Triethylamine reacts with iodoethane to give tetraethylammonium iodide, an SN2 reaction in which neutral reactants form an ion pair (the Menshutkin reaction). For the same concentrations and temperature, the rate is highest in

The transition state has partial positive and partial negative charge, so it is more polar than the neutral reactants, and a polar solvent lowers its energy more than that of the reactants. Acetonitrile (dielectric constant about 37) therefore speeds the reaction most. This is the opposite behaviour from an anionic nucleophile attacking a neutral substrate, where charge is dispersed in the transition state and polar protic solvents slow the reaction.