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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
| ABSTRACT - Internet Electron. J. Mol. Des. August 2004, Volume 3, Number 8, 474-498 |
Reaction Constants Derived from Activation Parameters for
the Evaluation of Substituent and Solvent Effects
Ferenc Ruff
Internet Electron. J. Mol. Des. 2004, 3, 474-498
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Abstract:
In order to separate the effect of substituents into two parts,
referring to the interaction of the reacting molecules and the
solvation, the δΔG‡,
δΔH‡ and
δΔS‡ reaction constants were
defined and determined from the dependence of ΔG‡,
ΔH‡ and
ΔS‡ 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. δΔG‡,
δΔH‡ and
δΔS‡ can be divided into internal
(δΔXint‡, X = G,
H, S) and external (δΔXext‡)
parts which refer
to the bond formation and the solvation, respectively. The
contribution of the substituents to the internal part of entropy of
activation (δΔSint‡),
and the external part of free energy of
activation (δΔGext‡),
originated from solvent reorganization
were supposed to be zero. Thus δΔG‡
and δΔS‡ present a good
approximation to δΔHint‡
and δΔSext‡, describing the effect of
substituents on the energy barrier of the reaction and on the
solvation, respectively. The δΔG‡ reaction constant is
interpreted in the same way as the ρ constant in the Hammett
equation. The δΔS‡ reaction constant reflects the change in
solvation with the substituents in the reaction. A tentative
interpretation of δΔS‡, 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-catalyzed reactions. A break of the δΔH‡
vs. σ and δΔS‡ vs. σ
plots at about σ ~ 0 is diagnostic for the change of solvation
with the electronic effect of the substituents. The δΔS‡ reaction
constants can be used to describe change of solvation with the
substituents for a reaction in a solvent.
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