UPSC Civil Services (Mains), Chemistry optional · 2025 · Paper II
Q1(a). Tropolone, fulvene and pseudo-aromaticity
Question
(i) Tropolone is aromatic, but fulvene is non-aromatic. Why? (ii) Explain with example pseudo-aromaticity.
Answer in brief
Tropolone is aromatic because its dipolar form is a 6π hydroxytropylium ring; fulvene is cross-conjugated and non-aromatic; pseudo-aromatic rings are cyclic and conjugated yet lack aromatic stabilisation.
Key points
- Tropolone: C=O polarises to C⁺–O⁻; the ring becomes a 6π tropylium cation.
- Evidence: μ ≈ 3.5 D; substitution (bromination, nitration, azo coupling), not addition.
- Fulvene: cross-conjugated; its dipolar form needs an unstabilised 1° carbocation; μ ≈ 0.4 D.
- Pseudo-aromatic: cyclic, fully conjugated and with Kekulé structures, yet no aromatic stabilisation (4n π electrons).
- Examples: pentalene (8π) dimerises above −100 °C; heptalene (12π) non-planar; cyclooctatetraene (8π) tub-shaped.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coverstropolonefulvenepseudo-aromaticityHückel rulepentalenecyclooctatetraene
All parts of Question 1
- 1(a) Tropolone, fulvene and pseudo-aromaticity10 marks
- 1(b) Missing reagent and intermediates10 marks
- 1(c) Neomenthyl chloride with sodium ethoxide10 marks
- 1(d) Spin state of a carbene; p-bromophenol with NaNH₂10 marks
- 1(e) Permanent and temporary denaturation of a protein10 marks