Separation of the Enthalpic and Entropic Contributions to Substituent Effects for the Equilibrium Constants of Aromatic Acids

Abstract

Separation of the substituent effect ρσ into entropie ρ_Sσ_S and enthalpic ρ_Hσ_H contributions is presented within the framework of the general Hammett equation. Analysis of the experimental data for aromatic acids shows that, in general, entropie effects are the major contribution to the total substituent effect, the entropic reaction constant being approximately equal to the normal Hammett ρ A comparison of the present results with those based on a separation of inductive and resonance effects shows that the entropie and inductive effects are directly related. On the other hand, strongly resonance interacting substituents were found to be enthalpy controlled. The substituent effect on enthalpy and entropy changes is discussed from a molecular point of view and related to inductive and resonance effects. A list of 16 enthalpic (σ_H) and entropie (σ_S) substituent constants are presented.