UPSC Civil Services (Mains), Chemistry optional · 2024 · Paper II
Q1(a)(i). Aromatic, non-aromatic or antiaromatic
Question
Classify the following as aromatic, non-aromatic or antiaromatic: (1) azulene, (2) pyridine, (3) sydnone, (4) cyclooctatetraene, (5) cyclopentadienyl cation.
Answer in brief
Azulene, pyridine and sydnone are aromatic; cyclooctatetraene is non-aromatic; the cyclopentadienyl cation is antiaromatic.
Key points
- Hückel rule: planar, cyclic, conjugated ring with (4n + 2) π electrons.
- Azulene, sydnone: dipolar forms give 6π rings (μ of azulene ≈ 1.0 D).
- Cyclooctatetraene: tub shape avoids 8π antiaromaticity; behaves as a polyene.
- C₅H₅⁺: planar 4π ring, triplet ground state, very unstable.
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.
CoversHückel rulearomaticityantiaromaticazulenesydnonecyclooctatetraene
All parts of Question 1
- 1(a)(i) Aromatic, non-aromatic or antiaromatic5 marks
- 1(a)(ii) Diphenylcyclopropenone resists nucleophilic addition5 marks
- 1(b) Intermediate in the amination of bromobenzene10 marks
- 1(c) (S)-2-Butanol to (S)-2-butyl chloride10 marks
- 1(d) Name and reagent for five conversions10 marks
- 1(e) Electrocyclic steps of cyclononatriene10 marks