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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(a). Aromaticity classifications

Question

Classify cyclooctatetraene, cyclobutadiene, the drawn benzofused cyclooctadienediyne and pyrene as aromatic, antiaromatic or nonaromatic. [structures given in the paper]

Answer in brief

Cyclooctatetraene: nonaromatic; cyclobutadiene: antiaromatic; the planar cyclooctadienediyne circuit: antiaromatic; pyrene: aromatic.

Key points

  • Cyclooctatetraene avoids a planar eight-pi-electron arrangement by adopting a tub conformation; continuous parallel overlap around the ring is lost.
  • Cyclobutadiene has a planar four-pi-electron cyclic system; bond-length alternation reduces but does not eliminate its antiaromatic character.
  • The cyclooctadienediyne has a planar eight-electron conjugated circuit; only one pi bond of each alkyne belongs to that circuit.
  • Pyrene is a fused aromatic hydrocarbon with delocalized benzenoid circuits; a monocyclic 4n + 2 count over all sixteen pi electrons would mislead.

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(a), 2021 Paper II: solution pages 1 and 2
Solution pages 1 and 2 of 40
Read the whole 2021 solution2021 question paper

Topic notes: aromaticity (Hückel rule, antiaromaticity, annulenes, azulene, tropolone, fulvene, sydnones), with every Civil Services, IFoS and Geo-Scientist PYQ →

CoversaromaticityantiaromaticitycyclooctatetraenecyclobutadienepyreneHückel rule

All parts of Question 1