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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 · 2017 · Paper I

Q8(c). Phosphazenes: ring and chain types; substitution reactions

  • 15 marks
  • Answered
Question

Define phosphazenes with formulae for ring and chain types. Write two examples of substitution reactions of cyclophosphazenes.

Answer in brief

Phosphazenes are compounds with alternating P and N atoms, built from the unit -N=PX₂⁻: cyclic (NPX₂)ₙ (n=3,4,…) and linear polymers –[N=PX₂]ₙ–. The P–Cl bonds of (NPCl₂)₃ are replaced by nucleophiles, for example (NPCl₂)₃ + 6NaOCH₃ → [NP(OCH₃)₂]₃ + 6NaCl.

Key points

  • rings and chains of alternating phosphorus and nitrogen atoms with two substituents (Cl, F, OR, NR₂, R) on each P. Formally P(V) and N(III), with the repeat unit R₂PN, analogous to the R₂SiO unit of siloxanes.
  • (NPX₂)_(n): trimer (NPCl₂)₃ (six-membered, planar ring), tetramer (NPCl₂)₄ (eight-membered, puckered), and larger rings.
  • –[N=PX₂]ₙ–, high polymers (polyphosphazenes), the "inorganic rubber" [NPCl₂]_(n).
  • n PCl₅ + n NH₄Cl → (Cl₂PN)_(n) + 4n HCl (chlorocarbon solvent, about 130°C: mainly n=3 and 4). Heating the trimer to about 290°C opens the ring to the polymer.

The full model answer

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

Definition and types

  1. Phosphazenes: rings and chains of alternating phosphorus and nitrogen atoms with two substituents (Cl, F, OR, NR₂, R) on each P. Formally P(V) and N(III), with the repeat unit R₂PN, analogous to the R₂SiO unit of siloxanes.
  2. Cyclic phosphazenes: (NPX₂)_(n): trimer (NPCl₂)₃ (six-membered, planar ring), tetramer (NPCl₂)₄ (eight-membered, puckered), and larger rings.
  3. Chain (linear) phosphazenes: –[N=PX₂]ₙ–, high polymers (polyphosphazenes), the "inorganic rubber" [NPCl₂]_(n).
  4. Preparation: n PCl₅ + n NH₄Cl → (Cl₂PN)_(n) + 4n HCl (chlorocarbon solvent, about 130°C: mainly n=3 and 4). Heating the trimer to about 290°C opens the ring to the polymer.
  5. Bonding: in the trimer all P–N bonds are equal and shorter than a single bond: the π electrons are delocalised over the ring (often described with P dπ–N pπ or ionic P⁺–N⁻ contributions). Unlike benzene, the ring is not aromatic in the Hückel sense.

Substitution reactions of cyclophosphazenes

  1. Alkoxide (methoxide): (NPCl₂)₃ + 6NaOCH₃ → [NP(OCH₃)₂]₃ + 6NaCl; likewise (NPCl₂)₄ + 8NaOCH₃ → [NP(OCH₃)₂]₄ + 8NaCl.
  2. Amines: (NPCl₂)₃ + 12(CH₃)₂NH → [NP(N(CH₃)₂)₂]₃ + 6(CH₃)₂NH₂Cl (half of the amine takes up the HCl).
  3. Fluorination and other nucleophiles: (NPCl₂)₃ + 6NaF → (NPF₂)₃ + 6NaCl; fluoroalkoxides such as CF₃CH₂O⁻ or CF₃CF₂O⁻ replace Cl in the same way, also on the polymer: (Cl₂PN)_(n) + 2n CF₃CF₂O⁻ → [(CF₃CF₂O)₂PN]_(n) + 2n Cl⁻.
  4. Mechanism: the P atoms are electrophilic (P–Cl bonds polarised by the ring N); substitution is nucleophilic (S_N2-like) at phosphorus. Stepwise substitution can be geminal (both Cl on one P) or non-geminal, depending on the nucleophile.

Application. substituted polyphosphazenes remain rubbery at low temperature and are used as flame-resistant elastomers and as biodegradable biomedical materials: implants, supports for bone regeneration and drug-delivery systems.

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Model answer to Q8(c), 2017 Paper I: solution page 36
Solution page 36 of 38
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