Phosphorus pentachloride

The structures for the phosphorus chlorides are invariably consistent with VSEPR theory. The structure of {{chem2|PCl5}} depends on its environment. Gaseous and molten {{chem2|PCl5}} is a neutral molecule with trigonal bipyramidal geometry and (D_3h) symmetry. The hypervalent nature of this species (as well as of {{chem2|[PCl6]-}}, 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 carbon disulfide and carbon tetrachloride.{{cite book| first = D. E. C. | last=Corbridge| title = Phosphorus: An outline of its chemistry, biochemistry, and uses| year = 1995| publisher = Elsevier Science | isbn = 0-444-89307-5}} In the solid state {{chem2|PCl5}} is an ionic compound called tetrachlorophosphonium hexachlorophosphate formulated {{chem2|[PCl4]+[PCl6]-}}.{{cite book| first1= A. F.|last1= Holleman|first2=E.|last2= Wiber |first3=N.|last3= Wiberg | title = Inorganic Chemistry| year = 2001| publisher = Academic Press| isbn = 978-0-12-352651-9}}

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In solutions of polar solvents, {{chem2|PCl5}} undergoes self-ionization.{{cite journal | last1= Suter |first1=R. W.|last2= Knachel |first2=H. C. |last3=Petro |first3=V. P. |last4=Howatson |first4=J. H. |last5= Shore|first5= S. G. |name-list-style=amp | title = Nature of Phosphorus(V) Chloride in Ionizing and Nonionizing Solvents | journal = Journal of the American Chemical Society | volume = 95 | year = 1978 | pages = 1474–1479 | doi = 10.1021/ja00786a021 | issue = 5}} Dilute solutions dissociate according to the following equilibrium:

:{{chem2|PCl5 ⇌ [PCl4]+ + Cl−}}

At higher concentrations, a second equilibrium becomes more prevalent:

:{{chem2|2 PCl5 ⇌ [PCl4]+ + [PCl6]-}}

The cation {{chem2|[PCl4]+}} and the anion {{chem2|[PCl6]-}} are tetrahedral and octahedral, respectively. At one time, {{chem2|PCl5}} in solution was thought to form a dimeric structure, {{chem2|P2Cl10}}, but this suggestion is not supported by Raman spectroscopic measurements.

Arsenic pentachloride and Antimony pentachloride 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).{{cite journal |last1=Haupt |first1=S. |last2=Seppelt |first2=K. | title = Solid State Structures of AsCl₅ and SbCl₅ | journal = Zeitschrift für anorganische und allgemeine Chemie | year = 2002 | volume = 628 |pages = 729–734 | doi = 10.1002/1521-3749(200205)628:4<729::AID-ZAAC729>3.0.CO;2-E | issue = 4| doi-access = free}} At low temperatures, {{chem2|SbCl5}} converts to the dimer, dioctahedral {{chem2|Sb2Cl10}}, structurally related to niobium pentachloride.

{{chem2|PCl5}} is prepared by the chlorination of {{chem2|PCl3}}.{{cite book|first=R. N.|last=Maxson|chapter=Phosphorus Pentachloride|date=1939|volume=1|pages=99–100|doi=10.1002/9780470132326.ch34|title=Inorganic Syntheses|isbn=9780470132326}} This reaction is used to produce around 10,000 tonnes of {{chem2|PCl5}} per year (as of 2000).

:{{chem2|PCl3 + Cl2 ⇌ PCl5}} {{pad|3em}} (ΔH = −124 kJ/mol)

{{chem2|PCl5}} exists in equilibrium with {{chem2|PCl3}} and chlorine, and at 180 °C the degree of dissociation is about 40%. Because of this equilibrium, samples of {{chem2|PCl5}} often contain chlorine, which imparts a greenish coloration.

