Phosphorus pentoxide

Phosphorus pentoxide
Other names Diphosphorus pentoxide Phosphorus(V) oxide Phosphoric anhydride
Identifiers
CAS number	1314-56-3 Y,  [16752-60-6] (P4O10)
PubChem	14812
ChemSpider	14128 Y
ChEBI	CHEBI:37376 Y
RTECS number	TH3945000
Jmol-3D images	Image 1
SMILES O=P13OP2(=O)OP(=O)(O1)OP(=O)(O2)O3
InChI InChI=1S/O10P4/c1-11-5-12(2)8-13(3,6-11)10-14(4,7-11)9-12 Y Key: DLYUQMMRRRQYAE-UHFFFAOYSA-N Y
Properties
Molecular formula	O10P4
Molar mass	283.89 g mol−1
Exact mass	283.889048 g/mol
Appearance	white powder very deliquescent pungent odour
Density	2.39 g/cm3
Melting point	340 °C, 613 K, 644 °F
Boiling point	360 °C (sublimes)
Solubility in water	exothermic hydrolysis
Vapor pressure	1 mmHg @ 384 °C
Hazards
MSDS	MSDS
EU classification	not listed
NFPA 704	1 3 3 W
Y (verify) (what is: Y/N?) Except where noted otherwise, 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.

Structure

Phosphorus pentoxide crystallizes in at least four forms or polymorphs. The most familiar one, 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^[1]). The structure of the P₄O₁₀ cage is reminiscent of adamantane with T_d symmetry point group.^[2] It is closely related to the corresponding anhydride of phosphorous acid, P₄O₆. The latter lacks terminal oxo groups. Its density is 2.30 g/cm³. It boils at 423 °C under atmospheric pressure; if heated more rapidly it can sublimate.

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. The O-form (density 3.05 g/cm³, melting point 580 °C), adopts a layered structure consisting of interconnected P₆O₆ rings, not unlike the structure adopted by certain polysilicates. A lower density phase, the so-called O' form, consists of a 3-dimensional framework is also known, density 2.72 g/cm³.^[3] 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 elemental phosphorus with sufficient supply of air:

P₄ + 5 O₂ → P₄O₁₀

For most of the 20th century, phosphorus pentoxide was used to provide a supply of concentrated pure phosphoric acid. In the thermal process, the phosphorus pentoxide obtained by burning white phosphorus was dissolved in dilute phosphoric acid to produce concentrated acid.^[4] Improvements in filter technology is leading to the "wet phosphoric acid process" taking over from the thermal process, obviating the need to produce white phosphorus as a starting material.^[5] The dehydration of phosphoric acid to give phosphorus pentoxide is not practicable; on heating, metaphosphoric acid will decompose before it loses water.

Applications

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

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 amides into nitriles:^[6]

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.

Supposedly, when combined with a carboxylic acid, the result is the corresponding anhydride:^[7]:

P₄O₁₀ + RCO₂H → P₄O₉(OH)₂ + [RC(O)]₂O

The "Onodera reagent", a solution of P₄O₁₀ in DMSO, is employed for the oxidation of alcohols.^[8] 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.

Related phosphorus oxides

Between the commercially important P₄O₆ and P₄O₁₀, phosphorus oxides are known with intermediate structures.^[9]

Hazards

Phosphorus pentoxide is not flammable. It reacts vigorously with water and water-containing substances like wood or cotton, liberates much heat and may even cause fire. It is corrosive to metal and is very irritating – may cause severe burn to the eye, skin, mucous membrane, and respiratory tract even at concentrations as low as 1 mg/m³.^[10]

Fiction

• In Anthony Burgess' The Wanting Seed, phosphorus pentoxide is a highly prized compound.^{[clarification needed]}
• In Detective Comics #825, Batman notices that phosphorus pentoxide was at the scene of a fire, indicating that the villain Dr. Phosphorus was involved.
• In Aldous Huxley's Point Counter Point, Lord Edward bemoans societal loss of phosphorous pentoxide to his assistant Illidge.
• In Aldous Huxley's Brave New World, Henry Foster tells Lenina about the recovery of phosphorus pentoxide.

See also

• Eaton's reagent

References

1. ^ Cruickshank, D.W.J. (1964). "Refinements of Structures Containing Bonds between Si, P, S or Cl and O or N: V. P₄O₁₀". Acta Cryst. 17 (6): 677–9. doi:10.1107/S0365110X64001669. 
2. ^ D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam. ISBN 0-444-89307-5.
3. ^ D. Stachel, I. Svoboda and H. Fuess (June 1995). "Phosphorus Pentoxide at 233 K". Acta Cryst. C51 (6): 1049–1050. doi:10.1107/S0108270194012126. 
4. ^ Threlfall, Richard E., (1951). The story of 100 years of Phosphorus Making: 1851 - 1951. Oldbury: Albright & Wilson Ltd
5. ^ Podger, Hugh (2002). Albright & Wilson: The Last 50 Years. Studley: Brewin Books. ISBN 1-85858-223-7
6. ^ Meier, M. S. "Phosphorus(V) Oxide" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289.
7. ^ Joseph C. Salamone, ed (1996). Polymeric materials encyclopedia: C, Volume 2. CRC Press. p. 1417. ISBN 084932470X. http://books.google.com/?id=pzRpQC7CO6sC&pg=PA1417. 
8. ^ Tidwell, T. T. "Dimethyl Sulfoxide–Phosphorus Pentoxide" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289.
9. ^ Luer, B.; Jansen, M. "Crystal Structure Refinement of Tetraphosphorus Nonaoxide, P₄O₉" Zeitschrift fur Kristallographie 1991, volume 197, pages 247-8.
10. ^ Phosphorus pentoxide MSDS

External links

• OSHA
• Spec sheet
• Definition
• Website of the Technische Universität Darmstadt and the CEEP about Phosphorus Recovery