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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.
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.
is a precursor to other phosphorus compound, undergoing oxidation
to e.g. phosphorus pentachloride
), thiophosphoryl chloride
), or phosphorus oxychloride
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 :
- 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 produces phosphonic acid and an alkyl chloride, according to the following stoichiometry:
- PCl3 + 3 C2H5OH → 3C2H5Cl + H3PO3
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 R3
P (sometimes called phosphanes) such as triphenylphosphine, Ph3
- 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
it forms a 1:1 adduct, Br3
. Metal complexes such as Ni(PCl3
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.
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.
is important indirectly as a precursor to PCl5
. which in turn enjoy many applications in herbicides
, 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.
is toxic, with a concentration of 600 ppm
being lethal in just a few minutes
is classified as very toxic
under EU Directive 67/548/EEC
, and the risk phrases
R14, R26/28, R35 and R48/20 are obligatory.
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