Ch12: Friedel-Crafts alkylation

Chapter 12 : Reactions of Arenes. Electrophilic Aromatic Substitution

Friedel-Crafts Alkylation of Benzene

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.

1,2-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 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

Step 1:
The alkyl halide reacts with the Lewis acid to form a a more electrophilic C, a carbocation

Step 2:
Loss of the halide to the Lewis acid forms the electrophilic alkyl carbocation.

Step 3:
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 4:
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.