Friedel-Crafts Alkylation Mechanism

Core Idea

Friedel-Crafts alkylation adds an alkyl group to a benzene ring via an alkyl carbocation intermediate, typically generated from an alkyl halide and a Lewis acid catalyst (AlCl₃). The reaction proceeds through electrophilic aromatic substitution with the benzene π-electrons attacking the carbocation. Rearrangement to more stable carbocations can occur, making primary alkyl halides problematic substrates.

Explainer

You already know the general mechanism of electrophilic aromatic substitution (EAS): an electrophile attacks the pi electron cloud of benzene, forming a resonance-stabilized carbocation intermediate (the arenium ion or sigma complex), followed by loss of a proton to restore aromaticity. Friedel-Crafts alkylation is a specific instance of EAS where the electrophile is a carbocation derived from an alkyl halide, and the result is a new C–C bond between the ring and an alkyl group.

The reaction begins with generating the electrophile. Aluminum chloride (AlCl₃), a strong Lewis acid, coordinates to the halide of the alkyl halide (say, CH₃CH₂CH₂Cl), polarizing the C–Cl bond and either forming a tight ion pair or fully generating the free carbocation. The electron-rich benzene ring then attacks this electrophilic carbon, forming the arenium ion intermediate. Deprotonation by AlCl₄⁻ (the aluminum ate complex) restores the aromatic ring and regenerates the AlCl₃ catalyst. The overall transformation: a hydrogen on benzene has been replaced by an alkyl group.

The most important complication is carbocation rearrangement. From your study of carbocation stability, you know that secondary carbocations are more stable than primary, and tertiary more stable than secondary. If a primary alkyl halide like 1-chloropropane forms a primary carbocation, it will rapidly rearrange via a 1,2-hydride shift to the more stable secondary carbocation before the benzene ring attacks. This means that attempting Friedel-Crafts alkylation with n-propyl chloride gives predominantly isopropylbenzene, not n-propylbenzene. This rearrangement problem limits the synthetic utility of the reaction — you cannot reliably install primary alkyl groups longer than methyl or ethyl.

There is a second limitation: polyalkylation. The alkyl group you just attached to the ring is an electron-donating group (via hyperconjugation and induction), which activates the ring toward further electrophilic attack. This means the monoalkylated product reacts *faster* than the starting benzene, making it difficult to stop at a single substitution. Using a large excess of benzene relative to the alkyl halide helps, but the selectivity is imperfect. A third limitation is that the reaction fails entirely with rings bearing strong electron-withdrawing groups (–NO₂, –CF₃, –COR) because the deactivated ring is not nucleophilic enough to attack the carbocation. These limitations — rearrangement, polyalkylation, and sensitivity to ring electronics — are why Friedel-Crafts acylation (followed by reduction) is often preferred when you need to install a straight-chain alkyl group without rearrangement.