-benzoyl-4-phenyl-1H-pyrimidine-2-thione (II), were investigated with respect to pH and time in aqueous methanol (5%, v/v methanol) and in pure methanol by electronic absorption spectra to elucidate the nature of the tautomeric interconversions and the role of carbonyl oxygen in I and thiocarbonyl sulfur in II. At room conditions, the carbonyl group at position 2 in I was found to undergo a significant spontaneous enolization (66%) at pH 1.0 in aqueous methanol medium in 8 h. On the other hand, thioketonization for the protonated form of the thiocarbonyl group in II was comparably small (37%) in pure methanol at pH 10.0 over a 5-day period. The interconversion mechanism was identified for each compound using the UV-vis data. The equilibrium constant for each compound was estimated in an ionic strength of 0.10 M (LiCl) at room temperature, from the ratios of molar absorptivities of the pure tautomeric forms. For I, the spontaneous enolization equilibrium constant (pKenol) was 0.291 in aqueous methanol solution, while the thioketonization equilibrium constant (pKthiocarbonyl) for II was 0.234 in pure methanol.

The UV-vis spectra of recently synthesized 1-amino-5-benzoyl-4-phenyl-1H-pyrimidine-2-one, (I), and 1-amino-5-benzoyl-4-phenyl-1H-pyrimidine-2-thione (II), were studied in aqueous methanol (5%, v/v, methanol) and pure methanol. The nature of the electronic transitions and the role of carbonyl oxygen of I and thiocarbonyl sulfur of II in the behavior of the observed UV-vis spectra were discussed. The carbonyl group at position 2 of I and the thiocarbonyl group of II were found to be enolized instead of protonation. Quantum chemical calculations showed agreement with the experimental evidence. However, the carbonyl group of the benzoyl moiety at position 5 of both compounds underwent neither enolization nor protonation. Acid-base equilibria of the compounds against varying pH have been examined in detail. The pKa values of all related equilibria were determined at room temperature and an ionic strength of 0.10 M from the pH-dependence of the absorbance values using the Henderson-Haselbalch equation and graphical logarithmic analysis. The mean acidity constants for the protonated forms of the compounds were determined as pKa1=4.214 and pKa2=6.678 for I and pKa1=3.739 and pKa2=6.258 for II. The mean acidity constants (pKa3) for the enol form of I and the thioenol form of II were determined as 11.278 and 11.063, respectively. The preferred dissociation mechanisms were discussed based on the data of UV-vis spectroscopy and a mechanism was proposed for each compound. The formation of intramolecular and intermolecular hydrogen bonding were found with I but not with II. The intramolecular bonding stabilizing the enol form was favoured at pH values corresponding to pKa1 and above. On the other hand, the intermolecular hydrogen bonding stabilizing the free form of the carbonyl group was favoured at all pH values.