Phosphorus pentachloride
Script error: No such module "redirect hatnote". Template:Chembox Phosphorus pentachloride is the chemical compound with the formula Template:Chem2. It is one of the most important phosphorus chlorides/oxychlorides, others being [[phosphorus trichloride|Template:Chem2]] and [[phosphorus oxytrichloride|Template:Chem2]]. Template:Chem2 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 Template:Chem2 depends on its environment. Gaseous and molten Template:Chem2 is a neutral molecule with trigonal bipyramidal geometry and (D3h) symmetry. The hypervalent nature of this species (as well as of Template:Chem2, 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 [[carbon disulfide|Template:Chem2]] and [[carbon tetrachloride|Template:Chem2]].[1] In the solid state Template:Chem2 is an ionic compound called tetrachlorophosphonium hexachlorophosphate formulated Template:Chem2.[2]
In solutions of polar solvents, Template:Chem2 undergoes self-ionization.[4] Dilute solutions dissociate according to the following equilibrium:
At higher concentrations, a second equilibrium becomes more prevalent:
The cation Template:Chem2 and the anion Template:Chem2 are tetrahedral and octahedral, respectively. At one time, Template:Chem2 in solution was thought to form a dimeric structure, Template:Chem2, but this suggestion is not supported by Raman spectroscopic measurements.
Related pentachlorides
[[Arsenic pentachloride|Template:Chem2]] and [[Antimony pentachloride|Template:Chem2]] 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).[5] At low temperatures, Template:Chem2 converts to the dimer, dioctahedral Template:Chem2, structurally related to niobium pentachloride.
Preparation
Template:Chem2 is prepared by the chlorination of Template:Chem2.[6] This reaction is used to produce around 10,000 tonnes of Template:Chem2 per year (as of 2000).[2]
- Template:Chem2 Template:Pad (ΔH = −124 kJ/mol)
Template:Chem2 exists in equilibrium with Template:Chem2 and chlorine, and at 180 °C the degree of dissociation is about 40%.[2] Because of this equilibrium, samples of Template:Chem2 often contain chlorine, which imparts a greenish coloration.
Reactions
Hydrolysis
In its most characteristic reaction, Template:Chem2 reacts upon contact with water to release hydrogen chloride and give phosphorus oxides. The first hydrolysis product is phosphorus oxychloride:
In hot water, hydrolysis proceeds completely to orthophosphoric acid:
Lewis acidity
Phosphorus pentachloride is a Lewis acid. This property underpins many of its characteristic reactions, autoionization, chlorinations, hydrolysis. A well studied adduct is Template:Chem2.[7]
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 Template:Chem2 from the reagent to the substrate. Substitutive chlorinations entail replacement of O or OH groups with chloride. Template:Chem2 can be used for both processes.
Upon treatment with Template:Chem2, carboxylic acids convert to the corresponding acyl chloride.[8] The following mechanism has been proposed:[9]
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 Template:Chem2.
Template:Chem2 reacts with a tertiary amides, such as dimethylformamide (DMF), to give dimethylchloromethyleneammonium chloride, which is called the Vilsmeier reagent, Template:Chem2. More typically, a related salt is generated from the reaction of DMF and Template:Chem2. 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.[10]
It is especially renowned for the conversion of C=O groups to Template:Chem2 groups.[11] For example, benzophenone and phosphorus pentachloride react to give the diphenyldichloromethane:[12]
The electrophilic character of Template:Chem2 is highlighted by its reaction with styrene to give, after hydrolysis, phosphonic acid derivatives.[13]
Both Template:Chem2 and Template:Chem2 convert Template:Chem2 groups to the chloride Template:Chem2. The pentachloride is however a source of chlorine in many reactions. It chlorinates allylic and benzylic CH bonds. Template:Chem2 bears a greater resemblance to [[sulfuryl chloride|Template:Chem2]], also a source of Template:Chem2. For oxidative chlorinations on the laboratory scale, sulfuryl chloride is often preferred over Template:Chem2 since the gaseous Template:Chem2 by-product is readily separated.
Chlorination of inorganic compounds
As for the reactions with organic compounds, the use of Template:Chem2 has been superseded by Template:Chem2. The reaction of phosphorus pentoxide and Template:Chem2 produces [[phosphorus oxychloride|Template:Chem2]] :[14]Script error: No such module "Unsubst".
Template:Chem2 chlorinates nitrogen dioxide to form unstable nitryl chloride:
Template:Chem2 is a precursor for lithium hexafluorophosphate, Template:Chem2. Lithium hexafluorophosphate is a commonly employed salt in electrolytes in lithium ion batteries.[15] Template:Chem2 is produced by the reaction of Template:Chem2 with lithium fluoride, with lithium chloride as a side product:
Safety
Template:Chem2 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.
History
Phosphorus pentachloride was first prepared in 1808 by the English chemist Humphry Davy.[16] Davy's analysis of phosphorus pentachloride was inaccurate;[17] the first accurate analysis was provided in 1816 by the French chemist Pierre Louis Dulong.[18]
See also
References
External links
- The period 3 chlorides
- International Chemical Safety Card 0544
- CDC - NIOSH Pocket Guide to Chemical Hazards
Template:Phosphorus compounds Template:Chlorides Template:Authority control
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- ↑ Script error: No such module "Citation/CS1". 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).
- ↑ Script error: No such module "Citation/CS1". On p. 257, Davy presented his empirical formula for phosphorus pentachloride: 1 portion of phosphorus to 3 portions of "oxymuriatic gas" (chlorine).
- ↑ Script error: No such module "Citation/CS1". 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).