Phosphorus pentachloride - Misplaced Pages

Phosphorus pentachloride is the chemical compound with the formula PCl₅. It is one of the most important phosphorus chlorides/oxychlorides, others being PCl₃ and POCl₃. PCl₅ finds use as a chlorinating reagent. It is a colourless, water-sensitive solid, although commercial samples can be yellowish and contaminated with hydrogen chloride.

Structure

The structures for the phosphorus chlorides are invariably consistent with VSEPR theory. The structure of PCl₅ depends on its environment. Gaseous and molten PCl₅ is a neutral molecule with trigonal bipyramidal geometry and (D_3h) symmetry. The hypervalent nature of this species (as well as of PCl
₆, see below) can be explained with the inclusion of non-bonding molecular orbitals (molecular orbital theory) or resonance (valence bond theory). This trigonal bipyramidal structure persists in nonpolar solvents, such as CS₂ and CCl₄. In the solid state PCl₅ is an ionic compound called tetrachlorophosphonium hexachlorophosphate formulated PCl
₄PCl
₆.

In solutions of polar solvents, PCl₅ undergoes self-ionization. Dilute solutions dissociate according to the following equilibrium:

PCl₅ ⇌ PCl
₄ + Cl

At higher concentrations, a second equilibrium becomes more prevalent:

2 PCl₅ ⇌ PCl
₄ + PCl
₆

The cation PCl
₄ and the anion PCl
₆ are tetrahedral and octahedral, respectively. At one time, PCl₅ in solution was thought to form a dimeric structure, P₂Cl₁₀, but this suggestion is not supported by Raman spectroscopic measurements.

Related pentachlorides

AsCl₅ and SbCl₅ also adopt trigonal bipyramidal structures. The relevant bond distances are 211 pm (As−Cl_eq), 221 pm (As−Cl_ax), 227 pm (Sb−Cl_eq), and 233.3 pm (Sb−Cl_ax). At low temperatures, SbCl₅ converts to the dimer, dioctahedral Sb₂Cl₁₀, structurally related to niobium pentachloride.

Preparation

PCl₅ is prepared by the chlorination of PCl₃. This reaction is used to produce around 10,000 tonnes of PCl₅ per year (as of 2000).

PCl₃ + Cl₂ ⇌ PCl₅ (ΔH = −124 kJ/mol)

PCl₅ exists in equilibrium with PCl₃ and chlorine, and at 180 °C the degree of dissociation is about 40%. Because of this equilibrium, samples of PCl₅ often contain chlorine, which imparts a greenish coloration.

Reactions

Hydrolysis

In its most characteristic reaction, PCl₅ reacts upon contact with water to release hydrogen chloride and give phosphorus oxides. The first hydrolysis product is phosphorus oxychloride:

PCl₅ + H₂O → POCl₃ + 2 HCl

In hot water, hydrolysis proceeds completely to orthophosphoric acid:

PCl₅ + 4 H₂O → H₃PO₄ + 5 HCl

Lewis acidity

Phosphorus pentachloride is a Lewis acid. This property underpins many of its characteristic reactions, autoionization, chlorinations, hydrolysis. A well studied adduct is PCl₅(pyridine).

Chlorination of organic compounds

In synthetic chemistry, two classes of chlorination are usually of interest: oxidative chlorinations and substitutive chlorinations. Oxidative chlorinations entail the transfer of Cl₂ from the reagent to the substrate. Substitutive chlorinations entail replacement of O or OH groups with chloride. PCl₅ can be used for both processes.

Upon treatment with PCl₅, carboxylic acids convert to the corresponding acyl chloride. The following mechanism has been proposed:

It also converts alcohols to alkyl chlorides. Thionyl chloride is more commonly used in the laboratory because the resultant sulfur dioxide is more easily separated from the organic products than is POCl₃.

PCl₅ reacts with a tertiary amides, such as dimethylformamide (DMF), to give dimethylchloromethyleneammonium chloride, which is called the Vilsmeier reagent, Cl. More typically, a related salt is generated from the reaction of DMF and POCl₃. Such reagents are useful in the preparation of derivatives of benzaldehyde by formylation and for the conversion of C−OH groups into C−Cl groups.

