A chemist wants to synthesize n-propylbenzene (benzene with a straight-chain propyl group attached). Why should they use Friedel-Crafts acylation followed by reduction rather than direct Friedel-Crafts alkylation with n-propyl chloride?
ADirect alkylation would give a product with an electron-withdrawing group that deactivates the ring, stopping the reaction before completion
BDirect alkylation would produce rearrangement products (e.g., isopropylbenzene) because the n-propyl carbocation can rearrange to a more stable secondary cation; acylation gives a non-rearranging acylium ion, and the ketone can be reduced to the straight-chain product
CAcylation requires only a catalytic amount of Lewis acid while alkylation requires stoichiometric amounts, making acylation cheaper
DDirect alkylation cannot add more than one carbon to the ring, so a three-carbon chain would require three separate reactions
The n-propyl carbocation (a primary cation) readily rearranges via hydride shift to form the more stable isopropyl (secondary) carbocation, giving the unwanted branched product. The acylium ion (CH₃CH₂CO⁺) is stabilized by resonance with the oxygen and does not rearrange. The straight-chain propanoyl group is installed cleanly, and subsequent Clemmensen or Wolff-Kishner reduction gives n-propylbenzene. Note that option C is backwards — it's acylation that requires stoichiometric Lewis acid.
Question 2 Multiple Choice
Why must Friedel-Crafts acylation use a full equivalent of AlCl₃, even though Lewis acids are traditionally described as catalysts in Friedel-Crafts reactions?
AThe acylium ion is so reactive that it consumes the AlCl₃ before the ring can react, requiring excess to drive the reaction forward
BThe carbonyl oxygen of the aromatic ketone product coordinates strongly to AlCl₃, forming a stable product-Lewis acid complex that sequesters the catalyst; one full equivalent of AlCl₃ is therefore consumed per mole of product
CAlCl₃ is catalytic in acylation, but practical reactions use excess because some AlCl₃ is destroyed by moisture during workup
DMultiple equivalents of acylium ion attack the ring before the product deactivates it, requiring excess Lewis acid to generate each electrophile
The carbonyl oxygen of the ketone product is a Lewis base that coordinates strongly to AlCl₃, forming a stable [ketone–AlCl₃] complex. This complex is thermodynamically downhill and essentially irreversible under the reaction conditions, which means the Lewis acid is no longer free to catalyze further reactions. The AlCl₃ must be stoichiometric (one equivalent per product molecule) and is only released — along with the free ketone — during aqueous workup. This is a key practical difference from other Lewis acid-catalyzed reactions where the catalyst is truly regenerated.
Question 3 True / False
Friedel-Crafts acylation is self-limiting: the reaction cleanly stops after a single substitution because the acyl product deactivates the ring toward further electrophilic attack.
TTrue
FFalse
Answer: True
The acyl group (RCO–) is an electron-withdrawing group through both inductive effects (the electronegative oxygen withdraws electron density through the sigma bond) and resonance effects (the carbonyl pi system draws electrons out of the ring). This deactivation raises the energy of the arenium ion intermediate for any subsequent electrophilic attack, effectively shutting down further substitution. This is one of acylation's practical advantages over alkylation: the product is inherently resistant to overreaction, giving the mono-substituted ketone in high selectivity.
Question 4 True / False
The acylium ion (RCO⁺) is an unstable intermediate that readily undergoes rearrangement to form a more stable carbocation, just like the carbocation intermediates in Friedel-Crafts alkylation.
TTrue
FFalse
Answer: False
This is the key mechanistic distinction that makes acylation more reliable than alkylation. The acylium ion is stabilized by resonance: the positive charge is delocalized between carbon (R–C⁺=O) and oxygen (R–C≡O⁺). This resonance stabilization means the acylium ion does not need to rearrange to find a lower-energy structure — it already is lower energy. In alkylation, an unstabilized carbocation intermediate (e.g., primary → secondary rearrangement) drives unwanted skeletal rearrangement. The resonance-stabilized acylium ion delivers the expected acyl group without rearrangement, making the reaction synthetically predictable.
Question 5 Short Answer
Why does Friedel-Crafts acylation produce a single monosubstituted product reliably, while Friedel-Crafts alkylation often gives a mixture of mono- and polysubstituted products?
Think about your answer, then reveal below.
Model answer: Acylation installs an electron-withdrawing acyl group that deactivates the ring toward further electrophilic attack — once one acyl group is added, the ring is less reactive than unsubstituted benzene, so polysubstitution does not occur. Alkylation installs an electron-donating alkyl group that activates the ring toward further substitution — the monoalkyl product is more reactive than the starting benzene, making a second (and third) substitution energetically favorable. This means alkylation tends to give mixtures, while acylation cleanly stops at mono-substitution.
The contrast stems from the electronic effects of the two groups. Alkyl groups donate electrons to the ring through hyperconjugation and induction, raising the ring's nucleophilicity and accelerating further EAS. The product is therefore more reactive than the starting material, leading to overalkylation. The carbonyl group does the opposite: it withdraws electron density via resonance and induction, deactivating the ring. The product is less reactive than benzene, and the reaction stops naturally. This built-in selectivity is why acylation is so synthetically useful — it delivers a pure monosubstituted product without requiring careful control of stoichiometry.