Counterion exchange Salt metathesis is a common technique for exchanging
counterions. The choice of reactants is guided by a
solubility chart or
lattice energy.
HSAB theory can also be used to predict the products of a metathesis reaction. Salt metathesis is often employed to obtain salts that are soluble in organic solvents. Illustrative is the conversion of
sodium perrhenate to the
tetrabutylammonium salt: :NaReO4 + N(C4H9)4Cl → N(C4H9)4[ReO4] + NaCl The tetrabutylammonium salt precipitates from the aqueous solution. It is soluble in
dichloromethane. Salt metathesis can be conducted in nonaqueous solution, illustrated by the conversion of
ferrocenium tetrafluoroborate to a more lipophilic salt containing the
tetrakis(pentafluorophenyl)borate anion: :[Fe(C5H5)2]BF4 + NaB(C6F5)4 → [Fe(C5H5)2]B(C6F5)4 + NaBF4 When the reaction is conducted in
dichloromethane, the salt NaBF4 precipitates and the B(C6F5)4- salt remains in solution. Metathesis reactions can occur between two
inorganic salts when one product is
insoluble in the reaction solvent. For example, the
precipitation of
silver chloride from a mixture of
silver nitrate and
cobalt hexammine chloride delivers the
nitrate salt of the cobalt complex: :3 + [Co(NH3)6]Cl3 → 3 AgCl + [Co(NH3)6](NO3)3 The reactants need not be highly soluble for metathesis reactions to take place. For example
barium thiocyanate forms when boiling a slurry of
copper(I) thiocyanate and
barium hydroxide in water: : + 2 → + 2CuOH
Mechanisms The mechanism of silver-based salt metathesis reactions are revealed with the use of , which contains a weakly coordinating carborane anion. With (
Vaska's Complex), the product is an adduct with a Ir-Ag bond. By contrast, AgClO4 simply delivers . The intermediate is observed in the reaction of with .
Alkylation Metal complexes are
alkylated via salt metathesis reactions. Illustrative is the
methylation of
titanocene dichloride to give the
Petasis reagent: :(C5H5)2TiCl2 + 2 ClMgCH3 → (C5H5)2Ti(CH3)2 + 2 MgCl2 The salt product typically precipitates from the reaction solvent.
Neutralization reaction A
neutralization reaction is a type of double replacement reaction. A neutralization reaction occurs when an
acid reacts with an equal amount of a
base. This reaction usually produces a salt. One example,
hydrochloric acid reacts with
disodium iron tetracarbonyl to produce the iron dihydride: : Reaction between an acid and a carbonate or bicarbonate salt yields
carbonic acid, which spontaneously decomposes into
carbon dioxide and water. The release of carbon dioxide gas from the reaction mixture drives the reaction to completion. For example, a common, science-fair "volcano" reaction involves the reaction of
hydrochloric acid with
sodium carbonate: : :
Salt-free metathesis reaction In contrast to salt metathesis reactions, which are driven by the precipitation of solid salts, are
salt-free reductions, which are driven by formation of silyl halides, Salt-free metathesis reactions proceed homogeneously. ==See also==