Phosphorus pentoxide

Phosphorus pentoxide is the chemical compound with the formula P4O10. This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant.

Structure
P2O5 crystallizes in at least four forms or polymorphs. The most familiar one shown in the figure, comprises molecules with the formulae 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 ). The structure of the P4O10 cage is reminiscent of adamantane with Td symmetry point group. It is closely related to the corresponding anhydride of phosphorous acid, P4O6. The latter lacks terminal oxo groups. Its density of 2.30 g/cm³. It can be boiled at 423 °C, if the sample is heated more rapidly than 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 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.

Preparation
P2O5 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.

Applications
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 used in desiccators.

Consistent with its strong desiccating power, P4O10 is used in organic synthesis for dehydration. The most important applicaton is for the conversion of amides into nitriles:
 * 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.

Apparently, when combined with a carboxylic acid, the result is the corresponding anhydride (however, what is noticeable is that no references have been provided to substantiate this apparent transformation - although it might happen at around 400C when used in conjunction with Aluminium Oxide):
 * 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 and P4O10, phosphorus oxides are known with intermediate structures.



Fiction
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.