$\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CH}{}_2-\mathrm{Br}$ is allowed to react with sodium cyanide at the same temperature in four different solvents. In which solvent is the SN2 reaction fastest?

Protic solvents hydrogen-bond to the small anion $\mathrm{CN}{}^{-}$ and lower its energy, which slows SN2. A polar aprotic solvent such as DMSO solvates the $\mathrm{Na}{}^{+}$ cation but leaves $\mathrm{CN}{}^{-}$ largely unsolvated and highly reactive, so DMSO gives the fastest reaction.