Q2(d). Skraup synthesis; Wolff–Kishner reduction
Identify (X) in the reactions given below:
(i) C₆H₅NH₂ + C₆H₅NO₂ + (X) → (H₂SO₄/FeSO₄, Δ) Quinoline
(ii) [structure] → (NH₂NH₂, OH⁻) (X)
Answer in brief
(i) X = glycerol (Skraup synthesis; acrolein is the real reactant). (ii) X = 1,1-dimethylcyclohexane (Wolff–Kishner reduction).
Key points
- Hot conc. H₂SO₄ dehydrates glycerol to acrolein (propenal).
- Aniline adds to acrolein (conjugate (Michael) addition); the aldehyde then attacks the benzene ring ortho to N (acid-catalysed cyclisation) and loses water to give 1,2-dihydroquinoline.
- Nitrobenzene oxidises the dihydroquinoline to quinoline (it is reduced to aniline, which re-enters the reaction). FeSO₄ moderates the otherwise violent, exothermic reaction.
- Hydrazine forms the hydrazone R₂C=N-NH₂. Hydroxide removes an N–H proton; the anion is protonated on carbon, a second deprotonation gives a diazenyl anion, which loses N₂ (driving force) to give a carbanion that takes a proton…
The full model answer
Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.
(i) Skraup synthesis: X = glycerol
Scheme: Glycerol supplies C-2, C-3 and C-4 of the pyridine ring.
- Hot conc. H₂SO₄ dehydrates glycerol to acrolein (propenal).
- Aniline adds to acrolein (conjugate (Michael) addition); the aldehyde then attacks the benzene ring ortho to N (acid-catalysed cyclisation) and loses water to give 1,2-dihydroquinoline.
- Nitrobenzene oxidises the dihydroquinoline to quinoline (it is reduced to aniline, which re-enters the reaction). FeSO₄ moderates the otherwise violent, exothermic reaction.
(ii) Wolff–Kishner reduction: X = 1,1-dimethylcyclohexane
Scheme: C=O becomes CH₂.
- Hydrazine forms the hydrazone R₂C=N-NH₂. Hydroxide removes an N–H proton; the anion is protonated on carbon, a second deprotonation gives a diazenyl anion, which loses N₂ (driving force) to give a carbanion that takes a proton from the solvent.
- Net change C=O → CH₂ under basic conditions: X = 1,1-dimethylcyclohexane (C₈H₁₆).
Conclusion. (i) X = glycerol (Skraup synthesis; acrolein is the real reactant). (ii) X = 1,1-dimethylcyclohexane (Wolff–Kishner reduction).
Application. Skraup and related quinoline syntheses give the ring of antimalarials (chloroquine, quinine analogues); Wolff–Kishner reduction (Huang-Minlon version) deoxygenates ketones that carry acid-sensitive groups.
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All parts of Question 2
- 2(a) Sulfonation of naphthalene: kinetic and thermodynamic control15 marks
- 2(b) Benzyne from anthranilic acid; trapping with furan and benzoic acid15 marks
- 2(c) Singlet and triplet carbenes with cis-pent-2-ene10 marks
- 2(d) Skraup synthesis; Wolff–Kishner reduction10 marks