Phosphonium

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Structure of PH4+, the parent phosphonium cation.

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]

Types of phosphonium cations[edit]

Phosphonium, PH+
4
[edit]

The parent phosphonium is PH+
4
. One example is phosphonium iodide PH+
4
I
. 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
2
OH)+
4
Cl

Protonated organophosphines[edit]

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[edit]

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
.

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 iodide gives methyltriphenylphosphonium iodide, the precursor to a Wittig reagent:[5]

PPh3 + CH3I → CH
3
PPh+
3
I

Phosphorus pentachloride and related halophosphonium compounds[edit]

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

PCl5PCl+
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[edit]

Textile finishes[edit]

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[edit]

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

Synthesis of phosphonium halides from triphenylphosphine and an alkyl halide

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

Reagents for organic synthesis[edit]

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-

See also[edit]

References[edit]

  1. ^ Corbridge, D. E. C. (1995). Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology (5th ed.). Amsterdam: Elsevier. ISBN 0-444-89307-5.
  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: 3796–3803. doi:10.1002/chem.200601484.
  3. ^ a b H. F. Klein (1978). "Trimethylphosphonium Methylide (Trimethyl Methylenephosphorane)". Inorganic Syntheses. XVIII: 138–140. doi:10.1002/9780470132494.ch23.
  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. ^ a b George Wittig U. Schoellkopf (1973). "Methylenecyclohexane (describes Ph3PCH2)". Organic Syntheses.; Collective Volume, 5, p. 751
  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". Chem. Commun. (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: 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