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

Q6(c). Woodward–Fieser rules: three dienones

4 marks
Topic: Spectroscopic methods of analysis
Answered

Question

Arrange the following compounds in order of increasing λmax. Provide λmax values also. [structures of three dienones I, II and III given in the paper]

Answer in brief

I (281 nm) < II (309 nm) < III (311 nm): calculated λmax of the π → π* band.

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Topic notes: UV spectroscopy and Woodward–Fieser rules, with every Civil Services, IFoS and Geo-Scientist PYQ →

CoversWoodward–Fieser rulesλmaxUV spectroscopyconjugated dienone

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