Phosphorus(III) Chloride

Phosphorus trichloride is an inorganic compound with the chemical formula PCl₃. A colorless liquid when pure, it is an important industrial chemical, being used for the manufacture of phosphites and other organophosphorus compounds. It is toxic and reacts readily with water to release hydrogen chloride.

History

Phosphorus trichloride was first prepared in 1808 by the French chemists Joseph Louis Gay-Lussac and Louis Jacques Thénard by heating calomel (Hg₂Cl₂) with phosphorus. Later during the same year, the English chemist Humphry Davy produced phosphorus trichloride by burning phosphorus in chlorine gas.

Preparation

World production exceeds one-third of a million tonnes. Phosphorus trichloride is prepared industrially by the reaction of chlorine with white phosphorus, using phosphorus trichloride as the solvent. In this continuous process PCl₃ is removed as it is formed in order to avoid the formation of PCl₅.

:P₄ + 6 Cl₂ → 4 PCl₃

Structure and spectroscopy

It has a trigonal pyramidal shape. Its ³¹P NMR spectrum exhibits a singlet around +220 ppm with reference to a phosphoric acid standard.

Reactions

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.

Oxidation

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

PCl₃ as an electrophile

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

PCl₃ reacts vigorously with water to form phosphorous acid (H₃PO₃) and hydrochloric acid:

:PCl₃ + 3 H₂O → H₃PO₃ + 3 HCl

A large number of similar substitution reactions are known, the most important of which is the formation of phosphites by reaction with alcohols and phenols. For example, with phenol, triphenyl phosphite is formed:

:3 PhOH + PCl₃ → P(OPh)₃ + 3 HCl

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

:PCl₃ + 3 EtOH + 3 R₃N → P(OEt)₃ + 3 R₃NH⁺Cl⁻

In the absence of base, however, the reaction proceeds with the following stoichiometry to give diethylphosphite:
:PCl₃ + 3 EtOH → (EtO)₂P(O)H + 2 HCl + EtCl

Secondary amines (R₂NH) form aminophosphines. For example, bis(diethylamino)chlorophosphine, (Et₂N)₂PCl, is obtained from diethylamine and PCl₃. 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 sequestering 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 or especially with bulky organic groups, similar reactions afford less substituted derivatives such as chlorodiisopropylphosphine.

PCl₃ as a nucleophile

Phosphorus trichloride has a lone pair, and therefore can act as a Lewis base, e.g., forming a 1:1 adduct Br₃B-PCl₃. Metal complexes such as Ni(PCl₃)₄ are known, again demonstrating the ligand properties of PCl₃.

This Lewis basicity is exploited in the Kinnear–Perren reaction to prepare alkylphosphonyl dichlorides (RP(O)Cl₂) and alkyl phosphonate esters (RP(O)(OR')₂). Alkylation of phosphorus trichloride is effected in the presence of aluminium trichloride give the alkyltrichlorophosphonium salts, which are versatile intermediates:
:PCl₃ + RCl + AlCl₃ → RPCl + AlCl
The RPCl product can then be decomposed with water to produce an alkylphosphonic dichloride RP(=O)Cl₂.

PCl₃ as a ligand

PCl₃, like the more popular phosphorus trifluoride, is a ligand in coordination chemistry. One example is Mo(CO)₅PCl₃.

Uses

PCl₃ is important indirectly as a precursor to PCl₅, POCl₃ and PSCl₃, which are used in many applications, including 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.

Safety

* 600 ppm is lethal in just a few minutes.
* 25 ppm is the US NIOSH "Immediately Dangerous to Life and Health" level
* 0.5 ppm is the US OSHA " permissible exposure limit" over a time-weighted average of 8 hours.
* 0.2 ppm is the US NIOSH " recommended exposure limit" over a time-weighted average of 8 hours.
* Under EU Directive 67/548/EEC, PCl₃ is classified as ''very toxic'' and '' corrosive'' , and the risk phrases R14, R26/28, R35 and R48/20 are obligatory.

Industrial production of phosphorus trichloride is controlled under the Chemical Weapons Convention, where it is listed in schedule 3, as it can be used to produce mustard agents.

See also

* Phosphorus pentachloride
* Phosphoryl chloride
*Phosphorus trifluorodichloride

References

{{Chemical agents

Inorganic phosphorus compounds
Phosphorus chlorides
Phosphorus(III) compounds
Pulmonary agents