Final answer:
The three main disadvantages of Friedel-Crafts alkylations are polyalkylation, carbocation rearrangement, and the formation of complex mixtures due to over-alkylation and side reactions.
Step-by-step explanation:
The three main disadvantages of Friedel-Crafts alkylations are polyalkylation, carbocation rearrangement, and the formation of complex mixtures with over-alkylation. Polyalkylation occurs because the alkylated product is more reactive than the original substrate, leading to multiple substitutions.
Carbocation rearrangement is possible because carbocations are unstable intermediates that can undergo hydride or alkyl shifts leading to unexpected products. The presence of strong Lewis acids required for the reaction can lead to complex mixtures due to over-alkylation and other side reactions, making the reaction difficult to control and requiring extensive purification.