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 choking 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 such as tricresyl phosphate.

Structure

Like phosphate, phosphoryl chloride is tetrahedral in shape. It features three P-Cl bonds and one strong P=O double bond, with an estimated bond dissociation energy of 533.5 kJ/mol. On the basis of bond length and electronegativity, the Schomaker-Stevenson rule suggests that the double bond form is very dominant (in contrast with POF₃). The P=O bond does not utilize the d-orbitals on phosphorus as is commonly described in older textbooks as quantum chemical calculations have shown that d-orbitals are not involved in main group chemical bonding (see Hypervalent molecule). More modern texts favour a description which involves the donation of the lone pair electrons from oxygen p-orbitals to the antibonding phosphorus-chlorine bonds thus constituting π bonding.