Phosphorus pentachloride

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Phosphorus pentachloride
Phosphorus pentachloride (gas phase structure)
Phosphorus pentachloride
Phosphorus-pentachloride-3D-vdW.png
Phosphorus pentachloride ampoule.jpg
Names
IUPAC names
Phosphorus pentachloride
Phosphorus(V) chloride
Other names
Pentachlorophosphorane
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.030.043
EC Number 233-060-3
RTECS number TB6125000
UNII
UN number 1806
Properties
Cl5P
Molar mass 208.22 g·mol−1
Appearance colourless crystals
Odor pungent, unpleasant[1]
Density 2.1 g/cm3
Melting point 160.5 °C (320.9 °F; 433.6 K)
Boiling point 166.8 °C (332.2 °F; 439.9 K) sublimation
reacts
Solubility soluble in CS2, chlorocarbons, benzene
Vapor pressure 1.11 kPa (80 °C)
4.58 kPa (100 °C)[2]
Structure
tetragonal
D3h (trigonal bipyramidal)
D
Thermochemistry
111.5 J/mol·K[2]
364.2 J/mol·K[2]
Hazards
Safety data sheet ICSC 0544
GHS pictograms The corrosion pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)The skull-and-crossbones pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)The health hazard pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)[3]
GHS signal word Danger
H302, H314, H330, H373[3]
P260, P280, P284, P305+351+338, P310[3]
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorus Special hazard W: Reacts with water in an unusual or dangerous manner. E.g., cesium, sodiumNFPA 704 four-colored diamond
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
660 mg/kg (rat, oral)[4]
205 mg/m3 (rat)[4]
1020 mg/m3 (mouse, 10 min)[4]
US health exposure limits (NIOSH):
PEL (Permissible)
TWA 1 mg/m3[1]
REL (Recommended)
TWA 1 mg/m3[1]
IDLH (Immediate danger)
70 mg/m3[1]
Related compounds
Related phosphorus pentahalides
Phosphorus pentafluoride
Phosphorus pentabromide
Phosphorus pentaiodide
Related compounds
Phosphorus trichloride
Phosphoryl chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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

Structure[edit]

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 (D3h) symmetry. The hypervalent nature of this species (as well as for 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.[5] In the solid state PCl5 is an ionic compound, formulated PCl+
4
PCl
6
.[6]

In solutions of polar solvents, PCl5 undergoes self-ionization.[7] Dilute solutions dissociate according to the following equilibrium:

PCl5PCl+
4
+ Cl

At higher concentrations, a second equilibrium becomes more prevalent:

2 PCl5PCl+
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[edit]

AsCl5 and SbCl5 also adopt trigonal bipyramidal structures. The relevant bond distances are 211 pm (As−Cleq), 221 pm (As−Clax), 227 pm (Sb−Cleq), and 233.3 pm (Sb−Clax).[8] At low temperatures, SbCl5 converts to the dimer, dioctahedral Sb2Cl10, structurally related to niobium pentachloride.

Preparation[edit]

PCl5 is prepared by the chlorination of PCl3.[9] This reaction is used to produce around 10,000 tonnes of PCl5 per year (as of 2000).[6]

PCl3 + Cl2 ⇌ PCl5  H = −124 kJ/mol)

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

Reactions[edit]

Hydrolysis[edit]

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

Chlorination of organic compounds[edit]

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]

Phosphorus pentachloride mechanism.png

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, [(CH3)2N=CClH]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[edit]

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[edit]

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][page needed]

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[edit]

PCl5 is a dangerous substance as it reacts violently with water.

See also[edit]

References[edit]

  1. ^ a b c d "NIOSH Pocket Guide to Chemical Hazards #0509". National Institute for Occupational Safety and Health (NIOSH). 
  2. ^ a b c 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), http://webbook.nist.gov (retrieved 2014-05-15)
  3. ^ a b c Phosphorus pentachloride
  4. ^ a b c "Phosphorus pentachloride". Immediately Dangerous to Life and Health. National Institute for Occupational Safety and Health (NIOSH). 
  5. ^ Corbridge, D. E. C. (1995). Phosphorus: An outline of its chemistry, biochemistry, and uses. Elsevier Science. ISBN 0-444-89307-5. 
  6. ^ a b c Holleman, A. F.; Wiber, E.; Wiberg, N. (2001). Inorganic Chemistry. Academic Press. ISBN 978-0-12-352651-9. 
  7. ^ 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. 
  8. ^ 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. 
  9. ^ Maxson, R. N. (1939). "Phosphorus Pentachloride". Inorganic Syntheses. 1: 99–100. doi:10.1002/9780470132326.ch34. 
  10. ^ Adams, R.; Jenkins, R. L. (1941). "p-Nitrobenzoyl chloride". Org. Synth. ; Coll. Vol., 1, p. 394 
  11. ^ Clayden, Jonathan (2005). Organic chemistry (Reprinted ed.). Oxford: Oxford University Press. ISBN 978-0-19-850346-0. 
  12. ^ Burks, J. E., Jr (2004). "Phosphorus(V) chloride". In Paquette, L. Encyclopedia of Reagents for Organic Synthesis. New York, NY: J. Wiley & Sons. doi:10.1002/047084289. 
  13. ^ Gross, H.; Rieche, A.; Höft, E.; Beyer, E. (1973). "Dichloromethyl methyl ether". Org. Synth. ; Coll. Vol., 5, p. 365 
  14. ^ 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. 
  15. ^ Schmutzler, R. (1973). "Styrylphosphonic dichloride". Org. Synth. ; Coll. Vol., 5, p. 1005 
  16. ^ Cotton, Frank Albert (1999). Advanced Inorganic Chemistry. Wiley-Interscience. ISBN 978-0-471-19957-1. 

External links[edit]