Phosphorus pentoxide

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 T_d 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, and 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.

part of an o′-(P2O5)∞ layer	o′-(P2O5)∞ layers stacking

P₄O₁₀ is prepared by burning white phosphorus with a sufficient supply of oxygen:[6]

P₄ + 5 O₂ → P₄O₁₀

The dehydration of phosphoric acid to give phosphorus pentoxide is not possible, as on heating it forms various polyphosphates but will not dehydrate sufficiently to form P₄O₁₀.

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

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:[7]

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:[8]

P₄O₁₀ + RCO₂H → P₄O₉(OH)₂ + [RC(O)]₂O

The "Onodera reagent", a solution of P₄O₁₀ in DMSO, is employed for the oxidation of alcohols.[9] 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.[10]

On observation it will be seen that double bonded oxygen in ${\displaystyle {\ce {P4O8}}}$ at 1,2 position or 1,3 position are identical and both positions have same steric hindrance. Cycle 12341 and ABCDA are identical.

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.[11]

• Eaton's reagent