UPSC Combined Geo-Scientist, Mains · 2022 · Mains Paper III

Q6(i). 3-Methyl- and 4-methylpentan-2-one by mass spectrometry

4 marks
Topic: Spectroscopic methods of analysis
Answered

Question

How can the following pairs of isomeric compounds be differentiated by mass spectrometry? [structures of 3-methylpentan-2-one and 4-methylpentan-2-one given in the paper]

Answer in brief

By the McLafferty peak: 3-methylpentan-2-one gives m/z 72 (loss of ethene, 28); 4-methylpentan-2-one gives m/z 58 (loss of propene, 42).

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

Coversmass spectrometryMcLafferty rearrangementα-cleavagemethylpentanone isomersm/z 58

All parts of Question 6

6(a) Cyclohexanone: self-condensation with NaOH, none with LDA4 marks
6(b) Ethyl acetoacetate with cyclohexanone: Knoevenagel condensation4 marks
6(c) Birch reduction of benzoic acid4 marks
6(d) HCl with 1-methyl-1-vinylcyclobutane: ring expansion4 marks
6(e) Cyclopentadiene with maleic anhydride: endo and exo adducts4 marks
6(f) UV absorption of aniline in neutral and in acidic solution4 marks
6(g) Aromatic proton shifts: NO₂, COOH and CF₃ as para substituents4 marks
6(h) Citral and citronellal by IR: conjugated and isolated C=O4 marks
6(i) 3-Methyl- and 4-methylpentan-2-one by mass spectrometry4 marks
6(j) 4-Bromobenzoyl chloride to 4-bromoaniline: Lossen rearrangement4 marks