Phosphorus pentoxide
From Wikipedia, the free encyclopedia
| Phosphorus pentoxide | |
|---|---|
   |
|
| General | |
| Other names | phosphoric anhydride |
| Empirical formula | P2O5 |
| Molecular formula | P4O10 |
| Molar mass | 283.889 g mol−1 |
| Appearance | white powder very deliquescent pungent odor |
| CAS number | [1314-56-3] |
| Properties | |
| Density and phase | 2.39 g cm−3, solid |
| Solubility in water | exothermic hydrolysis |
| Melting point | 300-360 °C |
| Boiling point | sublimes at 250 °C |
| Vapor pressure | 1 mmHg @ 384°C |
| Hazards | |
| EU classification | not listed |
| NFPA 704 |
1
3
3
|
| 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 acid anhydride of phosphoric acid. It is a powerful desiccant.
Contents |
[edit] Structure
P2O5 crystallizes in at least four forms or polymorphs. The most familiar one shown in the figure, is comprised of molecules with the formulae P4O10. These molecules are packed in a hexagonal lattice, the molecules being held together by weak van der Waal's forces. The structure of the P4O10 cage is reminiscent of adamantane with Td symmetry point group.[1] 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 then 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.
[edit] Preparation
P4O10 is prepared by burning elemental phosphorus:
- 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.[2] 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.[3]
[edit] Uses
Phosphorus pentoxide is an extremely good dehydrating agent.
- P4O10 (am) + 2H2O (lq) → 4HPO3 (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:[4]
- 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.
When combined with a carboxylic acid, the result is the corresponding anhydride:
- P4O10 + RCO2H → P4O9(OH)2 + [RC(O)]2O
The "Onodera reagent", a solution of P4O10 in DMSO, is employed for the oxidation of alcohols.[5] 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.
[edit] Fiction
In Anthony Burgess' The Wanting Seed, phosphorus pentoxide is a highly prized compound.
In Detective Comics #825, Batman notices that phosohorus pentoxide was at the scene of a fire, indicating that the villain Dr. Phosphorus was involved.
[edit] References
- ^ 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.
