Friedel–Crafts Alkylation and Acylation

The flagship Friedel–Crafts acylation: benzene + acetyl chloride, catalyzed by AlCl₃, gives acetophenone — one clean product.

1. Both reactions attach a carbon group to benzene using AlCl₃

Friedel–Crafts reactions are two members of the same family: they are electrophilic aromatic substitutions (EAS) that replace a ring hydrogen with a carbon group, forging a new C–C bond. Benzene is a poor nucleophile, so it will not attack an ordinary alkyl or acyl chloride. The job of the Lewis acid catalyst — almost always aluminum chloride, AlCl₃ — is to pull chloride off the reagent and unmask a far more aggressive electrophile that even benzene’s lazy pi cloud can attack.

The difference between the two reactions is only what you feed in. Give the ring an alkyl halide and you run alkylation, planting an alkyl group. Give it an acyl halide (RCOCl) and you run acylation, planting a ketone. Same catalyst, same EAS mechanism at the ring, very different electrophiles — and, as we will see, very different reliability.

2. Alkylation generates a carbocation electrophile

In alkylation, AlCl₃ coordinates to the chlorine of an alkyl chloride and yanks it away as AlCl₄⁻, leaving behind a carbocation (or a strongly polarized complex that behaves like one). That cation is the electrophile: benzene’s pi system attacks it, giving an arenium (sigma-complex) intermediate, and loss of a proton restores aromaticity. The net result is an alkylbenzene.

For example, methyl chloride plus AlCl₃ methylates benzene to toluene. This is genuinely useful — it is one of the few ways to make a fresh C–C bond directly onto an aromatic ring. But the moment the carbocation forms, all of the baggage of carbocation chemistry comes with it, and that baggage is exactly why alkylation is the less trustworthy of the two reactions.

Alkylation: AlCl₃ strips chloride from CH₃Cl to give a methyl cation, which the ring attacks to form toluene.

3. Alkylation has three problems you must plan around

The carbocation intermediate causes three predictable failures. Problem 1 — rearrangement. Carbocations slide toward greater stability by 1,2-hydride and methyl shifts. Feed benzene n-propyl chloride and you expect propylbenzene; instead the primary cation rearranges to the more stable secondary cation, and the major product is isopropylbenzene (cumene). You asked for a straight chain and got a branched one.

Rearrangement in action: n-propyl chloride gives mostly the branched isopropyl product, not the straight-chain one.

Problem 2 — over-alkylation. The alkyl group you add is electron-donating, so the product ring is more reactive than the benzene you started with. It out-competes the starting material and reacts again, giving di-, tri-, and poly-alkylated messes. Problem 3 — dead rings. Strongly deactivated rings simply refuse to react. A ring bearing a nitro group, for instance, is so electron-poor that Friedel–Crafts alkylation fails outright, so you cannot alkylate nitrobenzene.

4. Acylation generates a resonance-stabilized acylium ion

Acylation solves these problems because it routes through a different electrophile. When AlCl₃ removes chloride from an acyl chloride (RCOCl), the cation that forms is an acylium ion, R–C≡O⁺. Unlike a bare carbocation, the acylium ion is resonance-stabilized: the positive charge is shared between carbon and oxygen, with the oxygen lone pair forming a full triple-bond resonance form. That extra stability is the whole story.

The ring attacks the acylium carbon, and after loss of a proton you get an aryl ketone. Propionyl chloride, for example, acylates benzene to propiophenone. The mechanism at the ring is identical to alkylation — it is still EAS — but the electrophile that arrives is calm and well-behaved.

Acylation: the resonance-stabilized acylium ion from propionyl chloride adds cleanly to give propiophenone.

5. Acylation is clean — no rearrangement, no over-reaction

Because the acylium ion is already stabilized by resonance, it has no reason to rearrange — there is no more stable cation to shift toward. Whatever carbon skeleton you put in the acyl chloride is the carbon skeleton you get on the ring. That directly cures Problem 1.

Acylation also cures Problem 2. The product is a ketone, and the carbonyl group is electron-withdrawing, so the acylated ring is less reactive than the starting benzene. Having reacted once, the ring is now deactivated and will not react a second time under the same conditions — so you cleanly stop at the mono-ketone instead of over-reacting. (One practical note: because the ketone product ties up a full equivalent of AlCl₃ as a complex, acylation needs a bit more than one equivalent of the Lewis acid.)

6. Acylate then reduce to install a straight chain cleanly

Here is the payoff that ties everything together. Suppose you actually want a straight-chain alkyl group — say, an n-propyl group — on benzene. Direct alkylation will betray you with rearrangement (Section 3). So take the two-step detour: acylate first, then reduce the ketone to a CH₂.

Acylation with propionyl chloride gives propiophenone with no rearrangement. Then a carbonyl-to-methylene reduction — Clemmensen (Zn(Hg), HCl) or Wolff–Kishner (N₂H₄, KOH) — converts the C=O into a CH₂, delivering the straight-chain propylbenzene you originally wanted. You got the linear chain that alkylation could never give you, by borrowing acylation’s good behavior.

The workaround: acylate to propiophenone, then Clemmensen-reduce the ketone to reach straight-chain propylbenzene without rearrangement.

7. Summary

Friedel–Crafts alkylation and acylation both use AlCl₃ to generate a carbon electrophile that a benzene ring attacks in an EAS mechanism. Alkylation makes a carbocation and inherits three problems: it rearranges (n-propyl becomes isopropyl), over-alkylates (the product is more reactive than benzene), and fails on deactivated rings like nitrobenzene. Acylation makes a resonance-stabilized acylium ion that does not rearrange and gives a deactivating ketone, so it stops cleanly at one substitution. The professional move for a straight-chain group is to acylate, then reduce the ketone with Clemmensen or Wolff–Kishner — sidestepping alkylation’s rearrangement entirely.