Phosphonium

The molecular ion analogous to the ammonium ion , in which the nitrogen atom has been replaced by a phosphorus atom, is called phosphonium . Analogous to the ammonium ion, the phosphonium cation is simply positively charged, the phosphorus atom has an oxidation number of −3. The simplest phosphonium ion is [H ₄ P] ⁺ . Its salts decompose on contact with water into hydrogen phosphide and the acid of the anion, so phosphonium chloride decomposes into hydrogen phosphide ( monophosphine , PH ₃ ) and hydrochloric acid (HCl). Phosphonium salts are very poisonous because of the splitting off of phosphine. They are used in pest control to generate hydrogen phosphide.

Organic phosphonium ions have the form [R ₄ P] ⁺ , where R stands for an organic radical (e.g. alkyl or aryl groups). Like the homologous nitrogen compounds, they are very stable. They can be produced from phosphines by reaction with alkyl and aryl halides, a catalyst being necessary when using aryl halides (e.g. NiCl ₂ ).

${\displaystyle \mathrm {PR_{3}+R'X\longrightarrow [R_{3}PR']^{\,+}X^{\,-}} }$

Phosphonium salts are of great importance in the Wittig reaction .

literature

• Jerry March: Advanced Organic Chemistry, John Wiley & Sons New York 1985, ISBN 0-471-88841-9 .
• Michael B. Smith: Organic Synthesis. 1st edition, McGraw-Hill, 1994, ISBN 0-07-113909-5 .

Individual evidence

1. ^ Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, p. 1058.
2. ^ G. Wittig, G. Geissler: Liebigs Ann. Chem. 1953 , 580 , 44.