UPSC Combined Geo-Scientist, Mains · 2023 · Mains Paper III
Q6(d). Benzaldehyde and ethane-1,2-dithiol: a 1,3-dithiolane
Question
Predict the product of the following reaction and give the plausible mechanism: benzaldehyde + ethane-1,2-dithiol with BF₃·OEt₂ in DCM gives ?. [Structures given in the paper]
Answer in brief
2-Phenyl-1,3-dithiolane, C₉H₁₀S₂, the cyclic dithioacetal of benzaldehyde, and water.
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Coversthioacetal1,3-dithiolaneBF3 etheratecarbonyl protectionthionium ion
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