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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

Q5(c). Acetylene to neoprene; why rubber perishes in air

Question

(i) 2 CH≡CH → A (Cu₂Cl₂, NH₄Cl); A + HCl → B. Identify A and B. The polymerisation of the rearranged product of B, unaided by any catalyst, gives rise to a ‘synthetic rubber’. Name this rubber along with its structural formula. (ii) When rubber balls and other objects made of rubber are exposed to the air for long periods of time, they turn brittle and crack. This does not happen to objects made of polyethylene. Explain.

Answer in brief

A = vinylacetylene; B = 4-chlorobuta-1,2-diene, which rearranges to chloroprene, giving neoprene; rubber perishes because its C=C and allylic C–H are attacked by O₂ and O₃, absent in saturated polyethylene.

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Coversneoprenechloroprenevinylacetylenerubber ageingautoxidationozonolysis

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