Effects of Solvent Polarity on Rates

We have already described briefly some of the effects of dipole-dipole association. For example, the more strongly solvated an ion or molecule is, the more difficult it is for desolvation to occur so that an active site is exposed. Reactions in which ions are produced as the transition state forms from reactants will usually be accelerated as the solvent dielectric constant and dipole moment increase for a series of solvents. The increased solvation of the ions that constitute the transition state will cause this effect. In contrast, reactions that involve the combination of ions to produce a transition state of low charge will be retarded by solvents that strongly solvate ions. In order for the ions to combine, they must be separated from the solvent molecules, which is energetically unfavorable. 

An early attempt to explain these factors was put forth by Sir Christopher Ingold and his coworkers in 1935. The cases considered involve charge neutralization as positive and negative ions react and charge dispersion as a 

Some solvents consisting of polar molecules solvate anions and cations to different degrees because of their molecule structure. For example, 

TABLE 5.4 Relative Rates of the Reaction Shown in Eq. (5.30) in Several Solvents (Ege, 1994).  

The data show that as the size of the solvent molecule increases, the solvent is less able to solvate the ion that is the entering group and the rate of the reaction increases. The number of cases where similar observations on solvent effects are encountered is enormous. Later in this chapter, those principles will be extended to include the effects that result from using solvents that have different solubility parameters. 

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The effect of solvent polarity on rates has been delineated for equation (45) in which R = H and Nu = c-CoH N . First the specific rate constant increased as the aziridine concentration increased, when the solvent was benzene. Then a fair proportionality between k and Ft, a measure of solvent polarity, was demonstrated. 

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