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.[3] It is mainly used to make phosphate esters such as tricresyl phosphate.

Phosphoryl chloride
Names
Preferred IUPAC name
Phosphoryl trichloride[1]
Other names
Phosphorus oxychloride
Phosphoric trichloride
Trichlorophosphate
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.030.030
EC Number
  • 233-046-7
RTECS number
  • TH4897000
UNII
UN number 1810
Properties
POCl3
Molar mass 153.33 g/mol
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)
Reacts
Solubility highly soluble in benzene, chloroform, CS2, CCl4
Vapor pressure 40 mmHg (27 °C)[2]
1.460
Structure
tetrahedral
2.54 D
Thermochemistry
84.35 J/mol K
Std enthalpy of
formation (ΔfH⦵298)
 -568.4 kJ/mol
Hazards
Safety data sheet See: data page
ICSC 0190
Very toxic (T+)
Harmful (Xn)
Corrosive (C)
R-phrases (outdated) R14, R22, R26, R35, R48/23
S-phrases (outdated) (S1/2), S7/8, S26, S36/37/39, S45
NFPA 704 (fire diamond)
Flammability code 0: Will not burn. E.g. waterHealth code 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasReactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g. white phosphorusSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g. sodium, sulfuric acid
0
3
2
Lethal dose or concentration (LD, LC):
36 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]
Related compounds
Related compounds
 Thiophosphoryl chloride
Phosphorus oxybromide
Phosphorus trichloride
Phosphorus pentachloride
Supplementary data page
Refractive index (n),
Dielectric constant (εr), etc.
Thermodynamic
data
 Phase behaviour
solid–liquid–gas
UV, IR, NMR, MS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Structure

Like phosphate, phosphoryl chloride is tetrahedral in shape.[4] 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 dominant, in contrast with the case of POF3. The P=O bond involves the donation of the lone pair electrons on oxygen p-orbitals to the antibonding combinations associated with phosphorus-chlorine bonds, thus constituting π bonding.[5]

Physical properties

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+,Cl−.

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, P2O3Cl4.[6]

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:

Cl3PO + TiCl4 → Cl3POTiCl4

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.[7]

By oxidation

The commercial method involves oxidation of phosphorus trichloride with oxygen:[8]

2  PCl3 + O2 → 2  POCl3

An alternative method involves the oxidation of phosphorus trichloride with potassium chlorate:[9]

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:[9]

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:[10]

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

The reaction of phosphorus pentoxide with sodium chloride is also reported:[10]

2 P2O5 + 3 NaCl → 3 NaPO3 + POCl3.

Uses

In one commercial application, phosphoryl chloride is used in the manufacture of phosphate esters. Triarylphosphates such as triphenyl phosphate and tricresyl phosphate are used as flame retardants and plasticisers for PVC. Trialkylphosphates such as tributyl phosphate 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.

As a reagent

In the laboratory, POCl3 is a reagent in dehydrations. One example involves conversion of primary amides to nitriles:[11]

RC(O)NH2 + POCl3 → RCN + "PO2Cl" + 2 HCl

In a related reaction, certain aryl-substituted amides can be cyclised 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.[12]

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.[13]

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. Toy, Arthur D. F. (1973). The Chemistry of Phosphorus. Oxford: Pergamon Press. ISBN 9780080187808. OCLC 152398514.
  4. Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann.
  5. 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.
  6. 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 fuer Anorganische und Allgemeine Chemie. 324: 1–14. doi:10.1002/zaac.19633240102.
  7. 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.
  8. Betterman, G.; Krause, W.; Riess, G.; Hofmann, T. (2005). "Phosphorus Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. Ullmann’s Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_527. ISBN 3527306730.CS1 maint: uses authors parameter (link).
  9. Pradyot, Patnaik (2003). Handbook of Inorganic Chemicals. New York: McGraw-Hill. p. 709. ISBN 0070494398.
  10. Lerner, Leonid (2011). Small-Scale Synthesis of Laboratory Reagents with Reaction Modeling. Boca Raton, Florida: CRC Press. pp. 169–177. ISBN 9781439813126.
  11. March, J. (1992). Advanced Organic Chemistry (4th ed.). New York, NY: Wiley. p. 723.
  12. Elderfield, R. C. (ed.). Heterocyclic Compound. 6. New York, NY: John Wiley & Sons. p. 265.
  13. 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.
  • Stecher, Paul G. (1960). The Merck Index of Chemicals and Drugs (7th ed.). Rahway: Merck & Co. OCLC 3653550.
  • Wade, L. G., Jr (2005). Organic Chemistry (6th ed.). Upper Saddle River, NJ: Pearson/Prentice Hall. p. 477.
  • Walker, B. J. (1972). Organophosphorus Chemistry. Harmondsworth: Penguin. pp. 101–116.
  • "CDC – NIOSH Pocket Guide to Chemical Hazards".
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