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Phosphorus pentoxide |
| Phosphorus pentoxide | |
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| Other names | Phosphorus(V) oxide Phosphoric anhydride |
| Identifiers | |
| CAS number | [1314-56-3] (P2O5) [16752-60-6] (P4O10) |
| Properties | |
| Molecular formula | O10P4 |
| Molar mass | 283.889 g mol−1 |
| Appearance | white powder very deliquescent pungent odour |
| Density | 2.39 g cm−3, solid |
| Melting point |
569 °C (At a higher atmosphere of pressure) |
| Boiling point |
sublimes at 360 °C (At standard atmosphere of pressure) |
| Solubility in water | exothermic hydrolysis |
| Vapor pressure | 1 mmHg @ 384 °C |
| Hazards | |
| EU classification | not listed |
| NFPA 704 |
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| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references |
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Phosphorus pentoxide is a chemical compound with formula P2O5. This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant.
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P2O5 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 packing1). 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 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, 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. The remaining polymorph is a glass or amorphous form; it can be made by fusing any of the others.
P2O5 is prepared by burning elemental phosphorus with sufficient supply of air :
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 The dehydration of phosphoric acid to give phosphorus pentoxide is not practicable; on heating, metaphosphoric acid will decompose before it loses water.
Phosphorus pentoxide is a potent dehydrating agent as indicated by the exothermic nature of its hydrolysis:
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:5
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 anhydridecitation needed:
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 N2O5; H2SO4 is converted to SO3; HClO4 is converted to Cl2O7.
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.
Non-flammable. Reacts vigorously with water and water-containing substances like wood or cotton, liberates much heat and may even cause fire. Corrosive to metal. Very irritating. May cause severe burn to eye, skin, mucosa and respiratory tract even at as low as 1mg/cm3.
Fire fighting measures: Wear full protective gear. Do NOT use water directly on fire. Use dry chemical, carbon dioxide, or alcohol-resistant foam.
First aid measures:
Inhalation: Remove from exposure to fresh air immediately. Get medical aid.
Eye: Flush eyes with plenty of water for at least 15 minutes. Get medical aid.
Skin: Flush skin with plenty of water. Get medical aid.
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