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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 I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2022 · Paper I

Q5(e). Lower cyclosiloxanes: synthesis and structure of (Me₂SiO)₃

Question

Write the general synthetic procedure of lower cyclosiloxanes like cyclic dimethylsiloxane trimer [(Me₂SiO)₃]. Draw its structure. Why MeSiCl₃ is not used as starting material for its synthesis?

Answer in brief

Hydrolysis of Me₂SiCl₂ gives Me₂Si(OH)₂, which condenses to rings (Me₂SiO)ₙ; (Me₂SiO)₃ is a planar six-membered Si₃O₃ ring; MeSiCl₃ is trifunctional and gives a cross-linked network, not rings.

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Topic notes: main group chemistry: boranes, borazine, phosphazenes, silicates, silicones, interhalogens, xenon and sulfur–nitrogen compounds, with every Civil Services, IFoS and Geo-Scientist PYQ →

CoverscyclosiloxanessiliconeshexamethylcyclotrisiloxaneRochow processdimethyldichlorosilanesilicone resin

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