UPSC Civil Services (Mains), Chemistry optional · 2022 · Paper II
Q1(a). Ionisation of 3,4-dichloro-1,2,3,4-tetramethylcyclobutene in SbF₅–SO₂
Question
Ionization of 3,4-dichloro-1,2,3,4-tetramethylcyclobutene in SbF₅–SO₂ at −75 °C produces a reaction intermediate of its own kind. Predict the intermediate and comment on its stability and aromaticity.
Answer in brief
The tetramethylcyclobutenyl dication, C₄(CH₃)₄²⁺: a cyclic 2π-electron system, aromatic by Hückel's rule (4n + 2, n = 0).
Key points
- Double ionisation: SbF₅ removes both Cl⁻ as SbF₅Cl⁻.
- Ring: planar, four sp² carbons, 2π electrons: Hückel aromatic (n = 0).
- Evidence: all four CH₃ equivalent: one ¹H NMR singlet (δ ≈ 3.7).
- Stabilised by four CH₃ (+I, hyperconjugation) and non-nucleophilic SbF₅Cl⁻.
- Limit: charge–charge repulsion; survives only in superacid at low temperature.
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.
Coverscyclobutenyl dicationHückel's rule2π aromaticsuperacid SbF₅carbocationaromaticity
All parts of Question 1
- 1(a) Ionisation of 3,4-dichloro-1,2,3,4-tetramethylcyclobutene in SbF₅–SO₂10 marks
- 1(b) (4-Nitrophenyl)oxirane with methoxide in methanol10 marks
- 1(c) The reaction that is not a condensation, and its mechanism10 marks
- 1(d) Acid-catalysed rearrangement of two phenyl-substituted 1,2-diols10 marks
- 1(e) Heating cyclopentadiene10 marks