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-sensitive solid, although commercial samples can be yellowish and contaminated with hydrogen chloride.

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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 PCl₆^-, see below) can be explained with three-center four-electron bonding model. This trigonal bipyramidal structure persists in non-polar solvents, such as CS₂ and CCl₄, the D_3h.^[1]

In solutions of polar solvents, however, PCl₅ undergoes "autoionization".^[2] Dilute solutions dissociate according to the following equilibrium:

PCl₅ [PCl₄⁺]Cl⁻

At higher concentrations, a second equilibrium becomes more important:

2 PCl₅ [PCl₄⁺][PCl₆⁻]

The cation PCl₄⁺ and the anion PCl₆⁻ 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 the Raman spectroscopic measurements.

Preparation

PCl₅ is prepared by the chlorination of PCl₃. This reaction was used to produce ca. 10,000,000 kg of PCl₅ in 2000.^[3]

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%.^[3] Because of this equilibrium, samples of PCl₅ often contain chlorine, which imparts a greenish colouration.

Hydrolysis

In its most characteristic reaction, PCl₅ react 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

Other reactions

Most often PCl₅ is used for chlorinations.^[4]

Chlorinations of organic compounds with PCl₅

In synthetic chemistry, two classes of chlorination are usually of interest. 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.

PCl₅ will convert carboxylic acids to the corresponding acyl chloride^[5] as well as 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 and PCl₃ bear some resemblance to SO₂Cl₂, as both serve often as sources of Cl₂. Again for oxidative chlorinations on the laboratory scale, SO₂Cl₂ is often preferred over PCl₅ since the gaseous SO₂ by-product is readily separated.

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.^[4]

In contrast to PCl₃, the pentachloride replaces allylic and benzylic CH bonds and is especially renown for the conversion of C=O groups to CCl₂ groups.^[6]

The electrophilic character of PCl₅ is highlighted by its reaction with styrene to give, after hydrolysis, phosphonic acid derivatives.^[7]

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:^[2]:

6 PCl₅ + P₄O₁₀ → 10 POCl₃

PCl₅ chlorinates nitrogen dioxide:

PCl₅ + 2 NO₂ → PCl₃ + 2 NO₂Cl

PCl₅ is a precursor for lithium hexafluorophosphate, LiPF₆, an electrolytes in lithium ion battery:

PCl₅ + 6 LiF → LiPF₆ + 5 LiCl

Arsenic and antimony pentachloride

AsCl₅ and SbCl₅ adopt trigonal bipyramidal structures. The relevant bond distances are 211 (As-Cl_eq) 221 (As-Cl_eq), 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.

Safety

PCl₅ is a dangerous substance as it reacts violently with water and is a source of both hydrogen chloride and chlorine.

See also

• Phosphorus halides

References

1. ^ D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam 1995. ISBN 0-444-89307-5.
2. ^ Suter, R. W.; Knachel, H. C.; Petro, V. P.; Howatson, J. H.; S. G. Shore, S. G. ”Nature of Phosphorus(V) Chloride in Ionizing and Nonionizing Solvents” Journal of the American Chemical Society 1973, volume 95, pp 1474 - 1479; DOI: 10.1021/ja00786a021
3. ^ ^{a b} Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
4. ^ ^{a b} 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.
5. ^ Adams, R.; Jenkins, R. L. “p-Nitrobenzoyl chloride” Organic Syntheses, Collected Volume 1, p.394 (1941).
6. ^ Gross, H.; Rieche, A.; Höft, E.; Beyer, E. “Dichloromethyl Methyl Ether” Organic Syntheses, Collected Volume 5, p.365 (1973).
7. ^ Schmutzler, R. ”Styrylphosphonic dichloride” Organic Syntheses, Collected Voume 5, p.1005 (1973).
8. ^ Haupt, S.; Seppelt, K., "Solid State Structures of AsCl₅ and SbCl₅", Zeitschrift für Anorganische und Allgemeine Chemie, 2002, volume 628, pages 729-734.