Phosphonium

The phosphonium (more obscurely: phosphinium) cation describes polyatomic cations with the chemical formula PR+
4 (R = H, alkyl, aryl, halide). They are tetrahedral and generally colorless.[1]

Structure of PH4+, the parent phosphonium cation.

Types of phosphonium cations

Phosphonium, PH+
4

The parent phosphonium is PH+
4. One example is phosphonium iodide PH+
4I−
. Salts of the parent 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 → P(CH
2OH)+
4Cl−

Protonated organophosphines

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

PR3 + H+ → HPR+
3

The basicity of phosphines follows the usual trends, with R = alkyl being more basic than R = aryl.[2]

Tetraorganophosphonium cations

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

Tetramethylphosphonium bromide[3]
Structure of solid phosphorus pentachloride, illustrating its autoionization[4]

Quaternary phosphonium cations (PR+
4) are produced by alkylation of organophosphines.[3] For example, the reaction of triphenylphosphine with methyl bromide gives methyltriphenylphosphonium bromide, the precursor to a Wittig reagent:[5]

PPh3 + CH3Br → CH
3PPh+
3Br−

Solid phosphorus pentachloride is an ionic compound, formulated PCl+
4PCl-
6 i.e. a salt containing tetrachlorophosphonium cation.[6][7] Dilute solutions dissociate according to the following equilibrium:

PCl5 ⇌ PCl+
4 + Cl−

Triphenylphosphine dichloride (Ph3PCl2) exists both as the pentacoordinate phosphorane and as the chlorotriphenylphosphonium chloride, depending on the medium.[8] 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.[9]

Uses

Textile finishes
Tetrakis(hydroxymethyl)phosphonium chloride 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.[10][11] A flame-retardant finish can be prepared from THPC by the Proban Process,[12] 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.[13]

Phase-transfer catalysts and precipitating agents

Organic phosphonium cations are lipophilic and can be useful in phase transfer catalysis, much like quaternary ammonium salts.

The cation tetraphenylphosphonium (PPh+
4) is a useful precipitating agent.

Reagents for organic synthesis

Wittig reagents are used in organic synthesis. They are derived from phosphonium salts, which is in turn prepared by deprotonation of alkylphosphonium 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 ylide is methylenetriphenylphosphorane (Ph3P=CH2).[5]

The compounds Ph3PX2 (X = Cl, Br) are used in the Kirsanov reaction.[14]

The Kinnear–Perren reaction is used to prepare alkylphosphonyl dichlorides (RP(O)Cl2) and alkylphosphonate esters (RP(O)(OR')2). Alkylation of phosphorus trichloride in the presence of aluminium trichloride give the alkyltrichlorophosphonium salts, which are versatile intermediates:[15]

RCl + PCl3 + AlCl3 → [RPCl3]+AlCl4−
gollark: `char` bad, `u8` good.
gollark: Do you *need* that?
gollark: Well, "char" is wrong in C?
gollark: Maybe I should rename ilmenite to apiary.
gollark: Seems fine to me.

See also

References
  1. Corbridge, D. E. C. (1995). Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology (5th ed.). Amsterdam: Elsevier. ISBN 978-0-444-89307-9.
  2. Li, T.; Lough, A. J.; Morris, R. H. (2007). "An Acidity Scale of Tetrafluoroborate Salts of Phosphonium and Iron Hydride Compounds in [D2]Dichloromethane". Chem. Eur. J. 13 (13): 3796–3803. doi:10.1002/chem.200601484. PMID 17245785.
  3. H. F. Klein (1978). Trimethylphosphonium Methylide (Trimethyl Methylenephosphorane). Inorganic Syntheses. XVIII. pp. 138–140. doi:10.1002/9780470132494.ch23. ISBN 9780470132494.
  4. Finch, A.; Fitch, A.N.; Gates, P.N. (1993). "Crystal and Molecular structure of a metastable modification of phosphorus pentachloride". Journal of the Chemical Society, Chemical Communications: 957–958.
  5. Wittig; Schoellkopf, U. (1960). "Methylenecyclohexane (describes Ph3PCH2". 40: 66. doi:10.15227/orgsyn.040.0066. Cite journal requires |journal= (help)
  6. Holleman, A. F.; Wiber, E.; Wiberg, N. (2001). Inorganic Chemistry. Academic Press. ISBN 978-0-12-352651-9.
  7. Suter, R. W.; Knachel, H. C.; Petro, V. P.; Howatson, J. H. & Shore, S. G. (1978). "Nature of Phosphorus(V) Chloride in Ionizing and Nonionizing Solvents". Journal of the American Chemical Society. 95 (5): 1474–1479. doi:10.1021/ja00786a021.
  8. S. M. Godfrey; C. A. McAuliffe; R. G. Pritchard; J. M. Sheffield (1996). "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". Chemical Communications (22): 2521–2522. doi:10.1039/CC9960002521.
  9. Jennings, EV; Nikitin, K; Ortin, Y; Gilheany, DG (2014). "Degenerate Nucleophilic Substitution in Phosphonium Salts". J. Am. Chem. Soc. 136 (46): 16217–16226. doi:10.1021/ja507433g. PMID 25384344.
  10. Weil, Edward D.; Levchik, Sergei V. (2008). "Flame Retardants in Commercial Use or Development for Textiles". J. Fire Sci. 26 (3): 243–281. doi:10.1177/0734904108089485.
  11. 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
  12. "Frequently asked questions: What is the PROBAN® process?". Rhodia Proban. Retrieved February 25, 2013.
  13. Reeves, Wilson A.; Guthrie, John D. (1956). "Intermediate for Flame-Resistant Polymers-Reactions of Tetrakis(hydroxymethyl)phosphonium Chloride". Industrial and Engineering Chemistry. 48 (1): 64–67. doi:10.1021/ie50553a021.
  14. 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
  15. Svara, J.; Weferling, N.; Hofmann, T. "Phosphorus Compounds, Organic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_545.pub2.CS1 maint: multiple names: authors list (link)
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