Potentiometric investigation of acid-base equilibria of two newv pyrimidine derivatives in various methanol–water media

The acid-base properties of 1-amino-5-benzoyl-4-phenyl-1H-pyrimidine-2-one (L1) and 1-amino-5-benzoyl-4-phenyl-1H-pyrimidine-2-thione (L2) were investigated potentiometrically at an ionic strength of 0.10M(LiCl) in 19.8, 33.6 and 55.9 % (v/v) methanola water mixtures at 25.0 ± 0.1 oC. The apparent dissociation constants (psKa) were calculated for the di-protonated form (L1H2+2 and L2H2+2) of pyrimidine bases, using a software package TITFIT, which were then extrapolated to pure water to derive the dissociation constants in aqueous solution (pKa). The aqueous pKa constants were found to be: L1, pKa1 = 3.76 and pKa2 = 6.95; L2, pKa1 = 3.57 and pKa1 = 6.90. At pH 2.00, the dominant species in solution were the protonated form of the amino group substituted at the 1-position, while at a pH around 5.00, they were the protonated form of the pyrimidine ring nitrogen at the 3-position. An effect of intramolecular hydrogen bonding on the psKa values was observed with L1 but not L2. The effects ofmolecular structure and solvent medium on the psKa values are also discussed.