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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 · 2021 · Paper II

Q2(a). Elimination from trans- and cis-1-chloro-2-methylcyclohexane

Question

Involving the stereochemical concept, compare the elimination behaviour of compounds A (trans-1-chloro-2-methylcyclohexane) and B (cis-1-chloro-2-methylcyclohexane) in the presence of base. Also give the product(s) of the reactions. [structures of A and B given in the paper]

Answer in brief

B (cis) reacts fast to give mainly 1-methylcyclohexene with some 3-methylcyclohexene; A (trans) reacts slowly to give only 3-methylcyclohexene, because E2 needs H and Cl trans-diaxial.

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Topic notes: elimination reactions (E1, E2, E1cB, Saytzeff and Hofmann, anti-periplanar, Chugaev, Cope), 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 →

CoversE2 eliminationanti-periplanartrans-diaxialSaytzeff rule1-methylcyclohexenementhyl chloride

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