Phosphorus pentachloride

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Phosphorus pentachloride
Phosphorus pentachloride (gas phase structure)
Phosphorus pentachloride Phosphorus-pentachloride-3D-vdW.png
Phosphorus pentachloride ampoule.jpg
Identifiers
CAS number 10026-13-8 YesY
PubChem 24819
EC number 233-060-3
UN number 1806
RTECS number TB6125000
Jmol-3D images Image 1
Properties
Molecular formula Cl5P
Molar mass 208.24 g mol−1
Appearance colourless crystals
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
Solubility in water decomposition
(exothermic)
Solubility soluble in CS2, chlorocarbons, benzene
Vapor pressure 1.11 kPa (80 °C)
4.58 kPa (100 °C)[1]
Structure
Crystal structure tetragonal
Coordination
geometry
D3h (trigonal bipyramidal)
Dipole moment 0 D
Thermochemistry
Specific
heat capacity
C
111.5 J/mol·K[1]
Std molar
entropy
So298
364.2 J/mol·K[1]
Hazards
MSDS 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)[2]
GHS signal word Danger
GHS hazard statements H302, H314, H330, H373[2]
GHS precautionary statements P260, P280, P284, P305+351+338, P310[2]
EU Index 015-008-00-X
EU classification Very Toxic T+
R-phrases R14, R22, R26, R34, R48/20
S-phrases (S1/2), S7/8, S26, S36/37/39, S45
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
LD50 660 mg/kg
Related compounds
Related phosphorus pentahalides Phosphorus pentafluoride
Phosphorus pentabromide
Phosphorus pentaiodide
Related compounds Phosphorus trichloride
Phosphoryl chloride
Except where noted otherwise, 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- 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 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+
4
PCl
6
.[4]

In solutions of polar solvents, PCl5 undergoes "autoionization".[5] Dilute solutions dissociate according to the following equilibrium:

PCl5 is in equilibrium with [PCl+
4
]Cl

At higher concentrations, a second equilibrium becomes more important:

2 PCl5 is in equilibrium with [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.

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[edit]

PCl5 is prepared by the chlorination of PCl3. This reaction was used to produce ca. 10,000,000 kg of PCl5 in 2000.[4]

PCl3 + Cl2 is in equilibrium with 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[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 ortho-phosphoric 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.

PCl5 will convert carboxylic acids to the corresponding acyl chloride[7] through the following mechanism:[8]

Phosphorus pentachloride mechanism.png

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.[9]

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.[10]

The electrophilic character of PCl5 is highlighted by its reaction with styrene to give, after hydrolysis, phosphonic acid derivatives.[11]

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:[12]

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 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)
  2. ^ a b c Phosphorus pentachloride
  3. ^ D. E. C. Corbridge (1995). Phosphorus: an outline of its chemistry, biochemistry, and uses. Elsevier Science Ltd. ISBN 0-444-89307-5. 
  4. ^ a b c Arnold F. Holleman; Egon Wiber; Nils Wiberg (2001). Inorganic Chemistry. Academic Press. ISBN 978-0-12-352651-9. 
  5. ^ Suter, R. W.; Knachel, H. C.; Petro, V. P.; Howatson, J. H.; S. G. Shore, S. G. (1978). "Nature of Phosphorus(V) Chloride in Ionizing and Nonionizing Solvents". J. Am. Chem. Soc. 95 (5): 1474–1479. doi:10.1021/ja00786a021. 
  6. ^ 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. 
  7. ^ Adams, R.; Jenkins, R. L. (1941), "p-Nitrobenzoyl chloride", Org. Synth. ; Coll. Vol. 1: 394 
  8. ^ Clayden, Jonathan (2005). Organic chemistry (Reprinted (with corrections). ed.). Oxford [u.a.]: Oxford Univ. Press. ISBN 978-0-19-850346-0. 
  9. ^ 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.
  10. ^ Gross, H.; Rieche, A.; Höft, E.; Beyer, E. (1973), "Dichloromethyl Methyl Ether", Org. Synth. ; Coll. Vol. 5: 365 
  11. ^ Schmutzler, R. (1973), "Styrylphosphonic dichloride", Org. Synth. ; Coll. Vol. 5: 1005 
  12. ^ Frank Albert Cotton (1999). Advanced inorganic chemistry. Wiley-Interscience. ISBN 978-0-471-19957-1. 

External links[edit]