Substituent effects on the thermodynamic functions of ionisation of benzoic acids

The thermodynamic acidity constants of benzoic acid, sixteen substituted benzoic acids, and methyl orange, have been measured in water over a wide range of temperature, by spectrophotometrically based techniques. The thermodynamic functions AG^^, and AS^^ of proton ionisation have been calculated by the Clarke and Glew procediere. An extensive review and recalculation of previously reported thermodynam^ic fixnctions of ionisation of the acids has been carried out. In contrast to the older data, the thermodynamic functions of ionisation of tiie present work show good linear relationships between the free energies, enthalpies and entropies for meta° and para-, substituted benzoic acids. The results are also discussed in termes of substituent effects on the reviewed quantities.