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.
- 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.
- 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).
- 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.
- Some functional groups (eg. NO2, C=O) are not compatible with the AlCl3.