THERMODYNAMIC FUNCTIONS OF IONIZATION OF BENZOIC AND SUBSTITUTED BENZOIC-ACIDS IN WATER AND FORMAMIDE - ADDITIVE SUBSTITUENT EFFECTS

Thermodynamic ionisation constants ((T)pK(a)m) of benzoic, 3-nitrobenzoic, 3,5-dinitrobenzoic, 3-chlorobenzoic, 3,5-dichlorobenzoic, 3-hydroxybenzoic, 3,5-dihydroxybenzoic, 3-methylbenzoic and 3,5-dimethylbenzoic acids have been determined in water and formamide by employing electromotive force measurements of buffer cells in the temperature range 5-45-degrees. Values of standard free energy (DELTAG-degrees), enthalpy (DELTAH-degrees) and entropy (DELTAS-degrees) changes associated with their ionisation are also calculated. Additivity of substituent effects for the ionisation of 3,5-disubstituted benzoic acids in both water and formamide have been discussed. The results indicate that the ionisation process is relatively insensitive to steric effects.