Phosphoryl chloride

Phosphoryl chloride (commonly called phosphorus oxychloride) is a colourless liquid with the formula POCl₃. 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.^[4] It is mainly used to make phosphate esters.

Phosphoryl chloride

Names
Preferred IUPAC name Phosphoryl trichloride[1]
Other names Phosphorus(V) oxychloride Phosphoric trichloride Trichlorophosphate Phosphorus(V) oxide trichloride
Identifiers
CAS Number	10025-87-3 Y
3D model (JSmol)	Interactive image Interactive image
ChEBI	CHEBI:30336 Y
ChemSpider	23198 Y
ECHA InfoCard	100.030.030
EC Number	233-046-7
Gmelin Reference	2272
PubChem CID	24813
RTECS number	TH4897000
UNII	9XM78OL22K Y
UN number	1810
CompTox Dashboard (EPA)	DTXSID5029710
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 [O-][P+](Cl)(Cl)Cl
Properties
Chemical formula	POCl3
Molar mass	153.32 g·mol−1
Appearance	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
Solubility	highly soluble in benzene, chloroform, carbon disulfide, carbon tetrachloride
Vapor pressure	40 mmHg (27 °C)[2]
Refractive index (nD)	1.460
Structure
Molecular shape	Tetrahedral at the P atom
Dipole moment	2.54 D
Thermochemistry[3]
Heat capacity (C)	138.8 J·mol−1·K−1 (liquid), 84.9 J·mol−1·K−1 (gas)
Std molar entropy (S⦵298)	222.5 J·mol−1·K−1 (liquid), 325.5 J·mol−1·K−1 (gas)
Std enthalpy of formation (ΔfH⦵298)	−597.1 kJ·mol−1 (liquid), −558.5 kJ·mol−1 (gas)
Gibbs free energy (ΔfG⦵)	−520.8 kJ·mol−1 (liquid), −512.9 kJ·mol−1(gas)
Enthalpy of fusion (ΔfH⦵fus)	13.1 kJ·mol−1
Enthalpy of vaporization (ΔfHvap)	38.6 kJ·mol−1
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Toxic and corrosive; releases HCl on contact with water[2]
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H302, H314, H330, H372
Precautionary statements	P260, P264, P270, P271, P280, P284, P301+P312, P301+P330+P331, P303+P361+P353, P304+P340, P305+P351+P338, P310, P314, P320, P321, P330, P363, P403+P233, P405, P501
NFPA 704 (fire diamond)	3 0 2 W
Lethal dose or concentration (LD, LC):
LD50 (median dose)	380 mg/kg (rat, oral)
NIOSH (US health exposure limits):
PEL (Permissible)	none[2]
REL (Recommended)	TWA 0.1 ppm (0.6 mg/m3) ST 0.5 ppm (3 mg/m3)[2]
IDLH (Immediate danger)	N.D.[2]
Safety data sheet (SDS)	ICSC 0190
Related compounds
Related compounds	Thiophosphoryl chloride Phosphorus oxybromide Phosphorus trichloride Phosphorus pentachloride
Supplementary data page
Phosphoryl chloride (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Structure

Unit cell of phosphoryl chloride.^[5]

Like phosphate, POCl₃ is tetrahedral in shape.^[6] 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 POF₃, 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.^{[citation needed]} 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.^[7]

Phosphoryl chloride exists as neutral POCl₃ molecules in the solid, liquid and gas states. This is unlike phosphorus pentachloride which exists as neutral PCl₅ molecules in the gas and liquid states but adopts the ionic form [PCl₄]⁺[PCl₆]⁻ (tetrachlorophosphonium hexachlorophosphate(V)) in the solid state. The average bond lengths in the crystal structure of POCl₃ are 1.98 Å for P–Cl and 1.46 Å for P=O.^[5]

Physical properties

It has a critical pressure of 3.4 atm.^[8] With a freezing point of 1 °C and boiling point of 106 °C, the liquid range of POCl₃ is rather similar to water. Also like water, POCl₃ autoionizes, owing to the reversible formation of [POCl₂]⁺ cations (dichlorooxophosphonium cations) and Cl⁻ anions.

Chemical properties

POCl₃ reacts with water to give hydrogen chloride and phosphoric acid:

O=PCl₃ + 3 H₂O → O=P(OH)₃ + 3 HCl

Intermediates in the conversion have been isolated, including pyrophosphoryl chloride, O(−P(=O)Cl₂)₂.^[9]

Upon treatment with excess alcohols and phenols, POCl₃ gives phosphate esters:

O=PCl₃ + 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.

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

POCl₃ + TiCl₄ → POCl₃·TiCl₄

The aluminium chloride adduct (POCl₃·AlCl₃) is quite stable, and so POCl₃ can be used to remove AlCl₃ from reaction mixtures, for example at the end of a Friedel-Crafts reaction.

POCl₃ reacts with hydrogen bromide in the presence of Lewis-acidic catalysts to produce POBr₃.

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.^[10]

By oxidation

The commercial method involves oxidation of phosphorus trichloride with oxygen:^[11]

2 PCl₃ + O₂ → 2 POCl₃

An alternative method involves the oxidation of phosphorus trichloride with potassium chlorate:^[12]

3 PCl₃ + KClO₃ → 3 POCl₃ + KCl

Oxygenations

The reaction of phosphorus pentachloride (PCl₅) with phosphorus pentoxide (P₄O₁₀).

