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 dativeπ 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, POCl3autoionizes, owing to the reversible formation of [POCl2]+cations (dichlorooxophosphonium cations) and Cl−anions.
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]
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]
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]
^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. ISBN978-1-4987-5428-6. OCLC930681942.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: others (link)
^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.
^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.