Q5(e). McLafferty rearrangement in pentan-2-one; base peak of pentan-3-one
2-Pentanone and 3-pentanone are structural isomers. Only one of the isomers exhibits McLafferty rearrangement. Identify and show the McLafferty fragmentation for this isomer. Also propose the base peak for the other isomer, which does not exhibit the McLafferty rearrangement.
Answer in brief
Pentan-2-one has a γ-hydrogen and gives the McLafferty ion at m/z 58 (loss of ethene); pentan-3-one has none and its base peak is the propanoyl ion at m/z 57 from α-cleavage.
Part of VKC Basic
Read the key points and the full model answer with VKC Basic
VKC Basic (₹999 for 6 months) opens every solved answer, the notes, the handbook and the daily answer archive. Every Mains test series includes it.
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.
CoversMcLafferty rearrangementmass spectrometrybase peakα-cleavageacylium ionpentanone
All parts of Question 5
- 5(a) Five ¹H NMR signals of pentane-2,4-dione10 marks
- 5(b)(i) Three para-substituted styrenes in anionic polymerisation5 marks
- 5(b)(ii) Structures and configurations of natural and synthetic rubber5 marks
- 5(c) Acetophenone with SeO₂; benzoic acid with Na in liquid NH₃10 marks
- 5(d) Ease of Norrish type II hydrogen abstraction in two groups of ketones10 marks
- 5(e) McLafferty rearrangement in pentan-2-one; base peak of pentan-3-one10 marks