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

Q6(d). Carbonyl stretching bands of three lactones

4 marks
Topic: Spectroscopic methods of analysis
Answered

Question

Arrange the following compounds in descending order of the carbonyl stretching absorptions. [structures of three lactones I, II and III given in the paper]

Answer in brief

I > III > II: about 1800 cm⁻¹ > 1770 cm⁻¹ > 1735 cm⁻¹.

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Topic notes: IR spectroscopy: group frequencies, carbonyl stretching and hydrogen bonding, with every Civil Services, IFoS and Geo-Scientist PYQ →

CoversIR spectroscopylactone carbonyl stretchring strainγ-butyrolactoneδ-valerolactone

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