Phosphorus pentoxide is a chemical compound with formula P2O5. This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant.
in at least four forms or polymorphs
. The most familiar one, shown in the figure, comprises molecules of P4
. 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). The structure of the P4
cage is reminiscent of adamantane
with Td symmetry point group
. It is closely related to the corresponding anhydride of phosphorous acid
. 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.
is prepared by burning elemental phosphorus
with sufficient supply of air :
- P4 + 5 O2 → 2 P2O5
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. 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.
Phosphorus pentoxide is a potent dehydrating
agent as indicated by the exothermic nature of its hydrolysis:
- P4O10 (am) + 6H2O (lq) → 4H3PO4 (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 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:
- P4O10 + RC(O)NH2 → P4O9(OH)2 + RCN
The indicated coproduct P4
is an idealized formula for undefined products resulting from the hydration of P4
Apparently, 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. 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.
Related phosphorus oxides
Between the commercially important P4O6
, phosphorus oxides are known with intermediate structures.
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.