It has been generally accepted that aprotic solvent is the best solvent condition for Sn2 reaction due to its ability to solvate the cation but leaving the nucleophile unencumbered. Polar aprotic solvents (e.g., DMSO, acetone, acetonitrile, DMF) are very effective in solvating cations, but not nearly so effective in solvating anions. Why is that so?