The acid-base properties of 1-amino-5-benzoyl-4-phenyl-1H-pyrimidine-2-one (L 1 ) and 1-amino-5-benzoyl-4-phenyl-1H-pyrimidine-2-thione (L 2 ) were investigated potentiometrically at an ionic strength of 0.10 M (LiCl) in 19.8, 33.6 and 55.9 % (v/v) methanol-water mixtures at 25.0 ± 0.1 ºC. The apparent dissociation constants (p s K a ) were calculated for the di-protonated form (L 1 H 2 +2 and L 2 H 2 +2 ) of pyrimidine bases, using a software package TITFIT, which were then extrapolated to

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... water to derive the dissociation constants in aqueous solution (pK a ). The aqueous pK a constants were found to be: L 1 , pK a1 = 3.76 and pK a2 = 6.95; L 2 , pK a1 = 3.57 and pK a2 = 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 p s K a values was observed with L 1 but not L 2 . The effects of molecular structure and solvent medium on the p s K a values are also discussed.