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 and moisture-sensitive solid, although commercial samples can be yellowish and contaminated with hydrogen chloride.
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||208.22 g·mol−1|
|Melting point||160.5 °C (320.9 °F; 433.6 K)|
|Boiling point||166.8 °C (332.2 °F; 439.9 K) sublimation|
|Solubility||soluble in CS2, chlorocarbons, benzene|
|Vapor pressure||1.11 kPa (80 °C)|
4.58 kPa (100 °C)
|D3h (trigonal bipyramidal)|
Heat capacity (C)
|Safety data sheet||ICSC 0544|
|GHS Signal word||Danger|
|H302, H314, H330, H373|
|P260, P280, P284, P305+351+338, P310|
|NFPA 704 (fire diamond)|
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|660 mg/kg (rat, oral)|
LC50 (median concentration)
|205 mg/m3 (rat)|
LCLo (lowest published)
|1020 mg/m3 (mouse, 10 min)|
|NIOSH (US health exposure limits):|
|TWA 1 mg/m3|
|TWA 1 mg/m3|
IDLH (Immediate danger)
Related phosphorus pentahalides
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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 geometry and (D3h) symmetry. The hypervalent nature of this species (as well as for 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 CS2 and CCl4. In the solid state PCl5 is an ionic compound, formulated PCl+
- PCl5 ⇌ PCl+
4 + Cl−
At higher concentrations, a second equilibrium becomes more prevalent:
- 2 PCl5 ⇌ PCl+
4 + PCl−
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.
AsCl5 and SbCl5 also adopt trigonal bipyramidal structures. The relevant bond distances are 211 pm (As−Cleq), 221 pm (As−Clax), 227 pm (Sb−Cleq), and 233.3 pm (Sb−Clax). At low temperatures, SbCl5 converts to the dimer, dioctahedral Sb2Cl10, structurally related to niobium pentachloride.
- 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.
- PCl5 + H2O → POCl3 + 2 HCl
In hot water, hydrolysis proceeds completely to orthophosphoric acid:
- PCl5 + 4 H2O → H3PO4 + 5 HCl
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 compoundsEdit
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.
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.
- (C6H5)2CO + PCl5 → (C6H5)2CCl2 + POCl3
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 compoundsEdit
- 6 PCl5 + P4O10 → 10 POCl3
- PCl5 + 2 NO2 → PCl3 + 2 NO2Cl
- 2 NO2Cl → 2 NO2 + Cl2
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
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.
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.
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- Phosphorus pentachloride
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- Davy, Humphry (1809). "The Bakerian Lecture. An account of some new analytical researches on the nature of certain bodies, particularly the alkalies, phosphorus, sulphur, carbonaceous matter, and the acids hitherto undecomposed; with some general observations on chemical theory". Philosophical Transactions of the Royal Society of London. 99: 39–104. doi:10.1098/rstl.1809.0005. On pp. 94–95, Davy mentioned that when he burned phosphorus in chlorine gas ("oxymuriatic acid gas"), he obtained a clear liquid (phosphorus trichloride) and a white solid (phosphorus pentachloride).
- Davy, Humphry (1810). "Researches on the oxymuriatic acid [i.e., chlorine], its nature and combinations; and on the elements of the muriatic acid [i.e., hydrogen chloride]. With some experiments on sulphur and phosphorus, made in the laboratory of the Royal Institution". Philosophical Transactions of the Royal Society of London. 100: 231–257. doi:10.1098/rstl.1810.0016. On p. 257, Davy presented his empirical formula for phosphorus pentachloride: 1 portion of phosphorus to 3 portions of "oxymuriatic gas" (chlorine).
- Dulong (1816). "Extrait d'un mémoire sur les combinaisons du phosphore avec l'oxigène" [Extract from a memoir on the compounds of phosphorus with oxygen]. Annales de Chimie et de Physique. 2nd series (in French). 2: 141–150. On p. 148, Dulong presented the correct analysis of phosphorus pentachloride (which is 14.9% phosphorus and 85.1% chlorine by weight, vs. Dulong's values of 15.4% and 84.6%, respectively).