Phosphonium


The phosphonium cation describes polyatomic cations with the chemical formula . They are tetrahedral and generally colorless.

Types of phosphonium cations

Phosphonium,

The parent phosphonium is. One example is phosphonium iodide. Salts of the parent are rarely encountered, but this ion is an intermediate in the preparation of the industrially useful tetrakisphosphonium chloride:

Protonated organophosphines

Many phosphonium salts are produced by protonation of primary, secondary, and tertiary phosphines:
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, 4P+ and tetramethylphosphonium.
Quaternary phosphonium cations are produced by alkylation of organophosphines. For example, the reaction of triphenylphosphine with methyl bromide gives methyltriphenylphosphonium bromide, the precursor to a Wittig reagent:

Phosphorus pentachloride and related halophosphonium compounds

Solid phosphorus pentachloride is an ionic compound, formulated PClPCl i.e. a salt containing tetrachlorophosphonium cation. Dilute solutions dissociate according to the following equilibrium:
Triphenylphosphine dichloride exists both as the pentacoordinate phosphorane and as the chlorotriphenylphosphonium chloride, depending on the medium. The situation is similar to that of PCl5. It is an ionic compound +Cl− in polar solutions and a molecular species with trigonal bipyramidal molecular geometry in apolar solution.

Uses

Textile finishes

has industrial importance in the production of crease-resistant and flame-retardant finishes on cotton textiles and other cellulosic fabrics. A flame-retardant finish can be prepared from THPC by the Proban Process, 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.

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 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:
One of the simplest ylide is methylenetriphenylphosphorane.
The compounds Ph3PX2 are used in the Kirsanov reaction.
The Kinnear–Perren reaction is used to prepare alkylphosphonyl dichlorides and alkylphosphonate esters. Alkylation of phosphorus trichloride in the presence of aluminium trichloride give the alkyltrichlorophosphonium salts, which are versatile intermediates: