Salt metathesis
A salt metathesis reaction (from the Greek μετάθεσις, "transposition"), sometimes called a double replacement reaction or double displacement reaction, is a chemical process involving the exchange of bonds between two reacting chemical species, which results in the creation of products with similar or identical bonding affiliations. This reaction is represented by the general scheme:
The bond between the reacting species can either be ionic or covalent. Classically, these reactions result in the precipitation of one product.
In older literature, the term double decomposition is frequently encountered. The term double decomposition is more specifically used when at least one of the substances does not dissolve in the solvent, as the ligand or ion exchange takes place in the solid state of the reactant, for example:
Salt metathesis is a common technique for exchanging counter ions. Starting from aqueous solutions of inorganic anions, one can use salt metathesis reaction to isolate salts that are soluble in organic solvents. Illustrative is the conversion of sodium perrhenate to the tetrabutylammonium salt:
The tetrabutylammonium salt precipitates from the aqueous solution. In contrast to sodium perrhenate, this salt is soluble in chloroform.
Illustrative is the exchange of the counteranion in ferrocenium tetrafluoroborate to a more lipophilic borate salt, which is soluble in aromatic solvents:
When the reaction is conducted in non-polar solvents such as dichloromethane, the salt NaBF4 precipitates, which helps drive these reactions.
Metal complexes are alkylated via salt metathesis reactions. Illustrative is the methylation of titanocene dichloride:
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