Phosphorus trichloride is a chemical compound of phosphorus and chlorine, having chemical formula PCl3. It has a trigonal pyramidal shape. It 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. It has a 31P NMR signal at around +220 ppm with reference to a phosphoric acid standard.
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 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 + 3 H2O → H3PO3 + 3 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
- 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.
- PCl3 + 3 C2H5OH → 3 C2H5Cl + H3PO3
- PCl3 + 3 iPrOH → iPr2PH=O + iPrCl + 2 HCl(g) where iPr = isopropyl, (CH3)2CH-
- 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 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 using an alkyl chloride and aluminium chloride:
- PCl3 + RCl + AlCl3 → RPCl+
3 + AlCl−
3 product can then be decomposed with water to produce an alkylphosphonic dichloride RP(=O)Cl2.
World production exceeds one-third of a million tonnes. 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. It is sufficiently inexpensive that it would not be synthesized for laboratory use.
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.
PCl3 is toxic, with a concentration of 600 ppm being lethal in just a few minutes. 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.
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