The Solvent Effect and Mechanism

For a reaction between two ions, of charge and z-g, the rate constant k (reduced to zero ionic strength) is given by 

There should be a linear plot of log vs /D with a negative slope if the charges of the ions are of the same sign and with a positive slope if the ions are oppositely charged. For the reaction of an ion and a polar molecule (common with solvolysis reactions), the linear plot of log A vs /D will have a positive slope irrespective of the charge of the ion since (1/r — 1/n) is positive. 

These expressions appear more applieable to nonpolar solvents or mixtures than to polar solvents. The nature of the solvation process (and the radii and so forth of the solvated reactants) may stay approximately constant in the first situation but almost certainly will not in the seeond. The function (E op A ) features in the reorganisation term Xq which is used for estimating rate constants for redox reactions (Eqn. 5.23). is the optical dielectric constant and Dj the static dielectric constant (= refractive index ). 

The rate of solvolysis (kj of ieri-butyl chloride in a particular solvent S compared with the rate (ka) in 80% v/v aqueous ethanol is used as a measure of that solvent s ionizing power, y  

For any other substrate acting by an S l mechanism, it might be expected that 

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