Thermodynamics of benzoic acid protonation

Abstract

The thermodynamic parameters ΔG°, ΔG_Thm, ΔH°, and ΔS° have been calculated using protonation constants for 12 meta- and para- substituted benzoic acids at 25, 40, and 60 °C and the van't Hoff model. The isokinetic temperature, β 327 K, was evaluated from regression analysis of the enthalpy and free energy estimates at the harmonic mean of the experimental temperatures (T_hm 314.17 K). The large difference between β and T_hm suggests a strong chemical compensation effect. The enthalpy, which is dominated by internal contributions, is comparable by an entropy contribution of equal sign to δΔG°. This finding essentially differs from the results for benzoic acid dissociation in water.