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Phosphorus pentoxide

Names
IUPAC names Tetraphosphorus decaoxide Tricyclotetraphosphoxane 1,3,5,7-tetraoxide
Systematic IUPAC name 2,4,6,8,9,10-Hexaoxa-1λ,3λ,5λ,7λ-tetraphosphatricyclodecane 1,3,5,7-tetraoxide
Other names Diphosphorus pentoxide Phosphorus(V) oxide Phosphoric anhydride Tetraphosphorus decaoxide Tetraphosphorus decoxide
Identifiers
CAS Number	1314-56-3 16752-60-6 (P4O10)
3D model (JSmol)	molecular form: Interactive image crystal o′ form: Interactive image
ChEBI	CHEBI:37376
ChemSpider	14128
ECHA InfoCard	100.013.852
PubChem CID	14812
RTECS number	TH3945000
UNII	51SWB7223J
CompTox Dashboard (EPA)	DTXSID701335584 DTXSID9047754, DTXSID701335584
InChI InChI=1S/O10P4/c1-11-5-12(2)8-13(3,6-11)10-14(4,7-11)9-12Key: DLYUQMMRRRQYAE-UHFFFAOYSA-N
SMILES molecular form: O=P13OP2(=O)OP(=O)(O1)OP(=O)(O2)O3 crystal o′ form: P12(=O)OP3(=O)OP4(=O)OP5(=O)OP6(=O)OP(=O)(O1)OP7(=O)OP(=O)OP(=O)OP(=O)(O2)OP(=O)OP(=O)OP(=O)(O3)OP(=O)OP(=O)OP(=O)(O4)OP(=O)OP(=O)OP(=O)(O5)OP(=O)OP(=O)OP(=O)(O6)OP(=O)OP(=O)(O7)O
Properties
Chemical formula	P4O10
Molar mass	283.9 g mol
Appearance	White powder Very deliquescent
Odor	Odorless
Density	2.39 g/cm
Melting point	340 °C (644 °F; 613 K)
Boiling point	360 °C (sublimes)
Solubility in water	exothermic hydrolysis
Vapor pressure	1 mmHg @ 385 °C (stable form)
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	reacts with water, strong dehydrating agent, corrosive
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H314
Precautionary statements	P280, P301+P330+P331, P303+P361+P353, P305+P351+P338, P310
NFPA 704 (fire diamond)	3 0 3W
Safety data sheet (SDS)	MSDS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). Infobox references

Phosphorus pentoxide is a chemical compound with molecular formula P₄O₁₀ (with its common name derived from its empirical formula, P₂O₅). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydrating agent.

Structure

Phosphorus pentoxide crystallizes in at least four forms or polymorphs. The most familiar one, a metastable form (shown in the figure), comprises molecules of P₄O₁₀. Weak van der Waals forces hold these molecules together in a hexagonal lattice (However, in spite of the high symmetry of the molecules, the crystal packing is not a close packing). The structure of the P₄O₁₀ cage is reminiscent of adamantane with T_d symmetry point group. It is closely related to the corresponding anhydride of phosphorous acid, P₄O₆. The latter lacks terminal oxo groups. Its density is 2.30 g/cm. It boils at 423 °C under atmospheric pressure; if heated more rapidly it can sublimate. This form can be made by condensing the vapor of phosphorus pentoxide rapidly, and the result is an extremely hygroscopic solid.

The other polymorphs are polymeric, but in each case the phosphorus atoms are bound by a tetrahedron of oxygen atoms, one of which forms a terminal P=O bond involving the donation of the terminal oxygen p-orbital electrons to the antibonding phosphorus-oxygen single bonds. The macromolecular form can be made by heating the compound in a sealed tube for several hours, and maintaining the melt at a high temperature before cooling the melt to the solid. The metastable orthorhombic "O"-form (density 2.72 g/cm, melting point 562 °C) adopts a layered structure consisting of interconnected P₆O₆ rings, not unlike the structure adopted by certain polysilicates. The stable form is a higher density phase, also orthorhombic, the so-called O' form. It consists of a 3-dimensional framework, density 3.5 g/cm. The remaining polymorph is a glass or amorphous form; it can be made by fusing any of the others.

Part of an o′-(P2O5)∞ layer	o′-(P2O5)∞ layers stacking

Preparation

P₄O₁₀ is prepared by burning white phosphorus with a sufficient supply of oxygen:

P₄ + 5 O₂ → P₄O₁₀

The dehydration of phosphoric acid to give phosphorus pentoxide is not possible, as on heating it forms various polyphosphates but will not dehydrate sufficiently to form P₄O₁₀.

