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

Abstract

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) methanol–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.

Keywords

• dissociation constant
• pKa
• potentiometry
• pyrimidine derivatives
• protonated amines
• nitrogen-protonated pyrimidine bases
• intramolecular hydrogen bonding