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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper II covers organic chemistry and spectroscopy.

UPSC Civil Services (Mains), Chemistry optional · 2023 · Paper II

Q1(a)(i). Acidity of cyclopentadiene

Question

pKa value of cyclopentadiene is almost similar to water. Explain.

Answer in brief

Cyclopentadiene (pKa ≈ 16) matches water (pKa 15.7): its conjugate base is an aromatic 6π anion.

Key points

  • Deprotonation of sp³ CH₂: planar, cyclic, conjugated anion.
  • Aromatic: 6π (4n + 2, n = 1); charge on five carbons.
  • Aromatic stabilisation: pKa about 27 units below propene.
  • Contrast: cycloheptatriene, pKa ≈ 36: 8π antiaromatic anion.

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)(i), 2023 Paper II: solution pages 1 and 2
Solution pages 1 and 2 of 26
Read the whole 2023 solution2023 question paper

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

Coverscyclopentadiene pKacyclopentadienyl anionaromaticityHückel rulecarbon acid

All parts of Question 1