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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 · 2022 · Paper II

Q8(a)(i). Carbonyl stretching frequency of two acetylcyclohexenes

Question

Among the following compounds I and II, which has more carbonyl stretching frequency in IR spectra? Explain. [structures of 1-acetylcyclohexene (I) and its 2-methyl analogue (II) given in the paper]

Answer in brief

II absorbs at the higher frequency (about 1690 cm⁻¹, against about 1665 cm⁻¹ for I): its 2-methyl group twists the acetyl group out of the plane of the C=C, so conjugation is reduced.

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Coverscarbonyl stretching frequencyIR spectroscopysteric inhibition of resonanceconjugated enoneforce constant

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