Everything about Phosphorus Oxychloride totally explained
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Phosphoryl chloride (commonly called
phosphorus oxychloride) is a colourless liquid with the formula
3. It hydrolyses in moist air to
phosphoric acid to release 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 very 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 POF
3). The P=O bond doesn't resemble the
π bond in a
carbonyl group as in a ketone. The appropriate description of the P-O interaction is a matter of long discussion. Older textbooks favor a description that invokes participation of the
d-orbitals on phosphorus. Some of these d-orbitals project toward the O atom, overlapping with
p-orbitals on oxygen. More modern texts seem to favor a description where the P-O
π bonding involves the sigma* components of the P-Cl bonds. These descriptions don't consider a role for d-orbitals.
» ::
where pm = picometers
Chemical properties
POCl
3 reacts with water and
alcohols to give
phosphoric acid or
phosphate esters, respectively, for example
» O=PCl
3 + 3 H
2O →
O=P(OH)3 + 3
HCl
If the water is replaced by an
alcohol, the trialkyl phosphate esters result. Such reactions are often performed in the presence of an HCl acceptor such as
pyridine or an
amine. If POCl
3 is heated with an excess of a
phenol (
ArOH) in the presence of a
Lewis acid catalyst such as
magnesium chloride a triaryl phosphate ester is formed, for example:
» 3
C6H5OH + O=PCl
3 →
O=P(OC6H5)3 + 3
HCl
POCl
3 can also act as a
Lewis base, forming s with a variety of Lewis acids such as
titanium tetrachloride:
» Cl
3P
+-O
− + TiCl
4 → Cl
3P
+-O-
−TiCl
4
The
aluminium chloride adduct (POCl
3·AlCl
3) is quite stable, and so POCl
3 can be used to remove AlCl
3 completely from reaction mixtures at the end of a
Friedel-Crafts reaction. POCl
3 reacts with
hydrogen bromide in the presence of AlCl
3 to produce POBr
3.
Preparation
Phosphoryl chloride can be prepared by the reaction of
phosphorus trichloride with
oxygen at 20-50 °C (air is ineffective):
2
PCl3 +
O2 → 2 O=PCl
3
An alternative synthesis involves the reaction of
phosphorus pentachloride and
phosphorus pentoxide. Since these compounds are both solids, a convenient way of performing the reaction is to
chlorinate a mixture of PCl
3 and P
4O
10, which generates the PCl
5 . As the PCl
3 is consumed, the POCl
3 becomes the reaction solvent.
6
PCl3 + 6
Cl2 → 6
PCl5
6
PCl5 +
P4O10 → 10 POCl
3
Phosphorus pentachloride also forms POCl
3 by reaction with water, but this reaction is less easily controlled than the above reaction.
Uses
The most important use for phosphoryl chloride is in the manufacture of triarylphosphate esters (as described
above) such as
triphenyl phosphate and
tricresyl phosphate. These esters have been used for many years as
flame retardants and
plasticisers for
PVC. Meanwhile trialkyl esters such as
tributyl phosphate (made similarly from
butan-1-ol) are used as
liquid-liquid extraction solvents in
nuclear reprocessing and elsewhere.
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.
In the laboratory, POCl
3 is widely used as a dehydrating agent, for example the conversion of
amides to
nitriles. Similarly, certain cyclic amides can be cyclised to dihydro
isoquinoline derivatives using the
Bischler-Napieralski reaction.
Such reactions are believed to go via an
imidoyl chloride; in certain cases where it's stable, the imidoyl chloride is the final product. For example
pyridones and pyrimidones can be converted to chloro- derivatives of
pyridines and
pyrimidines, which are important intermediates in the pharmaceutical industry. Likewise
barbituric acid is converted to 2,4,6-trichloropyrimidine.
[8] by reaction with POCl
3 at 140 °C.
Related to this chemistry is the use of POCl
3 in acylation of activated
aromatic rings via the
Vilsmeier-Haack reaction to produce
aryl aldehydes and
ketones. The reaction most often uses a formamide such as
DMF or
N-phenyl-
N-methylformamide, and it produces an iminium salt which is easily hydrolysed to the aldehyde upon workup. For example
anthracene gives 9-anthraldehyde:
Further Information
Get more info on 'Phosphorus Oxychloride'.
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