The Mechanistic Probe of Increasing Electron Demand in the Study of Solvolytic Reactions

Abstract

Among the most striking developments in the history of physical organic chemistry have been methods for the quantitative correlation of rate and equilibrium constants in terms of relative free energy changes.1 One of the best known and most useful of these is an equation proposed by Hammett, which relates equilibrium and rate constants for the reactions of meta- and para-substituted benzene derivatives.1,2 The Hammett relationship is based on the fact that as the substituent is varied, the logarithms of the rate constants for aromatic side-chain reactions are linearly related to one another. Thus, a rate or equilibrium constant for a compound is related to the rate or equilibrium constant for the $$\log \frac{k}{{{k_0}}} = \rho \sigma$$ (1) unsubstituted compound in the same process through two parameters, ρ and σ. In the case of rate constants, the relationship is as shown in equation 1, where ko is the rate constant for the unsubstituted compound. Of these two parameters, the substituent constant, σ, is characteristic only of the substituent on an aryl group, while ρ is a reaction constant and is a function of the reaction being considered. Thus the reaction constant, or ρ value, is a measure of the sensitivity of a particular reaction to the substituents.