Phosphorus trichloride

Phosphorus trichloride
Names
IUPAC name
Phosphorus trichloride
Other names
Phosphorus(III) chloride
Phosphorous chloride
Identifiers
7719-12-2 YesY
ChEBI CHEBI:30334 YesY
ChemSpider 22798 YesY
EC Number 231-749-3
Jmol 3D model Interactive image
PubChem 24387
RTECS number TH3675000
UN number 1809
Properties
PCl3
Molar mass 137.33 g/mol
Appearance Colorless to yellow fuming liquid[1]
Odor like hydrochloric acid[1]
Density 1.574 g/cm3
Melting point −93.6 °C (−136.5 °F; 179.6 K)
Boiling point 76.1 °C (169.0 °F; 349.2 K)
hydrolysis
Solubility in other solvents soluble in benzene, CS2, ether, chloroform, CCl4, halogenated organic solvents
reacts with ethanol
Vapor pressure 13.3 kPa
1.5122 (21 °C)
Viscosity 0.65 cP (0 °C)
0.438 cP (50 °C)
0.97 D
Thermochemistry
−319.7 kJ/mol
Hazards
Safety data sheet See: data page
ICSC 0696
T+ Xn C
R-phrases R14-R26/28-R35-R48/20
S-phrases (S1/2)-S7/8-S26-S36/37/39-S45
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 4: Very short exposure could cause death or major residual injury. E.g., VX gas Reactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorus Special hazard W: Reacts with water in an unusual or dangerous manner. E.g., cesium, sodiumNFPA 704 four-colored diamond
0
4
2
Lethal dose or concentration (LD, LC):
18 mg/kg (rat, oral)[2]
104 ppm (rat, 4 hr)
50 ppm (guinea pig, 4 hr)[2]
US health exposure limits (NIOSH):
PEL (Permissible)
 TWA 0.5 ppm (3 mg/m3)[1]
REL (Recommended)
 TWA 0.2 ppm (1.5 mg/m3) ST 0.5 ppm (3 mg/m3)[1]
IDLH (Immediate danger)
 25 ppm[1]
Related compounds
Related phosphorus chlorides
 Phosphorus pentachloride
Phosphorus oxychloride
Diphosphorus tetrachloride
Related compounds
 Phosphorus trifluoride
Phosphorus tribromide
Phosphorus triiodide
Supplementary data page
Refractive index (n),
Dielectric constant (εr), etc.
Thermodynamic
data
 Phase behaviour
solid–liquid–gas
UV, IR, NMR, MS
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Infobox references

Phosphorus trichloride is a chemical compound of phosphorus and chlorine, having the 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 is a precursor to other phosphorus compounds, undergoing oxidation to 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 P(III) 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:

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

where "Ph" stands for phenyl group, -C6H5. Alcohols such as ethanol react similarly in the presence of a base such as a tertiary amine:[3]

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.

In the absence of base, however, the reaction produces phosphonic acid and an alkyl chloride, according to the following stoichiometry:

PCl3 + 3 C2H5OH → 3 C2H5Cl + H3PO3[4]

Alternatively, under certain conditions the dialkyl phosphonate (dialkyl phosphite) may be isolated, as well as some alkyl chloride:[5]

PCl3 + 3 iPrOH → (iPrO)2PH=O + iPrCl + 2 HCl(g) where iPr = isopropyl, (CH3)2CH-

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[6] 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−
4

The RPCl+
3 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.[7] 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 (in order to avoid the formation of PCl5).

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.[8] 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 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.[9]

Precautions

PCl3 is toxic, with a concentration of 600 ppm being lethal in just a few minutes.[10] 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.

Government agencies in the United States have set occupational exposure limits for PCl3. The Occupational Safety and Health Administration has set a permissible exposure limit at 0.5 ppm over a time-weighted average of 8 hours, while the National Institute for Occupational Safety and Health has set a recommended exposure limit at 0.2 ppm over a time-weighted average of 8 hours.[11] Additionally, PCl3 has been designated IDLH with a maximum exposure limit at 25 ppm.[12]

References

  1. 1 2 3 4 5 "NIOSH Pocket Guide to Chemical Hazards #0511". National Institute for Occupational Safety and Health (NIOSH).
  2. 1 2 "Phosphorus trichloride". Immediately Dangerous to Life and Health. National Institute for Occupational Safety and Health (NIOSH).
  3. ↑ A. H. Ford-Moore and B. J. Perry (1963). "Triethyl Phosphite". Org. Synth.; Coll. Vol. 4, p. 955
  4. ↑ Clark, Jim (2008). "Replacing the OH in alcohols by a halogen". Retrieved October 9, 2008.
  5. ↑ Pedrosa, Leandro (2011). "Esterification of Phosphorus Trichloride with Alcohols; Diisopropyl phosphonate". ChemSpider Synthetic Pages (Royal Society of Chemistry): 488. doi:10.1039/SP488.
  6. ↑ R. R. Holmes (1960). "An examination of the basic nature of the trihalides of phosphorus, arsenic and antimony,". Journal of Inorganic and Nuclear Chemistry 12 (3-4): 266–275. doi:10.1016/0022-1902(60)80372-7.
  7. ↑ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.
  8. ↑ M. C. Forbes, C. A. Roswell, R. N. Maxson (1946). "Phosphorus(III) Chloride". Inorg. Synth. 2: 145–7. doi:10.1002/9780470132333.ch42.
  9. ↑ L. G. Wade, Jr. (2005). Organic Chemistry (6th ed.). Upper Saddle River, New Jersey, USA: Pearson/Prentice Hall. p. 477.
  10. ↑ A. D. F. Toy (1973). The Chemistry of Phosphorus. Oxford, UK: Pergamon Press.
  11. ↑ CDC - NIOSH Pocket Guide to Chemical Hazards
  12. ↑ Documentation for Immediately Dangerous To Life or Health Concentrations (IDLHs)
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