Ultraviolet-visible study on acid-base equilibria of 2-substituted-4(3H)-quinazolinones

The apparent dissociation constants of 2-substituted-4(3H)-quinazolinones were determined in water and methanol, at constant ionic strength (0.1 M NaCl) and 35 °C. Ab initio quantum chemical calculations were performed for all possible protonation and tautomeric structures for 2-substituted-4(3H)-quinazolinones in the gas phase, and in water and methanol using a continuum solvent model. The observed UV-vis spectra clearly revealed the number of dissociation constants to be determined. The energy calculations thus indicated that the dominant tautomeric form is 2-substituted-4-oxo-N(3H)-quinazolinones. The results obtained are discussed in terms of average macroscopic properties of the mixed solvents. To our knowledge, this is the first paper addressing the acid dissociation and tautomerization of 2-substituted-4(3H)-quinazolinones in polar solvent environments. The results showed a good correlation between experimentally determined pKa values and theoretically calculated energies for the ionization processes of 2-substituted-4(3H)-quinazolinones.