Applications

Phosphorus pentoxide is a potent dehydrating agent as indicated by the exothermic nature of its hydrolysis producing phosphoric acid:

P₄O₁₀ + 6 H₂O → 4 H₃PO₄   (–177 kJ)

However, its utility for drying is limited somewhat by its tendency to form a protective viscous coating that inhibits further dehydration by unspent material. A granular form of P₄O₁₀ is used in desiccators.

Consistent with its strong desiccating power, P₄O₁₀ is used in organic synthesis for dehydration. The most important application is for the conversion of primary amides into nitriles:

P₄O₁₀ + RC(O)NH₂ → P₄O₉(OH)₂ + RCN

The indicated coproduct P₄O₉(OH)₂ is an idealized formula for undefined products resulting from the hydration of P₄O₁₀.

Alternatively, when combined with a carboxylic acid, the result is the corresponding anhydride:

P₄O₁₀ + RCO₂H → P₄O₉(OH)₂ + ₂O

The "Onodera reagent", a solution of P₄O₁₀ in DMSO, is employed for the oxidation of alcohols. This reaction is reminiscent of the Swern oxidation.

The desiccating power of P₄O₁₀ is strong enough to convert many mineral acids to their anhydrides. Examples: HNO₃ is converted to N₂O₅;  H₂SO₄ is converted to SO₃;  HClO₄ is converted to Cl₂O₇;  CF₃SO₃H is converted to (CF₃)₂S₂O₅.

As a proxy measurement

P₂O₅ content is often used by industry as proxy value for all the phosphorus oxides in a material. For example, fertilizer grade phosphoric acid can also contain various related phosphorous compounds which are also of use. All these compounds are described collectively in terms of 'P₂O₅ content' to allow convenient comparison of the phosphorous content of different products. Despite this, phosphorus pentoxide is not actually present in most samples as it is not stable in aqueous solutions.

Related phosphorus oxides

Between the commercially important P₄O₆ and P₄O₁₀, phosphorus oxides are known with intermediate structures.

On observation it will be seen that double bonded oxygen in ${\displaystyle {\ce {P4O8}}}$ at 1,2 position or 1,3 position are identical and both positions have same steric hindrance. Cycle 12341 and ABCDA are identical.

Hazards

Phosphorus pentoxide itself is not flammable. Just like sulfur trioxide, it reacts vigorously with water and water-containing substances like wood or cotton, liberates much heat and may even cause fire due to the highly exothermic nature of such reactions. It is corrosive to metal and is very irritating – it may cause severe burns to the eye, skin, mucous membrane, and respiratory tract even at concentrations as low as 1 mg/m.

See also

• Eaton's reagent

References

1. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
2. Cruickshank, D. W. J. (1964). "Refinements of Structures Containing Bonds between Si, P, S or Cl and O or N: V. P₄O₁₀". Acta Crystallogr. 17 (6): 677–9. doi:10.1107/S0365110X64001669.
3. D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam. ISBN 0-444-89307-5.
4. ^ .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.
5. D. Stachel, I. Svoboda and H. Fuess (June 1995). "Phosphorus Pentoxide at 233 K". Acta Crystallogr. C. 51 (6): 1049–1050. doi:10.1107/S0108270194012126.
6. Threlfall, Richard E., (1951). The story of 100 years of Phosphorus Making: 1851 - 1951. Oldbury: Albright & Wilson Ltd
7. Meier, M. S. "Phosphorus(V) Oxide" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289X.
8. Joseph C. Salamone, ed. (1996). Polymeric materials encyclopedia: C, Volume 2. CRC Press. p. 1417. ISBN 0-8493-2470-X.
9. Tidwell, T. T. "Dimethyl Sulfoxide–Phosphorus Pentoxide" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289X.
10. Luer, B.; Jansen, M. "Crystal Structure Refinement of Tetraphosphorus Nonaoxide, P₄O₉" Zeitschrift für Kristallographie 1991, volume 197, pages 247-8.
11. Phosphorus pentoxide MSDS

External links

• Inorganic phosphorus compounds
• Acid anhydrides
• Acidic oxides
• Glass compositions
• Dehydrating agents
• Adamantane-like molecules
• Phosphorus oxides
• Phosphorus(V) compounds
• Deliquescent materials