Phosphorus pentoxide

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Phosphorus pentoxide
Phosphorus pentoxide Phosphorus-pentoxide-2D-dimensions.png
Phosphorus pentoxide
Phosphorus pentoxide Phosphorus-pentoxide-3D-balls.png
Phosphorus pentoxide
Sample of Phosphorus pentoxide.jpg
Names
Other names
Diphosphorus pentoxide
Phosphorus(V) oxide
Phosphoric anhydride
Tetraphosphorus decaoxide
Tetraphosphorus decoxide
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
PubChem CID
RTECS number
  • TH3945000
Properties
P4O10
Molar mass 283.9 g mol−1
Appearancewhite powder
very deliquescent
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
Safety data sheet MSDS
NFPA 704 (fire diamond)
Flammability code 0: Will not burn. E.g. waterHealth code 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasReactivity code 3: Capable of detonation or explosive decomposition but requires a strong initiating source, must be heated under confinement before initiation, reacts explosively with water, or will detonate if severely shocked. E.g. hydrogen peroxideSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g. sodium, sulfuric acidPhosphorus pentoxide
0
3
3
W
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Phosphorus pentoxide is a chemical compound with molecular formula P 4 O 10 (with its common name derived from its empirical formula, P2O5). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydrating agent.

Contents

Structure

Phosphorus pentoxide crystallizes in at least four forms or polymorphs. The most familiar one, a metastable form, [1] 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 packing [2] ). The structure of the P4O10 cage is reminiscent of adamantane with Td symmetry point group. [3] 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. This form can be made by condensing the vapor of phosphorus pentoxide rapidly, 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 a glass or amorphous form; it can be made by fusing any of the others.

Phosphorus-pentoxide-sheet-from-xtal-3D-balls.png
Phosphorus-pentoxide-xtal-3D-balls.png
part of an o′-(P2O5) layer
o′-(P2O5) layers stacking

Preparation

P4O10 is prepared by burning tetraphosphorus with sufficient supply of oxygen:

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. [6] 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. [7] The dehydration of phosphoric acid to give phosphorus pentoxide is not possible as on heating metaphosphoric acid will boil without losing all its water.

Applications

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

P4O10 + 6 H2O → 4 H3PO4  (–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 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 primary amides into nitriles: [8]

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.

Alternatively, when combined with a carboxylic acid, the result is the corresponding anhydride: [9]

P4O10 + RCO2H → P4O9(OH)2 + [RC(O)]2O

The "Onodera reagent", a solution of P4O10 in DMSO, is employed for the oxidation of alcohols. [10] 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;  CF3SO3H is converted to (CF3)2S2O5.

Between the commercially important P4O6 and P4O10, phosphorus oxides are known with intermediate structures. [11]

Structures of phosphorus oxides.png

Hazards

Phosphorus pentoxide itself is not flammable. Just like sulfur 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 highly exothermic nature of such reactions. It is corrosive to metal and is very irritating – it may cause severe burns to the eye, skin, mucous membrane, and respiratory tract even at concentrations as low as 1 mg/m3. [12]

Trivia


See also

Related Research Articles

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Dinitrogen pentoxide chemical compound

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Phosphorus pentachloride chemical compound

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Phosphorus oxoacids are oxoacids of phosphorus. Phosphorus exhibits oxidation states from +1 to +5. Oxygen may be in oxidation state -2 or -1, depending on whether a compound contains the peroxide group.

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Phosphoryl chloride chemical compound

Phosphoryl chloride (commonly called phosphorus oxychloride) is a colourless liquid with the formula POCl3. It hydrolyses in moist air releasing phosphoric acid and fumes of hydrogen chloride. It is manufactured industrially on a large scale from phosphorus trichloride and oxygen or phosphorus pentoxide. It is mainly used to make phosphate esters such as tricresyl phosphate.

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Phosphorus trioxide chemical compound

Phosphorus trioxide is the chemical compound with the molecular formula P4O6. Although the molecular formula suggests the name tetraphosphorus hexoxide, the name phosphorus trioxide preceded the knowledge of the compound's molecular structure, and its usage continues today. This colorless solid is structurally related to adamantane. It is formally the anhydride of phosphorous acid, H3PO3, but cannot be obtained by the dehydration of the acid. It is a white, waxy, crystalline and highly toxic solid with garlic odour.

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Iodine pentoxide chemical compound

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Phosphorus oxide can refer to:

Hypophosphoric acid chemical compound

Hypophosphoric acid is a mineral acid with the formula H4P2O6, with phosphorus in a formal oxidation state of +4. In the solid state it is present as the dihydrate, H4P2O6·2H2O. In hypophosphoric acid the phosphorus atoms are identical and joined directly with a P−P bond. There is an isomeric form isohypophosphoric acid which has a different structure with non-identical phosphorus atoms, one of which has a directly bonded H atom and an oxidation state of +3, which is joined by a P−O−P bridge to the second phosphorus atom which has an oxidation state of +5.

References

  1. 1 2 Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN   978-0-08-037941-8.
  2. Cruickshank, D.W.J. (1964). "Refinements of Structures Containing Bonds between Si, P, S or Cl and O or N: V. P4O10". Acta Crystallogr. 17 (6): 677–9. doi:10.1107/S0365110X64001669.
  3. D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam. ISBN   0-444-89307-5.
  4. 1 2 .Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 15: The group 15 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 473. ISBN   978-0-13-175553-6.
  5. D. Stachel, I. Svoboda and H. Fuess (June 1995). "Phosphorus Pentoxide at 233 K". Acta Crystallogr. C. 51 (6): 1049–1050. doi:10.1107/S0108270194012126.
  6. Threlfall, Richard E., (1951). The story of 100 years of Phosphorus Making: 1851 - 1951. Oldbury: Albright & Wilson Ltd
  7. Podger, Hugh (2002). Albright & Wilson: The Last 50 Years. Studley: Brewin Books. ISBN   1-85858-223-7
  8. 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.
  9. Joseph C. Salamone, ed. (1996). Polymeric materials encyclopedia: C, Volume 2. CRC Press. p. 1417. ISBN   0-8493-2470-X.
  10. 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.
  11. Luer, B.; Jansen, M. "Crystal Structure Refinement of Tetraphosphorus Nonaoxide, P4O9" Zeitschrift fur Kristallographie 1991, volume 197, pages 247-8.
  12. Phosphorus pentoxide MSDS