18.9
Electrophilic Aromatic Substitution: Friedel–Crafts Alkylation of Benzene
Friedel–Crafts reactions were developed in 1877 by the French chemist Charles Friedel and the American chemist James Crafts. Friedel–Crafts alkylation…
Friedel–Crafts alkylation of aromatic compounds was discovered by Charles Friedel and James Crafts.
It is a typical electrophilic aromatic substitution in which the hydrogen of an aromatic ring is replaced with an alkyl group.
For example, 2-chlorobutane reacts with benzene in the presence of aluminum chloride, a Lewis acid catalyst, to form 2-butylbenzene.
The alkylation starts with a Lewis acid–base reaction in which the alkyl halide reacts with aluminum chloride. The complex then dissociates to form a carbocation.
As an electrophile, the carbocation reacts with the π electron cloud of benzene, forming a resonance-stabilized arenium ion.
Finally, deprotonation of the arenium ion restores aromaticity, giving 2-butylbenzene and regenerating the catalyst.
Notably, with secondary and tertiary halides, the carbocation is the reacting electrophile.
With primary alkyl halides, the free primary carbocation is relatively unstable.
Therefore, a complex of an alkyl halide with aluminum chloride serves as the electrophile.
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Q1: What is Friedel-Crafts alkylation and how does it work?
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.
Q2: What role does aluminum chloride play in Friedel-Crafts alkylation?
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.
Q3: How do primary and secondary alkyl halides differ in Friedel-Crafts alkylation?
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.
Q4: What is an arenium ion and why is it important in this reaction?
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.
Q5: What is the mechanism for converting 2-chlorobutane and benzene to 2-butylbenzene?
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.
Q6: Why is aromaticity restored after the arenium ion intermediate forms?
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.
Q7: How does the Lewis acid-base reaction initiate Friedel-Crafts alkylation?
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.