Potassium fluoride is almost insoluble in acetonitrile, and 1-bromooctane is converted into 1-fluorooctane only extremely slowly by it. Addition of a catalytic amount of 18-crown-6 makes the reaction fast. The best explanation is that the crown ether

1. Aconverts 1-bromooctane into a carbocation that is trapped by fluoride
2. Bhydrogen bonds to fluoride and thereby makes it a stronger nucleophile
3. Cprotonates fluoride to give $\mathrm{HF}$, a stronger nucleophile than $\mathrm{F}{}^{-}$
4. Dbinds $\mathrm{K}{}^{+}$ in its cavity, brings $\mathrm{KF}$ into solution and leaves a poorly solvated, highly nucleophilic fluoride ion