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| Names | |||
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| IUPAC names Phosphorus pentachloride Pentachloro-λ5-phosphane | |||
| Other names Pentachlorophosphorane | |||
| Identifiers | |||
3D model (JSmol) | |||
| ChemSpider |
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| ECHA InfoCard | 100.030.043 | ||
| EC Number |
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| RTECS number |
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| UNII | |||
| UN number | 1806 | ||
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| Properties | |||
| PCl5 | |||
| Molar mass | 208.22 g·mol−1 | ||
| Appearance | Colorless crystals, but commercial samples often yellowish white crystals | ||
| Odor | pungent, unpleasant[1] | ||
| Density | 2.1 g/cm3 | ||
| Melting point | 160.5 °C (320.9 °F; 433.6 K) | ||
| Boiling point | 166.8 °C (332.2 °F; 439.9 K) sublimation | ||
| reacts | |||
| Solubility | soluble inCS2,chlorocarbons,benzene | ||
| Vapor pressure | 1.11 kPa (80 °C) 4.58 kPa (100 °C)[2] | ||
| Structure | |||
| tetragonal | |||
| D3h (trigonal bipyramidal) | |||
| 0 D | |||
| Thermochemistry | |||
| 111.5 J/(mol·K)[2] | |||
Std molar entropy(S⦵298) | 364.2 J/(mol·K)[2] | ||
| Hazards | |||
| GHS labelling: | |||
   [3] | |||
| Danger | |||
| H302,H314,H330,H373[3] | |||
| P260,P280,P284,P305+P351+P338,P310[3] | |||
| NFPA 704 (fire diamond) | |||
| Flash point | Non-flammable | ||
| Lethal dose or concentration (LD, LC): | |||
LD50 (median dose) | 660 mg/kg (rat, oral)[4] | ||
LC50 (median concentration) | 205 mg/m3 (rat)[4] | ||
LCLo (lowest published) | 1020 mg/m3 (mouse, 10 min)[4] | ||
| NIOSH (US health exposure limits): | |||
PEL (Permissible) | TWA 1 mg/m3[1] | ||
REL (Recommended) | TWA 1 mg/m3[1] | ||
IDLH (Immediate danger) | 70 mg/m3[1] | ||
| Safety data sheet (SDS) | ICSC 0544 | ||
| Related compounds | |||
Related phosphorus pentahalides | |||
Related compounds | |||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Phosphorus pentachloride is thechemical compound with the formulaPCl5. It is one of the most importantphosphorus chlorides/oxychlorides, others beingPCl3 andPOCl3.PCl5 finds use as achlorinating reagent. It is a colourless, water-sensitivesolid, although commercial samples can be yellowish and contaminated withhydrogen chloride.
The structures for the phosphorus chlorides are invariably consistent withVSEPR theory. The structure ofPCl5 depends on its environment. Gaseous and moltenPCl5 is a neutral molecule withtrigonal bipyramidal geometry and (D3h)symmetry. Thehypervalent nature of this species (as well as of[PCl6]−, see below) can be explained with the inclusion of non-bondingmolecular orbitals (molecular orbital theory) orresonance (valence bond theory). This trigonal bipyramidal structure persists in nonpolar solvents, such asCS2 andCCl4.[5] In the solid statePCl5 is anionic compound called tetrachlorophosphonium hexachlorophosphate formulated[PCl4]+[PCl6]−.[6]
In solutions of polar solvents,PCl5 undergoes self-ionization.[8] Dilute solutions dissociate according to the following equilibrium:
At higher concentrations, a second equilibrium becomes more prevalent:
The cation[PCl4]+ and the anion[PCl6]− aretetrahedral andoctahedral, respectively. At one time,PCl5 in solution was thought to form a dimeric structure,P2Cl10, but this suggestion is not supported byRaman spectroscopic measurements.
AsCl5 andSbCl5 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).[9] At low temperatures,SbCl5 converts to the dimer, dioctahedralSb2Cl10, structurally related toniobium pentachloride.
PCl5 is prepared by thechlorination ofPCl3.[10] This reaction is used to produce around 10,000 tonnes ofPCl5 per year (as of 2000).[6]
PCl5 exists in equilibrium withPCl3 andchlorine, and at 180 °C the degree of dissociation is about 40%.[6] Because of this equilibrium, samples ofPCl5 often contain chlorine, which imparts a greenish coloration.
In its most characteristic reaction,PCl5reacts upon contact withwater to releasehydrogen chloride and give phosphorus oxides. The first hydrolysis product isphosphorus oxychloride:
In hot water, hydrolysis proceeds completely toorthophosphoric acid:
Phosphorus pentachloride is a Lewis acid. This property underpins many of its characteristic reactions, autoionization, chlorinations, hydrolysis. A well studied adduct isPCl5(pyridine).[11]
In synthetic chemistry, two classes of chlorination are usually of interest: oxidative chlorinations and substitutive chlorinations. Oxidative chlorinations entail the transfer ofCl2 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 withPCl5,carboxylic acids convert to the correspondingacyl chloride.[12] The following mechanism has been proposed:[13]
It also convertsalcohols toalkyl chlorides.Thionyl chloride is more commonly used in the laboratory because the resultantsulfur dioxide is more easily separated from the organic products than isPOCl3.
PCl5 reacts with a tertiary amides, such asdimethylformamide (DMF), to give dimethylchloromethyleneammonium chloride, which is called theVilsmeier reagent,[(CH3)2N=CClH]+Cl−. More typically, a related salt is generated from the reaction of DMF andPOCl3. Such reagents are useful in the preparation of derivatives ofbenzaldehyde by formylation and for the conversion of C−OH groups into C−Cl groups.[14]
It is especially renowned for the conversion ofC=O groups toCCl2 groups.[15] For example,benzophenone and phosphorus pentachloride react to give thediphenyldichloromethane:[16]
Theelectrophilic character ofPCl5 is highlighted by its reaction withstyrene to give, afterhydrolysis,phosphonic acid derivatives.[17]
BothPCl3 andPCl5 convertR3COH groups to the chlorideR3CCl. The pentachloride is however a source of chlorine in many reactions. It chlorinates allylic andbenzylic CH bonds.PCl5 bears a greater resemblance toSO2Cl2, also a source ofCl2. For oxidative chlorinations on the laboratory scale, sulfuryl chloride is often preferred overPCl5 since the gaseousSO2 by-product is readily separated.
As for the reactions with organic compounds, the use ofPCl5 has been superseded bySO2Cl2. The reaction ofphosphorus pentoxide andPCl5 producesPOCl3 :[18][page needed]
PCl5 chlorinatesnitrogen dioxide to form unstablenitryl chloride:
PCl5 is a precursor forlithium hexafluorophosphate,Li[PF6]. Lithium hexafluorophosphate is a commonly employed salt inelectrolytes inlithium ion batteries.[19]Li[PF6] is produced by the reaction ofPCl5 withlithium fluoride, withlithium chloride as a side product:
PCl5 is a dangerous chemical as it reacts violently with water. It is also corrosive when in contact with skin. It is toxic and can be fatal when inhaled.
Phosphorus pentachloride was first prepared in 1808 by the English chemistHumphry Davy.[20] Davy's analysis of phosphorus pentachloride was inaccurate;[21] the first accurate analysis was provided in 1816 by the French chemistPierre Louis Dulong.[22]
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