phosphonium

The parent phosphonium is {{Chem|PH|4|+}} as found in the iodide salt, phosphonium iodide. Salts of the parent {{Chem|PH|4|+}} are rarely encountered, but this ion is an intermediate in the preparation of the industrially useful tetrakis(hydroxymethyl)phosphonium chloride:

:PH₃ + HCl + 4 CH₂O → {{chem|P(CH|2|OH)|4|+|Cl|−}}

Many organophosphonium salts are produced by protonation of primary, secondary, and tertiary phosphines:

:PR₃ + H⁺ → {{chem|HPR|3|+}}

The basicity of phosphines follows the usual trends, with R = alkyl being more basic than R = aryl.{{cite journal|author1=Li, T. |author2=Lough, A. J. |author3=Morris, R. H. |title=An Acidity Scale of Tetrafluoroborate Salts of Phosphonium and Iron Hydride Compounds in [D₂]Dichloromethane|journal=Chem. Eur. J.|year=2007|volume=13|issue=13|pages=3796–3803|doi=10.1002/chem.200601484|pmid=17245785}}

The most common phosphonium compounds have four organic substituents attached to phosphorus. The quaternary phosphonium cations include tetraphenylphosphonium, (C₆H₅)₄P⁺ and tetramethylphosphonium {{chem|P(CH|3|)|4|+}}.

File:Me4PBr.png{{cite book|chapter=Trimethylphosphonium Methylide (Trimethyl Methylenephosphorane)|author=H.-F. Klein|title=Inorganic Syntheses|year=1978|volume=18|pages=138–140|doi=10.1002/9780470132494.ch23|series=Inorganic Syntheses|isbn=9780470132494}}]]

File:EntryWithCollCode76731.png to tetrachlorophosphonium.{{cite journal|author1=Finch, A. |author2=Fitch, A.N. |author3=Gates, P.N. |title=Crystal and Molecular structure of a metastable modification of phosphorus pentachloride|journal=Journal of the Chemical Society, Chemical Communications|year=1993|issue=11 |pages= 957–958|doi=10.1039/c39930000957 }}]]

Quaternary phosphonium cations ({{Chem|PR|4|+}}) are produced by alkylation of organophosphines. For example, the reaction of triphenylphosphine with methyl bromide gives methyltriphenylphosphonium bromide:

:PPh₃ + CH₃Br → [CH₃PPh₃]⁺Br⁻

The methyl group in such phosphonium salts is mildly acidic, with a pK_a estimated to be near 15:{{cite journal |doi=10.1039/C39900000662|title=Measurement of pK_a Values for Phosphonium Salts via the Kinetics of Proton Transfer to an Electrogenerated Base|year=1990|last1=Ling-Chung|first1=Sim|last2=Sales|first2=Keith D.|last3=Utley|first3=James H. P.|journal=Journal of the Chemical Society, Chemical Communications|issue=9|page=662}}

:[CH₃PPh₃]⁺ + base → CH₂=PPh₃ + [Hbase]⁺

This deprotonation reaction gives Wittig reagents.

Solid phosphorus pentachloride is an ionic compound, formulated {{chem2|[PCl4]+[PCl6]-}} (tetrachlorophosphonium hexachlorophosphate(V)), that is, a salt containing the tetrachlorophosphonium cation.{{cite book| first1= A. F.|last1= Holleman|first2=E.|last2= Wiber |first3=N.|last3= Wiberg | title = Inorganic Chemistry| year = 2001| publisher = Academic Press| isbn = 978-0-12-352651-9}}{{cite journal | last1= Suter |first1=R. W.|last2= Knachel |first2=H. C. |last3=Petro |first3=V. P. |last4=Howatson |first4=J. H. |last5= Shore|first5= S. G. |name-list-style=amp | title = Nature of Phosphorus(V) Chloride in Ionizing and Nonionizing Solvents | journal = Journal of the American Chemical Society | volume = 95 | year = 1978 | pages = 1474–1479 | doi = 10.1021/ja00786a021 | issue = 5}} Dilute solutions dissociate according to the following equilibrium:

