Why is #"AlF"_3# not-as-often used a good Lewis acid catalyst?
Because (paradoxically), the Lewis acidic centre in
The experimental fact is that both aluminum chloride and aluminum bromide are stronger Lewis acids that aluminum fluoride; in fact
How to rationalize this observation?
While fluorine is a more electronegative element than both chlorine and bromine, the fluoride ion, bound to aluminum, has lone pairs of the right size and the right shape to shunt electron density to the aluminum centre, thus moderating its Lewis acidity. The corresponding lone pairs of
A similar rationale applies to the reduced Lewis acidity of