Additive substituent effects on the thermodynamic functions of proton ionization processes. Part II. 3,5-Disubstituted anilinium ions

Abstract

Thermodynamic acidity constants of 3,5-dimethylanilinium, 3,5-dimethoxyanilinium, 3,5-dichloroanilinium, 3,5-dibromoanilinium, 3,5-di-iodoanilinium, and 3,5-dinitroanilinium ions have been measured spectrophotometrically within the temperature range 5–60 °C and the standard enthalpies and entropies of proton ionization have been calculated. These results in conjunction with others measured previously indicate that for this reaction series substituent effects on both the free energies and the enthalpies of proton ionization are precisely additive. A general hypothesis for additivity in such systems is formulated and the results are discussed in terms of Hepler's dichotomy of thermodynamic functions into internal and external components.