Phosphoryl chloride

Phosphoryl chloride

Phosphoryl chloride
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Names
IUPAC names Phosphoryl trichloride Phosphorus trichloride oxide
Other names Phosphorus oxychloride Phosphoric trichloride Trichlorophosphate
Identifiers
CAS Number	10025-87-3 Y
ChEBI	CHEBI:30336 Y
ChemSpider	23198 Y
EC Number	233-046-7
Jmol 3D model	Interactive image
PubChem	24813
RTECS number	TH4897000
UN number	1810
InChI InChI=1S/Cl3OP/c1-5(2,3)4 Y Key: XHXFXVLFKHQFAL-UHFFFAOYSA-N Y InChI=1/Cl3OP/c1-5(2,3)4 Key: XHXFXVLFKHQFAL-UHFFFAOYAS
SMILES O=P(Cl)(Cl)Cl
Properties
Chemical formula	POCl3
Molar mass	153.33 g/mol
Appearance	Clear, colourless liquid, fumes in moist air
Odor	pungent and musty
Density	1.645 g/cm3, liquid
Melting point	1.25 °C (34.25 °F; 274.40 K)
Boiling point	105.8 °C (222.4 °F; 378.9 K)
Solubility in water	Reacts
Vapor pressure	40 mmHg (27°C)
Structure
Molecular shape	tetrahedral
Dipole moment	2.54 D
Vapor pressure	{{{value}}}
Related compounds
Related compounds	Thiophosphoryl chloride Phosphorus oxybromide Phosphorus trichloride Phosphorus pentachloride
Supplementary data page
Structure and properties	Refractive index (n), Dielectric constant (εr), etc.
Thermodynamic data	Phase behaviour solid–liquid–gas
Spectral data	UV, IR, NMR, MS
Y verify (what is YN ?)
Infobox references

Phosphoryl chloride (commonly called phosphorus oxychloride) is a colourless liquid with the formula POCl₃. With the freezing point of 1 °C and boiling point 106 °C, the liquid range is, incidentally, rather similar to water. 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.

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where pm = picometers

Chemical properties

POCl₃ reacts with water and alcohols to give hydrogen chloride and phosphoric acid or phosphate esters, respectively:

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 such as triphenyl phosphate 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 various methods, notable examples include:

• The reaction of phosphorus trichloride with oxygen at 20–50°C (air is ineffective):

2 PCl₃ + O₂ → 2 POCl₃

• The reaction of phosphorus pentachloride (PCl
  ₅) with phosphorus pentoxide (P
  ₄O
  ₁₀). The reaction can be simplified by chlorinating a mixture of PCl₃ and P₄O₁₀, generating the PCl₅ in situ. As the PCl₃ is consumed, the POCl₃ products become a solvent for the unreacted solid starting reagents:

6 PCl₃ + 6 Cl₂ → 6 PCl₅

6 PCl₅ + P₄O₁₀ → 10 POCl₃

• The reaction of phosphorus pentachloride with boric acid or oxalic acid:

3 PCl₅ + 2 B(OH)₃ → 3 POCl₃ + B₂O₃ + 6 HCl

PCl₅ + (COOH)₂ → POCl₃ + CO + CO₂ + 2 HCl

• The oxidation of phosphorus trichloride with potassium chlorate:

3 PCl₃ + KClO₃ → 3 POCl₃ + KCl

• The reaction of phosphorus pentoxide with sodium chloride:

2 P₂O₅ + 3 NaCl → 3 NaPO₃ + POCl₃.

• Reduction of tricalcium phosphate with carbon in the presence of chlorine gas:

Ca₃(PO₄)₂ + 6 C + 6 Cl₂ → 3 CaCl₂ + 6 CO + 2 POCl₃

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 primary amides to nitriles. Similarly, certain aryl amides can be cyclised to dihydroisoquinoline derivatives using the Bischler-Napieralski reaction.

Two uses for phosphorus oxychloride in organic chemistry

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.

Another common laboratory use for POCl₃ is in the Vilsmeier-Haack reaction, where it 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.

Further reading

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. CDC - NIOSH Pocket Guide to Chemical Hazards