Questions: Friedel-Crafts Acylation and Aromatic Ketones

Question 1

A chemist needs to attach a straight-chain propyl group to benzene without any rearrangement. Which approach reliably achieves this?

• A. React benzene with 1-chloropropane and AlCl₃ (direct Friedel-Crafts alkylation)
• B. React benzene with propanoyl chloride and AlCl₃, then reduce the ketone to a methylene group
• C. React benzene with 2-chloropropane and AlCl₃ to get the secondary carbocation, then quench
• D. React benzene with propionic acid and AlCl₃ directly

Answer: (B)

Direct alkylation with 1-chloropropane would generate a primary carbocation that readily rearranges to a secondary carbocation, yielding isopropylbenzene instead of propylbenzene. Acylation with propanoyl chloride generates a resonance-stabilized acylium ion (no rearrangement possible), installs the correct three-carbon skeleton as a ketone, and the ketone is then cleanly reduced (Clemmensen or Wolff-Kishner) to the desired propyl group. Options C and D are either rearrangement-prone or chemically incorrect — carboxylic acids do not react under standard Friedel-Crafts conditions.

Question 2

Why does Friedel-Crafts acylation stop after one acyl group is installed, whereas Friedel-Crafts alkylation often gives polysubstituted products?

• A. The acylium ion is too bulky to attack a substituted ring a second time
• B. AlCl₃ is consumed in the first reaction and unavailable for a second substitution
• C. The ketone product is electron-withdrawing and deactivates the ring toward further electrophilic attack
• D. The acylation product is insoluble and precipitates, removing it from reaction

Answer: (C)

The ketone product contains a carbonyl group directly attached to the ring — a strong electron-withdrawing group that pulls electron density away from the aromatic π system. This deactivation makes the ring far less reactive toward electrophilic aromatic substitution, so a second acylation does not occur under normal conditions. By contrast, an alkyl group is electron-donating, activating the ring and making the first substitution product even more reactive than benzene, leading to polysubstitution. Option B is partially true (AlCl₃ does complex with the product), but this is not the correct reason for selectivity — the electronic deactivation is.

Question 3

The acylium ion (RCO⁺) undergoes rearrangement to a more stable carbocation before attacking the aromatic ring.

• T. True
• F. False

Answer: False

This is false. The acylium ion is resonance-stabilized: the positive charge is delocalized between carbon and oxygen (R–C≡O⁺ ↔ R–C=O⁺), making it already stabilized without needing to rearrange. Simple carbocations (primary, secondary, tertiary) can rearrange via hydride or methyl shifts to reach a lower-energy structure, but the acylium ion's resonance stabilization removes the thermodynamic incentive to rearrange. This is precisely why Friedel-Crafts acylation gives predictable carbon skeletons while alkylation often does not.

Question 4

Friedel-Crafts acylation requires a full stoichiometric equivalent of AlCl₃, not merely a catalytic amount.

• T. True
• F. False

Answer: True

True. Unlike in some Lewis acid-catalyzed reactions, the AlCl₃ is not truly regenerated in Friedel-Crafts acylation. After the reaction, AlCl₃ forms a stable 1:1 complex with the ketone product (via the lone pair on the carbonyl oxygen). This complex must be destroyed in the aqueous workup to liberate the ketone and AlCl₃. Because one equivalent of AlCl₃ is sequestered per equivalent of product, a full stoichiometric amount is required — making the reaction more wasteful and costly than a catalytic process.

Question 5

Explain why the acylium ion does not undergo carbocation rearrangement, whereas a simple primary carbocation does.

Model answer: The acylium ion is resonance-stabilized: the positive charge is shared between carbon and oxygen through the π bond of the carbonyl (R–C≡O⁺ ↔ R–C=O). This delocalization makes the acylium ion thermodynamically stable without rearranging. A simple primary carbocation has no such stabilization — it is a localized, high-energy species with a strong thermodynamic driving force to rearrange (via hydride or methyl migration) to reach a lower-energy secondary or tertiary carbocation. The acylium ion's resonance removes that driving force entirely.

The mechanism of carbocation rearrangement is driven by enthalpy: a primary carbocation rearranges because the product secondary or tertiary carbocation is significantly more stable. The acylium ion avoids this by achieving stability through a different mechanism — resonance delocalization of the positive charge onto electronegative oxygen. With no lower-energy structure available via rearrangement, the acylium ion attacks the aromatic ring with its original carbon skeleton intact, guaranteeing the expected product.