Explain each of the following observations.

(a) When benzyl bromide (Ph—CH₂—Br) is added to a suspension of potassium fluoride in benzene, no reaction occurs. However, when a catalytic amount of the crown ether [18]-crown-6 (Sec. 8.7B) is added to the solution, benzyl fluoride can be isolated in high yield.

(b) If lithium fluoride is substituted for potassium fluoride, no reaction occurs even in the presence of the crown ether.