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
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
A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...
, 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
In chemistry, the lattice energy is the energy change (released) upon formation of one mole of a crystalline compound from its infinitely separated constituents, which are assumed to initially be in the gaseous state at 0 K. It is a measure of ...
.
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
In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...
:
:NaReO
4 + N(C
4H
9)
4Cl → N(C
4H
9)
4 eO4 + 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:
:
e(C5H5)2F
4 + NaB(C
6F
5)
4 →
e(C5H5)2(C
6F
5)
4 + NaBF
4
When the reaction is conducted in
dichloromethane, the salt NaBF
4 precipitates and the B(C
6F
5)
4- salt remains in solution.
Metathesis reactions can occur between two
inorganic
An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bondsthat is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemistry''.
Inor ...
salt
In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...
s when one product is
insoluble in the reaction solvent. For example, the
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...
of
silver chloride from a mixture of
silver nitrate and
cobalt hexammine chloride delivers the
nitrate
Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wa ...
salt of the cobalt complex:
:3 +
o(NH3)6l
3 → 3 AgCl +
o(NH3)6NO
3)
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
Titanocene dichloride is the organotitanium compound with the formula (hapticity, ''η''5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid t ...
to give the
Petasis reagent
The Petasis reagent, named after Nicos A. Petasis, is an organotitanium compound with the formula Cp2Ti(CH3)2. It is an orange-colored solid.
Preparation and use
The Petasis reagent is prepared by the salt metathesis reaction of methylmagnesiu ...
:
:(C
5H
5)
2TiCl
2 + 2 ClMgCH
3 → (C
5H
5)
2Ti(CH
3)
2 + 2 MgCl
2
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
An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...
reacts with an equal amount of a
base. This reaction usually produces a salt. One example,
hydrochloric acid
Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...
reacts with
disodium iron tetracarbonyl to produce the iron dihydride:
:
Reaction between an acid and a carbonate or bicarbonate salt yields
carbonic acid
Carbonic acid is a chemical compound with the chemical formula . The molecule rapidly converts to water and carbon dioxide in the presence of water. However, in the absence of water, it is quite stable at room temperature. The interconversion ...
, which spontaneously decomposes into
carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
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
Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...
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
*
Single displacement reaction
References
{{Authority control
Chemical reactions
Inorganic reactions