Phosphorus pentoxide
2229678
221756950
2008-06-25T23:26:02Z
201.27.209.105
/* Hazard */
{{Chembox new
| Name = Phosphorus pentoxide
| ImageFile = Phosphorus-pentoxide-2D-dimensions.png
<!-- | ImageSize = 200px -->
| ImageName = Phosphorus pentoxide
| ImageFile1 = Phosphorus-pentoxide-3D-balls.png
<!-- | ImageSize1 = 150px -->
| ImageName1 = Phosphorus pentoxide
| OtherNames = Phosphorus(V) oxide<br />Phosphoric anhydride<!--
| [[Empirical formula]]
| P<sub>2</sub>O<sub>5</sub>
|-
-->
| Section1 = {{Chembox Identifiers
| CASOther = [1314-56-3] (P<sub>2</sub>O<sub>5</sub>)<br />[16752-60-6] (P<sub>4</sub>O<sub>10</sub>)
}}
| Section2 = {{Chembox Properties
| Formula = O<sub>10</sub>P<sub>4</sub>
| MolarMass = 283.889 g mol<sup>−1</sup>
| Appearance = white powder<br />very [[deliquescent]]<br />pungent odour
| Density = 2.39 g cm<sup>−3</sup>, solid
| Solubility = [[exothermic]] hydrolysis
| MeltingPt = 569 °C
| BoilingPt = sublimes at 360 °C
| VaporPressure = 1 mmHg @ 384 °C
}}
| Section7 = {{Chembox Hazards
| EUClass = not listed
| NFPA-H = 3
| NFPA-R = 3
| NFPA-F = 1
| NFPA-O = W
}}
}}
'''Phosphorus pentoxide''' is a [[chemical compound]] with formula [[Phosphorus|P]]<sub>2</sub>[[Oxygen|O]]<sub>5</sub>. This white crystalline solid is the [[anhydride]] of [[phosphoric acid]]. It is a powerful [[desiccant]].
==Structure==
P<sub>2</sub>O<sub>5</sub> '''crystallizes''' in at least four forms or [[polymorphism (materials science)|polymorphs]]. The most familiar one, shown in the figure, comprises molecules of P<sub>4</sub>O<sub>10</sub>. 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<ref>Cruickshank, D.W.J. "Refinements of Structures Containing Bonds between Si, P, S or Cl and O or N: V. P<sub>4</sub>O<sub>10</sub>" Acta Cryst. 1964, volume 17, pages 677-9.</ref>). The structure of the P<sub>4</sub>O<sub>10</sub> cage is reminiscent of [[adamantane]] with T<sub>d</sub> [[symmetry point group]].<ref>D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam. ISBN 0-444-89307-5.</ref> It is closely related to the corresponding anhydride of [[phosphorous acid]], P<sub>4</sub>O<sub>6</sub>. 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 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 P<sub>6</sub>O<sub>6</sub> rings, not unlike the structure adopted by certain poly[[silicate]]s. 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.
==Preparation==
P<sub>2</sub>O<sub>5</sub> is prepared by burning elemental [[phosphorus]] with sufficient supply of air :
: P<sub>4</sub> + 5 O<sub>2</sub> → 2 P<sub>2</sub>O<sub>5</sub>
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.<ref>Threlfall, Richard E., (1951). ''The story of 100 years of Phosphorus Making: 1851 - 1951''. Oldbury: Albright & Wilson Ltd</ref> 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.<ref>Podger, Hugh (2002). ''Albright & Wilson: The Last 50 Years''. Studley: Brewin Books. ISBN 1-85858-223-7</ref>
==Applications==
Phosphorus pentoxide is a potent [[dehydration reaction|dehydrating]] agent as indicated by the exothermic nature of its hydrolysis:
:P<sub>4</sub>O<sub>10</sub> (am) + 6H<sub>2</sub>O (lq) → 4H<sub>3</sub>PO<sub>4</sub> (c) (-177 [[joule|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 P<sub>4</sub>O<sub>10</sub> is used in [[desiccator]]s.
