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Indian Forest Service (Mains)

Chemistry optional

  • 2 papers
  • 200 marks each
  • 3 h per paper

Paper II covers organic chemistry and spectroscopy.

Indian Forest Service (Mains), Chemistry optional · 2021 · Paper II

Q1(c). Isotope-labelled acetate pyrolysis

Question

Predict products and mechanisms when the two drawn stereoisomeric PhCH(OH)CH(D)Ph alcohols are heated with acetic anhydride. [stereostructures given in the paper]

Answer in brief

Acetylation followed by syn ester pyrolysis gives predominantly H-elimination products: the upper structure gives E-deuteriostilbene and the lower gives Z-deuteriostilbene.

Key points

  • Acetic anhydride converts OH to OAc without breaking the stereogenic C–O bond, so the carbon configuration is retained during acetylation.
  • On heating, the ester eliminates through a cyclic six-membered transition state: acetate carbonyl O accepts the syn β-H and the alkyl C–O bond breaks.
  • Rotating the β-H into the same plane and face as OAc places the phenyls opposite (E) for the upper substrate and together (Z) for the lower.
  • Preferential C–H over C–D cleavage retains D on the alkene; competing D-elimination gives the opposite geometries without D.

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

Model answer to Q1(c), 2021 Paper II: solution pages 3 and 4
Solution pages 3 and 4 of 40
Read the whole 2021 solution2021 question paper

Topic notes: methods of studying mechanisms (isotopic labelling, crossover, trapping, kinetic isotope effect, kinetic and thermodynamic control), with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversacetate pyrolysissyn eliminationEi mechanismdeuterium labellingstilbene stereochemistry

All parts of Question 1