PHOSPHAZENES (PN Compounds)
Inorganic Polymers & Heterocycles
JEE Advanced │ IIT-JAM │ GATE │ CSIR-NET │ IISc │ TIFR │ BARC
1. Introduction
- Phosphazenes are inorganic heterocyclic/polymeric compounds containing alternating P and N atoms with general formula (NPX2)n
- Backbone: –P=N–P=N– (similar to siloxane –Si–O–Si–O–)
- Also called phosphonitrilic compounds
- Most important examples: cyclic trimer (X2P=N)3 and tetramer (X2P=N)4, and linear high polymers [NPX2]n
2. Preparation of Cyclic Phosphazenes
n PCl5 + n NH4Cl → [NPCl2]n + 4n HCl
Most common reaction (in chlorobenzene or tetrachloroethane at 130–140°C)
Gives mixture of cyclic trimer (n=3), tetramer (n=4) and higher polymers
(NPCl2)3 and (NPCl2)4 can be prepared by ammonolysis of PCl5
3PCl5 + 3NH3 → (NPCl2)3 + 9HCl
4PCl5 + 4NH3 → (NPCl2)4 + 12HCl
Most common reaction (in chlorobenzene or tetrachloroethane at 130–140°C)
Gives mixture of cyclic trimer (n=3), tetramer (n=4) and higher polymers
(NPCl2)3 and (NPCl2)4 can be prepared by ammonolysis of PCl5
3PCl5 + 3NH3 → (NPCl2)3 + 9HCl
4PCl5 + 4NH3 → (NPCl2)4 + 12HCl
Separation: Trimer & tetramer are distilled out; high polymer remains as rubbery mass
3. Important Cyclic Phosphazenes
| Name | Formula | Structure | Physical State |
|---|---|---|---|
| Hexachlorocyclotriphosphazene | (NPCl2)3 | 6-membered ring | Colourless crystalline solid |
| Octachlorocyclotetraphosphazene | (NPCl2)4 | 8-membered ring | White solid |
| High polymer | [NPCl2]n | Linear/cross-linked | Rubbery elastomer |
Most important & most asked: (NPCl2)3 – called phosphonitrilic chloride trimer
4. Structure & Bonding
- All rings are planar
- P–N bond length ≈ 158 pm (intermediate between single & double bond)
- Bonding explained by three theories (very important for CSIR-NET):
- Island model (Dewar) → dπ–pπ bonding + hyperconjugation
- Three-centre four-electron bonds (heteromorphic bonding)
- Polarised π-bonding
- All P atoms are sp³ hybridised while N atoms are sp²
- Delocalised 6π or 8π electrons in cyclic systems → aromatic character
5. Reactions of (NPCl2)3 & (NPCl2)4 (Substitution Reactions)
All six/eight Cl atoms are highly reactive and can be replaced step-wise:
(NPCl2)3 + 6 NaOR → (NPRO2)3 + 6 NaCl
(NPCl2)3 + 6 NH3 → (NPNH2)3 + 6 HCl
(NPCl2)3 + 6 CH3MgBr → (NP(CH3)2)3 + 6 MgBrCl
(NPCl2)3 + 6 NH3 → (NPNH2)3 + 6 HCl
(NPCl2)3 + 6 CH3MgBr → (NP(CH3)2)3 + 6 MgBrCl
Key point: Geminal substitution preferred initially, then non-geminal.
6. Polyphosphazenes (Inorganic Rubber)
[NPCl2]n + 2n NaOR → [NPRO2]n + 2n NaCl
- When R = –OCH2CF3 or –OC6H5 → thermally stable, hydrophobic, flame-retardant elastomers
- Remain flexible even at –70°C
- Used as O-rings, gaskets, fuel hoses in aerospace & military
- Biocompatible → used in drug delivery & tissue engineering
Most famous commercial name: Poly(difluorophosphazene) & Poly(aryloxyphosphazene)
7. Key Exam Points (Repeatedly Asked)
- (NPCl2)3 is more stable than (NPCl2)4
- Trimer has chair conformation in solid state
- All Cl atoms are equivalent in (NPCl2)3
- Phosphazenes are hydrolysed by water (slowly) but very rapidly by alkali
- Unlike (SiO2)n, phosphazenes are flexible due to easy rotation about P–N bond
- High thermal stability + low glass transition temperature → “Inorganic rubber”