Eas Friedel Crafts Acylation

Aromatic Reactions: Friedel–Crafts Acylation (RCOCl/(RCO)₂O, AlX₃)

Aromatic Reactions: Friedel–Crafts Acylation (RCOCl or (RCO)₂O / AlCl₃, AlBr₃)

Friedel–Crafts acylation installs an acyl group (–COR) onto benzene and substituted arenes via electrophilic aromatic substitution (EAS). A Lewis acid such as AlCl₃ (AlBr₃ behaves similarly) activates an acyl chloride (RCOCl) or acid anhydride ((RCO)₂O) to generate the acylium ion (R–C≡O⁺ ↔ R–C⁺=O). The arene attacks this electrophile, forming a σ-complex that rearomatizes after deprotonation; aqueous workup breaks the Ar–COR·AlX₃ complex to release the aryl ketone. Because –COR is deactivating and meta-directing, the reaction typically stops at mono-acylation. This reliability makes acylation followed by carbonyl reduction the go-to workaround for the rearrangement pitfalls of Friedel–Crafts alkylation.


Key Emphasis (Teaching Pivots)

  • Electrophile identity: Acylium ion (R–C≡O⁺ ↔ R–C⁺=O) generated from RCOCl or (RCO)₂O plus AlCl₃/AlBr₃. Always show this activation before the arene attack.
  • Canonical EAS pattern: π-attack → σ-complex (Wheland) → deprotonation → aromaticity restored. The product initially remains complexed to the Lewis acid until aqueous workup.
  • No rearrangements: The acylium is resonance-stabilized, so the carbon skeleton stays intact—unlike Friedel–Crafts alkylation.
  • Mono-acylation default: –COR is deactivating/meta-directing, so additional acylations are disfavored.
  • Acylation–reduction hack: Install –COR, then reduce (Clemmensen or Wolff–Kishner) to obtain un-rearranged alkylbenzenes.

Quick Summary

  • Reagents/conditions: RCOCl or (RCO)₂O with AlCl₃/AlBr₃ in dry CH₂Cl₂, CS₂, or CCl₄ (0–25 °C for activated rings; warmer for sluggish ones).
  • Electrophile: Acylium ion (R–C≡O⁺) generated by coordination and halide/acetate departure.
  • Outcome: Ar–H → Ar–COR; product carbonyl is deactivating and meta-directing, so mono-acylation dominates.
  • Selectivity: Ortho/para directors lead to o/p acylation (para favored when sterics block ortho); meta directors yield meta if the substrate is reactive enough.
  • Limitations: Strong EDG (–NH₂, –OH) must be protected (acetanilide, silyl ether); strong EWGs (–NO₂, –CF₃, –SO₃H) prevent acylation.
  • Strategic use: Combine with reduction (Clemmensen, Wolff–Kishner) to build alkylbenzenes without rearrangements.

Mechanism — Friedel–Crafts Acylation (5 Frames; arrows A–F)

Each frame below uses benzene and acetyl chloride as the reference substrate.

Step 1 acylium generation
**Step 1 — Generate acylium ion (A, B):** The acyl chloride (or anhydride) coordinates to AlCl₃/AlBr₃, then the C–Cl (or C–O) bond donates to the Lewis acid, expelling AlX₄⁻ and leaving the resonance-stabilized acylium (R–C≡O⁺).
Step 2 pi attack to sigma complex
**Step 2 — π attack → σ-complex (C, D):** Benzene donates a π pair to the acylium carbon (C), forming the arenium ion. The C=O bond shifts toward oxygen (D), underscoring resonance stabilization while the Wheland intermediate forms.
Step 3 sigma complex snapshot
**Step 3 — σ-complex snapshot:** The Wheland intermediate shows the benzylic H explicit and the positive charge delocalized across the ring while AlX₄⁻ lines up to remove the proton.
Step 4 deprotonation
**Step 4 — Deprotonation/rearomatization (E):** AlX₄⁻ (or X⁻) removes the benzylic proton. The C–H bond collapses back into the π system, restoring aromaticity while the carbonyl oxygen remains coordinated to AlX₃.
Step 5 aqueous workup
**Step 5 — Aqueous workup (F):** H₂O/H⁺ hydrolyzes the Ar–COR·AlX₃ complex, freeing the aryl ketone and regenerating the Lewis acid (plus HCl/Al(OH)₃).

