|Other names||Phosphorus(V) oxide|
|Molar mass||283.889 g mol−1|
|Appearance||white powder |
|CAS number||[1314-56-3] (P2O5)|
|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|
|Except where noted otherwise, data are given for|
materials in their standard state (at 25 °C, 100 kPa)
Infobox disclaimer and references
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). The structure of the P4O10 cage is reminiscent of adamantane with Td symmetry point group. 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. 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.
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.
- 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
Related phosphorus oxides
- 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.
- D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam. ISBN 0-444-89307-5.
- Threlfall, Richard E., (1951). The story of 100 years of Phosphorus Making: 1851 - 1951. Oldbury: Albright & Wilson Ltd
- Podger, Hugh (2002). Albright & Wilson: The Last 50 Years. Studley: Brewin Books. ISBN 1-85858-223-7
- Meier, M. S. "Phosphorus(V) Oxide" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
- 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.
- Luer, B.; Jansen, M. "Crystal Structure Refinement of Tetraphosphorus Nonaoxide, P4O9" Zeitschrift fur Kristallographie 1991, volume 197, pages 247-8.
- Spec sheet
- Website of the Technische Universität Darmstadt and the CEEP about Phosphorus Recovery