Phosphorus pentachloride
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| Phosphorus pentachloride | |
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| General | |
| Systematic name | Phosphorus(V) chloride |
| Other names | Pentachlorophosphorus |
| Molecular formula | PCl5 |
| SMILES | ClP(Cl)(Cl)(Cl)Cl |
| Molar mass | 208.22 g mol−1 |
| Appearance | colorless crystals |
| CAS number | [10026-13-8] |
| Properties | |
| Density and phase | 1.6 g cm−3 |
| Solubility in water | decomposition (violent) |
| Other solvents | carbon disulfide, chlorocarbons, benzene |
| Melting point | 179–181 °C |
| Boiling point | sublimation 70-80 °C (vacuum) |
| Structure | |
| Coordination geometry |
trigonal bipyramidal |
| Crystal structure | |
| Dipole moment | 0 D |
| Hazards | |
| MSDS | External MSDS |
| Main hazards | HCl source |
| NFPA 704 | |
| R/S statement | R: 14-22-26-34-48/20 S: 26-36/37/39-45-7/8 |
| RTECS number | TB6125000 |
| Supplementary data page | |
| Structure and properties |
n, εr, etc. |
| Thermodynamic data |
Phase behaviour Solid, liquid, gas |
| Spectral data | Raman: 456 cm−1 (PCl4+) 354 cm−1 (PCl6−) 393 cm-1 (PCl5) |
| Related compounds | |
| Related compounds | POCl3, PCl3, PF5 |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references |
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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, which adopts a variety of structures under various conditions.
Contents |
[edit] Structure
Gaseous and molten PCl5 is a neutral molecule with trigonal bipyramidal (D3h) symmetry. The structure of “PCl5” in solution, however, depends on the solvent and on the concentration.[1] In polar solvents, dilute solutions dissociate according to the following equilibrium:
- PCl5
[PCl4+]Cl−
At higher concentrations, a second equilibrium becomes more important:
- 2 PCl5 [PCl4+][PCl6−]
The cation PCl4+ and the anion PCl6− are tetrahedral and octahedral, respectively. The structures for the phosphorus chlorides are invariably consistent with VSEPR theory.
In non-polar solvents, such as CS2 and CCl4, the D3h structure seen in gaseous and liquid remains intact.[2]
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.
The hypervalent nature of PCl5 can be explained using the three-center four-electron bonding model.
[edit] Preparation
PCl5 is prepared by the chlorination of PCl3. This reaction was used to produce ca. 10,000,000 kg of PCl5 in 2000.[3]
PCl3 + Cl2 PCl5 ΔH = −124 kJ/mol
PCl5 exists in equilibrium with PCl3 and chlorine, at 180 °C, the degree of dissociation is ca. 40%.[3] Because of this equilibrium, samples of PCl5 often contain chlorine, which imparts a greenish coloration.
[edit] 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
[edit] Other reactions
Most often PCl5 is used for chlorinations.[4]
[edit] 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[5] 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/PCl3 bears 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.[4]
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.[6]
The electrophilic character of PCl5 is highlighted by its reaction with styrene to give, after hydrolysis, phosphonic acid derivatives.[7]
[edit] 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 → 10 POCl3
PCl5 chlorinates nitrogen dioxide:
- PCl5 + 2 NO2 → PCl3 + 2 NO2Cl
PCl5 is a precursor for lithium hexafluorophosphate, LiPF6, an electrolytes in lithium ion battery:
[edit] 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 ).[8] At low temperatures, SbCl5 converts to the dimer, bioctahedral Sb2Cl10, structurally related to niobium pentachloride.
[edit] Safety
PCl5 is a dangerous substance as it reacts violently with water and is a source of both hydrogen chloride and chlorine.
[edit] See also
[edit] References
- ^ Suter, R. W.; Knachel, H. C.; Petro, V. P.; Howatson, J. H.; S. G. Shore, S. G. ”Nature of Phosphorus(V) Chloride in Ionizing and Nonionizing Solvents” Journal of the American Chemical Society 1973, volume 95, pp 1474 - 1479; DOI: 10.1021/ja00786a021
- ^ D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam 1995. ISBN 0-444-89307-5.
- ^ a b Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
- ^ a b Burks, Jr., J. E. “Phosphorus(V) Chloride” in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
- ^ Adams, R.; Jenkins, R. L. “p-Nitrobenzoyl chloride” Organic Syntheses, Collected Volume 1, p.394 (1941).
- ^ Gross, H.; Rieche, A.; Höft, E.; Beyer, E. “Dichloromethyl Methyl Ether” Organic Syntheses, Collected Volume 5, p.365 (1973).
- ^ Schmutzler, R. ”Styrylphosphonic dichloride” Organic Syntheses, Collected Voume 5, p.1005 (1973).
- ^ Haupt, S.; Seppelt, K., "Solid State Structures of AsCl5 and SbCl5", Zeitschrift für Anorganische und Allgemeine Chemie, 2002, volume 628, pages 729-734.
