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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 investi- gated 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 con- stants (psKa) were calculated for the di-protonated form (L1H2+2 and L2H2+2 )o f py- rimidine bases, using a software package TITFIT, which were then extrapolated to pure water to derive the dissociation constants in aqueous solution (pKa). The aque- ous pKa constants were found to be: L1 ,p Ka1 =3 .76 and pKa2 = 6.95; L2 ,p Ka1 =3 .57 and pKa2 = 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 of molecular structure and solvent medium on the psKa values are also discussed.

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