phosphorus trichloride

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.{{cite journal|last1=Gay-Lussac|last2=Thénard|title=Extrait de plusieurs notes sur les métaux de la potasse et de la soude, lues à l'Institut depuis le 12 janvier jusqu'au 16 mai|journal=Gazette Nationale, Ou le Moniteur Universel|date=27 May 1808|volume=40|issue=148|pages=581–582|url=https://books.google.com/books?id=GRROAAAAcAAJ&pg=PA581|trans-title=Extracts from several notes on the metals potassium and sodium, read at the Institute from the 12th of January to the 16th of May|language=fr}} From p. 582: "Seulement ils ont rapporté qu'en traitant le mercure doux par le phosphure, dans l'espérance d'avoir de l'acide muriatique bien sec, il ont trouvé une liqueur nouvelle très limpide, sans couleur, répandant de fortes vapeurs, s'enflammant spontanément lorsqu'on en imbibe le papier joseph; laquelle ne paraît être qu'une combinaison de phosphore, d'oxigène et d'acide muriatique, et par conséquent analogue à cette qu'on obtient en traitant le soufre par le gas acide muriatique oxigèné." (Only they reported that by treating calomel with phosphorus, in the hope of obtaining very dry hydrogen chloride, they found a new, very clear liquid, colorless, giving off strong vapors, spontaneously igniting when one soaks filter paper in it; which seems to be only a compound of phosphorus, oxygen, and hydrochloric acid, and thus analogous to what one obtains by treating sulfur with chlorine gas.) Later during the same year, the English chemist Humphry Davy produced phosphorus trichloride by burning phosphorus in chlorine gas.{{cite journal|last1=Davy|first1=Humphry|title=The Bakerian Lecture. An account of some new analytical researches on the nature of certain bodies, particularly the alkalies, phosphorus, sulphur, carbonaceous matter, and the acids hitherto undecomposed; with some general observations on chemical theory|journal=Philosophical Transactions of the Royal Society of London|date=1809|volume=99|pages=39–104|url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015034564347;view=1up;seq=53|doi=10.1098/rstl.1809.0005|s2cid=98814859|url-access=subscription}} On pp. 94–95, Davy mentioned that when he burned phosphorus in chlorine gas ("oxymuriatic acid gas"), he obtained a clear liquid (phosphorus trichloride) and a white solid (phosphorus pentachloride).

World production exceeds one-third of a million tonnes.{{Greenwood&Earnshaw2nd}} 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

It has a trigonal pyramidal shape. Its {{chem2|^{31}P}} NMR spectrum exhibits a singlet around +220 ppm with reference to a phosphoric acid standard.{{fact|date=February 2022}}

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.{{Cite web |title=Phosphorus trichloride (PCl3) |url=https://www.simply.science/images/content/chemistry/metals_and_non_metals/nitrogen_family/conceptmap/Phosphorus_chloride.html |access-date=2023-02-15 |website=www.simply.science}}

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

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

:PCl₃ + 3 H₂O → H₃PO₃ + 3 HCl

Phosphorus trichloride is the precursor to organophosphorus compounds. It reacts with phenol to give triphenyl phosphite:

:{{chem2|1=3 PhOH + PCl3 → P(OPh)3 + 3 HCl (Ph = C6H5)}}

Alcohols such as ethanol react similarly in the presence of a base such as a tertiary amine:{{OrgSynth | author = A. H. Ford-Moore | author2 = B. J. Perry | name-list-style=amp | title = Triethyl Phosphite | collvol = 4 | collvolpages = 955 | year = 1963 | prep = CV4P0955}}

:{{chem2|PCl3 + 3 EtOH + 3 R₃N → P(OEt)3 + 3 R3NH(+)Cl(-)}}

With one equivalent of alcohol and in the absence of base, the first product is alkoxyphosphorodichloridite:{{cite journal |doi=10.1039/JR9530001920|title=394. Interaction of phosphorus trichloride with alcohols and with hydroxy-esters |year=1953 |last1=Gerrard |first1=W. |last2=Isaacs |first2=M. J. D. |last3=Machell |first3=G. |last4=Smith |first4=K. B. |last5=Wyvill |first5=P. L. |journal=Journal of the Chemical Society (Resumed) |page=1920 }}

:{{chem2|PCl3 + EtOH → PCl2(OEt) + HCl}}

In the absence of base, however, with excess alcohol, phosphorus trichloride converts to diethylphosphite:{{cite book|chapter=Diethyl phosphite|author=Malowan, John E.|title=Inorganic Syntheses|year=1953|volume=4|pages=58–60|doi=10.1002/9780470132357.ch19|isbn=9780470132357}}{{cite journal|last = Pedrosa|first = Leandro|year = 2011|url = http://cssp.chemspider.com/488|title = Esterification of Phosphorus Trichloride with Alcohols; Diisopropyl phosphonate|journal = ChemSpider Synthetic Pages|pages=488|doi = 10.1039/SP488|publisher = Royal Society of Chemistry|doi-access = free}}

: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

The 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.

:{{chem2|3 RMgBr + PCl3 -> R3P + 3 MgBrCl}}

Triphenylphosphine is produced industrially by the reaction between phosphorus trichloride, chlorobenzene, and sodium:{{cite book | author = Corbridge, D. E. C. | title = Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology | year = 1995 | edition = 5th | publisher = Elsevier | location = Amsterdam | isbn = 0-444-89307-5}}

:{{chem2|PCl3 + 3 PhCl + 6 Na -> PPh3 + 6 NaCl}}, where Ph = {{chem2|C6H5}}

Under controlled conditions or especially with bulky R groups, similar reactions afford less substituted derivatives such as chlorodiisopropylphosphine.

