Electrophilic Aromatic Substitution: Friedel–Crafts Alkylation of Benzene

Friedel-Crafts alkylation is an electrophilic aromatic substitution reaction where a hydrogen atom on an aromatic ring is replaced with an alkyl group. Discovered by Charles Friedel and James Crafts, the reaction uses a Lewis acid catalyst like aluminum chloride to activate an alkyl halide, forming a carbocation that attacks the benzene ring's π electron cloud, creating a resonance-stabilized arenium ion before deprotonation restores aromaticity.

Aluminum chloride acts as a Lewis acid catalyst that initiates the reaction by forming a complex with the alkyl halide. This Lewis acid-base reaction generates an electrophilic carbocation or carbocation-like complex. The catalyst is regenerated at the end of the reaction cycle, allowing it to be reused. Without this catalyst, the alkyl halide cannot effectively generate the electrophile needed for aromatic substitution.

With secondary and tertiary alkyl halides, a stable free carbocation forms as the electrophile and directly attacks the aromatic ring. Primary alkyl halides generate unstable free carbocations, so instead, the complex of the primary alkyl halide with aluminum chloride serves as the electrophile. This complex is more stable and reactive enough to attack the benzene ring's π electron cloud effectively.

An arenium ion is a resonance-stabilized intermediate formed when the carbocation attacks the π electron cloud of the aromatic ring. This intermediate is crucial because its resonance stabilization makes the reaction feasible. Deprotonation of the arenium ion restores the aromatic character of the ring, yielding the final alkylated product and regenerating the Lewis acid catalyst for subsequent reactions.

The reaction begins when aluminum chloride reacts with 2-chlorobutane in a Lewis acid-base interaction, forming a carbocation. This carbocation attacks benzene's π electron cloud, generating a resonance-stabilized arenium ion. Deprotonation of the arenium ion restores aromaticity and produces 2-butylbenzene while regenerating the aluminum chloride catalyst for reuse.

Aromaticity is restored through deprotonation of the arenium ion, which removes a hydrogen from the carbon that was attacked by the carbocation. This deprotonation regenerates the stable aromatic π electron system in the benzene ring. The process converts the temporary arenium ion intermediate back into an aromatic compound, making the overall substitution thermodynamically favorable and completing the reaction cycle.

The Lewis acid-base reaction occurs when aluminum chloride, acting as a Lewis acid, accepts electron density from the alkyl halide, acting as a Lewis base. This interaction weakens the carbon-halogen bond and generates an electrophilic carbocation or carbocation-aluminum chloride complex. This electrophile is then reactive enough to attack the aromatic ring's π electrons, initiating the substitution mechanism.