In its most characteristic reaction, {{chem2|PCl5}} reacts upon contact with water to release hydrogen chloride and give phosphorus oxides. The first hydrolysis product is phosphorus oxychloride:

:{{chem2|PCl5 + H2O → POCl3 + 2 HCl}}

In hot water, hydrolysis proceeds completely to orthophosphoric acid:

:{{chem2|PCl5 + 4 H2O → H3PO4 + 5 HCl}}

Phosphorus pentachloride is a Lewis acid. This property underpins many of its characteristic reactions, autoionization, chlorinations, hydrolysis. A well studied adduct is {{chem2|PCl5(pyridine)}}.{{cite journal|title=Neutral Six-Coordinate Phosphorus|first1=Chih Y.|last1=Wong|first2=Dietmar K.|last2=Kennepohl|first3=Ronald G.|last3=Cavell|journal=Chemical Reviews|year=1996|volume=96|issue=6|pages=1917–1952|doi=10.1021/cr9410880|pmid=11848816}}

In synthetic chemistry, two classes of chlorination are usually of interest: oxidative chlorinations and substitutive chlorinations. Oxidative chlorinations entail the transfer of {{chem2|Cl2}} from the reagent to the substrate. Substitutive chlorinations entail replacement of O or OH groups with chloride. {{chem2|PCl5}} can be used for both processes.

Upon treatment with {{chem2|PCl5}}, carboxylic acids convert to the corresponding acyl chloride.{{OrgSynth | authorlink = Roger Adams|last1=Adams |first1=R. |last2=Jenkins |first2=R. L. | title = p-Nitrobenzoyl chloride | collvol = 1 | collvolpages = 394 | prep = cv1p0394 | year = 1941}} The following mechanism has been proposed:{{cite book|last=Clayden|first=Jonathan|title=Organic chemistry|year=2005|publisher=Oxford University Press|location=Oxford|isbn=978-0-19-850346-0|edition=Reprinted|url-access=registration|url=https://archive.org/details/organicchemistry00clay_0}}

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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 {{chem2|POCl3}}.

{{chem2|PCl5}} reacts with a tertiary amides, such as dimethylformamide (DMF), to give dimethylchloromethyleneammonium chloride, which is called the Vilsmeier reagent, {{chem2|[(CH3)2N\dCClH]+Cl-}}. More typically, a related salt is generated from the reaction of DMF and {{chem2|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.{{cite encyclopedia|last=Burks Jr. |first=J. E. |title=Encyclopedia of Reagents for Organic Synthesis |chapter=Phosphorus(V) chloride |editor-first=L. |editor-last=Paquette |date=2004 |publisher=J. Wiley & Sons |location=New York, NY |doi=10.1002/047084289X.rp158|isbn=0471936235}}

It is especially renowned for the conversion of C=O groups to {{chem2|CCl2}} groups.{{OrgSynth | last1= Gross |first1=H. |last2=Rieche |first2=A. |last3=Höft |first3=E. |last4=Beyer |first4=E. | title = Dichloromethyl methyl ether | collvol = 5 | collvolpages = 365 | prep = cv5p0365 | year = 1973}} For example, benzophenone and phosphorus pentachloride react to give the diphenyldichloromethane:{{cite journal|last1=Spaggiari|first1=A.|first2=D. |last2=Vaccari |first3=P. |last3=Davoli |first4=G. |last4=Torre |first5=F. |last5=Prati |year=2007|title=A Mild Synthesis of Vinyl Halides and gem-Dihalides Using Triphenyl Phosphite−Halogen-Based Reagents|journal=The Journal of Organic Chemistry|volume=72|issue=6|pages=2216–2219|issn=0022-3263|pmid=17295542|doi=10.1021/jo061346g}}

:{{chem2|(C6H5)2CO + PCl5 → (C6H5)2CCl2 + POCl3}}

The electrophilic character of {{chem2|PCl5}} is highlighted by its reaction with styrene to give, after hydrolysis, phosphonic acid derivatives.{{OrgSynth | last= Schmutzler |first=R. | title = Styrylphosphonic dichloride | collvol = 5 | collvolpages = 1005 | prep = cv5p1005 | year = 1973}}

Both {{chem2|PCl3}} and {{chem2|PCl5}} convert {{chem2|R3COH}} groups to the chloride {{chem2|R3CCl}}. The pentachloride is however a source of chlorine in many reactions. It chlorinates allylic and benzylic CH bonds. {{chem2|PCl5}} bears a greater resemblance to sulfuryl chloride, also a source of {{chem2|Cl2}}. For oxidative chlorinations on the laboratory scale, sulfuryl chloride is often preferred over {{chem2|PCl5}} since the gaseous {{chem2|SO2}} by-product is readily separated.

