Phosphorus pentachloride

Preparation

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

PCl₃ + Cl₂ ⇌ PCl₅ (ΔH = −124 kJ/mol)

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

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 orthophosphoric acid:

PCl₅ + 4 H₂O → H₃PO₄ + 5 HCl

Lewis acidity

Phosphorus pentachloride is a Lewis acid. This property underpins many of its characteristic reactions, autoionization, chlorinations, hydrolysis. A well studied adduct is PCl₅(pyridine).

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.

Upon treatment with PCl₅, carboxylic acids convert to the corresponding acyl chloride. The following mechanism has been proposed:

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.

It is especially renowned for the conversion of C=O groups to CCl₂ groups. For example, benzophenone and phosphorus pentachloride react to give the diphenyldichloromethane:

(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.

Comparison with related reagents

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

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 :

6 PCl₅ + P₄O₁₀ → 10 POCl₃

PCl₅ chlorinates nitrogen dioxide to form unstable nitryl chloride:

PCl₅ + 2 NO₂ → PCl₃ + 2 NO₂Cl

2 NO₂Cl → 2 NO₂ + Cl₂

PCl₅ is a precursor for lithium hexafluorophosphate, LiPF₆. Lithium hexafluorophosphate is a commonly employed salt in electrolytes 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. It is also corrosive when in contact with skin and can be fatal when inhaled.

History

Phosphorus pentachloride was first prepared in 1808 by the English chemist Humphry Davy. Davy's analysis of phosphorus pentachloride was inaccurate; the first accurate analysis was provided in 1816 by the French chemist Pierre Louis Dulong.