It is especially renowned for the conversion of C=O groups to CCl₂ groups. For example, benzophenone and phosphorus pentachloride react to give the diphenyldichloromethane:

(C₆H₅)₂CO + PCl₅ → (C₆H₅)₂CCl₂ + POCl₃

The electrophilic character of PCl₅ is highlighted by its reaction with styrene to give, after hydrolysis, phosphonic acid derivatives.

Comparison with related reagents

Both PCl₃ and PCl₅ convert R₃COH groups to the chloride R₃CCl. The pentachloride is however a source of chlorine in many reactions. It chlorinates allylic and benzylic CH bonds. PCl5 bears a greater resemblance to SO₂Cl₂, also a source of Cl₂. For oxidative chlorinations on the laboratory scale, sulfuryl chloride is often preferred over PCl₅ since the gaseous SO₂ by-product is readily separated.

Chlorination of inorganic compounds

As for the reactions with organic compounds, the use of PCl₅ has been superseded by SO₂Cl₂. The reaction of phosphorus pentoxide and PCl₅ produces POCl₃ :

6 PCl₅ + P₄O₁₀ → 10 POCl₃

PCl₅ chlorinates nitrogen dioxide to form unstable nitryl chloride:

PCl₅ + 2 NO₂ → PCl₃ + 2 NO₂Cl

2 NO₂Cl → 2 NO₂ + Cl₂

PCl₅ is a precursor for lithium hexafluorophosphate, LiPF₆. Lithium hexafluorophosphate is a commonly employed salt in electrolytes in lithium ion batteries. LiPF
₆ is produced by the reaction of PCl
₅ with lithium fluoride, with lithium chloride as a side product:

PCl₅ + 6 LiF → LiPF₆ + 5 LiCl

Safety

PCl₅ is a dangerous substance as it reacts violently with water. It is also corrosive when in contact with skin and can be fatal when inhaled.

History

Phosphorus pentachloride was first prepared in 1808 by the English chemist Humphry Davy. Davy's analysis of phosphorus pentachloride was inaccurate; the first accurate analysis was provided in 1816 by the French chemist Pierre Louis Dulong.

