Salt metathesis reaction

A salt metathesis reaction, sometimes called a double replacement 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.[1] This reaction is represented by the general scheme:

A-B + C-D → A-D + C-B
 ${\displaystyle {\overset {+1}{\color {Orange}{\ce {Ag}}}}{\overset {-1}{\color {Blue}{\ce {NO3}}}}+{\overset {+1}{\color {Green}{\ce {H}}}}{\overset {-1}{\color {Magenta}{\ce {Cl}}}}\rightarrow {\overset {+1}{\color {Green}{\ce {H}}}}{\overset {-1}{\color {Blue}{\ce {NO3}}}}+{\overset {+1}{\color {Orange}{\ce {Ag}}}}{\overset {-1}{\color {Magenta}{\ce {Cl(v)}}}}}$

The bond between the reacting species can be either 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:

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:[2]

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:[3]

[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 water. 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 to 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
+ 2CuCNSBa(CNS)
2
+ 2CuOH

Alkylation

Metal complexes are alkylated via salt metathesis reactions. Illustrative is the methylation of titanocene dichloride to give the Petasis reagent:[4]

(C5H5)2TiCl2 + 2 ClMgCH3 → (C5H5)2Ti(CH3)2 + 2 MgCl2

The salt product typically precipitates from the reaction solvent.

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 usually creates a solution of a salt and water. For example, hydrochloric acid reacts with sodium hydroxide to produce sodium chloride and water:

HCl + NaOH → NaCl + H
2
O

Reaction between an acid and a carbonate or bicarbonate always yields carbonic acid as a product, 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 acetic acid with sodium bicarbonate:

CH
3
COOH
+ NaHCO
3
CH
3
COONa
+ CO
2
+ H
2
O

Salt-free metathesis reaction

In contrast to salt metathesis reactions, which benefit from the precipitation of solid salts, are salt-free metathesis reactions, which benefit from the formation of silyl halides, which have high Si-halide bond energies. Salt-free metathesis reactions proceed homogeneously.[5]

See also

• Alkane metathesis
• Alkyne metathesis
• Olefin metathesis
• Single displacement reaction

References

1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006) "metathesis". doi:10.1351/goldbook.M03878.
2. J. R. Dilworth, W. Hussain, A. J. Hutson, C. J. Jones, F. S. Mcquillan "Tetrahalo Oxorhenate Anions" Inorganic Syntheses 1997, volume 31, pages 257–262. doi:10.1002/9780470132623.ch42
3. J. Le Bras, H. Jiao, W. E. Meyer, F. Hampel and J. A. Gladysz, "Synthesis, Crystal Structure, and Reactions of the 17-Valence-Electron Rhenium Methyl Complex [(η5-C5Me5)Re(NO)(P(4-C6H4CH3)3)(CH3)]+B(3,5-C6H3(CF3)2)4: Experimental and Computational Bonding Comparisons with 18-Electron Methyl and Methylidene Complexes", J. Organomet. Chem. 2000 volume 616, 54-66. doi:10.1016/S0022-328X(00)00531-3
4. Payack, J. F.; Hughes, D. L.; Cai, D.; Cottrell, I. F.; Verhoeven, T. R. (2002). "Dimethyltitanocene". Organic Syntheses. 79: 19.CS1 maint: multiple names: authors list (link)
5. Mashima, Kazushi (2020). "Redox-Active α-Diimine Complexes of Early Transition Metals: From Bonding to Catalysis". Bulletin of the Chemical Society of Japan. 93 (6): 799–820. doi:10.1246/bcsj.20200056.
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