Phosphorus pentoxide

Phosphorus pentoxide is a chemical compound with molecular formula P₄O₁₀ (with its common name derived from its empirical formula, P₂O₅). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydrating agent.

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

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:

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:

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;  CF₃SO₃H is converted to (CF₃)₂S₂O₅.