Phosphorus pentachloride

Phosphorus pentachloride is the chemical compound with the formula PCl₅. It is one of the most important phosphorus chlorides, others being PCl₃ and POCl₃. PCl₅ finds use as a chlorinating reagent. It is a colourless, water- and moisture-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 (D_3h) symmetry. The hypervalent nature of this species (as well as for PCl−
6, see below) can be explained with the inclusion of non-bonding MOs (Molecular orbital theory) or resonance (Valence bond theory). This trigonal bipyramidal structure persists in non-polar solvents, such as CS₂ and CCl₄.^[5] In the solid state PCl₅ is ionic, formulated PCl+
4PCl−
6.^[6]

In solutions of polar solvents, PCl₅ undergoes "autoionization".^[7] 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][PCl₆⁻]

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 (As-Cl_eq), 221 (As-Cl_ax), 227 (Sb-Cl_eq), and 233.3 pm (Sb-Cl_ax ).^[8] At low temperatures, SbCl₅ converts to the dimer, bioctahedral Sb₂Cl₁₀, structurally related to niobium pentachloride.

Preparation

PCl₅ is prepared by the chlorination of PCl₃.^[9] This reaction is used to produce ca. 10,000,000 kg/y of PCl₅ (as of 2000).^[6]

PCl₃ + Cl₂ ⇌ PCl₅ (ΔH = −124 kJ/mol)

PCl₅ exists in equilibrium with PCl₃ and chlorine, and at 180 °C the degree of dissociation is ca. 40%.^[6] Because of this equilibrium, samples of PCl₅ often contain chlorine, which imparts a greenish colouration.

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 ortho-phosphoric acid:

PCl₅ + 4 H₂O → H₃PO₄ + 5 HCl

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.^[10] The following mechanism has been proposed:^[11]

It also converts alcohols to alkyl chloride. Thionyl chloride is more commonly used in the laboratory because the SO₂ is more easily separated from the organic products than is POCl₃.

PCl₅ reacts with a tertiary amides, such as DMF, to give dimethylchloromethyleneammonium chloride, which is called the Vilsmeier reagent, [(CH₃)₂NCClH]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.^[12]

It is especially renowned for the conversion of C=O groups to CCl₂ groups.^[13] For example, benzophenone and phosphorus pentachloride react to give the diphenyldichloromethane:^[14]

(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.^[15]

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:^[16]

6 PCl₅ + P₄O₁₀ → 10 POCl₃

PCl₅ chlorinates nitrogen dioxide to form nitronium chloride:

PCl₅ + 2 NO₂ → PCl₃ + 2 NO₂Cl

PCl₅ is a precursor for lithium hexafluorophosphate, LiPF₆, an electrolyte 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.

References

1 2 3 4 "NIOSH Pocket Guide to Chemical Hazards #0509". National Institute for Occupational Safety and Health (NIOSH).
1 2 3 Phosphorus pentachloride in Linstrom, P.J.; Mallard, W.G. (eds.) NIST Chemistry WebBook, NIST Standard Reference Database Number 69. National Institute of Standards and Technology, Gaithersburg MD. http://webbook.nist.gov (retrieved 2014-05-15)
1 2 3 Phosphorus pentachloride
1 2 3 "Phosphorus pentachloride". Immediately Dangerous to Life and Health. National Institute for Occupational Safety and Health (NIOSH).
↑ D. E. C. Corbridge (1995). Phosphorus: an outline of its chemistry, biochemistry, and uses. Elsevier Science Ltd. ISBN 0-444-89307-5.
1 2 3 Arnold F. Holleman; Egon Wiber; Nils Wiberg (2001). Inorganic Chemistry. Academic Press. ISBN 978-0-12-352651-9.
↑ Suter, R. W.; Knachel, H. C.; Petro, V. P.; Howatson, J. H.; S. G. Shore, S. G. (1978). "Nature of Phosphorus(V) Chloride in Ionizing and Nonionizing Solvents". J. Am. Chem. Soc. 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.
↑ R. N. Maxson,"Phosphorus Pentachloride" Inorganic Syntheses 1939, vol. 1, pp. 99–100. doi:10.1002/9780470132326.ch34
↑ Adams, R.; Jenkins, R. L. (1941). "p-Nitrobenzoyl chloride". Org. Synth. ; Coll. Vol., 1, p. 394
↑ Clayden, Jonathan (2005). Organic chemistry (Reprinted (with corrections). ed.). Oxford [u.a.]: Oxford Univ. Press. ISBN 978-0-19-850346-0.
↑ Burks, Jr., J. E. "Phosphorus(V) Chloride" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289.
↑ Gross, H.; Rieche, A.; Höft, E.; Beyer, E. (1973). "Dichloromethyl Methyl Ether". Org. Synth. ; Coll. Vol., 5, p. 365
↑ Spaggiari, Alberto; Daniele Vaccari; Paolo Davoli; Giovanni Torre; Fabio Prati (2007). "A Mild Synthesis of Vinyl Halides andgem-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". Org. Synth. ; Coll. Vol., 5, p. 1005
↑ Frank Albert Cotton (1999). Advanced inorganic chemistry. Wiley-Interscience. ISBN 978-0-471-19957-1.

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Phosphorus compounds

PBr₃ PBr₅ PCl₃ PCl₅ PF₃ PF₅ PI₃ InP PH₃ PN P₃N₅ P₂O₃ P₂O₄ P₂O₅ P₄S₃ P₄S₁₀ AlP HfP FeP Fe₃P BP Ca₃P₂ CuP₂

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