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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper II covers organic chemistry and spectroscopy.

UPSC Civil Services (Mains), Chemistry optional · 2025 · Paper II

Q8(b)(ii). C₆H₁₂ alkenes with peaks at m/z 42 and 56

Question

Two isomeric alkenes with the same molecular formula C₆H₁₂ show strong peaks at m/z 42 and 56 in the mass spectrum. Propose a fragmentation pattern for both the peaks.

Answer in brief

m/z 42 comes from hex-1-ene and m/z 56 from 2-methylpent-1-ene, both by McLafferty-type rearrangement.

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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 rearrangementalkene fragmentationhex-1-ene2-methylpent-1-enemass spectrometry

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