6 PCl₅ + P₄O₁₀ → 10 POCl₃

The reaction can be simplified by chlorinating a mixture of PCl₃ and P₄O₁₀, generating the PCl₅ in situ. The reaction of phosphorus pentachloride with boric acid or oxalic acid:^[12]

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

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

Other methods

Reduction of tricalcium phosphate with carbon in the presence of chlorine gas:^[13]

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

The reaction of phosphorus pentoxide with sodium chloride is also reported:^[13]

2 P₂O₅ + 3 NaCl → 3 NaPO₃ + POCl₃

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.^[11] POCl₃ is also used in the production of organophosphate insecticides.

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.

As a reagent

In the laboratory, POCl₃ is a reagent in dehydrations. One example involves conversion of formamides to isonitriles (isocyanides);^[14] primary amides to nitriles:^[15]

RC(O)NH₂ + POCl₃ → 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.^[16]

In the Vilsmeier-Haack reaction, POCl₃ 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.^[17]

References

1. Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. p. 926. doi:10.1039/9781849733069-FP001. ISBN 978-0-85404-182-4.
2. NIOSH Pocket Guide to Chemical Hazards. "#0508". National Institute for Occupational Safety and Health (NIOSH).
3. CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data. William M. Haynes, David R. Lide, Thomas J. Bruno (2016-2017, 97th ed.). Boca Raton, Florida. 2016. ISBN 978-1-4987-5428-6. OCLC 930681942.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: others (link)
4. Toy, Arthur D. F. (1973). The Chemistry of Phosphorus. Oxford: Pergamon Press. ISBN 978-0-08-018780-8. OCLC 152398514.
5. Olie, K. (1971). "The crystal structure of POCl₃". Acta Crystallogr. B. 27 (7): 1459–1460. doi:10.1107/S0567740871004138.
6. Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann.
7. Chesnut, D. B.; Savin, A. (1999). "The Electron Localization Function (ELF) Description of the PO Bond in Phosphine Oxide". Journal of the American Chemical Society. 121 (10): 2335–2336. doi:10.1021/ja984314m. ISSN 0002-7863.
8. "Phosphoryl chloride".
9. Grunze, Herbert (1963). "Über die Hydratationsprodukte des Phosphoroxychlorides. III. Darstellung von Pyrophosphorylchlorid aus partiell hydrolysiertem Phosphoroxychlorid (Hydration products of phosphorus oxychloride. III. Preparation of pyrophosphoryl chloride from partially hydrolyzed phosphorus oxychloride)". Zeitschrift für Anorganische und Allgemeine Chemie. 324: 1–14. doi:10.1002/zaac.19633240102.
10. Wurtz, Adolphe (1847). "Sur l'acide sulfophosphorique et le chloroxyde de phosphore" [On monothiophosphoric acid and phosphoryl chloride]. Annales de Chimie et de Physique. 3rd series (in French). 20: 472–481.; see Chloroxyde de phosphore, pp. 477–481. (Note: Wurtz's empirical formulas are wrong because, like many chemists of his day, he used the wrong atomic mass for oxygen.)Roscoe, Henry Enfield; Schorlemmer, Carl; Cannell, John, eds. (1920). A Treatise on Chemistry. Vol. 1 (5th ed.). London, England: Macmillan and Co. p. 676.
11. Bettermann, Gerhard; Krause, Werner; Riess, Gerhard; Hofmann, Thomas (2000). "Phosphorus Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_527. ISBN 978-3527306732..
12. Pradyot, Patnaik (2003). Handbook of Inorganic Chemicals. New York: McGraw-Hill. p. 709. ISBN 0-07-049439-8.
13. Lerner, Leonid (2011). Small-Scale Synthesis of Laboratory Reagents with Reaction Modeling. Boca Raton, Florida: CRC Press. pp. 169–177. ISBN 978-1-4398-1312-6.
14. Patil, Pravin; Ahmadian-Moghaddam, Maryam; Dömling, Alexander (29 September 2020). "Isocyanide 2.0". Green Chemistry. 22 (20): 6902–6911. doi:10.1039/D0GC02722G.
15. March, J. (1992). Advanced Organic Chemistry (4th ed.). New York, NY: Wiley. p. 723. ISBN 978-0-471-60180-7.
16. Elderfield, R. C. (ed.). Heterocyclic Compound. Vol. 6. New York, NY: John Wiley & Sons. p. 265.
17. Hurd, Charles D.; Webb, Carl N. (1925). "p-Dimethylaminobenzophenone". Organic Syntheses. 7: 24. doi:10.15227/orgsyn.007.0024.

Further reading

• Handbook of Chemistry and Physics (71st ed.). Ann Arbor, MI: CRC Press. 1990.^{[ISBN missing]}
• Stecher, Paul G. (1960). The Merck Index of Chemicals and Drugs (7th ed.). Rahway: Merck & Co. OCLC 3653550.^{[ISBN missing]}
• Wade, L. G. Jr (2005). Organic Chemistry (6th ed.). Upper Saddle River, NJ: Pearson/Prentice Hall. p. 477.^{[ISBN missing]}
• Walker, B. J. (1972). Organophosphorus Chemistry. Harmondsworth: Penguin. pp. 101–116.^{[ISBN missing]}
• "CDC – NIOSH Pocket Guide to Chemical Hazards".