Phosphoryl chloride

Phosphoryl chloride (commonly called phosphorus oxychloride) is a colourless liquid with the formula POCl₃. It hydrolyses in moist air releasing phosphoric acid and fumes of hydrogen chloride. It is manufactured industrially on a large scale from phosphorus trichloride and oxygen or phosphorus pentoxide. It is mainly used to make phosphate esters.

Like phosphate, POCl₃ is tetrahedral in shape. It features three P−Cl bonds and one strong P–O bond, with an estimated bond dissociation energy of 533.5 kJ/mol. Unlike in the case of POF₃, the Schomaker-Stevenson rule predicts appropriate bond length for the P–O bond only if the P–O bond is treated as a double bond, P=O.^{[citation needed]} More modern treatments explain the tight P–O bond as a combination of lone pair transfer from the phosphorus to the oxygen atom and a dative π back-bond that produces an effective [P⁺]-[O⁻] configuration.

Phosphoryl chloride exists as neutral POCl₃ molecules in the solid, liquid and gas states. This is unlike phosphorus pentachloride which exists as neutral PCl₅ molecules in the gas and liquid states but adopts the ionic form [PCl₄]⁺[PCl₆]⁻ (tetrachlorophosphonium hexachlorophosphate(V)) in the solid state. The average bond lengths in the crystal structure of POCl₃ are 1.98 Å for P–Cl and 1.46 Å for P=O.

It has a critical pressure of 3.4 atm. With a freezing point of 1 °C and boiling point of 106 °C, the liquid range of POCl₃ is rather similar to water. Also like water, POCl₃ autoionizes, owing to the reversible formation of [POCl₂]⁺ cations (dichlorooxophosphonium cations) and Cl⁻ anions.

POCl₃ reacts with water to give hydrogen chloride and phosphoric acid:

Intermediates in the conversion have been isolated, including pyrophosphoryl chloride, O(−P(=O)Cl₂)₂.

Upon treatment with excess alcohols and phenols, POCl₃ gives phosphate esters:

Such reactions are often performed in the presence of an HCl acceptor such as pyridine or an amine.