Orientation is handled by the global EDG/EWG map: activators (alkyl, alkoxy, amide) → ortho/para (para favored when sterically open); deactivators (carbonyls, –NO₂, –CF₃) push toward meta. The newly installed –COR behaves as a strong meta director for any subsequent EAS.


Mechanistic Checklist (Exam Focus)

  • Always show acylium formation first; the electrophile is R–C≡O⁺, not an acyl chloride attacking directly.
  • The σ-complex formation (π attack) is rate-determining; depict the Wheland intermediate with the benzylic H explicit.
  • No carbocation rearrangements occur—acylium ions are resonance-stabilized.
  • Product –COR groups deactivate and direct meta, so a single acylation is typical.
  • Strong donors (–NH₂, –OH) bind or protonate; protect them (acetanilide, silyl ethers) or expect failure.
  • Acid anhydrides mimic acyl chlorides mechanistically while avoiding HCl generation.

Worked Examples

Benzene → acetophenone (acetyl chloride/AlCl₃)
Benzene reactant Acetyl chloride/AlCl₃ reagent button Acetophenone product

Classic FC acylation: benzene forms acetophenone cleanly; mono-acylation suffices because –COCH₃ deactivates the ring.

Toluene (o/p director) → p-propiophenone (major)
Toluene reactant Acylation reagent button Para/ortho acylation products

Toluene’s methyl group directs ortho/para; sterics make the para ketone dominant, with a smaller ortho fraction.

Nitrobenzene + acetyl chloride/AlCl₃ → no reaction
Nitrobenzene reactant Acylation reagent button No reaction callout

Strong meta directors like –NO₂ deactivate the ring; Friedel–Crafts acylation fails under standard conditions.

Acylation–reduction strategy
Acylation followed by reduction to un-rearranged alkylbenzene

Acylate first, then reduce the carbonyl (Clemmensen or Wolff–Kishner) to obtain un-rearranged alkylbenzenes—a common workaround for rearrangement-prone alkylations.


Scope & Limitations

  • Works well: Benzene, alkylbenzenes, mildly activated rings. Anhydrides can be gentler for sensitive substrates.
  • Challenging: Strong EDG (–OH, –NH₂) unless protected; strong EWGs (–NO₂, –CF₃, –SO₃H) often halt the reaction.
  • Reagent choice: Acyl chlorides are common; symmetrical anhydrides avoid HCl formation.
  • Selectivity: Para favored over ortho with bulky o/p directors. Meta outcomes occur only when set by existing meta directors.
  • Formylation caveat: HCOCl is unstable—use Gattermann–Koch or Vilsmeier–Haack for formylation.

Practical Tips

  • Keep everything dry—AlCl₃/AlBr₃ hydrolyze instantly.
  • Add the acyl chloride slowly to cooled arene + AlCl₃ to control the exotherm.
  • Expect a persistent Ar–COR·AlCl₃ complex; quench with ice then acid to release the ketone.
  • For sluggish rings, switch to acid anhydrides or increase temperature, but monitor for Lewis-acid damage to protecting groups.
  • Plan protection for amines/phenols (e.g., acetanilide) before attempting ring acylation.

Exam-Style Summary

RCOCl or (RCO)₂O with AlCl₃/AlBr₃ generates an acylium electrophile. Benzene attacks to give the σ-complex, AlX₄⁻ removes the proton to restore aromaticity, and aqueous workup liberates Ar–COR. The product carbonyl deactivates and directs meta, so mono-acylation predominates. No carbocation rearrangements occur; use acylation followed by reduction to build un-rearranged alkylbenzenes.


Interactive Toolbox

  • Mechanism Solver — Animate acylium formation → σ-complex → deprotonation → workup while toggling acyl source (RCOCl vs anhydride) and Lewis acid (AlCl₃/AlBr₃).
  • Reaction Solver — Predict ortho/meta/para outcomes for substituted arenes under Friedel–Crafts acylation and flag incompatibilities (e.g., unprotected –NH₂/–OH, strongly deactivated rings).
  • IUPAC Namer — Practice naming aryl ketone products such as acetophenone, p-propiophenone, and cyclic acylation products.