Phosphorus pentachloride

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 (D3h) 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 D3h.

In solutions of polar solvents, however, PCl5 undergoes "autoionization". Dilute solutions dissociate according to the following equilibrium:
 * PCl5 $$\overrightarrow{\leftarrow}$$ [PCl4+]Cl−

At higher concentrations, a second equilibrium becomes more important:
 * 2 PCl5 $$\overrightarrow{\leftarrow}$$ [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.
 * PCl3 + Cl2 $$\overrightarrow{\leftarrow}$$ PCl5     &Delta;H = −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 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.

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 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.

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.

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

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 &rarr; 10 POCl3

PCl5 chlorinates nitrogen dioxide:
 * PCl5 +  2 NO2 &rarr;  PCl3 +  2 NO2Cl

PCl5 is a precursor for lithium hexafluorophosphate, LiPF6, an electrolytes in lithium ion battery:
 * PCl5 +  6 LiF  &rarr;  LiPF6 +  5 LiCl

Arsenic and antimony pentachloride
AsCl5 and SbCl5 adopt trigonal bipyramidal structures. The relevant bond distances are 211 (As-Cleq) 221 (As-Cleq), 227 (Sb-Cleq), and 233.3 pm (Sb-Clax ). 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.