Phosphoryl chloride

File:Phosphoryl-chloride-unit-cell-down-c-axis-3D-bs-17.png of phosphoryl chloride.{{ cite journal | first1 = K. | last1 = Olie | title = The crystal structure of POCl₃ | journal = Acta Crystallogr. B | year = 1971 | volume = 27 | issue = 7 | pages = 1459–1460 | doi = 10.1107/S0567740871004138 }}]]

Like phosphate, {{chem2|POCl3}} is tetrahedral in shape.{{cite book|first1=N. N.|last1=Greenwood|first2=A.|last2=Earnshaw|title=Chemistry of the Elements|edition=2nd|publisher=Butterworth-Heinemann|location=Oxford|date=1997}} 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 phosphoryl fluoride, 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.{{cite needed|date=August 2023}} 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.{{cite journal|last1=Chesnut|first1=D. B.|last2=Savin|first2=A.|date=1999|title=The Electron Localization Function (ELF) Description of the PO Bond in Phosphine Oxide|journal=Journal of the American Chemical Society|volume=121|issue=10|pages=2335–2336|doi=10.1021/ja984314m|issn=0002-7863}}

File:POCl3 structure.png

Phosphoryl chloride exists as neutral {{chem2|POCl3}} molecules in the solid, liquid and gas states. This is unlike phosphorus pentachloride which exists as neutral {{chem2|PCl5}} molecules in the gas and liquid states but adopts the ionic form {{chem2|[PCl4]+[PCl6]−}} (tetrachlorophosphonium hexachlorophosphate(V)) in the solid state. The average bond lengths in the crystal structure of {{chem2|POCl3}} are 1.98 Å for P–Cl and 1.46 Å for P=O.

It has a critical pressure of 3.4 atm.{{cite web | url=https://www.stenutz.eu/chem/solv6.php?name=phosphoryl+chloride | title=Phosphoryl chloride }} With a freezing point of 1 °C and boiling point of 106 °C, the liquid range of {{chem2|POCl3}} is rather similar to water. Also like water, {{chem2|POCl3}} autoionizes, owing to the reversible formation of {{chem2|[POCl2]+}} cations (dichlorooxophosphonium cations) and chloride anions.

{{chem2|POCl3}} reacts with water to give hydrogen chloride and phosphoric acid:

:{{chem2|O\dPCl3 + 3 H2O → O\dP(OH)3 + 3 HCl}}

Intermediates in the conversion have been isolated, including pyrophosphoryl chloride, {{chem2|O(\sP(\dO)Cl2)2}}.{{cite journal|title=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)|author=Grunze, Herbert|journal=Zeitschrift für Anorganische und Allgemeine Chemie|year=1963|volume=324|pages=1–14|doi=10.1002/zaac.19633240102}}

Upon treatment with excess alcohols and phenols, {{chem2|POCl3}} gives phosphate esters:

:{{chem2|O\dPCl3 + 3 ROH → O\dP(OR)3 + 3 HCl}}

Such reactions are often performed in the presence of an HCl acceptor such as pyridine or an amine.

{{chem2|POCl3}} can also act as a Lewis base, forming adducts with a variety of Lewis acids such as titanium tetrachloride:

:{{chem2|POCl3 + TiCl4 → POCl3*TiCl4}}

The aluminium chloride adduct ({{chem2|POCl3*AlCl3}}) is quite stable, and so {{chem2|POCl3}} can be used to remove {{chem2|AlCl3}} from reaction mixtures, for example at the end of a Friedel-Crafts reaction.

{{chem2|POCl3}} reacts with hydrogen bromide in the presence of Lewis-acidic catalysts to produce {{chem2|POBr3}}.

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.{{cite journal|last1=Wurtz|first1=Adolphe|title=Sur l'acide sulfophosphorique et le chloroxyde de phosphore|journal=Annales de Chimie et de Physique|date=1847|volume=20|pages=472–481|url=https://babel.hathitrust.org/cgi/pt?id=hvd.hx3dxy;view=1up;seq=480|series=3rd series|trans-title=On monothiophosphoric acid and phosphoryl chloride|language=French}}; 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.){{cite book|editor1-last=Roscoe|editor1-first=Henry Enfield|editor2-last=Schorlemmer|editor2-first=Carl|editor3-last=Cannell|editor3-first=John|title=A Treatise on Chemistry|date=1920|publisher=Macmillan and Co.|location=London, England|volume=1|page=676|edition=5th|url=https://books.google.com/books?id=uRJDAAAAIAAJ&pg=PA676}}

The commercial method involves oxidation of phosphorus trichloride with oxygen:{{Ullmann |doi=10.1002/14356007.a19_527|title=Phosphorus Compounds, Inorganic|year=2000|last1=Bettermann|first1=Gerhard|last2=Krause|first2=Werner|last3=Riess|first3=Gerhard|last4=Hofmann|first4=Thomas}}.