Consistent with its strong desiccating power, P<sub>4</sub>O<sub>10</sub> is used in [[organic synthesis]] for dehydration. The most important application is for the conversion of [[amide]]s into [[nitriles]]:<ref>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.</ref>
:P<sub>4</sub>O<sub>10</sub> + RC(O)NH<sub>2</sub> → P<sub>4</sub>O<sub>9</sub>(OH)<sub>2</sub> + RCN
The indicated coproduct P<sub>4</sub>O<sub>9</sub>(OH)<sub>2</sub> is an idealized formula for undefined products resulting from the hydration of P<sub>4</sub>O<sub>10</sub>.
Apparently, when combined with a [[carboxylic acid]], the result is the corresponding [[anhydride]] {{Fact|date=February 2008}}:
:P<sub>4</sub>O<sub>10</sub> + RCO<sub>2</sub>H → P<sub>4</sub>O<sub>9</sub>(OH)<sub>2</sub> + [RC(O)]<sub>2</sub>O
The "Onodera reagent", a solution of P<sub>4</sub>O<sub>10</sub> in [[DMSO]], is employed for the oxidation of [[alcohol]]s.<ref>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.</ref> This reaction is reminiscent of the [[Swern oxidation]].
The desiccating power of P<sub>4</sub>O<sub>10</sub> is strong enough to convert many mineral acids to their anhydrides. Examples: [[nitric acid|HNO<sub>3</sub>]] is converted to [[dinitrogen pentoxide|N<sub>2</sub>O<sub>5</sub>]]; [[sulfuric acid|H<sub>2</sub>SO<sub>4</sub>]] is converted to [[sulfur trioxide|SO<sub>3</sub>]]; [[perchloric acid|HClO<sub>4</sub>]] is converted to [[dichlorine heptoxide|Cl<sub>2</sub>O<sub>7</sub>]].
==Related phosphorus oxides==
Between the commercially important [[phosphorus trioxide|P<sub>4</sub>O<sub>6</sub>]] and P<sub>4</sub>O<sub>10</sub>, phosphorus oxides are known with intermediate structures.<ref>Luer, B.; Jansen, M. "Crystal Structure Refinement of Tetraphosphorus Nonaoxide, P<sub>4</sub>O<sub>9</sub>" Zeitschrift fur Kristallographie 1991, volume 197, pages 247-8.</ref>
[[Image:Structures of phosphorus oxides.png|600px|Phosphorus oxides: P<sub>4</sub>O<sub>6</sub>, P<sub>4</sub>O<sub>7</sub>, P<sub>4</sub>O<sub>8</sub>, P<sub>4</sub>O<sub>9</sub>, and P<sub>4</sub>O<sub>10</sub>.]]
==Fiction==
In [[Anthony Burgess]]' ''[[The Wanting Seed]]'', phosphorus pentoxide is a highly prized [[Chemical compound|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.
==Hazard==
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.
==References==
{{reflist}}
==See also==
*[[Eaton's reagent]]
==External links==
* [http://www.osha.gov/dts/chemicalsampling/data/CH_262830.html OSHA]
* [http://www.chemicalland21.com/arokorhi/industrialchem/inorganic/PHOSPHORUS%20PENTOXIDE.htm Spec sheet]
* [http://www.thermphos.com/pages/phos_pent.htm Definition]
* [http://www.phosphorus-recovery.tu-darmstadt.de Website of the Technische Universität Darmstadt and the CEEP about Phosphorus Recovery]
[[Category:Oxides]]
[[Category:Phosphorus compounds]]
[[Category:Acidic oxides]]
[[Category:Desiccants]]
[[Category:Glass types]]
[[cs:Oxid fosforečný]]
[[de:Phosphorpentoxid]]
[[fr:Pentoxyde de phosphore]]
[[it:Anidride fosforica]]
[[lv:Fosfora pentoksīds]]
[[ja:五酸化二リン]]
[[pl:Tlenek fosforu(V)]]
[[pt:Pentóxido de fósforo]]
[[sv:Fosforpentoxid]]
[[uk:Фосфатний ангідрид]]
[[zh:五氧化二磷]]