References

^ NIOSH Pocket Guide to Chemical Hazards. "#0509". National Institute for Occupational Safety and Health (NIOSH).
^ Phosphorus pentachloride in Linstrom, Peter J.; Mallard, William G. (eds.); NIST Chemistry WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, Gaithersburg (MD) (retrieved 2014-05-15)
^ Phosphorus pentachloride
^ "Phosphorus pentachloride". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
Corbridge, D. E. C. (1995). Phosphorus: An outline of its chemistry, biochemistry, and uses. Elsevier Science. ISBN 0-444-89307-5.
^ Holleman, A. F.; Wiber, E.; Wiberg, N. (2001). Inorganic Chemistry. Academic Press. ISBN 978-0-12-352651-9.
Finch, A.; Fitch, A.N.; Gates, P.N. (1993). "Crystal and Molecular structure of a metastable modification of phosphorus pentachloride". Journal of the Chemical Society, Chemical Communications (11): 957–958. doi:10.1039/C39930000957.
Suter, R. W.; Knachel, H. C.; Petro, V. P.; Howatson, J. H. & Shore, S. G. (1978). "Nature of Phosphorus(V) Chloride in Ionizing and Nonionizing Solvents". Journal of the American Chemical Society. 95 (5): 1474–1479. doi:10.1021/ja00786a021.
Haupt, S.; Seppelt, K. (2002). "Solid State Structures of AsCl₅ and SbCl₅". Zeitschrift für anorganische und allgemeine Chemie. 628 (4): 729–734. doi:10.1002/1521-3749(200205)628:4<729::AID-ZAAC729>3.0.CO;2-E.
Maxson, R. N. (1939). "Phosphorus Pentachloride". Inorganic Syntheses. Vol. 1. pp. 99–100. doi:10.1002/9780470132326.ch34. ISBN 9780470132326.
Wong, Chih Y.; Kennepohl, Dietmar K.; Cavell, Ronald G. (1996). "Neutral Six-Coordinate Phosphorus". Chemical Reviews. 96 (6): 1917–1952. doi:10.1021/cr9410880. PMID 11848816.
Adams, R.; Jenkins, R. L. (1941). "p-Nitrobenzoyl chloride". Organic Syntheses; Collected Volumes, vol. 1, p. 394.
Clayden, Jonathan (2005). Organic chemistry (Reprinted ed.). Oxford: Oxford University Press. ISBN 978-0-19-850346-0.
Burks Jr., J. E. (2004). "Phosphorus(V) chloride". In Paquette, L. (ed.). Encyclopedia of Reagents for Organic Synthesis. New York, NY: J. Wiley & Sons. doi:10.1002/047084289X.rp158. ISBN 0471936235.
Gross, H.; Rieche, A.; Höft, E.; Beyer, E. (1973). "Dichloromethyl methyl ether". Organic Syntheses; Collected Volumes, vol. 5, p. 365.
Spaggiari, A.; Vaccari, D.; Davoli, P.; Torre, G.; Prati, F. (2007). "A Mild Synthesis of Vinyl Halides and gem-Dihalides Using Triphenyl Phosphite−Halogen-Based Reagents". The Journal of Organic Chemistry. 72 (6): 2216–2219. doi:10.1021/jo061346g. ISSN 0022-3263. PMID 17295542.
Schmutzler, R. (1973). "Styrylphosphonic dichloride". Organic Syntheses; Collected Volumes, vol. 5, p. 1005.
Cotton, Frank Albert (1999). Advanced Inorganic Chemistry. Wiley-Interscience. ISBN 978-0-471-19957-1.
Bushkova, O. V.; Yaroslavtseva, T. V.; Dobrovolsky, Yu. A. (4 August 2017). "New lithium salts in electrolytes for lithium-ion batteries (Review)". Russian Journal of Electrochemistry. 53 (7): 677–699. doi:10.1134/S1023193517070035. S2CID 103854243.
Davy, Humphry (1809). "The Bakerian Lecture. An account of some new analytical researches on the nature of certain bodies, particularly the alkalies, phosphorus, sulphur, carbonaceous matter, and the acids hitherto undecomposed; with some general observations on chemical theory". Philosophical Transactions of the Royal Society of London. 99: 39–104. doi:10.1098/rstl.1809.0005. S2CID 98814859. On pp. 94–95, Davy mentioned that when he burned phosphorus in chlorine gas ("oxymuriatic acid gas"), he obtained a clear liquid (phosphorus trichloride) and a white solid (phosphorus pentachloride).
Davy, Humphry (1810). "Researches on the oxymuriatic acid [i.e., chlorine], its nature and combinations; and on the elements of the muriatic acid [i.e., hydrogen chloride]. With some experiments on sulphur and phosphorus, made in the laboratory of the Royal Institution". Philosophical Transactions of the Royal Society of London. 100: 231–257. doi:10.1098/rstl.1810.0016. S2CID 95219058. On p. 257, Davy presented his empirical formula for phosphorus pentachloride: 1 portion of phosphorus to 3 portions of "oxymuriatic gas" (chlorine).
Dulong (1816). "Extrait d'un mémoire sur les combinaisons du phosphore avec l'oxigène" [Extract from a memoir on the compounds of phosphorus with oxygen]. Annales de Chimie et de Physique. 2nd series (in French). 2: 141–150. On p. 148, Dulong presented the correct analysis of phosphorus pentachloride (which is 14.9% phosphorus and 85.1% chlorine by weight, vs. Dulong's values of 15.4% and 84.6%, respectively).

External links

v t e Phosphorus compounds
Phosphides	AlP AsP BP BiP Cs₂P₅ Ca₃P₂ Cd₃P₂ Cu₃P DyP ErP EuP GdP AuP FeP Fe₃P HfP HoP InP LaP Li₃P LuP MoP MoP₂ Mo₃P NbP NdP NpP Np₃P₄ OsP₂ Ru₂P PtP₂ PrP PrP₅ PuP ScP SmP SmP₅ Sr₃P₂ TbP SnP₃ ThP₇ TmP YP YbP ZnP₂ Zn₃P₂ ZrP ZrP₂
Other compounds	PBr₃ PBr₅ PBr₇ PCl₃ PCl₅ P₂Cl₄ PF₃ PF₅ PI₃ PH₃ PN P₃N₅ PO P₂O₃ P₂O₄ P₂O₅ P₄S₃ P₄S_x P₄S₁₀