Questions: Friedel-Crafts Alkylation and Limitations

Question 1

A chemist attempts to synthesize n-propylbenzene by reacting benzene with 1-chloropropane and AlCl₃. What is the major product actually formed?

• A. n-propylbenzene — the intended straight-chain product
• B. isopropylbenzene — the rearranged product via a more stable secondary carbocation
• C. allylbenzene — formed by elimination before ring attack
• D. No reaction — primary alkyl halides cannot form carbocations with AlCl₃

Answer: (B)

The AlCl₃ generates an incipient primary carbocation from 1-chloropropane, which rapidly rearranges via a 1,2-hydride shift to the more stable secondary carbocation. That secondary carbocation is the actual electrophile that attacks benzene, giving isopropylbenzene. Primary carbocations are too unstable to persist, so rearrangement is essentially unavoidable with primary alkyl halides.

Question 2

Why does polyalkylation occur in Friedel-Crafts alkylation, even when benzene is the sole starting material?

• A. Each alkyl group added makes the AlCl₃ catalyst progressively more active
• B. The installed alkyl group donates electron density to the ring, making it more nucleophilic than unreacted benzene
• C. The carbocation electrophile preferentially attacks the more substituted product
• D. Polyalkylation is a side reaction caused by moisture contaminating the AlCl₃

Answer: (B)

Alkyl groups are electron-donating via hyperconjugation and inductive effects, activating the ring toward electrophilic substitution. A monoalkylated product is therefore a better nucleophile than benzene itself — it reacts faster with the carbocation electrophile. The practical fix is using a large excess of benzene so most carbocations statistically encounter unreacted benzene rather than the already-alkylated product.

Question 3

Friedel-Crafts alkylation can be performed successfully on nitrobenzene if excess AlCl₃ is used to overcome the deactivating effect of the nitro group.

• T. True
• F. False

Answer: False

No amount of excess AlCl₃ rescues Friedel-Crafts alkylation on strongly deactivated rings. A nitro group withdraws electron density so aggressively that the ring is too electron-poor to attack the carbocation electrophile — the reaction simply fails. This is a fundamental mechanistic limitation, not a kinetic problem that can be overcome by increasing catalyst loading.

Question 4

Polyalkylation in Friedel-Crafts reactions occurs because each successive alkyl substitution deactivates the ring, so subsequent reactions occur at a different position on the ring rather than attacking the same molecule again.

• T. True
• F. False

Answer: False

This inverts the correct logic. Each alkyl group activates the ring, making the monosubstituted product MORE reactive than benzene — not less. Polyalkylation is not about regioselectivity on one ring; it is about the already-alkylated molecule being a faster-reacting species than the starting benzene in solution.

Question 5

Why must Friedel-Crafts acylation (followed by reduction) be used instead of direct alkylation when a straight-chain alkyl group is needed on an aromatic ring?

Model answer: Friedel-Crafts alkylation generates a carbocation intermediate that rearranges: primary carbocations shift to secondary or tertiary before attacking the ring, yielding branched products. Acylation produces a resonance-stabilized acylium ion that cannot rearrange because any shift would generate a less stable species. The resulting ketone retains the correct straight-chain carbon skeleton, which is then reduced (Clemmensen or Wolff-Kishner) to the desired alkyl group.

The deeper point is that this limitation is mechanistic, not a matter of conditions. Because the electrophile in alkylation is a carbocation, rearrangement is inherent whenever a primary (or certain secondary) alkyl halide is used. Acylation sidesteps this by using a fundamentally different electrophile that preserves the carbon skeleton.