UPSC Combined Geo-Scientist, Mains · 2023 · Mains Paper III
Q6(h). Mass spectrum of 3-butyn-2-ol: the ion of m/z 55
Question
The mass spectrum of 3-butyn-2-ol shows a large peak at m/z 55. Draw the structure of fragment and explain why it is particularly stable.
Answer in brief
m/z 55 = M − 15: the ion HC≡C–CH=OH⁺ (C₃H₃O⁺), formed by loss of •CH₃; its charge is shared by O, C-1 and C-3.
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Coversmass spectrometryα-cleavage3-butyn-2-oloxocarbenium ionpropargyl cation
All parts of Question 6
- 6(a) Chlorocarbene from CH₂Cl₂; ring expansion of 3-methylindole4 marks
- 6(b) Ozonolysis of camphene, then Wolff–Kishner reduction4 marks
- 6(c) Reductive amination, then intramolecular aza-Michael addition4 marks
- 6(d) Benzaldehyde and ethane-1,2-dithiol: a 1,3-dithiolane4 marks
- 6(e) Amidomercuration–demercuration of hex-1-ene in acetonitrile4 marks
- 6(f) Dimerisation of cyclopentadiene: endo rule by frontier orbitals4 marks
- 6(g) Two isomeric esters C₇H₁₄O₂ from ¹H NMR4 marks
- 6(h) Mass spectrum of 3-butyn-2-ol: the ion of m/z 554 marks
- 6(i) Acetone: solvent shift of the n → π* band4 marks
- 6(j) Butyrophenone: McLafferty rearrangement and α-cleavage4 marks