Phosphorus pentachloride
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2008-06-20T01:34:08Z
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{{Chembox new
| Name = Phosphorus pentachloride
| ImageFile = Phosphorus-pentachloride-2D-dimensions.png
<!-- | ImageSize = 150px -->
| ImageName = Phosphorus pentachloride (gas phase structure)
| ImageFile1 = Phosphorus-pentachloride-3D-balls.png
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| ImageName1 = Phosphorus pentachloride
| IUPACName = Phosphorus(V) chloride
| OtherNames = Phosphorus pentachloride
| Section1 = {{Chembox Identifiers
| SMILES = ClP(Cl)(Cl)(Cl)Cl
| CASNo = 10026-13-8
| RTECS = TB6125000
}}
| Section2 = {{Chembox Properties
| Formula = PCl<sub>5</sub>
| MolarMass = 208.22 g mol<sup>−1</sup>
| Appearance = colourless crystals
| Density = 1.6 g cm<sup>−3</sup>
| Solubility = decomposition (exothermic)
| Solvent = other solvents
| SolubleOther = [[carbon disulfide]],<br />[[haloalkane|chlorocarbons]],<br />[[benzene]]
| MeltingPt = 179–181 °C
| BoilingPt = sublimation 70-80 °C<br />(vacuum)
}}
| Section3 = {{Chembox Structure
| Coordination = trigonal bipyramidal
| CrystalStruct =
| Dipole = 0 [[Debye|D]]
| Coordination = D<sub>3h</sub>
}}
| Section7 = {{Chembox Hazards
| ExternalMSDS =
| MainHazards = HCl source
| RPhrases = 14-22-26-34-48/20
| SPhrases = 26-36/37/39-45-7/8
}}
| Section8 = {{Chembox Related
| OtherCpds = POCl<sub>3</sub>,<br />PCl<sub>3</sub>,<br />PF<sub>5</sub>
}}
}}
'''Phosphorus pentachloride''' is the [[chemical compound]] with the formula PCl<sub>5</sub>. It is one of the most important phosphorus chlorides, others being [[phosphorus trichloride|PCl<sub>3</sub>]] and [[phosphorus oxytrichloride|POCl<sub>3</sub>]]. PCl<sub>5</sub> 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 PCl<sub>5</sub> depends on its environment. Gaseous and molten PCl<sub>5</sub> is a neutral molecule with trigonal bipyramidal (''D''<sub>3h</sub>) [[Symmetry group|symmetry]]. The [[hypervalent]] nature of this species (as well as for PCl<sub>6</sub><sup>−</sup>, see below) can be explained with [[three-center four-electron bond]]ing model. This trigonal bipyramidal structure persists in non-polar solvents, such as [[carbon disulfide|CS<sub>2</sub>]] and [[carbon tetrachloride|CCl<sub>4</sub>]].<ref>D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam 1995. ISBN 0-444-89307-5.</ref>
In solutions of polar solvents, however, PCl<sub>5</sub> undergoes "autoionization".<ref>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</ref> Dilute solutions dissociate according to the following equilibrium:
:PCl<sub>5</sub><math>\overrightarrow{\leftarrow}</math> [PCl<sub>4</sub><sup>+</sup>]Cl<sup>−</sup>
At higher concentrations, a second equilibrium becomes more important:
:2 PCl<sub>5</sub><math>\overrightarrow{\leftarrow}</math> [PCl<sub>4</sub><sup>+</sup>][PCl<sub>6</sub><sup>−</sup>]
The cation PCl<sub>4</sub><sup>+</sup> and the anion PCl<sub>6</sub><sup>−</sup> are tetrahedral and octahedral, respectively. At one time, PCl<sub>5</sub> in solution was thought to form a dimeric structure, P<sub>2</sub>Cl<sub>10</sub>, but this suggestion is not supported by [[Raman spectroscopy|Raman spectroscopic]] measurements.
==Preparation==
PCl<sub>5</sub> is prepared by the [[chlorination]] of PCl<sub>3</sub>. This reaction was used to produce ca. 10,000,000 kg of PCl<sub>5</sub> in 2000.<ref name=Holleman>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.</ref>
:PCl<sub>3</sub> + Cl<sub>2</sub><math>\overrightarrow{\leftarrow}</math> PCl<sub>5</sub> ΔH = −124 kJ/mol
PCl<sub>5</sub> exists in equilibrium with PCl<sub>3</sub> and [[chlorine]], and at 180 °C the degree of dissociation is ca. 40%.<ref name=Holleman/> Because of this equilibrium, samples of PCl<sub>5</sub> often contain chlorine, which imparts a greenish colouration.
==Hydrolysis==
In its most characteristic reaction, PCl<sub>5</sub> [[chemical reaction|react]] upon contact with [[water (molecule)|water]] to release [[hydrogen chloride]] and give phosphorus oxides.
