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Phosphorus pentoxide

Phosphorus pentoxide

Other names Phosphorus(V) oxide
Phosphoric anhydride
Empirical formula P2O5
Molecular formula O10P4
Molar mass 283.889 g mol−1
Appearance white powder
very deliquescent
pungent odour
CAS number [1314-56-3] (P2O5)
[16752-60-6] (P4O10)
Density and phase 2.39 g cm−3, solid
Solubility in water exothermic hydrolysis
Melting point 569 °C
Boiling point sublimes at 360 °C
Vapor pressure 1 mmHg @ 384 °C
EU classification not listed
NFPA 704
Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)
Infobox disclaimer and references

Phosphorus pentoxide is the chemical compound with the formula P4O10. This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant.

Additional recommended knowledge



P2O5 crystallizes in at least four forms or polymorphs. The most familiar one shown in the figure, comprises molecules with the formulae 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 Td symmetry point group.[2] It is closely related to the corresponding anhydride of phosphorous acid, P4O6. The latter lacks terminal oxo groups. Its density of 2.30 g/cm³. It can be boiled at 423 °C, if the sample is heated more rapidly than it can sublime.

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, m.p. 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.


P2O5 is prepared by burning elemental phosphorus with sufficient supply of air :

P4 + 5 O2 → 2 P2O5

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.[3] 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.[4]


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

P4O10 (am) + 6H2O (lq) → 4H3PO4 (c)   (-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 used in desiccators.

Consistent with its strong desiccating power, P4O10 is used in organic synthesis for dehydration. The most important applicaton is for the conversion of amides into nitriles:[5]

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.

Apparently, when combined with a carboxylic acid, the result is the corresponding anhydride (however, what is noticeable is that no references have been provided to substantiate this apparent transformation - although it might happen at around 400C when used in conjunction with Aluminium Oxide):

P4O10 + RCO2H → P4O9(OH)2 + [RC(O)]2O

The "Onodera reagent", a solution of P4O10 in DMSO, is employed for the oxidation of alcohols.[6] 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 N2O5H2SO4 is converted to SO3HClO4 is converted to Cl2O7.

Related phosphorus oxides

Between the commercially important P4O6 and P4O10, phosphorus oxides are known with intermediate structures.[7]


In Anthony Burgess' The Wanting Seed, phosphorus pentoxide is a highly prized compound.

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.


  1. ^ Cruickshank, D.W.J. "Refinements of Structures Containing Bonds between Si, P, S or Cl and O or N: V. P4O10" Acta Cryst. 1964, volume 17, pages 677-9.
  2. ^ D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam. ISBN 0-444-89307-5.
  3. ^ Threlfall, Richard E., (1951). The story of 100 years of Phosphorus Making: 1851 - 1951. Oldbury: Albright & Wilson Ltd
  4. ^ Podger, Hugh (2002). Albright & Wilson: The Last 50 Years. Studley: Brewin Books. ISBN 1-85858-223-7
  5. ^ 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.
  6. ^ 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.
  7. ^ Luer, B.; Jansen, M. "Crystal Structure Refinement of Tetraphosphorus Nonaoxide, P4O9" Zeitschrift fur Kristallographie 1991, volume 197, pages 247-8.

See also

  • Eaton's reagent
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Phosphorus_pentoxide". A list of authors is available in Wikipedia.
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