SUBSTITUENT EFFECTS ON THERMODYNAMIC FUNCTIONS OF PROTON DISSOCIATION OF ORTHO-SUBSTITUTED ANILINIUM IONS

Thermodynamic acidity constants of the ortho-methoxyanilinium, ortho-chloroanilinium. ortho-bromoanilinium, ortho-iodoanilinium, and ortho-nitroanilinium ions have been measured by a combined e.m.f.- spectrophotometric technique within the temperature range 10-60° and the standard enthalpies and entropies of proton dissociation have been calculated. An analysis of substituent effects on the thermodynamic functions and pK a values is made in terms of two free-energy relationships and by comparison with the results for meta- and pars-substituted anilinium anions.