Chapter 12 :
Reactions of Arenes. Electrophilic Aromatic Substitution
Friedel-Crafts Alkylation of Benzene
Reaction type: Electrophilic Aromatic Substitution
- 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
- 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 such as H2SO4
- from the protonation of alkenes by acid such as H2SO4
- Alkylation products can also be obtained by the reduction of Friedel-Crafts
acylation products (more details)
|Summary of Limitations of Friedel-Crafts alkylations:
- The halide must be either an alkyl halide.
Vinyl or aryl halides do not react (their intermediate carbocations
are too unstable).
- Alkylation reactions are prone to carbocation rearrangements.
- Deactivated benzenes are not reactive to Friedel-Crafts conditions, the
benzene needs to be as or more reactive than a mono-halobenzene (see substituent effects)
- Over alkylation can be a problem since the product is more reactive than
the starting material. This can usually be controlled with an excess of the benzene.
- The Lewis acid catalyst AlCl3 often complexes to aryl amines
making them very unreactive.
MECHANISM FOR THE FRIEDEL-CRAFTS ALKYLATION OF BENZENE
The alkyl halide reacts with the Lewis acid to form a a more electrophilic
C, a carbocation
Loss of the halide to the Lewis acid forms the electrophilic alkyl carbocation.
The p electrons of the aromatic C=C act as a nucleophile, attacking
the electrophilic C+. This step destroys the aromaticity giving the cyclohexadienyl
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.