:PCl₅ {{eqm}} {{chem|PCl|4|+}} + Cl⁻

Triphenylphosphine dichloride (Ph₃PCl₂) exists both as the pentacoordinate phosphorane and as the chlorotriphenylphosphonium chloride, depending on the medium.{{cite journal | journal = Chemical Communications | year = 1996 | pages = 2521–2522 | doi = 10.1039/CC9960002521 |author1=S. M. Godfrey |author2=C. A. McAuliffe |author3=R. G. Pritchard |author4=J. M. Sheffield | title = An X-ray crystallorgraphic study of the reagent Ph₃PCl₂; not charge-transfer, R₃P–Cl–Cl, trigonal bipyramidal or [R₃PCl]Cl but an unusual dinuclear ionic species, [Ph₃PCl⁺⋯Cl^–⋯⁺CIPPh₃]Cl containing long Cl–Cl contacts | issue = 22}} The situation is similar to that of PCl₅. It is an ionic compound (PPh₃Cl)⁺Cl⁻ in polar solutions and a molecular species with trigonal bipyramidal molecular geometry in apolar solution.{{Cite journal|title = Degenerate Nucleophilic Substitution in Phosphonium Salts|date = 2014|journal = J. Am. Chem. Soc.|doi = 10.1021/ja507433g|pmid = 25384344|volume=136 |issue = 46|pages=16217–16226 | last1 = Jennings | first1 = EV | last2 = Nikitin | first2 = K | last3 = Ortin | first3 = Y | last4 = Gilheany | first4 = DG}}

The Michaelis–Arbuzov reaction is the chemical reaction of a trivalent phosphorus ester with an alkyl halide to form a pentavalent phosphorus species and another alkyl halide. Commonly, the phosphorus substrate is a phosphite ester (P(OR)₃) and the alkylating agent is an alkyl iodide.{{cite journal |author1=Bhattacharya, A. K. |author2=Thyagarajan, G. | journal = Chem. Rev. | year = 1981 | volume = 81 | pages = 415–430 | title = Michaelis–Arbuzov rearrangement | doi = 10.1021/cr00044a004 | issue = 4}}

file:Michaelis-Arbuzov Reaction Mechanism.png

File:Tetrakis(hydroxymethyl)phosphonium chloride.png is used in production of textiles.]]

Tetrakis(hydroxymethyl)phosphonium chloride has industrial importance in the production of crease-resistant and flame-retardant finishes on cotton textiles and other cellulosic fabrics.{{cite journal |author1=Weil, Edward D. |author2=Levchik, Sergei V. | title = Flame Retardants in Commercial Use or Development for Textiles | journal = J. Fire Sci. | year = 2008 | volume = 26 | issue = 3 | pages = 243–281 | doi = 10.1177/0734904108089485|s2cid=98355305 }}Svara, Jürgen; Weferling, Norbert ; Hofmann, Thomas. Phosphorus Compounds, Organic. Ullmann's Encyclopedia of Industrial Chemistry. John Wiley & Sons, Inc., 2008 {{doi|10.1002/14356007.a19_545.pub2}} A flame-retardant finish can be prepared from THPC by the Proban Process,{{cite web|url=http://www.rhodia-proban.com/uk/faq.asp|title=Frequently asked questions: What is the PROBAN® process?|publisher=Rhodia Proban|access-date=February 25, 2013|archive-date=December 7, 2012|archive-url=https://web.archive.org/web/20121207081548/http://www.rhodia-proban.com/uk/faq.asp|url-status=dead}} in which THPC is treated with urea. The urea condenses with the hydroxymethyl groups on THPC. The phosphonium structure is converted to phosphine oxide as the result of this reaction.{{cite journal |author1=Reeves, Wilson A. |author2=Guthrie, John D. | title = Intermediate for Flame-Resistant Polymers-Reactions of Tetrakis(hydroxymethyl)phosphonium Chloride | journal = Industrial and Engineering Chemistry | year = 1956 | volume = 48 | issue = 1 | pages = 64–67 | doi = 10.1021/ie50553a021}}

