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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

Q5(e). NMR at two field strengths; mass spectrum of a ketone

Question

(i) In a 100 MHz NMR instrument, a particular set of protons absorbs at δ = 3.0 with J = 4.5 Hz. Find the chemical shift (in Hz) and the coupling constant J in a 500 MHz instrument for the same set of protons. (5 marks) (ii) The mass spectrum of n-butyl phenyl ketone (C₆H₅COCH₂CH₂CH₂CH₃) shows peaks at m/z 162, 120, 105 and 85. Predict the fragmentation pattern. (5 marks)

Answer in brief

At 500 MHz the signal is 1500 Hz from TMS (δ still 3.0) and J stays 4.5 Hz; m/z 162 is M⁺•, 120 McLafferty, 105 base peak PhCO⁺, 85 C₄H₉CO⁺.

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Topic notes: ¹H and ¹³C NMR: chemical shift, spin–spin coupling and coupling constants, with every Civil Services, IFoS and Geo-Scientist PYQ →

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

Coverschemical shiftcoupling constantMcLafferty rearrangementα-cleavagebenzoyl cationvalerophenone

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