In chemistry, the term phosphonium (more obscurely: phosphinium) describes polyatomic ion with the chemical formula (where R is a hydrogen or an alkyl, aryl, organyl or halogen group). These cations have tetrahedral structures. The salts are generally colorless or take the color of the anions.
Types of phosphonium cations
Protonated phosphines
The parent
phosphonium is as found in the iodide salt, phosphonium iodide. Salts of the parent are rarely encountered, but this ion is an intermediate in the preparation of the industrially useful tetrakis(hydroxymethyl)phosphonium chloride:
- PH3 + HCl + 4 CH2O →
Many organophosphonium salts are produced by protonation of primary, secondary, and tertiary phosphines:
- PR3 + H+ →
The basicity of phosphines follows the usual trends, with R = alkyl being more basic than R = aryl.
Tetraorganophosphonium cations
The most common phosphonium compounds have four organic substituents attached to phosphorus. The quaternary phosphonium cations include tetraphenylphosphonium, (C
6H
5)
4P
+ and tetramethylphosphonium .
Quaternary phosphonium cations () 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 p Ka estimated to be near 15:
- CH3PPh3+ + base → CH2=PPh3 + Hbase+
This deprotonation reaction gives Wittig reagents.[
]
Phosphorus pentachloride and related compounds
Solid phosphorus pentachloride is an ionic compound, formulated (tetrachlorophosphonium hexachlorophosphate(V)), that is, a salt containing the tetrachlorophosphonium cation.
- PCl5 + Cl−
Triphenylphosphine dichloride (Ph3PCl2) exists both as the pentacoordinate phosphorane and as the chlorotriphenylphosphonium chloride, depending on the medium.
Alkoxyphosphonium salts: Arbuzov reaction
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.
Uses
Textile finishes
Tetrakis(hydroxymethyl)phosphonium chloride has industrial importance in the production of crease-resistant and flame-retardant finishes on cotton textiles and other cellulosic fabrics.[Svara, Jürgen; Weferling, Norbert ; Hofmann, Thomas. Phosphorus Compounds, Organic. Ullmann's Encyclopedia of Industrial Chemistry. John Wiley & Sons, Inc., 2008 ] A flame-retardant finish can be prepared from THPC by the Proban Process,
Phase-transfer catalysts and precipitating agents
Organic phosphonium cations are lipophilic and can be useful in phase transfer catalysis, much like quaternary ammonium salts. Salts or inorganic anions and tetraphenylphosphonium () are soluble in polar organic solvents. One example is the perrhenate (PPh4ReO4).
Reagents for organic synthesis
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+CH2RX− + C4H9Li → Ph3P=CHR + LiX + C4H10
One of the simplest ylides is methylenetriphenylphosphorane (Ph3P=CH2).[. 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 ]
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:
- RCl + PCl3 + AlCl3 → RPCl3+
Ammonia production for "green hydrogen"
The main industrial procedure for the production of ammonia today is the thermal Haber 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.[ 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.[ Nitrogen reduction to ammonia at high efficiency and rates based on a phosphonium proton shuttle]
See also
