Chapter 21 — Quiz

Twenty-five questions on electrophilic aromatic substitution. ∗ marks questions answered in the answer key.

Multiple choice

1.∗ Electrophilic aromatic substitution (EAS): (a) adds two groups across the C=C of benzene (alkene-like addition) (b) substitutes a ring H for an electrophile (preserves aromaticity) (c) destroys the aromatic ring (d) does nothing

2.∗ The intermediate in EAS is: (a) a free carbocation (off the ring) (b) an arenium ion (a cyclic σ-complex with sp³ ring C and positive charge delocalized over 3 ring atoms) (c) a free radical (d) a stable carbene

3.∗ Nitration uses: (a) HNO₃ + HBr (b) HNO₃ + H₂SO₄ (generates NO₂⁺) (c) HNO₂ (d) H₂SO₄ alone

4.∗ Friedel-Crafts alkylation problems include: (a) carbocation rearrangement and polyalkylation (b) too slow (c) doesn't work on benzene (d) only works at low temperature

5.∗ Friedel-Crafts acylation: (a) gives polyacylation (multiple groups on ring) (b) clean mono-acylation (no rearrangement; acyl group deactivates ring) (c) doesn't work on benzene (d) photochemical only

6.∗ Electrophile for halogenation of benzene: (a) X⁻ (b) X⁺ (generated from X₂ + Lewis acid like FeBr₃) (c) X• (d) HX

7.∗ Sulfonation: (a) reversible (SO₃H can be removed with hot water + acid) (b) irreversible (c) requires Pd catalyst (d) photochemical

8.∗ Friedel-Crafts alkylation of nitrobenzene: (a) gives the alkyl product (b) doesn't work (NO₂ deactivates the ring too much) (c) gives a free radical (d) gives an addition product

9.∗ Why does benzene substitute rather than add? (a) addition would destroy the aromatic stabilization (~36 kcal/mol) (b) addition is thermodynamically favored (c) random (d) kinetic preference only

10.∗ To install a linear alkyl chain on benzene: (a) FC alkylation with the alkyl halide (b) FC acylation with the acyl chloride, then reduce the C=O to CH₂ (Clemmensen or Wolff-Kishner) (c) Pd cross-coupling only (d) impossible

11.∗ The acylium ion (R-C≡O⁺) is: (a) resonance-stabilized; doesn't rearrange like alkyl carbocation (b) extremely unstable (c) only forms with FeBr₃ (d) photochemical

12.∗ Why does the alkyl group make the ring more reactive (activated) for further EAS? (a) electron-donating effect of alkyl groups stabilizes the positive arenium ion (b) random (c) it slows down EAS (d) only true for nitro

13.∗ Industrial production of cumene (isopropylbenzene): (a) benzene + propylene + acid catalyst (FC alkylation-style; gives isopropyl, not n-propyl) (b) benzene + 1-chloropropane only (c) benzene + propanal (d) random

14.∗ Cumene → phenol + acetone (the Hock process): (a) cumene oxidation gives cumene hydroperoxide; hydrolysis gives phenol + acetone (b) only with light (c) only at low T (d) not industrially used

15.∗ Vilsmeier-Haack formylation: (a) gives aldehyde (ArCHO) from ArH + DMF + POCl₃; mild conditions (b) only on phenols (c) only with Pd catalyst (d) only photochemical

16.∗ Reimer-Tiemann formylation: (a) phenol + CHCl₃ + NaOH → o-hydroxybenzaldehyde (b) only with HCl (c) only on aromatic rings without OH (d) random

17.∗ Kolbe-Schmitt reaction: (a) phenoxide + CO₂ → o-hydroxybenzoic acid (salicylic acid; precursor to aspirin) (b) only with HCl (c) doesn't work (d) random

18.∗ Industrial EAS produces: (a) dyes, drugs, polymers, detergents, pesticides, explosives — all involve EAS at some step (b) only fragrances (c) only fuels (d) only minor products

19.∗ Aromatic substitution patterns by IR (out-of-plane bending peaks 700-900 cm⁻¹): (a) characteristic for mono, ortho, meta, para (b) all the same (c) random (d) only at low T

20.∗ ¹H NMR aromatic H signals: (a) δ 0-2 ppm (b) δ 4-6 ppm (c) δ 7-8 ppm (deshielded by ring current) (d) δ 12-14 ppm

Short answer

21. Sketch the full mechanism of: benzene + Br₂/FeBr₃ → bromobenzene + HBr. Identify the arenium ion intermediate.

22. Compare the electrophiles in: (a) halogenation (b) nitration (c) sulfonation (d) Friedel-Crafts alkylation (e) Friedel-Crafts acylation

How is each generated?

23. Why does Friedel-Crafts alkylation often give carbocation rearrangement? Predict the product of: benzene + 1-bromopropane + AlCl₃.

24. Design a synthesis of n-propylbenzene from benzene. Why must you use FC acylation + reduction (not direct FC alkylation)?

