Thermodynamics of Ionization of Benzoic Acid and Substituted Benzoic Acids in Relation to the Hammett Equation

Abstract

We have made both ampoule and titration calorimetric measurements leading to ΔH0 of ionization of aqueous benzoic acid at 298 K, and have considered our results in relation to those from several earlier investigations in an effort to select the "best" value for this important quantity. We have also made calorimetric measurements leading to ΔH0 values for ionization of several substituted benzoic acids, and have selected "best" ΔH0 values for 15 meta- and para-substituted benzoic acids. It is found that there are good linear relationships between ΔG0, ΔH0, and ΔS0 values for most of these substituted benzoic acids. It is also found that the Hammett equation accounts well for ionization constants over the temperature range 278–318 K for all of these acids and accounts very well for ionization constants of those acids with positive σ substituent parameters.