UPSC Civil Services (Mains), Chemistry optional · 2025 · Paper II
Q1(b). Missing reagent and intermediates
Question
Identify the missing reagent and intermediates in the following chemical conversion: cyclohexyl methyl ketone → (PCl₅, 0 °C, −POCl₃) M → (1) 3 equiv. NaNH₂, heat; 2) HA) N → (NaNH₂, then C₂H₅Br) O → (P) (E)-1-cyclohexylbut-1-ene. [reaction scheme given in the paper]
Answer in brief
M = (1,1-dichloroethyl)cyclohexane; N = ethynylcyclohexane; O = 1-cyclohexylbut-1-yne; P = Na / liq. NH₃, −33 °C.
Key points
- M: PCl₅ converts C=O into a gem-dichloride; POCl₃ is the by-product.
- N: two E2 losses of HCl; the third NaNH₂ gives the acetylide; HA protonates it.
- O: acetylide (pKa of ≡C–H ≈ 25) attacks C₂H₅Br by SN2.
- P works through radical anion → trans vinyl radical → vinyl anion → (E)-alkene.
- Lindlar H₂ would add syn and give the (Z)-alkene, so P must be a dissolving-metal reduction.
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.
Coversgem-dichloridePCl5double dehydrohalogenationacetylide alkylationNa/liq. NH3 reductiontrans-alkene
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