FriedelCraftsalkylation

Friedel–Crafts reactions are a class of electrophilic aromatic substitution reactions that introduce an alkyl or acyl group onto an aromatic ring using Lewis acid catalysts such as aluminum chloride. Named after Charles Friedel and James Crafts, the reactions proceed via the formation of a potent electrophile (carbocation or acylium ion) from an alkyl halide or acyl halide, which then reacts with the aromatic substrate, followed by deprotonation to restore aromaticity.

Friedel–Crafts alkylation involves reaction of an alkyl halide with an arene in the presence of a Lewis acid catalyst to form an alkylated arene. Drawbacks include carbocation rearrangements and polyalkylation; alkylation can be difficult to stop after one substitution, and electron-rich rings are more reactive, while strongly deactivating rings may react slowly or not at all.

Friedel–Crafts acylation uses an acyl halide (or anhydride) with a Lewis acid to form an acylium ion that adds to the arene, yielding an aryl ketone. The reaction generally avoids rearrangements and is less prone to polyacylation because the acyl group deactivates the ring, limiting further substitution. Catalysts commonly include AlCl3 or FeCl3, with solvents such as dichloromethane. Workups typically remove the Lewis acid to furnish the ketone product.

Applications and limitations: Friedel–Crafts reactions are foundational in organic synthesis, enabling construction of substituted arenes for dyes, fragrances, pharmaceuticals, and polymers. Limitations include incompatibility with strongly deactivated rings, potential rearrangements in alkylation, and the need to control polysubstitution. Modern approaches employ alternative catalysts and milder conditions to improve selectivity and scalability.