Potassium fluoride is nearly unreactive toward 1-bromobutane in acetonitrile, but reacts rapidly after adding 18-crown-6. This is because the crown ether

1. Ahydrogen bonds to the bromide leaving group
2. Bbinds $\mathrm{F}{}^{-}$ and delivers it to the alkyl halide
3. Cprotonates the alkyl bromide to make it more electrophilic
4. Dencapsulates $\mathrm{K}{}^{+}$, freeing a poorly solvated, highly reactive $\mathrm{F}{}^{-}$ and bringing KF into solution