Phosphorus chlorides
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Several phosphorus chlorides are known. The most important are phosphorus trichloride (PCl3) and phosphorus pentachloride (PCl5). They are reactive chemicals used as parent compounds for a wide range of phosphorus containing compounds. They also find use as chlorinating reagents. They are applied on industrial and laboratory scale. They are both hazardous and toxic and neither is found in nature. They both react upon contact with water to produce corrosive and toxic hydrogen chloride gas. At room temperature and pressure, phosphorus trichloride is a liquid and phosphorus pentachloride is a solid.
(PCl3) contains a P atom with a formal charge of 3+. It is the parent compound for organophosphorus compounds which contain one or more P atoms with this charge, most notably phosphites and phosphonates. These compounds do not usually contain the chlorine atoms found in (PCl3). These are lost as hydrogen chloride (HCl) during their synthesis. (PCl3) is however the usual starting compound for their synthesis, since it is the most versatile and cheapest source of (P3+).
Both phosphorus chlorides will chlorinate carboxylic acids to their corresponding acyl chloride derivatives and both will chlorinate alcohols by substituting the hydroxyl group with a chlorine atom to yield an alkyl chloride. The use of thionyl chloride could provide similar results in such cases, except that the byproducts sulfur dioxide and hydrogen chloride are both gases which normally separate from the reaction product mixture.
Phosphorus trichloride is produced on a large scale by the reaction of chlorine with excess phosphorus, and it is used in a variety of applications such as manufacture of herbicides. The worldwide use of (PCl3) was several hundred thousand tons in 2005. (PCl5) world use is considerably less.
PCl5 is made by the reaction of more chlorine with PCl3. PCl5 is in equilibrium with PCl3 and free chlorine. Depending on the polarity of the solvent the compounds are crystallized from, it is obtained either as neutral PCl5 or ionic (PCl4+)(PCl6-).
If an electric discharge is passed through a mixture of PCl3 vapour and hydrogen gas, a rare chloride of phosphorus is formed, diphosphorus tetrachloride (P2Cl4).
Some reactions with reactants PCl3 or PCl5 can produce POCl3, called phosphorus oxytrichloride, phosphorus oxychloride, or phosphoryl chloride.
For example, POCl3 can be obtained by oxidation of PCl3 by oxygen as follows:[2]
2 PCl3 + O2 → 2 POCl3
POCl3 is used in the manufacture of phosphate esters, especially triesters which are not easily accessible using phosphorus pentoxide (P2O5) or polyphosphoric acid. These have many applications, including flame retardants and plasticizers.
PCl5 reacts with cold water to give POCl3 as follows:[3]
PCl5 + H2O → POCl3 + 2 HCl
In hot water, PCl5 is hydrolyzed all the way to ortho-phosphoric acid as follows:[3]
PCl5 + 4 H2O → H3PO4 + 5 HCl
Reacting the following phosphorus oxide with PCl5 can produce POCl3:[2]
P4O10 + 6 PCl5 → 10 POCl3
PCl5 is a mild chlorination agent, used in pharmaceuticals synthesis. It is also a precursor for lithium hexafluorophosphate (LiPF6), a key ingredient in lithium ion battery electrolytes.
Other phosphorus oxyhalides
There are phosphorus oxytrihalides with fluorine, chlorine, bromine, and iodine. They can react with water to form the ortho-phosphoric acid with the corresponding hydrogen halide. They can react with alcohols to form phosphate esters plus the corresponding hydrogen halide. They can also react with Grignard reagents to form trialkyl phosphoryl compounds (R3PO, also called phosphine oxides). They also complex with various metals. Some complex phosphorus oxohalides are also known for fluorine and chlorine.[2]
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References
- Appendix B, p. 897. William W. Porterfield, Inorganic Chemistry - A Unified Approach, 2nd Edition, (1993). Academic Press, Inc., San Diego.
- F. A. Cotton, G. Wilkinson, C. A. Murillo, and M. Bochmann (April 1999). Advanced Inorganic Chemistry, 6th Edition. Wiley-VCH. ISBN 0-471-19957-5
- [Hydrogen Chloride]
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