Friedel-Crafts Alkylation of Benzene

Summary.

• Overall transformation :  Ar-H to Ar-R
• Named after Friedel and Crafts who discovered the reaction in 1877.
• Reagent : normally the alkyl halide (e.g. R-Br or R-Cl) with aluminum trichloride, AlCl₃, a Lewis acid catalyst
• The AlCl₃ enhances the electrophilicity of the alkyl halide by complexing with the halide
• Electrophilic species : the carbocation (i.e. R ⁺) formed by the "removal" of the halide by the Lewis acid catalyst
• The reactive electrophile, the carbocation is prone to rearrangement to a more stable carbocation which will then undergo the alkylation reaction.

• Friedel-Crafts reactions are limited to arenes as or more reactive than mono-halobenzenes
• Other Lewis acids such as BF₃, FeCl₃ or ZnCl₂ can also be used
• Other sources of carbocations can also be used:
• from loss of water from alcohols treated with acid
• from the protonation of alkenes by acid
• Alkylation products can also be obtained by the reduction of Friedel-Crafts acylation products (more details)

MECHANISM FOR THE FRIEDEL-CRAFTS ALKYLATION OF BENZENE
Step 1: The alkyl halide reacts with the Lewis acid to form a more electrophilic C, a carbocation
Step 2: The p electrons of the aromatic C=C act as a nucleophile, attacking the electrophilic C+. This step destroys the aromaticity giving the cyclohexadienyl cation intermediate.
Step 3: Removal of the proton from the sp3 C bearing the alkyl- group reforms the C=C and the aromatic system, generating HCl and regenerating the active catalyst.

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