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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 · 2017 · Paper II

Q5(d). Regiospecific reaction; aniline to p-phenylenediamine

  • 10 marks
  • Answered
Question

Define a regio specific reaction. How will you convert aniline into p-phenylene diamine?

Answer in brief

Regiospecific: only one positional isomer is formed. Aniline Ac₂O acetanilide HNO₃/H₂SO₄ p-nitroacetanilide H₃O⁺ p-nitroaniline Sn/HCl p-phenylenediamine.

Key points

  • aniline + acetic anhydride gives acetanilide. Direct nitration of aniline is useless: HNO₃ oxidises it, and in acid the -NH₃⁺ group directs meta.
  • -NHCOCH₃ is a moderate o,p-director; its size blocks the ortho positions, so NO₂⁺ enters almost only para: p-nitroacetanilide.
  • acid (or base) hydrolysis of the amide gives p-nitroaniline.
  • Sn/HCl (or Fe/HCl, or H₂/Pd) reduces NO₂ to NH₂: p-phenylenediamine.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

Introduction. A reaction is regiospecific when, of two or more possible constitutional (positional) isomers, only one is formed (100%); it is regioselective when one isomer predominates. Example: hydroboration–oxidation of propene gives propan-1-ol with essentially no propan-2-ol.

Scheme: Protection, para-nitration, deprotection, reduction.

  1. Protect NH₂: aniline + acetic anhydride gives acetanilide. Direct nitration of aniline is useless: HNO₃ oxidises it, and in acid the -NH₃⁺ group directs meta.
  2. Nitrate: -NHCOCH₃ is a moderate o,p-director; its size blocks the ortho positions, so NO₂⁺ enters almost only para: p-nitroacetanilide.
  3. Deprotect: acid (or base) hydrolysis of the amide gives p-nitroaniline.
  4. Reduce: Sn/HCl (or Fe/HCl, or H₂/Pd) reduces NO₂ to NH₂: p-phenylenediamine.
  5. Alternative: diazotise aniline, couple with aniline to p-aminoazobenzene (para coupling), then cleave the N=N with SnCl₂/HCl to give p-phenylenediamine + aniline.

Conclusion. Regiospecific: only one positional isomer is formed. Aniline Ac₂O acetanilide HNO₃/H₂SO₄ p-nitroacetanilide H₃O⁺ p-nitroaniline Sn/HCl p-phenylenediamine.

Application. p-Phenylenediamine is a monomer of Kevlar (with terephthaloyl chloride) and the base of permanent hair dyes.

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Model answer to Q5(d), 2017 Paper II: solution page 16
Solution page 16 of 23
Read the whole 2017 solution2017 question paper

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