Phosphorus trioxide

Phosphorus trioxide is the chemical compound with the molecular formula P₄O₆. Although the molecular formula suggests the name tetraphosphorus hexoxide, the name phosphorus trioxide preceded the knowledge of the compound's molecular structure, and its usage continues today. This colorless solid is structurally related to adamantane. It is formally the anhydride of phosphorous acid, H₃PO₃, but cannot be obtained by the dehydration of the acid. It is a white, waxy, crystalline and highly toxic solid with garlic odour.[1]

Phosphorus(III) oxide

Phosphorus in orange, oxygen in red
Names
Other names Phosphorus sesquioxide Phosphorous anhydride Tetraphosphorous hexoxide
Identifiers
CAS Number	1314-24-5^Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:37372^Y
ChemSpider	109897^Y
ECHA InfoCard	100.032.414
PubChem CID	123290
UNII	0LTR52K7HK^Y
CompTox Dashboard (EPA)	DTXSID80907588
InChI InChI=1S/O6P4/c1-7-2-9-4-8(1)5-10(3-7)6-9^Y Key: VSAISIQCTGDGPU-UHFFFAOYSA-N^Y InChI=1/O6P4/c1-7-2-9-4-8(1)5-10(3-7)6-9 Key: VSAISIQCTGDGPU-UHFFFAOYAV
SMILES O1P3OP2OP(OP1O2)O3
Properties
Chemical formula	P4O6
Molar mass	219.88 g mol^−1
Appearance	colourless monoclinic crystals or liquid
Density	2.135 g/cm^3
Melting point	23.8 °C (74.8 °F; 296.9 K)
Boiling point	173.1 °C (343.6 °F; 446.2 K)
Solubility in water	reacts
Acidity (pKa)	9.4
Structure
Molecular shape	See Text
Dipole moment	0
Hazards
EU classification (DSD) (outdated)	Toxic (T+)
NFPA 704 (fire diamond)	0 3 0
Related compounds
Other anions	Phosphorus trisulfide
Other cations	Dinitrogen trioxide Arsenic trioxide Antimony trioxide
Related compounds	Phosphorus pentoxide Phosphorous acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Preparation

It is obtained by the combustion of phosphorus in a limited supply of air at low temperature.

P₄ + 3 O₂ → P₄O₆

By-products include red phosphorus suboxide.[1]

Chemical Properties

Phosphorus trioxide reacts with water to form phosphorous acid, reflecting the fact that it is the anhydride of that acid.[2]

P₄O₆ + 6 H₂O → 4 H₃PO₃

It reacts with hydrogen chloride to form H₃PO₃ and phosphorus trichloride.

P₄O₆ + 6 HCl → 2 H₃PO₃ + 2 PCl₃

With chlorine or bromine it forms the corresponding phosphoryl halide, and it reacts with iodine in a sealed tube to form diphosphorus tetraiodide.[1]

P₄O₆ reacts with ozone at 195 K to give the unstable compound P₄O₁₈.[3]

P₄O₁₈ decomposes above 238 K in solution with the release of O₂ gas. Decomposition of dry P₄O₁₈ is explosive.

In a disproportionation reaction, P₄O₆ is converted into the mixed P(III)P(V) species P₄O₈ when heated in a sealed tube at 710 K, with the side product being red phosphorus.[3]

As a ligand

Structure of P₄O₆·Fe(CO)₄.

P₄O₆ is a ligand for transition metals, comparable to phosphite. An illustrative complex is P₄O₆·Fe(CO)₄.[4] With BH₃, a dimeric adduct is produced:[3]

Structure of P₈O₁₂(BH₃)₂.

References

1. A. F. Holleman; Wiberg, Egon; Wiberg, Nils (2001). Inorganic Chemistry. Boston: Academic Press. ISBN 0-12-352651-5.
2. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
3. .Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 15: The group 15 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 473. ISBN 978-0-13-175553-6.
4. M. Jansen & J. Clade (November 1996). "Tetracarbonyl(tetraphosphorus hexaoxide)iron". Acta Crystallogr. C. 52 (11): 2650–2652. doi:10.1107/S0108270196004398.