:{{chem2|2 PCl3 + O2 → 2 POCl3}}

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

:{{chem2|3 PCl3 + KClO3 → 3 POCl3 + KCl}}

The reaction of phosphorus pentachloride ({{chem2|PCl5}}) with phosphorus pentoxide ({{chem2|P4O10}}).

:{{chem2|6 PCl5 + P4O10 → 10 POCl3}}

The reaction can be simplified by chlorinating a mixture of {{chem2|PCl3}} and {{chem2|P4O10}}, generating the {{chem2|PCl5}} in situ.

The reaction of phosphorus pentachloride with boric acid or oxalic acid:{{cite book|last1=Pradyot|first1=Patnaik|title=Handbook of Inorganic Chemicals|date=2003|publisher=McGraw-Hill|location=New York|isbn=0-07-049439-8|page=709|url=https://books.google.com/books?id=Xqj-TTzkvTEC&q=0070494398}}

:{{chem2|3 PCl5 + 2 B(OH)3 → 3 POCl3 + B2O3 + 6 HCl}}

:{{chem2|PCl5 + (COOH)2 → POCl3 + CO + CO2 + 2 HCl}}

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

:{{chem2|Ca3(PO4)2 + 6 C + 6 Cl2 → 3 CaCl2 + 6 CO + 2 POCl3}}

The reaction of phosphorus pentoxide with sodium chloride is also reported:{{cite book|last1=Lerner|first1=Leonid|title=Small-Scale Synthesis of Laboratory Reagents with Reaction Modeling|date=2011|publisher=CRC Press|location=Boca Raton, Florida|isbn=978-1-4398-1312-6|pages=169–177|url=https://books.google.com/books?id=KaIzmQEACAAJ&q=9781439813126}}

:{{chem2|2 P2O5 + 3 NaCl → 3 NaPO3 + POCl3}}

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. POCl₃ is also used in the production of organophosphate insecticides.

In the semiconductor industry, {{chem2|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, {{chem2|POCl3}} is a reagent in dehydrations. One example involves conversion of formamides to isonitriles (isocyanides);{{cite journal |last1=Patil |first1=Pravin |last2=Ahmadian-Moghaddam |first2=Maryam |last3=Dömling |first3=Alexander |title=Isocyanide 2.0 |journal=Green Chemistry |date=29 September 2020 |volume=22 |issue=20 |pages=6902–6911 |doi=10.1039/D0GC02722G |doi-access=free }} primary amides to nitriles:{{cite book|first=J.|last=March|title=Advanced Organic Chemistry|url=https://archive.org/details/advancedorganicc00marc_339|url-access=limited|edition=4th|page=[https://archive.org/details/advancedorganicc00marc_339/page/n739 723]|publisher=Wiley|location=New York, NY|date=1992|isbn=978-0-471-60180-7}}

:{{chem2|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.

:File:Bischler-Napieralski Reaction Scheme.png

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.{{cite book|editor-first=R. C.|editor-last=Elderfield|title=Heterocyclic Compound|volume=6|publisher=John Wiley & Sons|location=New York, NY|page=265}}

In the Vilsmeier-Haack reaction, {{chem2|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.{{cite journal|first1=Charles D.|last1=Hurd|first2=Carl N.|last2=Webb|year=1925|title=p-Dimethylaminobenzophenone|journal=Organic Syntheses|volume=7|page=24|doi=10.15227/orgsyn.007.0024}}

{{Reflist|2}}

{{Refbegin}}

• {{cite book|title=Handbook of Chemistry and Physics|edition=71st|publisher=CRC Press|location=Ann Arbor, MI|date=1990}}{{ISBN missing|date=March 2018}}
• {{cite book|title=The Merck Index of Chemicals and Drugs|last=Stecher|first=Paul G.|date=1960|edition=7th|publisher=Merck & Co|location=Rahway|oclc=3653550}}{{ISBN missing|date=March 2018}}
• {{cite book|first=L. G. Jr|last=Wade|title=Organic Chemistry|edition=6th|page=477|publisher=Pearson/Prentice Hall|location=Upper Saddle River, NJ|date=2005}}{{ISBN missing|date=March 2018}}
• {{cite book|first=B. J.|last=Walker|title=Organophosphorus Chemistry|pages=101–116|publisher=Penguin|location=Harmondsworth|date=1972}}{{ISBN missing|date=March 2018}}
• {{cite web|url=https://www.cdc.gov/niosh/npg/npgd0508.html|title=CDC – NIOSH Pocket Guide to Chemical Hazards}}

{{Refend}}

{{Chemical agents}}

Category:Dehydrating agents

Category:Oxychlorides

Category:Phosphorus oxohalides

Category:Phosphorus(V) compounds

Category:Phosphine oxides