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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 · 2016 · Paper II

Q6(a). Which ketone undergoes the Norrish type II reaction

  • 10 marks
  • Answered
Question

Among the following, identify the compound which can undergo Norrish type II reaction. Write the mechanism:
(i) CH₃COCH₂CH₃ (ii) CH₃COCH₂CH₂CH(CH₃)₂ (iii) CH₃COCH₂CH₂C(CH₃)₃ (iv) C₆H₅COCH₂CH₃

Answer in brief

Only (ii), CH₃COCH₂CH₂CH(CH₃)₂, has a γ-H and undergoes Norrish type II: γ-H abstraction → 1,4-biradical → acetone + 2-methylpropene (or a cyclobutanol).

Key points

  • Absorption (n→π*) gives the excited singlet, which partly crosses to the triplet. The half-filled n orbital on oxygen makes the oxygen radical-like.
  • The oxygen abstracts the γ-hydrogen through a six-membered cyclic transition state (an intramolecular 1,5-H shift), giving a 1,4-biradical (here a tertiary radical at the γ-carbon, which favours the step).
  • Cleavage of the C_α–C_β bond gives the enol of acetone (CH₂=C(OH)CH₃), which tautomerises to acetone, and 2-methylpropene (CH₂=C(CH₃)₂).
  • Alternatively the two radical centres combine (Yang cyclisation) to give 1,2,2-trimethylcyclobutan-1-ol.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

Introduction. In the Norrish type II reaction the n→π* excited carbonyl oxygen abstracts a hydrogen from the γ-carbon through a six-membered cyclic transition state. The essential structural condition is therefore at least one γ-hydrogen.

Compoundγ-carbon (C=O carbon counted as C-1, then α, β, γ)
(i)butan-2-onechain ends at β; no γ-C: no type II
(ii)5-methylhexan-2-oneγ-C is the CH of CH(CH₃)₂: one tertiary γ-H: type II
(iii)5,5-dimethylhexan-2-oneγ-C is quaternary C(CH₃)₃: no γ-H: no type II
(iv)propiophenonechain ends at β (CH₃); no γ-C: no type II

Scheme: Step 1: the excited C=O oxygen takes the γ-H.

Scheme: Step 2: the biradical splits at the C_α–C_β bond, or closes to a cyclobutanol (Yang cyclisation).

  1. Absorption (n→π*) gives the excited singlet, which partly crosses to the triplet. The half-filled n orbital on oxygen makes the oxygen radical-like.
  2. The oxygen abstracts the γ-hydrogen through a six-membered cyclic transition state (an intramolecular 1,5-H shift), giving a 1,4-biradical (here a tertiary radical at the γ-carbon, which favours the step).
  3. Cleavage of the C_α–C_β bond gives the enol of acetone (CH₂=C(OH)CH₃), which tautomerises to acetone, and 2-methylpropene (CH₂=C(CH₃)₂).
  4. Alternatively the two radical centres combine (Yang cyclisation) to give 1,2,2-trimethylcyclobutan-1-ol.
  5. Compounds (i), (iii) and (iv) lack a γ-H; they react only by Norrish type I (α-cleavage).

Conclusion. Only (ii), CH₃COCH₂CH₂CH(CH₃)₂, has a γ-H and undergoes Norrish type II: γ-H abstraction → 1,4-biradical → acetone + 2-methylpropene (or a cyclobutanol).

Application. Norrish type II cleavage makes ketone-containing polymers (ethylene–carbon monoxide copolymers) photodegradable, and the McLafferty rearrangement in mass spectra is its ionic analogue.

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Model answer to Q6(a), 2016 Paper II: solution page 20
Solution page 20 of 25
Read the whole 2016 solution2016 question paper

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