EAS-Alkylation

Alkylation (Friedel-Crafts Alkylation)

Alkyation results in the substitution of an alkyl group with an H atom.

The electrophile is a carbocation, so all learned about carbocations are important (i.e stability and rearrangement).  The carbonation (R+) is generated by the Lewis acid/base reaction of the alkyhalide (R-Cl) with AlCl3.

Here is a real example

The mechanism proceeds as follows.

Limitations

  1. Only alkyl halides undergo Friedel-Crafts alkylation, since only alkyl halides can form stable carbocations.  For a stable cation to form the halide atom must be bonded to an sp3 C atom.  

    For example, the following reaction will not proceed since chlorobenzene can not form the required carbocation.


  2. Be careful of carbocation rearrangements.

    For example if you were to attempt the following Friedel-Crafts a rearrangement would occur.

    The following carbocation rearrangment occurs, yielding a isopropyl radical since it is secondary carbocation (i.e. more stable).


  3. Addition of an alkyl group activates the ring for further alkylation.  Its difficult to achieve mono-alkylation since the mono-alkylated product is more reactive.

    Alkyl groups are considered EDG (Electron Donating Groups) and activate the ring.  In general methyl, ethyl, t-butyl and other alkyl groups will activate the ring.
  4. Some functional groups (eg. NO2, C=O) are not compatible with the AlCl3.