# Phosphorus pentachloride

Phosphorus pentachloride is the chemical compound with the formula PCl$_5$. It is one of the most important phosphorus chlorides/oxychlorides, others being PCl$_3$ and POCl$_3$. PCl$_5$ 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 PCl$_5$ depends on its environment. Gaseous and molten PCl$_5$ is a neutral molecule with trigonal bipyramidal geometry and (D$_{3h}$) symmetry. The hypervalent nature of this species (as well as of PCl$^−$ $_6$, see below) can be explained with the inclusion of non-bonding molecular orbitals (molecular orbital theory) or resonance (valence bond theory). This trigonal bipyramidal structure persists in nonpolar solvents, such as CS$_2$ and CCl$_4$. In the solid state PCl$_5$ is an ionic compound called tetrachlorophosphonium hexachlorophosphate formulated PCl$^+$ $_4$PCl$^−$ $_6$.
In solutions of polar solvents, PCl$_5$ undergoes self-ionization. Dilute solutions dissociate according to the following equilibrium:
    PCl$_5$ ⇌ PCl$^+$ $_4$ + Cl$^−$
At higher concentrations, a second equilibrium becomes more prevalent:
    2 PCl$_5$ ⇌ PCl$^+$ $_4$ + PCl$^−$ $_6$
The cation PCl$^+$ $_4$ and the anion PCl$^−$ $_6$ are tetrahedral and octahedral, respectively. At one time, PCl$_5$ in solution was thought to form a dimeric structure, P$_2$Cl$_{10}$, but this suggestion is not supported by Raman spectroscopic measurements.

### Related pentachlorides

AsCl$_5$ and SbCl$_5$ also adopt trigonal bipyramidal structures. The relevant bond distances are 211 pm (As−Cl$_{eq}$), 221 pm (As−Cl$_{ax}$), 227 pm (Sb−Cl$_{eq}$), and 233.3 pm (Sb−Cl$_{ax}$). At low temperatures, SbCl$_5$ converts to the dimer, dioctahedral Sb$_2$Cl$_{10}$, structurally related to niobium pentachloride.

## Preparation

PCl$_5$ is prepared by the chlorination of PCl$_3$. This reaction is used to produce around 10,000 tonnes of PCl$_5$ per year (as of 2000).
    PCl$_3$ + Cl$_2$ ⇌ PCl$_5$ (ΔH = −124 kJ/mol)
PCl$_5$ exists in equilibrium with PCl$_3$ and chlorine, and at 180 °C the degree of dissociation is about 40%. Because of this equilibrium, samples of PCl$_5$ often contain chlorine, which imparts a greenish coloration.

## Reactions
### Hydrolysis

In its most characteristic reaction, PCl$_5$ reacts upon contact with water to release hydrogen chloride and give phosphorus oxides. The first hydrolysis product is phosphorus oxychloride:
    PCl$_5$ + H$_2$O → POCl$_3$ + 2 HCl
In hot water, hydrolysis proceeds completely to orthophosphoric acid:
    PCl$_5$ + 4 H$_2$O → H$_3$PO$_4$ + 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 PCl$_5$(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 Cl$_2$ from the reagent to the substrate. Substitutive chlorinations entail replacement of O or OH groups with chloride. PCl$_5$ can be used for both processes.
Upon treatment with PCl$_5$, 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 POCl$_3$.
PCl$_5$ reacts with a tertiary amides, such as dimethylformamide (DMF), to give dimethylchloromethyleneammonium chloride, which is called the Vilsmeier reagent, [(CH$_3$)$_2$N=CClH]Cl. More typically, a related salt is generated from the reaction of DMF and POCl$_3$. 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 CCl$_2$ groups. For example, benzophenone and phosphorus pentachloride react to give the diphenyldichloromethane:
    (C$_6$H$_5$)$_2$CO + PCl$_5$ → (C$_6$H$_5$)$_2$CCl$_2$ + POCl$_3$
The electrophilic character of PCl$_5$ is highlighted by its reaction with styrene to give, after hydrolysis, phosphonic acid derivatives.

### Comparison with related reagents

Both PCl$_3$ and PCl$_5$ convert R$_3$COH groups to the chloride R$_3$CCl. The pentachloride is however a source of chlorine in many reactions. It chlorinates allylic and benzylic CH bonds. PCl5 bears a greater resemblance to SO$_2$Cl$_2$, also a source of Cl$_2$. For oxidative chlorinations on the laboratory scale, sulfuryl chloride is often preferred over PCl$_5$ since the gaseous SO$_2$ by-product is readily separated.

### Chlorination of inorganic compounds

As for the reactions with organic compounds, the use of PCl$_5$ has been superseded by SO$_2$Cl$_2$. The reaction of phosphorus pentoxide and PCl$_5$ produces POCl$_3$ :
    6 PCl$_5$ + P$_4$O$_{10}$ → 10 POCl$_3$
PCl$_5$ chlorinates nitrogen dioxide to form unstable nitryl chloride:
    PCl$_5$ + 2 NO$_2$ → PCl$_3$ + 2 NO$_2$Cl
    2 NO$_2$Cl → 2 NO$_2$ + Cl$_2$
PCl$_5$ is a precursor for lithium hexafluorophosphate, LiPF$_6$. 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:
    PCl$_5$ + 6 LiF → LiPF$_6$ + 5 LiCl

## Safety

PCl$_5$ 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.