In general, when the transition state is (partially) ionic by nature and the reactants are neutral, a solvent with larger dielectric constant, indicating greater polarity, or those with stronger hydrogen bond basicity, meaning they are more potent hydrogen bond acceptors, can lower the free energy of the transition state more than that lowered for the reactants, thereby reducing the overall free energy barrier. It is also known that non-basic, polar aprotic solvents are preferred as they do not solvate the nucleophile strongly, making it more reactive and available for the reaction.