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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

Q3(a)(i). Hydrogen exchange in cyclopropyl ketones

Question

Why does the indicated α-hydrogen of saturated cyclopropyl ketone A exchange about 6 × 10³ times faster than that of cyclopropenyl ketone B? [structures of A and B given in the paper]

Answer in brief

Deprotonating B would place a carbanion in conjugation with a cyclopropene double bond, introducing unfavourable four-π-electron cyclic character; A avoids that penalty.

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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 →

Coversantiaromaticitycyclopropenyl anionH/D exchangecarbanion stabilityenolate

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