UPSC Civil Services (Mains), Chemistry optional · 2024 · Paper II
Q1(a)(ii). Diphenylcyclopropenone resists nucleophilic addition
Question
Though the following compound contains a keto group, it does not undergo nucleophilic addition reactions. Explain. [structure of diphenylcyclopropenone given in the paper]
Answer in brief
The C=O is polarised into an aromatic 2π cyclopropenyl cation; addition would destroy this aromaticity.
Key points
- Dipolar form C⁺–O⁻ dominates: ring is a cyclopropenyl cation.
- Charge spread over three ring carbons: carbonyl carbon is a poor electrophile.
- Addition makes that carbon sp³: aromaticity lost, so no addition.
- Evidence: μ ≈ 5.1 D; slow ¹⁸O exchange.
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.
Coversdiphenylcyclopropenonecyclopropenyl cationaromaticitynucleophilic additionHückel rule
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