Q3(b). Cyclopropenyl cation; acidity of indene and cyclopentadiene; tropolones and sydnones
(i) Write the structure of the reaction product between 3-chlorocyclopropene and SbCl₅. What is the unique feature of this product in its ¹H NMR spectrum? (ii) Which one of compounds A (indene) and B (cyclopentadiene) is more acidic and why? [structures given in the paper] (iii) Predict the aromaticity of tropolones and sydnones.
Answer in brief
(i) C₃H₃⁺ SbCl₆⁻, a 2π aromatic cation giving one singlet at δ ≈ 11.1; (ii) cyclopentadiene (pKa 18.0 vs 20.1) is more acidic; (iii) both are aromatic 6π systems.
Part of VKC Basic
Read the key points and the full model answer with VKC Basic
VKC Basic (₹999 for 6 months) opens every solved answer, the notes, the handbook and the daily answer archive. Every Mains test series includes it.
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.
Coverscyclopropenyl cationHückel aromaticitycyclopentadiene acidityindenetropolonesydnone mesoionic
All parts of Question 3
- 3(a)(i) 1-(Hydroxydiphenylmethyl)cyclopentanol with conc. H₂SO₄10 marks
- 3(a)(ii) 1-Acetyl-1-chloro-4-methylcyclohexane with ethoxide10 marks
- 3(b) Cyclopropenyl cation; acidity of indene and cyclopentadiene; tropolones and sydnones15 marks
- 3(c)(i) meso-2,3-Dibromobutane with iodide and with ethoxide5 marks
- 3(c)(ii) 3-Iodo-2,2-dimethylbutane with silver nitrate in hot ethanol10 marks