As for the reactions with organic compounds, the use of {{chem2|PCl5}} has been superseded by {{chem2|SO2Cl2}}. The reaction of phosphorus pentoxide and {{chem2|PCl5}} produces phosphorus oxychloride :{{cite book| first= Frank Albert|last= Cotton| title = Advanced Inorganic Chemistry| year = 1999| publisher = Wiley-Interscience| isbn = 978-0-471-19957-1}}{{page needed|date=September 2017}}

:{{chem2|6 PCl5 + P4O10 → 10 POCl3}}

{{chem2|PCl5}} chlorinates nitrogen dioxide to form unstable nitryl chloride:

:{{chem2|PCl5 + 2 NO2 → PCl3 + 2 NO2Cl}}

:{{chem2|2 NO2Cl → 2 NO2 + Cl2}}

{{chem2|PCl5}} is a precursor for lithium hexafluorophosphate, {{chem2|Li[PF6]}}. Lithium hexafluorophosphate is a commonly employed salt in electrolytes in lithium ion batteries.{{cite journal |last1=Bushkova |first1=O. V. |last2=Yaroslavtseva |first2=T. V. |last3=Dobrovolsky |first3=Yu. A. |title=New lithium salts in electrolytes for lithium-ion batteries (Review) |journal=Russian Journal of Electrochemistry |date=4 August 2017 |volume=53 |issue=7 |pages=677–699 |doi=10.1134/S1023193517070035|s2cid=103854243}} {{chem2|Li[PF6]}} is produced by the reaction of {{chem2|PCl5}} with lithium fluoride, with lithium chloride as a side product:

:{{chem2|PCl5 + 6 LiF → Li[PF6] + 5 LiCl}}

{{chem2|PCl5}} is a dangerous chemical as it reacts violently with water. It is also corrosive when in contact with skin. It is toxic and can be fatal when inhaled.

Phosphorus pentachloride was first prepared in 1808 by the English chemist Humphry Davy.{{cite journal|last1=Davy|first1=Humphry|title=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|journal=Philosophical Transactions of the Royal Society of London|date=1809|volume=99|pages=39–104|url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015034564347;view=1up;seq=53|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's analysis of phosphorus pentachloride was inaccurate;{{cite journal|last1=Davy|first1=Humphry|title=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|journal=Philosophical Transactions of the Royal Society of London|date=1810|volume=100|pages=231–257|url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015034564339;view=1up;seq=301|doi=10.1098/rstl.1810.0016|doi-access=|s2cid=95219058}} On p. 257, Davy presented his empirical formula for phosphorus pentachloride: 1 portion of phosphorus to 3 portions of "oxymuriatic gas" (chlorine). the first accurate analysis was provided in 1816 by the French chemist Pierre Louis Dulong.{{cite journal|last1=Dulong|title=Extrait d'un mémoire sur les combinaisons du phosphore avec l'oxigène|journal=Annales de Chimie et de Physique|date=1816|volume=2|pages=141–150|url=https://babel.hathitrust.org/cgi/pt?id=hvd.hx3dvb;view=1up;seq=147|series=2nd series|trans-title=Extract from a memoir on the compounds of phosphorus with oxygen|language=fr}} 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).

• Phosphorus halides
• Phosphorus trichloride
• Phosphoryl chloride
• Phosphorus trifluorodichloride

{{Reflist|30em}}

{{Commons category|Phosphorus pentachloride}}

• [http://www.chemguide.co.uk/inorganic/period3/chlorides.html The period 3 chlorides]
• [http://www.inchem.org/documents/icsc/icsc/eics0544.htm International Chemical Safety Card 0544]
• [https://www.cdc.gov/niosh/npg/npgd0509.html CDC - NIOSH Pocket Guide to Chemical Hazards]

{{Phosphorus compounds}}

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Category:Phosphorus chlorides

Category:Hypervalent molecules

Category:Phosphorus(V) compounds