Friedel-Crafts acylation
/ FREE-del KRAFTS; AY-sil-ay-shun /
Friedel-Crafts acylation attaches an acyl group — a carbon double-bonded to oxygen carrying an R group, R-C(=O)- — onto a benzene ring, turning benzene into an aryl ketone. Think of it as the well-behaved sibling of Friedel-Crafts alkylation: it forms the same kind of new carbon-carbon bond to the ring, but without the two headaches that plague alkylation.
The electrophile is an acylium ion, R-C(=O)(+), generated from an acyl chloride (R-COCl) or an acid anhydride with the Lewis acid AlCl3. The acylium ion is special because its positive charge is shared between carbon and oxygen by resonance, making it stable and, crucially, unable to rearrange — so the group you attach is always exactly the group you drew. The ring attacks the acylium carbon, forms the arenium ion, loses a proton, and you get an aryl ketone. There is also no over-reaction: the carbonyl group you just installed is deactivating, so it makes the ring less reactive and stops at exactly one acyl group.
Because of these two virtues — no rearrangement, no polyacylation — Friedel-Crafts acylation is the chemist's preferred route to a clean, single, straight-chain alkyl group on a ring. The strategy is a two-step combo: acylate to put on R-C(=O)-, then reduce the resulting ketone all the way down to -CH2-R (using Clemmensen or Wolff-Kishner reduction). This pair of steps gives you, for example, propylbenzene cleanly, which a direct alkylation would botch through rearrangement. One practical note: you need a bit more than one equivalent of AlCl3, because the product ketone's oxygen binds and ties up the catalyst.
Benzene + CH3CH2-COCl with AlCl3 -> phenyl ethyl ketone (propiophenone). Follow it with a Clemmensen reduction and the C=O becomes CH2, giving clean propylbenzene — the product a direct alkylation could not deliver.
Acylate then reduce — the reliable way to a single, unrearranged alkyl chain.
The acylium ion cannot rearrange and the product is deactivated against further reaction, so acylation avoids both of alkylation's flaws. But like alkylation it still fails on strongly deactivated rings (e.g. nitrobenzene), which are too electron-poor to attack the electrophile.