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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 · 2021 · Paper II

Q5(a). Five ¹H NMR signals of pentane-2,4-dione

Question

2,4-Pentanedione exhibits five signals in its ¹H NMR spectrum. Account for the observation. Also write the approximate chemical shifts.

Answer in brief

It is a slowly interconverting mixture of keto form (CH₃ δ 2.2, CH₂ δ 3.6) and hydrogen-bonded enol form (CH₃ δ 2.0, =CH δ 5.5, O–H δ 15.5).

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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 →

Coversketo–enol tautomerismacetylacetone NMRpentane-2,4-dioneintramolecular hydrogen bondNMR time scale

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