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

Abstract

Separation of the substituent effect p o into entropic psos and enthalpic pHoH contributions is presented within the framework of the general Hammett equation. Analysis of the experimental data for aromatic acids shows that, in general, entropic effects are the major contribution to the total substituent effect, the entropic reaction constant being approximately equal to the normal Hammett p. A comparison of the present results with those based on a separation of inductive and resonance effects shows that the entropic 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 (uH) and entropic tos) substituent constants are presented.

Effects of substituents on activation parameters in SN2 reactions at aliphatic carbon in solution

“…Therefore, it is possible to believe that in Eqn (8) the internal part of the activation entropy is independent of the substituent in the absence of steric effects ( δ Δ S ≈ 0) and δ Δ S ≠ ≈ δ Δ S 36, 37, 40, 53, 57. In this case, the magnitude of δ Δ H can be calculated by Eqn (9) 36–40, 53, 57 …”

“…[36,37,40,53,57] In this case, the magnitude of dDH 6 ¼ ext can be calculated by Eqn (9). [36][37][38][39][40]53,57]…”

“…It is known that the substituent effect ρσ in the framework of the general Hammett equation can be separated into enthalpic and entropic contributions 36–47. Such a separation was made for both the σ substituent constants38–40 and the ρ reaction constants 36, 37, 41–47.…”

“…[35] It is known that the substituent effect rs in the framework of the general Hammett equation can be separated into enthalpic and entropic contributions. [36][37][38][39][40][41][42][43][44][45][46][47] Such a separation was made for both the s substituent constants [38][39][40] and the r reaction constants. [36,37,[41][42][43][44][45][46][47] Separate consideration of enthalpic and entropic contributions to the effects of substituents is useful for obtaining more conclusive information about the structure of the transition state, its connection with the r value and the interactions with the environment.…”

Parameters and Methods in Quantitative Structure-Activity Relationships

“…Assuming that the free energy AG° is separable into inductive and resonance free energies, we can write An illuminating thermodynamic analysis of the separation of the free energy relationships into enthalpic and entropic contributions was presented by Krygowski and Fawcett (22). They assumed that the linear dependence of the free energy on substitutions can be represented by contributions from enthalpic and entropic linear relationships as follows.…”