Medium effects on the acid dissociation constants of some diimine heterocyclic bases

Autor: Mohamed R. Mahmoud, M. M. A. Hamed, Elham M. Abd-Alla, S. A. El-Gyar
Rok vydání: 1993
Předmět:
Zdroj: Monatshefte f�r Chemie Chemical Monthly. 124:127-133
ISSN: 1434-4475
0026-9247
DOI: 10.1007/bf00808670
Popis: The acid dissociation constants of the protonated form of diimine heterocyclic compounds (imidazole, 2-methylimidazole, 2,2′-bipyridyl, and 1,10-phenanthroline) were determinedpH-meterically in aqueous media containing different amounts of organic solvents, viz. amphiprotic (methanol, ethanol), dipolar aportic (DMSO), and low basic aprotic (acetonitrile) at 25±0.1 °C. It was observed that by increasing the amount of alcohol orDMSO in the aqueous medium thepKa, values of the investigated compounds decreased. On the other hand, thepKa values increased as the amount of acetonitrile in the medium was increased. These results are discussed in terms of various solvent characteristics. It is concluded that solvent effects, viz. differences in stabilization of the free base by dispersion forces and of the proton by its interaction with solvent molecules in amphiprotic or dipolar aprotic solvent-aqueous media relative to that in pure aqueous one, as well as the basicity effect of acetonitrile play a vital role in the ionisation equilibria of the investigated compounds.
Databáze: OpenAIRE
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