Reaction Constants Derived from Activation Parameters for the Evaluation of Substituent and Solvent Effects

Internet Electron. J. Mol. Des. 2003, 1, 000–000 Abstract In order to separate the effect of substituents into two parts, referring to the interaction of the reacting molecules and the solvation, the δ∆G, δ∆H and δ∆S reaction constants were defined and determined from the dependence of ∆G, ∆H and ∆S activation parameters on the σ substituent constants, by analogy with the Hammett equation. The new reaction constants give the effect of the substituents on the reaction in energy units. δ∆G, δ∆H and δ∆S can be divided into internal (δ∆Xint, X = G, H, S) and external (δ∆Xext,) parts which refer to the bond formation and the solvation, respectively. The contribution of the substituents to the internal part of entropy of activation (δ∆Sint), and the external part of free energy of activation (δ∆Gext), originated from solvent reorganization were supposed to be zero. Thus δ∆G and δ∆S present a good approximation to δ∆Hint and δ∆Sext, describing the effect of substituents on the energy barrier of the reaction and on the solvation, respectively. The δ∆G reaction constant is interpreted in the same way as the ρ constant in the Hammett equation. The δ∆S reaction constant reflects the change in solvation with the substituents in the reaction. A tentative interpretation of δ∆S, based on the solvation of charged species in organic solvents and the rearrangement of the solvent structure in water containing mixtures is discussed for some nucleophilic addition, nucleophilic substitution and acid-catalysed reactions. A break of the δ∆H vs. σ and δ∆S vs. σ plots at about σ ~ 0 is diagnostic for the change of solvation with the electronic effect of the substituents.