Phosphorus pentachloride

Phosphorus pentachloride
IUPAC name Phosphorus pentachloride Phosphorus(V) chloride
Other names Pentachlorophosphorane
Identifiers
CAS number	10026-13-8 Y
PubChem	24819
EC number	233-060-3
UN number	1806
RTECS number	TB6125000
Jmol-3D images	Image 1
SMILES ClP(Cl)(Cl)(Cl)Cl
Properties
Molecular formula	PCl5
Molar mass	208.24 g/mol
Appearance	colourless crystals
Density	2.1 g/cm3
Melting point	166.8 °C, 440.0 K, 332.2 °F
Boiling point	160.5 °C, 434 K, 321 °F (sublimation)
Solubility in water	decomposition (exothermic)
Solubility	soluble in CS2, chlorocarbons, benzene
Structure
Crystal structure	tetragonal
Coordination geometry	D3h
Dipole moment	0 D
Hazards
MSDS	ICSC 0544
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	0 3 2 W
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
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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 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 (D_3h) symmetry. The hypervalent nature of this species (as well as for PCl−
6, see below) can be explained with three-center four-electron bonding model. This trigonal bipyramidal structure persists in non-polar solvents, such as CS₂ and CCl₄.^[1] In the solid state PCl₅ is ionic, formulated PCl+
4PCl−
6.^[2]

In solutions of polar solvents, PCl₅ undergoes "autoionization".^[3] Dilute solutions dissociate according to the following equilibrium:

PCl₅ [PCl+
4]Cl⁻

At higher concentrations, a second equilibrium becomes more important:

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.

AsCl₅ and SbCl₅ also adopt trigonal bipyramidal structures. The relevant bond distances are 211 (As-Cl_eq), 221 (As-Cl_eq), 227 (Sb-Cl_eq), and 233.3 pm (Sb-Cl_ax ).^[4] At low temperatures, SbCl₅ converts to the dimer, bioctahedral Sb₂Cl₁₀, structurally related to niobium pentachloride.

Preparation

PCl₅ is prepared by the chlorination of PCl₃. This reaction was used to produce ca. 10,000,000 kg of PCl₅ in 2000.^[2]

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%.^[2] Because of this equilibrium, samples of PCl₅ often contain chlorine, which imparts a greenish colouration.

Reactions

Hydrolysis

In its most characteristic reaction, PCl₅ react 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

PCl₅ is most often used for chlorinations of organic and inorganic compounds.^[5]

Chlorination of organic compounds

In synthetic chemistry, two classes of chlorination are usually of interest. 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^[6] as well as 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.^[5]

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.^[7]

The electrophilic character of PCl₅ is highlighted by its reaction with styrene to give, after hydrolysis, phosphonic acid derivatives.^[8]

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:^[9]

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. LiPF₆ is produced by the reaction of PCl₅ 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

• Phosphorus halides

References

1. ^ D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam 1995. ISBN 0-444-89307-5.
2. ^ ^{a b c} Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
3. ^ Suter, R. W.; Knachel, H. C.; Petro, V. P.; Howatson, J. H.; S. G. Shore, S. G. (1973). "Nature of Phosphorus(V) Chloride in Ionizing and Nonionizing Solvents". J. Am. Chem. Soc. 95: 1474–1479. doi:10.1021/ja00786a021. 
4. ^ Haupt, S.; Seppelt, K. (2002). "Solid State Structures of AsCl₅ and SbCl₅". Zeitschrift für anorganische und allgemeine Chemie 628: 729–734. doi:10.1002/1521-3749(200205)628:4<729::AID-ZAAC729>3.0.CO;2-E. 
5. ^ ^{a b} 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.
6. ^ Adams, R.; Jenkins, R. L. (1941), "p-Nitrobenzoyl chloride", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv1p0394 ; Coll. Vol. 1: 394 
7. ^ Gross, H.; Rieche, A.; Höft, E.; Beyer, E. (1973), "Dichloromethyl Methyl Ether", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv5p0365 ; Coll. Vol. 5: 365 
8. ^ Schmutzler, R. (1973), "Styrylphosphonic dichloride", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv5p1005 ; Coll. Vol. 5: 1005 
9. ^ F. A. Cotton, G. Wilkinson, C. A. Murillo, and M. Bochmann (April 1999). Advanced Inorganic Chemistry, 6th Edition. Wiley-VCH. ISBN 0-471-19957-5

External links

• The period 3 chlorides
• International Chemical Safety Card 0544