The first hydrolysis product is [[phosphorus oxychloride]]:
:PCl<sub>5</sub> + H<sub>2</sub>O → POCl<sub>3</sub> + 2 HCl
In hot water, hydrolysis proceeds completely to ''ortho''-[[phosphoric acid]]:
:PCl<sub>5</sub> + 4 H<sub>2</sub>O → H<sub>3</sub>PO<sub>4</sub> + 5 HCl</center>
==Other reactions==
Most often PCl<sub>5</sub> is used for chlorinations.<ref name=Burks>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.</ref>
===Chlorinations of organic compounds with PCl<sub>5</sub>===
In synthetic chemistry, two classes of chlorination are usually of interest. Oxidative chlorinations entail the transfer of Cl<sub>2</sub> from the reagent to the substrate. Substitutive chlorinations entail replacement of O or OH groups with chloride. PCl<sub>5</sub> can be used for both processes.
PCl<sub>5</sub> will convert [[carboxylic acid]]s to the corresponding [[acyl chloride]]<ref>[[Roger Adams|Adams]], R.; Jenkins, R. L. “p-Nitrobenzoyl chloride” Organic Syntheses, Collected Volume 1, p.394 (1941).</ref> as well as [[alcohol]]s to [[alkyl halide|alkyl chloride]]. [[Thionyl chloride]] is more commonly used in the laboratory because the SO<sub>2</sub> is more easily separated from the organic products than is POCl<sub>3</sub>.
PCl<sub>5</sub> and PCl<sub>3</sub> bear some resemblance to [[sulfuryl chloride|SO<sub>2</sub>Cl<sub>2</sub>]], as both serve often as sources of Cl<sub>2</sub>. Again for oxidative chlorinations on the laboratory scale, [[sulfuryl chloride|SO<sub>2</sub>Cl<sub>2</sub>]] is often preferred over PCl<sub>5</sub> since the gaseous SO<sub>2</sub> by-product is readily separated.
PCl<sub>5</sub> reacts with a tertiary amides, such as [[DMF]], to give dimethylchloromethyleneammonium chloride, which is called the [[Vilsmeier-Haack reaction|Vilsmeier reagent]], [(CH<sub>3</sub>)<sub>2</sub>NCClH]Cl. More typically, a related salt is generated from the reaction of DMF and POCl<sub>3</sub>. 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.<ref name=Burks/>
In contrast to PCl<sub>3</sub>, the pentachloride replaces allylic and benzylic CH bonds and is especially renown for the conversion of C=O groups to CCl<sub>2</sub> groups.<ref>Gross, H.; Rieche, A.; Höft, E.; Beyer, E. “Dichloromethyl Methyl Ether” Organic Syntheses, Collected Volume 5, p.365 (1973).</ref>
The [[electrophilic]] character of PCl<sub>5</sub> is highlighted by its reaction with [[styrene]] to give, after [[hydrolysis]], phosphonic acid derivatives.<ref>Schmutzler, R. ”Styrylphosphonic dichloride” Organic Syntheses, Collected Voume 5, p.1005 (1973).</ref>
===Chlorination of inorganic compounds===
As for the reactions with organic compounds, the use of PCl<sub>5</sub> has been superseded by SO<sub>2</sub>Cl<sub>2</sub>. The reaction of [[phosphorus pentoxide]] and PCl<sub>5</sub> produces [[phosphorus oxychloride|POCl<sub>3</sub>]]:<sup>[2]</sup>:
:6 PCl<sub>5</sub> + P<sub>4</sub>O<sub>10</sub> → 10 POCl<sub>3</sub></center>
PCl<sub>5</sub> chlorinates [[nitrogen dioxide]]:
:PCl<sub>5</sub> + 2 NO<sub>2</sub> → PCl<sub>3</sub> + 2 [[nitronium|NO<sub>2</sub>]]Cl
PCl<sub>5</sub> is a precursor for lithium [[hexafluorophosphate]], LiPF<sub>6</sub>, an [[electrolytes]] in [[lithium ion battery]]:
:PCl<sub>5</sub> + 6 [[Lithium fluoride|LiF]] → LiPF<sub>6</sub> + 5 [[Lithium chloride|LiCl]]
==Arsenic and antimony pentachloride==<!--in the future when these cmpds have their own articles, we could move this stuff-->
AsCl<sub>5</sub> and SbCl<sub>5</sub> adopt trigonal bipyramidal structures. The relevant bond distances are 211 (As-Cl<sub>eq</sub>)
221 (As-Cl<sub>eq</sub>), 227 (Sb-Cl<sub>eq</sub>), and 233.3 pm (Sb-Cl<sub>ax</sub> ).<ref>Haupt, S.; Seppelt, K., "Solid State Structures of AsCl<sub>5</sub> and SbCl<sub>5</sub>", Zeitschrift für Anorganische und Allgemeine Chemie, 2002, volume 628, pages 729-734.</ref> At low temperatures, SbCl<sub>5</sub> converts to the dimer, bioctahedral Sb<sub>2</sub>Cl<sub>10</sub>, structurally related to [[niobium pentachloride]].
==Safety==
PCl<sub>5</sub> 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==
<references/>
[[Category:Phosphorus compounds]]
[[Category:Chlorides]]
[[Category:Nonmetal halides]]
[[de:Phosphorpentachlorid]]
[[es:Cloruro de fósforo (V)]]
[[fr:Pentachlorure de phosphore]]
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[[pt:Pentacloreto de fósforo]]
[[zh:五氯化磷]]