Phosphorus pentachloride

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Phosphorus pentachloride
Phosphorus pentachloride (gas phase structure)
Phosphorus pentachloride
100px
200px
Names
IUPAC names
Phosphorus pentachloride
Phosphorus(V) chloride
Other names
Pentachlorophosphorane
Identifiers
10026-13-8 YesY
ChemSpider 23204 N
EC Number 233-060-3
Jmol 3D model Interactive image
PubChem 24819
RTECS number TB6125000
UN number 1806
  • InChI=1S/Cl5P/c1-6(2,3,4)5 N
    Key: UHZYTMXLRWXGPK-UHFFFAOYSA-N N
  • InChI=1/Cl5P/c1-6(2,3,4)5
    Key: UHZYTMXLRWXGPK-UHFFFAOYAP
  • ClP(Cl)(Cl)(Cl)Cl
Properties
Cl5P
Molar mass 208.22 g·mol−1
Appearance colourless crystals
Odor pungent, unpleasant[1]
Density 2.1 g/cm3
Melting point 166.8 °C (332.2 °F; 439.9 K)
Boiling point 160.5 °C (320.9 °F; 433.6 K) sublimation
decomposition
(exothermic)
Solubility soluble in CS2, chlorocarbons, benzene
Vapor pressure 1.11 kPa (80 °C)
4.58 kPa (100 °C)[2]
Structure
tetragonal
D3h (trigonal bipyramidal)
0 D
Thermochemistry
111.5 J/mol·K[2]
364.2 J/mol·K[2]
Vapor pressure {{{value}}}
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).
N verify (what is YesYN ?)
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- 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 (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 MOs (Molecular orbital theory) or resonance (Valence bond theory). This trigonal bipyramidal structure persists in non-polar solvents, such as CS2 and CCl4.[3] In the solid state PCl5 is ionic, formulated PCl+
4PCl−
6.[4]

In solutions of polar solvents, PCl5 undergoes "autoionization".[5] 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-Cleq), 221 (As-Clax), 227 (Sb-Cleq), and 233.3 pm (Sb-Clax ).[6] At low temperatures, SbCl5 converts to the dimer, bioctahedral Sb2Cl10, structurally related to niobium pentachloride.

Preparation

PCl5 is prepared by the chlorination of PCl3.[7] This reaction is used to produce ca. 10,000,000 kg/y of PCl5 (as of 2000).[4]

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%.[4] 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.

PCl5 will convert carboxylic acids to the corresponding acyl chloride[8] through the following mechanism:[9]

450px

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 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.[10]

In contrast to PCl3, the pentachloride replaces allylic and benzylic CH bonds and is especially renowned for the conversion of C=O groups to CCl2 groups.[11]

The electrophilic character of PCl5 is highlighted by its reaction with styrene to give, after hydrolysis, phosphonic acid derivatives.[12]

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:[13]

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.

See also

References

  1. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named PGCH
  2. ↑ 2.0 2.1 2.2 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)
  3. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  4. ↑ 4.0 4.1 4.2 Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  6. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  7. ↑ R. N. Maxson,"Phosphorus Pentachloride" Inorganic Syntheses 1939, vol. 1, pp. 99–100. doi:10.1002/9780470132326.ch34
  8. ↑ Lua error in package.lua at line 80: module 'strict' not found.; Lua error in package.lua at line 80: module 'strict' not found.
  9. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  10. ↑ 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.
  11. ↑ Lua error in package.lua at line 80: module 'strict' not found.; Lua error in package.lua at line 80: module 'strict' not found.
  12. ↑ Lua error in package.lua at line 80: module 'strict' not found.; Lua error in package.lua at line 80: module 'strict' not found.
  13. ↑ Lua error in package.lua at line 80: module 'strict' not found.

External links

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