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

Q6(h). McLafferty rearrangement in a ketone, an acid and an ester

6 marks
Topic: Spectroscopic methods of analysis
Answered

Question

Explain the peculiar fragmentation pattern(s) observed in the mass spectrum of the following: (i) 2-heptanone; (ii) hexanoic acid; (iii) methyl hexanoate.

Answer in brief

Each has a C=O and γ-hydrogens, so each shows the McLafferty ion at an even mass by loss of but-1-ene (56): m/z 58 (ketone), m/z 60 (acid), m/z 74 (methyl ester).

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Topic notes: mass spectrometry: molecular ion, metastable peak and McLafferty rearrangement, with every Civil Services, IFoS and Geo-Scientist PYQ →

CoversMcLafferty rearrangementmass spectrometryα-cleavagefragmentation pattern

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