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Friedel-Crafts Alkylation of Benzene

alkylation of arenes

Reaction type: Electrophilic Aromatic Substitution

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.
alkylation with rearrangement1,2-hydride shift during a Friedel-Crafts 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 
MECHANISM FOR THE FRIEDEL-CRAFTS ALKYLATION OF BENZENE
Step 1:
The alkyl halide reacts with the Lewis acid to form a a more electrophilic C, a carbocation
Friedel-Crafts alkylation of benzene
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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