Everything about Phosphorus Trichloride totally explained
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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.
Chemical properties
The
phosphorus in PCl
3 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
PCl
3 is a precursor to other phosphorus compound, undergoing
oxidation to for example
phosphorus pentachloride (PCl
5),
thiophosphoryl chloride (PSCl
3), or
phosphorus oxychloride (POCl
3).
If an
electric discharge is passed through a mixture of PCl
3 vapour and
hydrogen gas, a rare chloride of phosphorus is formed,
diphosphorus tetrachloride (P
2Cl
4).
PCl3 as an electrophile
Phosphorus trichloride is the precursor to
organophosphorus compounds that contain one or more (P
3+) atoms, most notably phosphites and phosphonates. These compounds don't usually contain the chlorine atoms found in PCl
3.
PCl
3 reacts rapidly and
exothermically with
water to form
phosphorous acid, H
3PO
3 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 + PCl
3 →
P(OPh)3 + 3 HCl
where "Ph" stands for
phenyl group, -C
6H
5.
Alcohols such as ethanol react similarly in the presence of a
base such as :
» PCl
3 + 3 EtOH + 3 R
3N → P(OEt)
3 + 3 R
3NH
+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:
» PCl
3 + 3 C
2H
5OH → (C
2H
5O)
2P(=O)H +
C2H5Cl + 2 HCl
Amines, R
2NH, form P(NR
2)
3, and
thiols (RSH) form P(SR)
3. An industrially relevant reaction of PCl
3 with amines is phosphonomethylation, which employs
formaldehyde:
» R
2NH + PCl
3 + CH
2O → (HO)
2P(O)CH
2NR
2 + 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 PCl
3 with
Grignard reagents and
organolithium reagents is a useful method for the preparation of organic
phosphines with the formula R
3P (sometimes called phosphanes) such as triphenylphosphine, Ph
3P.
» 3 PhMgBr + PCl
3 →
Ph3P + 3 MgBrCl
Under controlled conditions PCl
3 can be used to prepare PhPCl
2 and Ph
2PCl.
PCl3 as a nucleophile
Phosphorus trichloride has a lone pair, and therefore can act as a
Lewis base, for example with the
Lewis acids BBr
3[5] it forms a 1:1 adduct, Br
3B
−−
+PCl
3. Metal complexes such as Ni(PCl
3)
4 are known. This Lewis basicity is exploited in one useful route to organophosphorus compounds:
» PCl
3 +
RCl +
AlCl3 → (RPCl
3)
+−AlCl
4
The (RPCl
3)
+ product can then be decomposed with water to produce an alkylphosphonic dichloride RP(=O)Cl
2.
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 PCl
3 as it's formed.
» P4 + 6
Cl2 → 4 PCl
3
Industrial production of phosphorus trichloride is controlled under the
Chemical Weapons Convention, where it's 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 wouldn't be synthesized for laboratory use.
Uses
PCl
3 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 PCl
3 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.
PCl
3 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.
PCl
3 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 PCl
3[8].
Precautions
PCl
3 is toxic, with a concentration of 600
ppm being lethal in just a few minutes
[7]. PCl
3 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.
Further Information
Get more info on 'Phosphorus Trichloride'.
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