Phosphoryl chloride

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 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 phosphate esters, respectively, for example

O=PCl₃ + 3 H₂O → 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 magnesium chloride a triaryl phosphate ester is formed, for example:

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 hydrogen bromide in the presence of AlCl₃ to produce POBr₃.

Preparation

Phosphoryl chloride can be prepared by the reaction of oxygen at 20-50 °C (air is ineffective):

2 O₂ → 2 O=PCl₃

An alternative synthesis involves the reaction of 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 PCl₅

6 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 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 isoquinoline 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 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 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 Organic Syntheses, Coll. Vol. 3, p.98; Vol. 20, p.11. (Article)