The drawbacks of Friedel Craft Alkylation, such as polyalkylation and rearrangements, are overcome by FC Acylation Reaction.
1) Polyalkylation
In Friedel Craft alkylation, alkyl groups in the product activate the ring for more reactions. The reaction doesn’t stop but carries forward until the starting material is polyalkylated, making polysubstitution a significant drawback of the Friedel Craft alkylation reaction. Therefore, a mixture of three might be obtained from the reaction instead of one product.
However, an FC acylated product is ring deactivating and does not undergo further substitution reaction. The reaction stops at monosubstituted stage.
The carbonyl (C=O) group has the advantage that it can be reduced to the mono-substituted alkyl Benzene. This makes acylation reaction a preferred way for introducing a -CH2- (methylene) group to form desired alkyl benzenes.
The reduction methods of reducing carbonyl to alkyl are Clemmenson, Wolff Kishner, Mozingo, and hydrogenation over Palladium.
2) Rearrangement
The acylium ion, the electrophile intermediate in the reaction, is not free carbocation but exists as a strong ion-pair complex.
Therefore, it does not undergo any rearrangement reaction and is stable. Both the acylium ion and carbocation intermediate are stabilized by resonance.
Other sources besides AlCl3 to generate acylium ions are acid anhydride and AlCl3, carboxylic acid and mineral acid, carboxylic acid anhydride, and mineral acid.
Read drawbacks of FC acylation reaction.