25. Explain why sulfonation is reversible but nitration is essentially irreversible.

Answer key

  1. b — EAS substitutes H for E.
  2. b — Arenium ion.
  3. b — Nitration generates NO₂⁺.
  4. a — FC alkylation: rearrangement + polyalkylation.
  5. b — FC acylation: clean mono.
  6. b — X⁺ from X₂ + Lewis acid.
  7. a — Sulfonation reversible.
  8. b — FC doesn't work on deactivated rings.
  9. a — Addition destroys aromaticity.
  10. b — Acylation + reduction.
  11. a — Acylium resonance-stabilized.
  12. a — Alkyl groups donate electrons; activate ring.
  13. a — Cumene from FC alkylation with propylene.
  14. a — Hock process.
  15. a — Vilsmeier formylation.
  16. a — Reimer-Tiemann.
  17. a — Kolbe-Schmitt.
  18. a — Industrial EAS broad applications.
  19. a — Substitution patterns by IR.
  20. c — Aromatic H δ 7-8.

21. Mechanism of Br₂/FeBr₃ + benzene: - Step 1: $Br_2 + FeBr_3 \to Br^+ + FeBr_4^-$ (Lewis acid activates Br₂; generates electrophile). - Step 2 (EAS step 1, slow): Benzene's π electrons attack Br⁺. One ring C becomes sp³ with H and Br attached; positive charge delocalized over 3 other ring atoms. This is the arenium ion (or σ-complex). - Step 3 (EAS step 2, fast): $FeBr_4^-$ acts as a base, removing the H from the sp³ ring C. The C-H σ electrons return to the ring, restoring aromaticity. Products: bromobenzene + HBr + FeBr₃ (catalyst regenerated). The arenium ion has 3 resonance structures with positive charge at C2 (ortho), C4 (para), or C6 (ortho on the other side).

22. EAS electrophiles: - Halogenation: $X^+$ (from Cl₂ + FeCl₃; Br₂ + FeBr₃; etc.). Lewis acid polarizes X-X bond. - Nitration: $NO_2^+$ (nitronium ion; from $HNO_3 + H_2SO_4$). H₂SO₄ protonates HNO₃, eliminates water. - Sulfonation: $SO_3$ (or H₂SO₄ at high concentration). The S of SO₃ is the electrophile. - FC alkylation: $R^+$ (from $R-X + AlCl_3$). Lewis acid pulls Cl off R-Cl. Tertiary R+ stable; primary rearranges. - FC acylation: $R-C \equiv O^+$ (acylium ion; from $R-COCl + AlCl_3$). Resonance-stabilized; doesn't rearrange.

23. Carbocation rearrangement in FC alkylation: - Benzene + 1-bromopropane + AlCl₃ initially gives a primary carbocation $CH_3-CH_2-CH_2^+$. - Primary carbocations are unstable. The system relieves the instability by 1,2-hydride shift: the H on C2 moves to C1 with the electrons. The new cation is on C2 — secondary, more stable. - This 2° cation attacks benzene → isopropylbenzene (cumene), NOT n-propylbenzene. - So the product is the rearranged isomer; you don't get the linear chain. This rearrangement is one of the major limitations of FC alkylation.

24. Synthesis of n-propylbenzene from benzene: Wrong way: Direct FC alkylation with 1-bromopropane + AlCl₃ → mostly cumene (rearranged; see Problem 23). Right way: - Step 1: Benzene + propanoyl chloride (CH₃CH₂COCl) + AlCl₃ → propanophenone (propiophenone; PhCOCH₂CH₃). The acylium ion is resonance-stabilized; doesn't rearrange. Product is the linear ketone. - Step 2: Reduce the ketone to the methylene group. Two options: - Clemmensen: Zn(Hg) + HCl + heat → propylbenzene (PhCH₂CH₂CH₃). - Wolff-Kishner: NH₂NH₂ + KOH + heat in glycol → propylbenzene. Both give the linear alkyl chain. The acyl group + reduction strategy is the standard way to install a linear alkyl group on benzene.

25. Sulfonation vs. nitration reversibility: Sulfonation reversible: - The C-SO₃H bond is relatively weak. - The reverse reaction (Ar-SO₃H + H₂O + heat → ArH + H₂SO₄) is feasible at modest conditions. - Under hot dilute acid conditions, the equilibrium shifts back to ArH (especially if SO₃ is removed/diluted). - Used as a reversible directing/blocking group: install, do other chemistry, then remove.

Nitration irreversible: - The C-N bond of Ar-NO₂ is strong. - Once the NO₂ is on the ring, removing it would require harsh conditions (like H₂/Pd to reduce to NH₂, or other reductions). It's not a simple equilibrium. - The electron-withdrawing NO₂ group also stabilizes the C-N bond against nucleophilic attack.

The difference is thermodynamic and kinetic: sulfonation has a small free energy change in the right direction; nitration goes far downhill and stays there.