Phosphorus pentachloride
IUPAC name	Phosphorus(V) chloride
Other names	Phosphorus pentachloride
Identifiers
CAS number	10026-13-8
RTECS number	TB6125000
SMILES	ClP(Cl)(Cl)(Cl)Cl
Properties
Molecular formula	PCl5
Molar mass	208.22 g mol−1
Appearance	colorless crystals
Density	1.6 g cm−3
Melting point	179–181 °C
Boiling point	sublimation 70-80 °C (vacuum)
Solubility in water	decomposition (exothermic)
Solubility in other solvents	carbon disulfide, chlorocarbons, benzene
Structure
Coordination geometry	trigonal bipyramidal
Dipole moment	0 D
Hazards
Main hazards	HCl source
R-phrases	14-22-26-34-48/20
S-phrases	26-36/37/39-45-7/8
Related Compounds
Related compounds	POCl3, PCl3, PF5
Except where noted otherwise, data are given for materials in their standard state (at 25 C, 100 kPa)

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 PCl₆^-, 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, the D_3h.^[1]

In solutions of polar solvents, however, PCl₅ undergoes "autoionization".^[2] Dilute solutions dissociate according to the following equilibrium:

PCl₅ [PCl₄⁺]Cl⁻

At higher concentrations, a second equilibrium becomes more important:

2 PCl₅ [PCl₄⁺][PCl₆⁻]

The cation PCl₄⁺ and the anion PCl₆⁻ 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 the Raman spectroscopic measurements.

Preparation

PCl₅ is prepared by the chlorination of PCl₃. This reaction was used to produce ca. 10,000,000 kg of PCl₅ in 2000.^[3]

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%.^[3] Because of this equilibrium, samples of PCl₅ often contain chlorine, which imparts a greenish coloration.

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

Other reactions

Most often PCl₅ is used for chlorinations.^[4]

Chlorinations of organic compounds with PCl₅

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^[5] 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.^[4]

In contrast to PCl₃, the pentachloride replaces allylic and benzylic CH bonds and is especially renown for the conversion of C=O groups to CCl₂ groups.^[6]

The electrophilic character of PCl₅ is highlighted by its reaction with styrene to give, after hydrolysis, phosphonic acid derivatives.^[7]

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:^[2]:

6 PCl₅ + P₄O₁₀ → 10 POCl₃

PCl₅ chlorinates nitrogen dioxide:

PCl₅ + 2 NO₂ → PCl₃ + 2 NO₂Cl

PCl₅ is a precursor for lithium hexafluorophosphate, LiPF₆, an electrolytes in lithium ion battery:

PCl₅ + 6 LiF → LiPF₆ + 5 LiCl

Arsenic and antimony pentachloride

AsCl₅ and SbCl₅ 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 ).^[8] At low temperatures, SbCl₅ converts to the dimer, bioctahedral Sb₂Cl₁₀, structurally related to niobium pentachloride.

Safety

PCl₅ is a dangerous substance as it reacts violently with water and is a source of both hydrogen chloride and chlorine.

See also

• Phosphorus halides

References