Salt metathesis reaction

A salt metathesis reaction (also called a double displacement reaction, double replacement reaction, or double decomposition) is a type of chemical reaction in which two ionic compounds in aqueous solution exchange their component ions to form two new compounds. Often, one of these new compounds is a precipitate, gas, or weak electrolyte, driving the reaction forward.

AB + CD -> AD + CB

$\stackrel{+1}{\mathrm{A}\mathrm{g}}\stackrel{-1}{\mathrm{N}\mathrm{O}{\phantom{A}}_3}+\stackrel{+1}{\mathrm{H}}\stackrel{-1}{\mathrm{C}\mathrm{l}}\to \stackrel{+1}{\mathrm{H}}\stackrel{-1}{\mathrm{N}\mathrm{O}{\phantom{A}}_3}+\stackrel{+1}{\mathrm{A}\mathrm{g}}\stackrel{-1}{\mathrm{C}\mathrm{l}(\mathrm{v})}$

In older literature, the term double decomposition is common. 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:

AX(aq) + BY(s) → AY(aq) + BX(s).

Types of reactions

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 AgNO
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:

Ba(OH)
₂ + 2CuCNS → Ba(CNS)
₂ + 2CuOH

Mechanisms

The mechanism of silver-based salt metathesis reactions are revealed with the use of AgCB₁₁H₁₂, which contains a weakly coordinating carborane anion. With IrCl(CO)(PPh₃)₂ (Vaska's Complex), the product is an adduct with a Ir-Ag bond. By contrast, AgClO₄ simply delivers Ir(ClO₄)(CO)(PPh₃)₂. The intermediate Fe(Cp)(CO)₂I·Ag(B₁₁CH₁₂) is observed in the reaction of AgCB₁₁H₁₂ with (C₅H₅)Fe(CO)₂I.

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:

2 HCl + Na₂Fe(CO)₄ → 2 NaCl + H₂Fe(CO)₄

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:

2 HCl + Na₂CO₃ → H₂CO₃ + 2 NaCl

H₂CO₃ → H₂O + CO₂

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

• Single displacement reaction