UPSC Combined Geo-Scientist, Mains · 2021 · Mains Paper III

Q6(e). Epoxidation of cholesterol with a peroxy acid

4 marks
Topic: Addition reactions
Answered

Question

Predict the product in the following reaction. Provide a suitable justification to your answer. Cholesterol, m-chloroperbenzoic acid, Δ → ? [structure of cholesterol given in the paper]

Answer in brief

The C5=C6 bond is epoxidised from the less hindered α-face: 5α,6α-epoxycholestan-3β-ol (cholesterol α-epoxide) is the major product.

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Topic notes: reagents for oxidation, reduction and hydroboration, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversepoxidationm-CPBAperoxy acidcholesterolα-face attack

All parts of Question 6

6(a) Reactivity of four alkenes towards electrophilic addition4 marks
6(b) Quaternisation, then Sommelet–Hauser rearrangement4 marks
6(c) Woodward–Fieser rules: three dienones4 marks
6(d) Carbonyl stretching bands of three lactones4 marks
6(e) Epoxidation of cholesterol with a peroxy acid4 marks
6(f) Norrish Type II reaction of two aryl alkyl ketones6 marks
6(g) Three isomeric esters C₄H₈O₂ from ¹H NMR6 marks
6(h) McLafferty rearrangement in a ketone, an acid and an ester6 marks
6(i) Stretching frequencies of six single bonds2 marks