Determination of dissociation constant of some quinazoline derivatives at different temperatures in DMF-water medium

- The dissociation constant of compound depends on substitution present in the compound.
- Dissociation of compound increases with increase in temperature.
- The flouro group containing compound exhibited most acidic character whereas methoxy group containing compound showed most basic character.
- The positive Gibb's free energy indicates that dissolution process is not spontaneous.
- The negative entropy in all the studied compounds suggests more ordered structure in these solutions.

The dissociation constant and some thermodynamic parameters of synthesized pyrazolo[5,1-b]quinazolines are determined by Calvin Bjerrum pH titration method in mixed organic-water (60:40 v/v) solvent systems. The evaluated data are discussed in terms of the electron donating or electron withdrawing nature of side chain substitutions. The compound containing 4-flouro group is the most acidic whereas the methoxy containing compound is found to be the most basic. To determine factors which affect and control the dissociation process at three different temperatures, the thermodynamic analysis, such as enthalpy (∆H°), Gibb's free energy (∆G°) and entropy (∆S°) of solutions are presented and discussed in the present study.