UPSC Civil Services (Mains), Chemistry optional · 2025 · Paper II
Q1(d). Spin state of a carbene; p-bromophenol with NaNH₂
Question
(i) Discuss in detail how the reaction of a carbene with cis-2-butene can be used to define the spin state (S/T) of carbene. (ii) para-Bromophenol on reaction with NaNH₂/NH₃ (l) followed by acidic workup yields one major product. Explain the reaction by writing the steps involved.
Answer in brief
Skell's rule: addition to cis-2-butene is stereospecific for a singlet carbene, non-stereospecific for a triplet; p-bromophenol gives m-aminophenol by elimination–addition through an aryne.
Key points
- Singlet: paired electrons; both σ bonds form together; cis-2-butene gives only cis-1,2-dimethylcyclopropane.
- Triplet: parallel spins; one bond first, giving a triplet 1,3-diradical; rotation beats spin inversion: cis + trans cyclopropanes.
- Acid–base: NaNH₂ removes O–H; the phenoxide reacts.
- Elimination: NH₂⁻ removes the H ortho to Br (C-3); Br⁻ leaves, giving the aryne.
- Addition: NH₂⁻ attacks C-3; the carbanion forms at C-4, far from O⁻; NH₂ ends meta to oxygen.
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.
CoversSkell's rulesinglet carbenetriplet carbenebenzynecine substitutionm-aminophenol
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