Phosphorus pentachloride

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 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. In the solid state PCl₅ is ionic, formulated PCl+
4PCl−
6.

In solutions of polar solvents, PCl₅ undergoes "autoionization". 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-Cl_eq), 221 (As-Cl_ax), 227 (Sb-Cl_eq), and 233.3 pm (Sb-Cl_ax ). 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 is used to produce ca. 10,000,000 kg/y of PCl₅ (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 ca. 40%. Because of this equilibrium, samples of PCl₅ often contain chlorine, which imparts a greenish colouration.

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.

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

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 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.

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 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.