Phosphorus pentachloride
Phosphorus pentachloride is the chemical compound with the formula. It is one of the most important phosphorus chlorides/oxychlorides, others being [phosphorus trichloride|] and [phosphorus oxytrichloride|]. 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 depends on its environment. Gaseous and molten is a neutral molecule with trigonal [bipyramidal molecular geometry|trigonal bipyramidal] geometry and symmetry. The hypervalent nature of this species can be explained with the inclusion of non-bonding molecular orbitals or resonance. This trigonal bipyramidal structure persists in nonpolar solvents, such as [carbon disulfide|] and [carbon tetrachloride|]. In the solid state is an ionic compound called tetrachlorophosphonium hexachlorophosphate formulated.In solutions of polar solvents, undergoes self-ionization. Dilute solutions dissociate according to the following equilibrium:
At higher concentrations, a second equilibrium becomes more prevalent:
The cation and the anion are tetrahedral and octahedral, respectively. At one time, in solution was thought to form a dimeric structure,, but this suggestion is not supported by Raman spectroscopic measurements.
Related pentachlorides
and [Antimony pentachloride|] also adopt trigonal bipyramidal structures. The relevant bond distances are 211 pm, 221 pm, 227 pm, and 233.3 pm. At low temperatures, converts to the dimer, dioctahedral, structurally related to niobium pentachloride.Preparation
is prepared by the chlorination of. This reaction is used to produce around 10,000 tonnes of per year.exists in equilibrium with and chlorine, and at 180 °C the degree of dissociation is about 40%. Because of this equilibrium, samples of often contain chlorine, which imparts a greenish coloration.
Reactions
Hydrolysis
In its most characteristic reaction, 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.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 from the reagent to the substrate. Substitutive chlorinations entail replacement of O or OH groups with chloride. can be used for both processes.Upon treatment with, carboxylic acids convert to the corresponding acyl chloride. The following mechanism has been proposed:
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.
reacts with a tertiary amides, such as dimethylformamide, to give dimethylchloromethyleneammonium chloride, which is called the Vilsmeier reagent,. More typically, a related salt is generated from the reaction of DMF and. 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.
It is especially renowned for the conversion of C=O groups to groups. For example, benzophenone and phosphorus pentachloride react to give the diphenyldichloromethane:
The electrophilic character of is highlighted by its reaction with styrene to give, after hydrolysis, phosphonic acid derivatives.
Comparison with related reagents
Both and convert groups to the chloride. The pentachloride is however a source of chlorine in many reactions. It chlorinates allylic and benzylic CH bonds. bears a greater resemblance to [sulfuryl chloride|], also a source of. For oxidative chlorinations on the laboratory scale, sulfuryl chloride is often preferred over since the gaseous by-product is readily separated.Chlorination of inorganic compounds
As for the reactions with organic compounds, the use of has been superseded by. The reaction of phosphorus pentoxide and produces [phosphorus oxychloride|] :chlorinates nitrogen dioxide to form unstable nitryl chloride:
is a precursor for lithium hexafluorophosphate,. Lithium hexafluorophosphate is a commonly employed salt in electrolytes in lithium ion batteries. is produced by the reaction of with lithium fluoride, with lithium chloride as a side product: