Solvent effects

For protic solvents (solvents capable of forming hydrogen bonds in solution), an increase in the solvent's polarity results in a decrease in the rate of SN2 reactions. This decrease occurs because protic solvents solvate the nucleophile, thus lowering its ground state energy. Because the energy of the activated complex is a fixed value, the energy of activation becomes greater and, therefore, the rate of reaction decreases. 
Polar aprotic solvents (solvents that cannot form hydrogen bonds in solution) do not solvate the nucleophile but rather surround the accompanying cation, thereby raising the ground state energy of the nucleophile. Because the energy of the activated complex is a fixed value, the energy of activation becomes less and, therefore, the rate of reaction increases. 

Figure illustrates the effect of solvent polarity on the energy of activation and, thus, the rate of reaction

The smaller activation energy leads to the more rapid reaction.