Organic phosphonium cations are lipophilic and can be useful in phase transfer catalysis, much like quaternary ammonium salts. Salts or inorganic anions and tetraphenylphosphonium ({{chem|PPh|4|+}}) are soluble in polar organic solvents. One example is the perrhenate (PPh₄[ReO₄]).{{cite book |doi=10.1002/9780470132623.ch42|chapter=Tetrahalo Oxorhenate Anions|series= Inorganic Syntheses|year=1996|last1=Dilworth|first1=J. R.|last2=Hussain|first2=W.|last3=Hutson|first3=A. J.|last4=Jones|first4=C. J.|last5=McQuillan|first5=F. S.|title=Inorganic Syntheses|pages=257–262|volume=XXXI|isbn=9780470132623}}

Wittig reagents are used in organic synthesis. They are derived from phosphonium salts. A strong base such as butyllithium or sodium amide is required for the deprotonation:

:[Ph₃P⁺CH₂R]X⁻ + C₄H₉Li → Ph₃P=CHR + LiX + C₄H₁₀

One of the simplest ylides is methylenetriphenylphosphorane (Ph₃P=CH₂).{{cite journal|last1=Wittig|first2=U.|last2=Schoellkopf |year=1960|title=Methylenecyclohexane|journal=Organic Syntheses|volume=40|pages=66|doi=10.15227/orgsyn.040.0066}}. Describes Ph₃P=CH₂.

The compounds Ph₃PX₂ (X = Cl, Br) are used in the Kirsanov reaction.Studies in Organophosphorus Chemistry. I. Conversion of Alcohols and Phenols to Halides by Tertiary Phosphine Dihalides G. A. Wiley, R. L. Hershkowitz, B. M. Rein, B. C. Chung J. Am. Chem. Soc., 1964, 86 (5), pp 964–965 {{doi|10.1021/ja01059a073}}

The Kinnear–Perren reaction is used to prepare alkylphosphonyl dichlorides (RP(O)Cl₂) and esters (RP(O)(OR′)₂). A key intermediate are alkyltrichlorophosphonium salts, obtained by the alkylation of phosphorus trichloride:{{Ullmann | author = Svara, J. | author2 = Weferling, N. | author3 = Hofmann, T. | title = Phosphorus Compounds, Organic | doi = 10.1002/14356007.a19_545.pub2}}

:RCl + PCl₃ + AlCl₃ → [RPCl₃]⁺{{chem|AlCl|4|−}}

The main industrial procedure for the production of ammonia today is the thermal Haber-Bosch process, which generally uses fossil gas as a source of hydrogen, which is then combined with nitrogen to produce ammonia. In 2021, Professor Doug MacFarlane and collaborators Alexandr Simonov and Bryan Suryanto of Monash University devised a method of producing green ammonia that has the potential to make Haber-Bosch plants obsolete.[https://lens.monash.edu/@science/2021/11/29/1383516/breakthrough-brings-green-ammonia-production-closer-to-reality Breakthrough brings green ammonia production closer to reality] Their process is similar to the electrolysis approach for producing hydrogen. While working with local company Verdant, which wanted to make bleach from saltwater by electrolysis, Suryanto discovered that a tetraalkyl phosphonium salt allowed the efficient production of ammonia at room temperature.[https://www.science.org/doi/10.1126/science.abg2371 Nitrogen reduction to ammonia at high efficiency and rates based on a phosphonium proton shuttle]

• Ammonium ({{chem|NH|4|+}})
• Arsonium ({{chem|AsH|4|+}})
• Hydronium (H₃O⁺)
• Onium compounds
• Organophosphorus chemistry

{{Reflist}}

Category:Cations

Category:Functional groups

Category:Phosphonium compounds