Friedel-Crafts alkylation
/ FREE-del KRAFTS /
All the EAS reactions so far put a heteroatom group onto the ring. Friedel-Crafts alkylation does something different and very valuable: it forms a new carbon-carbon bond, attaching an alkyl group (like -CH3 or -CH2CH3) directly to a benzene ring. It is the classic way to build the carbon framework of a substituted aromatic from benzene.
The electrophile is a carbocation. You take an alkyl halide (R-X) and a Lewis acid catalyst, almost always AlCl3. The aluminum grabs the halide and pulls it off the carbon, generating a carbocation R(+) (or a strongly polarized complex acting like one). The ring's pi cloud attacks that cation, forms the arenium ion, loses a proton, and the alkyl group is now bonded to the ring. Because the active species is a carbocation, this reaction carries two famous hazards: the cation can rearrange to a more stable structure before the ring attacks it (so 1-chloropropane gives mostly isopropylbenzene, not propylbenzene), and the product is more reactive than the starting material, so it tends to get alkylated again — you often get multiple alkyl groups instead of just one.
Those two problems — rearrangement and polyalkylation — are exactly why chemists reach for Friedel-Crafts acylation instead whenever they want a single, unrearranged straight chain (acylate, then reduce). One more limit: Friedel-Crafts alkylation fails on rings that are strongly deactivated (for instance a ring already carrying -NO2), because such a ring is too electron-poor to attack the carbocation, and it also fails if a basic group like -NH2 is present, since that group ties up the Lewis acid.
Benzene + (CH3)3C-Cl with AlCl3 -> tert-butylbenzene. Here the tert-butyl cation is already stable, so there is no rearrangement; but benzene + CH3CH2CH2-Cl gives mostly isopropylbenzene, because the propyl cation rearranges to the more stable secondary cation first.
Watch for carbocation rearrangement — the chain you draw is often not the chain you get.
Two pitfalls make this reaction unreliable for clean products: the carbocation can rearrange to a more stable isomer, and the alkylated product is activated so it reacts again to give over-alkylation. When you need an unrearranged straight-chain group, do a Friedel-Crafts acylation and then reduce the ketone instead.