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.

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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^[1]). The structure of the P₄O₁₀ cage is reminiscent of adamantane with T_d symmetry point group.^[2] 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.^[3] 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.^[4]

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:^[5]

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.^[6] 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.^[7]

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.

References

See also

• Eaton's reagent