VKC ScienceChemistry for written exams Telegram

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 · 2016 · Paper II

Q1(c). Quinuclidine against triethylamine with ethyl iodide

Question

Quinuclidine reacts much faster with ethyl iodide than triethyl amine in SN2 reaction. Explain.

Answer in brief

Same basicity, but quinuclidine's alkyl groups are tied back , leaving an unhindered lone pair for the SN2 TS; in Et₃N the rotating ethyl groups hinder it.

Key points

  • In the SN2 TS the attacked carbon is five-coordinate and the N approaches it closely: any group near N raises the energy of this crowded TS.
  • In quinuclidine the three CH₂ arms are pinned back in the cage, so the lone pair is fully exposed: low steric barrier, fast reaction.
  • In Et₃N the CH₃ ends of the ethyl groups rotate into the space in front of N and clash with the ethyl group of EtI: higher Eₐ.
  • Rates with MeI: quinuclidine is about 60 times faster than Et₃N. With EtI (an extra CH₃ on the attacked carbon) the clash in Et₃N is worse, so the gap is larger.

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), 2016 Paper II: solution pages 3 and 4
Solution pages 3 and 4 of 31
Read the whole 2016 solution2016 question paper

Topic notes: nucleophilic substitution (SN1, SN2, SNi, neighbouring group participation, solvent effects, rearrangement), with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversfive-coordinatequinuclidinehigher Eₐ

All parts of Question 1