Phosphorus trichloride is commonly used to convert primary and secondary alcohols to the corresponding chlorides.{{cite book |doi=10.1002/9780470771259.ch11|chapter=Displacement of Hydroxyl Groups |title=The Hydroxyl Group (1971) |year=1971 |last1=Brown |first1=Geoffrey W. |series=PATai's Chemistry of Functional Groups |pages=593–639 |isbn=9780470771259 }} As discussed above, the reaction of alcohols with phosphorus trichloride is sensitive to conditions. The mechanism for the ROH →RCl conversion involves the reaction of HCl with phosphite esters:

:{{chem2|P(OR)3 + HCl <-> HP(OR)3(+)Cl(-)}}

:{{chem2|HP(OR)3(+)Cl(-) -> RCl + HOP(OR)2}}.

:{{chem2| HOP(OR)2 + HCl <-> H2OP(OR)2(+)Cl(-)}}

:{{chem2|H2OP(OR)2(+)Cl(-) -> RCl + (HO)2P(OR)}}

:{{chem2| (HO)2P(OR) + HCl <-> H(HO)2P(OR)(+)Cl(-)}}

:{{chem2|H(HO)2P(OR)(+)Cl(-) -> RCl + (HO)3P}}

The first step proceeds with nearly ideal stereochemistry but the final step far less so owing to an SN1 pathway.

Phosphorus trichloride undergoes a variety of redox reactions:

:{{chem2|3PCl3 + 2 CrO3 -> 3POCl3 + Cr2O3 }}

:{{chem2|PCl3 + SO3 -> POCl3 + SO2}}

:{{chem2|3 PCl3 + SO2 -> 2POCl3 + PSCl3}}

Phosphorus trichloride has a lone pair, and therefore can act as a Lewis base,{{cite journal | author = R. R. Holmes | journal = Journal of Inorganic and Nuclear Chemistry | volume = 12 | issue = 3–4 | pages = 266–275 | year = 1960 | doi = 10.1016/0022-1902(60)80372-7 | title = An examination of the basic nature of the trihalides of phosphorus, arsenic and antimony}} 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 alkylphosphonate 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:{{ Ullmann | author = Svara, J. | author2 = Weferling, N. | author3 = Hofmann, T. | title = Phosphorus Compounds, Organic | doi = 10.1002/14356007.a19_545.pub2}}

:PCl₃ + RCl + AlCl₃ → RPCl{{su|b=3|p=+}} + AlCl{{su|b=4|p=−}}

The RPCl{{su|b=3|p=+}} product can then be decomposed with water to produce an alkylphosphonic dichloride RP(=O)Cl₂.

PCl₃, like the more popular phosphorus trifluoride, is a ligand in coordination chemistry. One example is Mo(CO)₅PCl₃.{{cite journal |doi=10.1021/om020311d|title=Nature of the Metal−Ligand Bond in M(CO)₅PX₃ Complexes (M = Cr, Mo, W; X = H, Me, F, Cl): Synthesis, Molecular Structure, and Quantum-Chemical Calculations|year=2002|last1=Frenking|first1=Gernot|last2=Wichmann|first2=Karin|last3=Fröhlich|first3=Nikolaus|last4=Grobe|first4=Joseph|last5=Golla|first5=Winfried|last6=Van|first6=Duc Le|last7=Krebs|first7=Bernt|last8=Läge|first8=Mechtild|journal=Organometallics|volume=21|issue=14|pages=2921–2930}}

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.{{cite book | author = L. G. Wade Jr. | title = Organic Chemistry | edition = 6th | page = 477 | publisher = Pearson/Prentice Hall | location = Upper Saddle River, New Jersey, USA | year = 2005}}

• 600 ppm is lethal in just a few minutes.{{cite book | author = A. D. F. Toy | title = The Chemistry of Phosphorus | publisher = Pergamon Press | location = Oxford, UK | year = 1973}}
• 25 ppm is the US NIOSH "Immediately Dangerous to Life and Health" level[https://www.cdc.gov/niosh/idlh/7719122.html Documentation for Immediately Dangerous To Life or Health Concentrations (IDLHs)]
• 0.5 ppm is the US OSHA "permissible exposure limit" over a time-weighted average of 8 hours.[https://www.osha.gov/chemicaldata/chemResult.html?RecNo=629 OSHA: Phosphorus Trichloride]
• 0.2 ppm is the US NIOSH "recommended exposure limit" over a time-weighted average of 8 hours.[https://www.cdc.gov/niosh/npg/npgd0511.html CDC - NIOSH Pocket Guide to Chemical Hazards]
• 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.{{cite book |author1=M. C. Forbes |author2=C. A. Roswell |author3=R. N. Maxson |title=Inorganic Syntheses |chapter=Phosphorus(III) Chloride | journal = Inorg. Synth. | volume = 2 | pages = 145–7 | doi = 10.1002/9780470132333.ch42 |date=2007 |isbn=9780470132333 }}

• Phosphorus pentachloride
• Phosphoryl chloride
• Phosphorus trifluorodichloride

{{Reflist}}

{{Phosphorus compounds}}

{{Chlorides}}

{{Chemical agents}}

Category:Inorganic phosphorus compounds

Category:Phosphorus chlorides

Category:Phosphorus(III) compounds

Category:Pulmonary agents