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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper III covers analytical and organic chemistry.

UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper III

Q9(b)(i). HBr and buta-1,3-diene: kinetic and thermodynamic control

Question

Complete the following reaction with suitable mechanism and explanation for the products formed X and Y: CH₂=CH–CH=CH₂ + HBr → X (−78 °C) and Y (60 °C).

Answer in brief

X = 3-bromobut-1-ene (1,2-adduct, kinetic product). Y = (E)-1-bromobut-2-ene (1,4-adduct, thermodynamic product).

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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 →

Covers1,2- and 1,4-additionkinetic vs thermodynamic controlallylic carbocationbuta-1,3-dieneHBr addition

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