Ultraviolet–visible study of tautomeric behavior of some carbonyl and thiocarbonyl pyrimidine derivatives: Experimental evidence of enolization and thioketonization

Abstract

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 (p K enol) was 0.291 in aqueous methanol solution, while the thioketonization equilibrium constant (p K thiocarbonyl) for II was 0.234 in pure methanol.