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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 · 2024 · Paper II

Q3(a)(ii). Bromine addition to cis- and trans-2-butene

Question

Addition of bromine to cis-2-butene gives racemic 2,3-dibromobutane while trans-2-butene yields meso-2,3-dibromobutane. Justify.

Answer in brief

Addition is anti and stereospecific because it passes through a bridged bromonium ion.

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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.

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Topic notes: addition to alkenes and alkynes (Markovnikov, stereochemistry, dienes), with every Civil Services, IFoS and Geo-Scientist PYQ →

Topic notes: stereochemistry and conformational analysis (R/S, E/Z, Fischer and Newman projections, cyclohexane chair), with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversbromonium ionanti additionstereospecificmeso-2,3-dibromobutaneracemic mixture2-butene

All parts of Question 3