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UPSC Combined Geo-Scientist

Chemist

3 descriptive papers
200 marks each
3 h per paper

Mains Paper III covers analytical and organic chemistry.

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

Q6(i). Stretching frequencies of six single bonds

2 marks
Topic: Spectroscopic methods of analysis
Answered

Question

Arrange, giving reasons, the following bonds in order of their decreasing vibrational frequencies: C–Cl, C–Br, C–C, C–O, C–H, C–I

Answer in brief

C–H > C–C > C–O > C–Cl > C–Br > C–I (highest to lowest stretching frequency).

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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.

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Read the whole 2021 solution 2021 question paper

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

CoversIR stretching frequencyHooke's lawreduced massforce constant

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