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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 · 2022 · Paper II

Q1(d). Dichlorocarbene trapping

Question

Give the products and mechanism for cis-but-2-ene with CHCl₃/OH⁻, and demonstrate the intermediate using a suitable trap.

Answer in brief

Singlet dichlorocarbene gives cis-1,1-dichloro-2,3-dimethylcyclopropane; the original methyl groups remain cis.

Key points

  • OH⁻ + CHCl₃ → CCl₃⁻ + H₂O, followed by α-elimination CCl₃⁻ → :CCl₂ + Cl⁻, generates dichlorocarbene.
  • The carbene adds to the alkene in a concerted, stereospecific step, making two C–C bonds; both methyls retain their same-face relationship.
  • An independent alkene trap such as cyclohexene gives 7,7-dichlorobicyclo[4.1.0]heptane, supporting carbene intermediacy.
  • The trap intercepts the reactive intermediate; it need not be present in the main product-preparation experiment.

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(d), 2022 Paper II: solution pages 5 and 6
Solution pages 5 and 6 of 34
Read the whole 2022 solution2022 question paper

Topic notes: reactive intermediates (carbocations, carbanions, radicals, carbenes, nitrenes, benzyne), with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversdichlorocarbenecarbene trappingstereospecific cyclopropanationα-eliminationsinglet carbene

All parts of Question 1