UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper III

Q6(i). McLafferty rearrangement: 2-pentanone and 3-pentanone

4 marks
Topic: Spectroscopic methods of analysis
Answered

Question

2-Pentanone and 3-pentanone are structural isomers. Only one of these compounds exhibits McLafferty rearrangement in mass spectrometry. Identify the same and provide the McLafferty fragmentation for the compound.

Answer in brief

2-Pentanone: it has γ-hydrogens, loses ethene (28) and gives m/z 58.

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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 spectrometrygamma hydrogen2-pentanonealpha cleavage

All parts of Question 6

6(a) Singlet and triplet carbenes; fluorenylidene with cis-but-2-ene4 marks
6(b) Bromine addition: styrene reacts faster than propene4 marks
6(c) Addition of HCl to an α,β-unsaturated acid4 marks
6(d) Michael addition of a cyclic 1,3-dione to cyclohex-2-enone4 marks
6(e) Hydroboration–oxidation of 1-methylcyclohexene4 marks
6(f) Cope rearrangement of a 1,5-diene4 marks
6(g) Diels–Alder reaction: product and reactants4 marks
6(h) IR distinction: hydrogen bonding and ring strain4 marks
6(i) McLafferty rearrangement: 2-pentanone and 3-pentanone4 marks
6(j) NMR assignment of 4-methylpent-1-yne4 marks