UPSC Civil Services (Mains), Chemistry optional · 2022 · Paper II
Q1(b). (4-Nitrophenyl)oxirane with methoxide in methanol
Question
Predict the structures of X and Y, and also mention the major product: (4-nitrophenyl)oxirane treated with MeO⁻ in MeOH gives X + Y. [structure of the chiral (4-nitrophenyl)oxirane given in the paper]
Answer in brief
X (major) = (R)-2-methoxy-1-(4-nitrophenyl)ethanol, from attack at the CH₂ carbon; Y (minor) = (S)-2-methoxy-2-(4-nitrophenyl)ethanol, from attack at the benzylic carbon with inversion.
Key points
- Basic medium: epoxide oxygen not protonated; strong nucleophile, pure SN2.
- Steric control: MeO⁻ prefers the unhindered CH₂ carbon: X major.
- p-NO₂ (−M) removes benzylic cation character: benzylic attack stays minor.
- X keeps the C–O bond at the stereocentre: configuration retained.
- Y forms by back-side attack at the stereocentre: Walden inversion.
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.
Coversepoxide ring openingSN2regioselectivityWalden inversionmethoxidestyrene oxide
All parts of Question 1
- 1(a) Ionisation of 3,4-dichloro-1,2,3,4-tetramethylcyclobutene in SbF₅–SO₂10 marks
- 1(b) (4-Nitrophenyl)oxirane with methoxide in methanol10 marks
- 1(c) The reaction that is not a condensation, and its mechanism10 marks
- 1(d) Acid-catalysed rearrangement of two phenyl-substituted 1,2-diols10 marks
- 1(e) Heating cyclopentadiene10 marks