Salt metathesis reaction
Template:Short description 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, 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:[1]
- 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:[2]
- [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 Page Module:Chem2/styles.css has no content.AgCB11H12, which contains a weakly coordinating carborane anion. With Page Module:Chem2/styles.css has no content.IrCl(CO)(PPh3)2 (Vaska's Complex), the product is an adduct with a Ir-Ag bond. By contrast, AgClO4 simply delivers Page Module:Chem2/styles.css has no content.Ir(ClO4)(CO)(PPh3)2. The intermediate Page Module:Chem2/styles.css has no content.Fe(Cp)(CO)2I·Ag(B11CH12) is observed in the reaction of Page Module:Chem2/styles.css has no content.AgCB11H12 with Page Module:Chem2/styles.css has no content.(C5H5)Fe(CO)2I.[3]
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.
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:
- Page Module:Chem2/styles.css has no content.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:
- Page Module:Chem2/styles.css has no content.2 HCl + Na2CO3 → H2CO3 + 2 NaCl
- Page Module:Chem2/styles.css has no content.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.[5]
See also
References
Page Template:Reflist/styles.css has no content.
- ^ J. R. Dilworth, W. Hussain, A. J. Hutson, C. Tetrahalo Oxorhenate Anions" Inorganic Syntheses 1997, volume 31, pages 257–262. Script error: No such module "CS1 identifiers".
- ^ 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. Script error: No such module "CS1 identifiers".
- ^ Page Module:Citation/CS1/styles.css has no content.Liston, David J.; Lee, Young Ja; Scheidt, W. Robert; Reed, Christopher A. (1989). "Observations on Silver Salt Metathesis Reactions with Very Weakly Coordinating Anions". Journal of the American Chemical Society. 111 (17): 6643–6648. Bibcode:1989JAChS.111.6643L. doi:10.1021/ja00199a025.
- ^ Page Module:Citation/CS1/styles.css has no content.Payack, J. F.; Hughes, D. L.; Cai, D.; Cottrell, I. F.; Verhoeven, T. R. (2002). "Dimethyltitanocene". Organic Syntheses. 79: 19
{{cite journal}}: CS1 maint: multiple names: authors list (link). - ^ Page Module:Citation/CS1/styles.css has no content.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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