Metathesis reaction

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Metathesis is a bimolecular process involving the exchange of bonds between the two reacting chemical species, which results in the creation of products with similar or identical bonding affiliations.[1] To illustrate, consider two chemical species, AB and CD, which react to give AD and CB:

AB + CD → AD + CB

These chemical species can either be ionic or covalent. When referring to precipitation reactions between solutions of ions in inorganic chemistry, it was formerly referred to as double displacement or double replacement reactions, though these terms are now discouraged.

Types of reaction

Neutralization

A neutralization reaction is a specific type of double displacement reaction. Neutralization occurs when equal amounts of acid react with equal amounts of a base. A neutralization reaction creates a solution of salt and water, for example:

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Aqueous metathesis

Metathesis reactions can occur between two inorganic salts when one product is insoluble in water, driving the reaction forward, as in the following examples:

AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
2AgNO3(aq) + CaCl2(aq) → 2AgCl(s) + Ca(NO3)2(aq)

One of the compounds formed is usually a precipitate, an insoluble gas that bubbles out of the solution or an insoluble solid, or a molecular compound, usually water. Since one of these conditions must always be met, a solubility chart (or general knowledge of solubility rules) can be used in advance to predict whether two aqueous reactants will react or not. HSAB theory can also be applied as a means of determining or prediction of products created from such a metathesis reaction

Acid and carbonates

Another subcategory of this reaction type features acids reacting with carbonates/bicarbonates. These reactions always yield carbonic acid as a product, which in turn automatically decomposes into carbon dioxide and water. An example below is the common science fair "volcano" reaction - acetic acid with sodium bicarbonate:

CH3COOH (aq) + NaHCO3(s) → CH3COONa(aq) + CO2(g) + H2O(l)

Olefin metathesis

File:Metathesis.gif
An example of metathesis reaction involving the redistribution of alkenes fractions, referred to as olefins within a metathesis reaction of this type.

An important and influential reaction within organic chemistry, which involves the principle of metathesis is the olefin metathesis reaction developed by Yves Chauvin, Richard R. Schrock and Robert H. Grubbs, who shared a Nobel Prize in Chemistry in 2005.

The principle of olefin metathesis is that an alkene double bond is cleaved and redistributed alongside the redistribution of an alkylidene under the presence of catalytic metals, such as ruthenium, nickel, or tungsten. It has several commercially significant applications, and it is especially useful due to the relatively small amounts of side products and hazardous waste products.

In this context, the otherwise abstract concepts of metathesis are especially well-visualized in the unique diplomas which were crafted for King Carl XIV Gustaf to present to the three chemists who shared the Nobel Prize in 2005.

Intramolecular metathesis of a diene

File:Intramolecular metathesis.gif
Intramolecular metathesis using 4-methyl-1,7-octadiene.

Ring formation occurs when a diene is used as the starting material. Any size ring is possible to make with metathesis of a diene. For example, in the presence of a metal catalyst, usually Grubbs catalyst, 4-methyl-1,7-octadiene forms 4-methylcyclohexene. When using terminal alkenes the by-product is always gaseous ethene.[2]

See also

References

  1. International Union of Pure and Applied Chemistry, (2006). IUPAC Gold Book, Compendium of Chemical Terminology IUPAC Publications
  2. Janice Gorzynski Smith. Organic Chemistry: Second Ed. 2008. pp 1016-1018.

Further reading

  • R. H. Grubbs (Ed.), Handbook of Metathesis, Wiley-VCH, Weinheim, 2003.

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