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What is the mechanism of Friedel-Crafts alkylation?

A Friedel-Crafts alkylation is a type of electrophilic aromatic substitution reaction in which an alkyl group becomes attached to an aromatic ring.

The mechanism consists of three steps.

Step 1.

The alkyl chloride or bromide forms an adduct with a Lewis acid catalyst such as AlCl₃, FeCl₃, BF₃, or ZnCl₂.

The positive charge on the Cl atom enhances the electrophilicity of the alkyl halide. For most practical purposes, you can consider the complex as if it were R⁺, Cl-AlCl₃⁻.

Step 2.

The π electrons in the aromatic ring attack the electrophilic C atom in the alkyl halide.

This forms AlCl₄⁻, destroys the aromaticity of the ring, and forms a cyclohexadienyl cation.

Step 3.

The AlCl₄⁻ removes the H atom from the sp³ C atom in the six-membered ring.

This re-establishes the aromatic system, forms hydrogen chloride, and regenerates the AlCl₃ catalyst.

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