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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 · 2017 · Paper II

Q8(b). Mass spectra of 4-phenylbutan-2-one and 4-ethylanisole

  • 10 marks
  • Answered
Question

Predict the M+ peaks and mass spectral fragmentation for the following compounds.
[structure] and [structure]

Answer in brief

4-Phenylbutan-2-one: M⁺ 148; 133, 105, 91 (tropylium), 65 and 43 (acetyl). 4-Ethylanisole: M⁺ 136; base peak 121 (M - CH₃), 91, 77, 65.

Key points

  • M⁺ = 148.
  • α-Cleavage on the alkyl side: CH₃C#O⁺, m/z 43; on the methyl side: loss of CH₃• gives PhCH₂CH₂C#O⁺, m/z 133.
  • C₆H₅CH₂⁺ rearranging to the tropylium ion, m/z 91 (strong); 91 loses C₂H₂ to give m/z 65.
  • Loss of the acetyl radical (CH₃CO•, 43) leaves PhCH₂CH₂⁺, m/z 105.

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. EI mass spectra show the molecular ion (M⁺˙) and fragments from the weakest bonds that give the most stable cations: α-cleavage next to C=O (acylium ions) and benzylic cleavage (benzyl/tropylium ions).

4-Phenylbutan-2-one, C₁₀H₁₂O

  1. M⁺ = 148.
  2. α-Cleavage on the alkyl side: CH₃C#O⁺, m/z 43; on the methyl side: loss of CH₃• gives PhCH₂CH₂C#O⁺, m/z 133.
  3. Benzylic cleavage of the CH₂-CH₂ bond: C₆H₅CH₂⁺ rearranging to the tropylium ion, m/z 91 (strong); 91 loses C₂H₂ to give m/z 65.
  4. Loss of the acetyl radical (CH₃CO•, 43) leaves PhCH₂CH₂⁺, m/z 105.
  5. A normal McLafferty rearrangement is not possible: the γ-carbon (on the carbonyl side towards the ring) is the ipso ring carbon, which carries no H.

4-Ethylanisole (1-ethyl-4-methoxybenzene), C₉H₁₂O

  1. M⁺ = 136.
  2. Benzylic cleavage loses CH₃• (M - 15) to give the 4-methoxybenzyl / methoxytropylium ion, m/z 121, the base peak (stabilised by the OMe lone pair).
  3. m/z 121 loses formaldehyde (CH₂O, 30) to give C₇H₇⁺, m/z 91; further fragments at m/z 77 (C₆H₅⁺) and 65.

Scheme: Main fragmentation path of 4-ethylanisole.

Conclusion. 4-Phenylbutan-2-one: M⁺ 148; 133, 105, 91 (tropylium), 65 and 43 (acetyl). 4-Ethylanisole: M⁺ 136; base peak 121 (M - CH₃), 91, 77, 65.

Application. Benzylic cleavage to m/z 91 (or 121 for anisyl compounds) is the fingerprint used to recognise alkylbenzenes and methoxyaryl drugs in GC–MS analysis.

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Model answer to Q8(b), 2017 Paper II: book edition page 32
Book edition, page 32
Model answer to Q8(b), 2017 Paper II: book edition page 33
Book edition, page 33
Read the whole 2017 solution2017 question paper

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