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| Names | |||
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| IUPAC names Tetraphosphorus decaoxide Tricyclo[3.3.1.13,7]tetraphosphoxane 1,3,5,7-tetraoxide | |||
| Systematic IUPAC name 2,4,6,8,9,10-Hexaoxa-1λ5,3λ5,5λ5,7λ5-tetraphosphatricyclo[3.3.1.13,7]decane 1,3,5,7-tetraoxide | |||
| Other names Diphosphorus pentoxide Phosphorus(V) oxide Phosphoric anhydride Tetraphosphorus decaoxide Tetraphosphorus decoxide | |||
| Identifiers | |||
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3D model (JSmol) |
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| ECHA InfoCard | 100.013.852 | ||
| RTECS number |
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| Properties | |||
| P4O10 | |||
| Molar mass | 283.9 g mol−1 | ||
| Appearance | White powder Verydeliquescent | ||
| Odor | Odorless | ||
| Density | 2.39 g/cm3 | ||
| Melting point | 340 °C (644 °F; 613 K) | ||
| Boiling point | 360 °C (sublimes) | ||
| exothermic hydrolysis | |||
| Vapor pressure | 1 mmHg @ 385 °C (stable form) | ||
| Hazards | |||
| Occupational safety and health (OHS/OSH): | |||
Main hazards | reacts with water, strongdehydrating agent, corrosive | ||
| GHS labelling: | |||
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| Danger | |||
| H314 | |||
| P280,P301+P330+P331,P303+P361+P353,P305+P351+P338,P310 | |||
| NFPA 704 (fire diamond) | |||
| Safety data sheet (SDS) | MSDS | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Phosphorus pentoxide is achemical compound with molecular formulaP4O10 (with its common name derived from itsempirical formula, P2O5). This white crystalline solid is theanhydride ofphosphoric acid. It is a powerfuldesiccant anddehydrating agent.
Phosphorus pentoxide crystallizes in at least four forms orpolymorphs. The most familiar one, a metastable form[1] (shown in the figure), comprises molecules of P4O10. Weakvan 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[2]). The structure of the P4O10 cage is reminiscent ofadamantane withTdsymmetry point group.[3] It is closely related to the corresponding anhydride ofphosphorous 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. This form can be made by condensing the vapor of phosphorus pentoxide rapidly, and the result is an extremely hygroscopic solid.[4]
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 involving the donation of the terminal oxygen p-orbital electrons to the antibonding phosphorus-oxygen single bonds. The macromolecular form can be made by heating the compound in a sealed tube for several hours, and maintaining the melt at a high temperature before cooling the melt to the solid.[4] The metastable orthorhombic "O"-form (density 2.72 g/cm3,melting point 562 °C) adopts a layered structure consisting of interconnected P6O6 rings, not unlike the structure adopted by certain polysilicates. The stable form is a higher density phase, also orthorhombic, the so-called O' form. It consists of a 3-dimensional framework, density 3.5 g/cm3.[1][5] The remaining polymorph is aglass or amorphous form; it can be made by fusing any of the others.
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| Part of an o′-(P2O5)∞ layer | o′-(P2O5)∞ layers stacking |
P4O10 is prepared by burningwhite phosphorus with a sufficient supply of oxygen:[6]
The dehydration ofphosphoric acid to give phosphorus pentoxide is not possible, as on heating it forms variouspolyphosphates but will not dehydrate sufficiently to form P4O10.
Phosphorus pentoxide is a potentdehydrating agent as indicated by the exothermic nature of its hydrolysis producingphosphoric acid:
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 indesiccators.
Consistent with its strong desiccating power, P4O10 is used inorganic synthesis for dehydration. The most important application is for the conversion of primaryamides intonitriles:[7]
The indicated coproduct P4O9(OH)2 is an idealized formula for undefined products resulting from the hydration of P4O10.
Alternatively, when combined with acarboxylic acid, the result is the correspondinganhydride:[8]
The "Onodera reagent", a solution of P4O10 inDMSO, is employed for the oxidation ofalcohols.[9] This reaction is reminiscent of theSwern oxidation.
The desiccating power of P4O10 is strong enough to convert many mineral acids to their anhydrides. Examples:HNO3 is converted toN2O5; H2SO4 is converted toSO3; HClO4 is converted toCl2O7; CF3SO3H is converted to(CF3)2S2O5.
P2O5 content is often used by industry as proxy value for all the phosphorus oxides in a material. For example, fertilizer grade phosphoric acid can also contain variousrelated phosphorous compounds which are also of use. All these compounds are described collectively in terms of 'P2O5 content' to allow convenient comparison of the phosphorous content of different products. Despite this, phosphorus pentoxide is not actually present in most samples as it is not stable in aqueous solutions.
Phosphorus pentoxide itself is not flammable. Just likesulfur trioxide, it reacts vigorously with water and water-containing substances like wood or cotton, liberates much heat and may even cause fire due to the highlyexothermic nature of such reactions. It iscorrosive to metal and is very irritating – it may cause severe burns to the eye, skin,mucous membrane, andrespiratory tract even at concentrations as low as 1 mg/m3.[10]
