Phosphoryl chloride

Phosphoryl chloride (commonly called phosphorus oxychloride) is a colourless liquid with the formula POCl₃. 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.

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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 POF₃). The P=O bond does not 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 do not consider a role for d-orbitals.

where pm = picometers

Chemical properties

POCl₃ reacts with water and alcohols to give phosphoric acid or phosphate esters, respectively, for example

O=PCl₃ + 3 H₂O → 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 POCl₃ 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=PCl₃ → O=P(OC₆H₅)₃ + 3 HCl

POCl₃ can also act as a Lewis base, forming adducts with a variety of Lewis acids such as titanium tetrachloride:

Cl₃P⁺-O⁻ + TiCl₄ → Cl₃P⁺-O-⁻TiCl₄

The aluminium chloride adduct (POCl₃·AlCl₃) is quite stable, and so POCl₃ can be used to remove AlCl₃ completely from reaction mixtures at the end of a Friedel-Crafts reaction. POCl₃ reacts with hydrogen bromide in the presence of AlCl₃ to produce POBr₃.

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=PCl₃

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 PCl₃ and P₄O₁₀, which generates the PCl₅ in situ. As the PCl₃ is consumed, the POCl₃ becomes the reaction solvent.

6 PCl₃ + 6 Cl₂ → 6 PCl₅

6 PCl₅ + P₄O₁₀ → 10 POCl₃

Phosphorus pentachloride also forms POCl₃ 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, POCl₃ 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, POCl₃ is widely used as a dehydrating agent, for example the conversion of amides to nitriles. Similarly, certain cyclic amides can be cyclised to dihydroisoquinoline derivatives using the Bischler-Napieralski reaction.

Such reactions are believed to go via an imidoyl chloride; in certain cases where it is 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 POCl₃ at 140 °C.

Related to this chemistry is the use of POCl₃ 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:

References

1. N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
2. Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
3. J. March, Advanced Organic Chemistry, 4th ed., p. 723, Wiley, New York, 1992.
4. The Merck Index, 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
5. A. D. F. Toy, The Chemistry of Phosphorus, Pergamon Press, Oxford, UK, 1973.
6. L. G. Wade, Jr., Organic Chemistry, 6th ed., p. 477, Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005.
7. B. J. Walker, Organophosphorus chemistry, p101-116, Penguin, Harmondsworth, UK, 1972.
8. R. C. Elderfield, Heterocyclic Compounds, Vol. 6, p 265-266, Wiley, New York, 1957. For trichloropyrimidine prep. see Gabriel & Colman, Berichte 37, 3657 (1904).
9. Formylation of anthracene. Organic Syntheses, Coll. Vol. 3, p.98; Vol. 20, p.11. (Article)