Solution effects
Effect of solvent polarity on the rate of substitution reactions
The effect of changing solvent polarity on reaction rate depends on the nature of the reactants and products in the rate-determining step. If the product(s) have higher charge, the rate is higher in a more polar solvent. This is because the energy of charged particles (and the transition state) is lowered by solvation.
For SN1 reaction of an uncharged nucleophile with an uncharged substrate
The rate-determining step is heterolytic cleavage of the C-Leaving Group bond in the uncharged substrate to give charged particles. Therefore increasing the polarity of the solvent increases the rate.
For SN2 reaction of an anionic nucleophile and an uncharged substrate
The products of the rate-determining step are also an anion and an uncharged product. Thus increasing the polarity of the solvent for an SN2 reaction has no effect on rate.
Substitution reactions are carried out in polar solvents so that the reactants dissolve. Polar solvents can classed as protic (ROH) and aprotic (examples at the right).
While SN2 reactions can occur in polar protic solvents, their rate is much enhanced if carried out in a polar aprotic solvent. This is because protic solvents form hydrogen-bonded complexes with anionic nucleophiles and make them less reactive.