1) Alkylation
Branched alkyl chlorides (20 or 3o) cannot be used as a reagent in FC acylation reaction. The branched alkyl chlorides on forming acylium ion undergo the loss of CO to create a stable alkyl cation. The 20 or 3o alkyl cation reacts with benzene to form alkyl benzene.
E.g., treatment of Pivaloyl chloride with AlCl3 and Benzene results in the formation of tert-butyl benzene. The loss of the CO group is due to the higher stability of tert-alkyl carbocation.
2) Only ketones and not aldehydes are obtained
FC acylation method introduces only the keto group and not the formyl group.
Formyl chloride is an unstable reagent, and formylation does not give an aldehyde product.
3) Reaction fails with electron-withdrawing substituents
Benzene attached to electron-withdrawing substituent does not undergo FC acylation reaction. E.g., nitrobenzene, aryl ketone, trifluoromethyl benzene.