Solvent effects

The effect of the solvent is not only because of its acidity or basicity but also because of its dielectric constant and its ability to preferentially solvate and thus stabilize certain species in acid-base equilibria.

Consider the following acid dissociation equilibrium: Water, being the most polar-solvent listed above, stabilizes the ionized species to a greater extent than does DMSO or Acetonitrile.

Thus, equilibrium-solvent effects are observed in reactions that tend to have sharp barriers and weakly dipolar, rapidly relaxing solvents.

In such cases involving strongly dipolar, slowly relaxing solvents, solvation of the transition state does not play a very large role in affecting the reaction rate.

Instead, dynamic contributions of the solvent (such as friction, density, internal pressure, or viscosity) play a large role in affecting the reaction rate.

[6][9] The effect of solvent on elimination and nucleophillic substitution reactions was originally studied by British chemists Edward D. Hughes and Christopher Kelk Ingold.

[6] Hughes and Ingold then made certain assumptions about the extent of solvation to be expected in these situations: The applicable effect of these general assumptions are shown in the following examples: The solvent used in substitution reactions inherently determines the nucleophilicity of the nucleophile; this fact has become increasingly more apparent as more reactions are performed in the gas phase.

The following table shows the relative solvolysis rates of tert-butyl chloride with acetic acid (CH3CO2H), methanol (CH3OH), and water (H2O).

In either case (SN1 or SN2), the ability to either stabilize the transition state (SN1) or destabilize the reactant starting material (SN2) acts to decrease the ΔG‡activation and thereby increase the rate of the reaction.

This arises from the fact that polar solvents stabilize the formation of the carbocation intermediate to a greater extent than the non-polar-solvent conditions.

Polar solvents stabilize the reactants to a greater extent than the non-polar-solvent conditions by solvating the negative charge on the nucleophile, making it less available to react with the electrophile.

The reactions involving charged transition metal complexes (cationic or anionic) are dramatically influenced by solvation, especially in the polar media.