# Phosphoryl chloride

Phosphoryl chloride (commonly called phosphorus oxychloride) is a colourless liquid with the formula POCl$_3$. 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.

## Structure

Like phosphate, POCl$_3$ 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 POF$_3$, 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. 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 POCl$_3$ molecules in the solid, liquid and gas states. This is unlike phosphorus pentachloride which exists as neutral PCl$_5$ molecules in the gas and liquid states but adopts the ionic form [PCl$_4$]$^+$[PCl$_6$]$^−$ (tetrachlorophosphonium hexachlorophosphate(V)) in the solid state. The average bond lengths in the crystal structure of POCl$_3$ are 1.98 Å for P–Cl and 1.46 Å for P=O.

## Physical properties

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 POCl$_3$ is rather similar to water. Also like water, POCl$_3$ autoionizes, owing to the reversible formation of [POCl$_2$]$^+$ cations (dichlorooxophosphonium cations) and Cl$^−$ anions.

## Chemical properties

POCl$_3$ reacts with water to give hydrogen chloride and phosphoric acid:
    O=PCl$_3$ + 3 H$_2$O → O=P(OH)$_3$ + 3 HCl
Intermediates in the conversion have been isolated, including pyrophosphoryl chloride, O(−P(=O)Cl$_2$)$_2$.
Upon treatment with excess alcohols and phenols, POCl$_3$ gives phosphate esters:
    O=PCl$_3$ + 3 ROH → O=P(OR)$_3$ + 3 HCl
Such reactions are often performed in the presence of an HCl acceptor such as pyridine or an amine.
POCl$_3$ can also act as a Lewis base, forming adducts with a variety of Lewis acids such as titanium tetrachloride:
    POCl$_3$ + TiCl$_4$ → POCl$_3$·TiCl$_4$
The aluminium chloride adduct (POCl$_3$·AlCl$_3$) is quite stable, and so POCl$_3$ can be used to remove AlCl$_3$ from reaction mixtures, for example at the end of a Friedel-Crafts reaction.
POCl$_3$ reacts with hydrogen bromide in the presence of Lewis-acidic catalysts to produce POBr$_3$.

## Preparation

Phosphoryl chloride can be prepared by many methods. Phosphoryl chloride was first reported in 1847 by the French chemist Adolphe Wurtz by reacting phosphorus pentachloride with water.

### By oxidation

The commercial method involves oxidation of phosphorus trichloride with oxygen:
    2 PCl$_3$ + O$_2$ → 2 POCl$_3$
An alternative method involves the oxidation of phosphorus trichloride with potassium chlorate:
    3 PCl$_3$ + KClO$_3$ → 3 POCl$_3$ + KCl

### Oxygenations

The reaction of phosphorus pentachloride (PCl$_5$) with phosphorus pentoxide (P$_4$O$_{10}$).
    6 PCl$_5$ + P$_4$O$_{10}$ → 10 POCl$_3$
The reaction can be simplified by chlorinating a mixture of PCl$_3$ and P$_4$O$_{10}$, generating the PCl$_5$ in situ. The reaction of phosphorus pentachloride with boric acid or oxalic acid:
    3 PCl$_5$ + 2 B(OH)$_3$ → 3 POCl$_3$ + B$_2$O$_3$ + 6 HCl
    PCl$_5$ + (COOH)$_2$ → POCl$_3$ + CO + CO$_2$ + 2 HCl

### Other methods

Reduction of tricalcium phosphate with carbon in the presence of chlorine gas:
    Ca$_3$(PO$_4$)$_2$ + 6 C + 6 Cl$_2$ → 3 CaCl$_2$ + 6 CO + 2 POCl$_3$
The reaction of phosphorus pentoxide with sodium chloride is also reported:
    2 P$_2$O$_5$ + 3 NaCl → 3 NaPO$_3$ + POCl$_3$

## Uses

Phosphoryl chloride is used on an industrial scale for the manufacture of phosphate esters (organophosphates). These have a wide range of uses, including as flame retardants (bisphenol A diphenyl phosphate, TCPP and tricresyl phosphate), plasticisers for PVC and related polymers ( 2-ethylhexyl diphenyl phosphate) and hydraulic fluids. POCl$_3$ is also used in the production of organophosphate insecticides.
In the semiconductor industry, POCl$_3$ is used as a safe liquid phosphorus source in diffusion processes. The phosphorus acts as a dopant used to create n-type layers on a silicon wafer.

### As a reagent

In the laboratory, POCl$_3$ is a reagent in dehydrations. One example involves conversion of formamides to isonitriles (isocyanides); primary amides to nitriles:
    RC(O)NH$_2$ + POCl$_3$ → RCN + P(O)OHCl + 2 HCl
In a related reaction, certain aryl-substituted amides can be cyclized using the Bischler-Napieralski reaction.
Such reactions are believed to proceed via an imidoyl chloride. In certain cases, the imidoyl chloride is the final product. For example, pyridones and pyrimidones can be converted to chloro- derivatives such as 2-chloropyridines and 2-chloropyrimidines, which are intermediates in the pharmaceutical industry.
In the Vilsmeier-Haack reaction, POCl$_3$ reacts with amides to produce a "Vilsmeier reagent", a chloro-iminium salt, which subsequently reacts with electron-rich aromatic compounds to produce aromatic aldehydes upon aqueous work-up.