What is the major problem with friedel crafts alkylation?

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 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 

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