UPSC Civil Services (Mains), Chemistry optional · 2025 · Paper II
Q1(c). Neomenthyl chloride with sodium ethoxide
Question
Write the structure of the major product when neomenthyl chloride is reacted with sodium ethoxide in ethanol. Justify your answer. [structure of neomenthyl chloride given in the paper]
Answer in brief
Major product: 3-menthene (≈ 75 %); minor product: 2-menthene (≈ 25 %), by anti-periplanar E2.
Key points
- E2: H and Cl must be anti-periplanar (trans-diaxial on a chair).
- Stable chair: isopropyl and CH₃ equatorial, Cl axial.
- Axial β-H anti to Cl: present on both C-2 (bearing i-Pr) and C-6.
- Saytzeff rule: loss of the C-2 H gives trisubstituted 3-menthene, the major product.
- Menthyl chloride must first flip to the all-axial chair; it reacts ≈ 200 times slower and gives only 2-menthene.
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.
Coversneomenthyl chloridementhyl chlorideE2 eliminationanti-periplanarSaytzeff rulementhene
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