Medium Effect on the Ionisation Constants of Some Hydroxy Derivatives of 1-Ethyl-2-styrylpyridinium and -quinolinium Iodides

Abstract

Abstract The pKa values of 1-ethyl-2-(4 or 2-hydroxystyryl)pyridinium and -quinolinium iodides in different organic solvent–water mixtures were determined. The organic solvents used are methanol, ethanol, acetone, DMF and DMSO. The ionisation of the compounds under investigation depends largely on both the proportion and the nature of the organic co-solvent used. The ionisation constant decreases with increase in the proportion of the organic co-solvent in the medium. The major important effect responsible for this is the difference in the stabilization of base by donor hydrogen bond from solvent molecules. The free energy of solvation of both acid and proton by electrostatic interaction and that of the base by donor hydrogen bond from solvent molecules were calculated. The effect of the molecular structure of the compound on pKa is also discussed.