Phosphorus trichloride

Phosphorus trichloride (formula PCl₃) is the most important of the three phosphorus chlorides. It is an important industrial chemical, being used for the manufacture of organophosphorus compounds for a wide variety of applications.

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Chemical properties

The phosphorus in PCl₃ is often considered to have the +3 oxidation state and the chlorine atoms are considered to be in the -1 oxidation state. Most of its reactivity is consistent with this description.

Redox reactions

PCl₃ is a precursor to other phosphorus compound, undergoing oxidation to e.g. phosphorus pentachloride (PCl₅), thiophosphoryl chloride (PSCl₃), or phosphorus oxychloride (POCl₃).

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).

PCl₃ as an electrophile

Phosphorus trichloride is the precursor to organophosphorus compounds that contain one or more (P³⁺) atoms, most notably phosphites and phosphonates. These compounds do not usually contain the chlorine atoms found in PCl₃.

PCl₃ reacts rapidly and exothermically with water to form phosphorous acid, H₃PO₃ and HCl. A large number of similar substitution reactions are known, the most important of which is the formation of phosphite esters by reaction with alcohols or phenols. For example, with phenol, triphenyl phosphite is formed:

3 PhOH + PCl₃ → P(OPh)₃ + 3 HCl

where "Ph" stands for phenyl group, -C₆H₅. Alcohols such as ethanol react similarly in the presence of a base such as :^[1]

PCl₃ + 3 EtOH + 3 R₃N → P(OEt)₃ + 3 R₃NH⁺Cl^-

Of the many related compounds can be prepared similarly, triisopropyl phosphite is an example (b.p. 43.5 °C/1.0 mm; CAS# 116-17-6).

In the absence of base, however, the reaction affords a dialkyl phosphonate and an alkyl chloride, according to the following stoichiometry:

PCl₃ + 3 C₂H₅OH → (C₂H₅O)₂P(=O)H + C₂H₅Cl + 2 HCl

Amines, R₂NH, form P(NR₂)₃, and thiols (RSH) form P(SR)₃. An industrially relevant reaction of PCl₃ with amines is phosphonomethylation, which employs formaldehyde:

R₂NH + PCl₃ + CH₂O → (HO)₂P(O)CH₂NR₂ + 3 HCl

Aminophosphonates are widely used as sequestring and antiscale agents in water treatment. The large volume herbicide glyphosate is also produced this way. The reaction of PCl₃ with Grignard reagents and organolithium reagents is a useful method for the preparation of organic phosphines with the formula R₃P (sometimes called phosphanes) such as triphenylphosphine, Ph₃P.

3 PhMgBr + PCl₃ → Ph₃P + 3 MgBrCl

Under controlled conditions PCl₃ can be used to prepare PhPCl₂ and Ph₂PCl.

PCl₃ as a nucleophile

Phosphorus trichloride has a lone pair, and therefore can act as a Lewis base, for example with the Lewis acids BBr₃^[5] it forms a 1:1 adduct, Br₃B⁻−⁺PCl₃. Metal complexes such as Ni(PCl₃)₄ are known. This Lewis basicity is exploited in one useful route to organophosphorus compounds:

PCl₃ + RCl + AlCl₃ → (RPCl₃)⁺⁻AlCl₄

The (RPCl₃)⁺ product can then be decomposed with water to produce an alkylphosphonic dichloride RP(=O)Cl₂.

Preparation

World production exceeds one-third of a million tonnes^[1]. Phosphorus trichloride is prepared industrially by the reaction of chlorine with a refluxing solution of white phosphorus in phosphorus trichloride, with continuous removal of PCl₃ as it is formed.

P₄ + 6 Cl₂ → 4 PCl₃

Industrial production of phosphorus trichloride is controlled under the Chemical Weapons Convention, where it is listed in schedule 3.In the laboratory it may be more convenient to use the less toxic red phosphorus^[6]. It is sufficiently inexpensive that it would not be synthesized for laboratory use.

Uses

PCl₃ is important indirectly as a precursor to PCl₅, POCl₃ and PSCl₃. which in turn enjoy many applications in herbicides, insecticides, plasticisers, oil additives, and flame retardants.

For example oxidation of PCl₃ gives POCl₃, which is used for the manufacture of triphenyl phosphate and tricresyl phosphate, which find application as flame retardants and plasticisers for PVC. They are also used to make insecticides such as diazinon. Phosphonates include the herbicide glyphosate.

PCl₃ is the precursor to triphenylphosphine for the Wittig reaction, and phosphite esters which may be used as industrial intermediates, or used in the Horner-Wadsworth-Emmons reaction, both important methods for making alkenes. It can be used to make trioctylphosphine oxide (TOPO), used as an extraction agent, although TOPO is usually made via the corresponding phosphine.

PCl₃ is also used directly as a reagent in organic synthesis. It is used to convert primary and secondary alcohols into alkyl chlorides, or carboxylic acids into acyl chlorides, although thionyl chloride generally gives better yields than PCl₃^[8].

Precautions

PCl₃ is toxic, with a concentration of 600 ppm being lethal in just a few minutes^[7]. PCl₃ is classified as very toxic and corrosive under EU Directive 67/548/EEC, and the risk phrases R14, R26/28, R35 and R48/20 are obligatory.

References

1. ^ A. H. Ford-Moore and B. J. Perry (1963). "Triethyl Phosphite". Org. Synth.; Coll. Vol. 4: 955. 

1. N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
2. Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
3. J. March, Advanced Organic Chemistry, 4th ed., p. 723, Wiley, New York, 1992.
4. The Merck Index, 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
5. R. R. Holmes, Journal of Inorganic and Nuclear Chemistry 12, 266-275 (1960).
6. M. C. Forbes, C. A. Roswell, R. N. Maxson, Inorganic Syntheses, Vol. II, 145-7 (1946).
7. A. D. F. Toy, The Chemistry of Phosphorus, Pergamon Press, Oxford, UK, 1973.
8. L. G. Wade, Jr., Organic Chemistry, 6th ed., p. 477, Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005.