Question

In which ever cases friedel crafts alkylation/acylation will not work

Answer 1

Summary of Limitations of Friedel-Crafts alkylations: The halide must be either an alkyl halide. Vinyl or aryl halides do not react (their intermediate carbocations are too unstable). Alkylation reactions are prone to carbocation rearrangements. Deactivated benzenes are not reactive to Friedel-Crafts conditions, the benzene needs to be as or more reactive than a mono-halobenzene (see substituent effects) Over alkylation can be a problem since the product is more reactive than the starting material. This can usually be controlled with an excess of the benzene. The Lewis acid catalyst AlCl3 often complexes to aryl amines making them very unreactive.

Summary of Limitations of Friedel-Crafts acylations: Acylation can only be used to give ketones. This is because HCOCl decomposes to CO and HCl under the reaction conditions. Deactivated benzenes are not reactive to Friedel-Crafts conditions, the benzene needs to be as or more reactive than a mono-halobenzene (see substituent effects) The Lewis acid catalyst AlCl3 often complexes to aryl amines making them very unreactive. Amines and alcohols can give competing N or O acylations rather than the require ring acylation.