Grignard connections

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A Grignard compound [ ɡriˈɲaːr- ] is an organometallic reagent which was named after Victor Grignard . For this discovery he received the Nobel Prize in Chemistry in 1912 . Grignard compounds are formed when an alkyl halide (e.g. bromomethane, BrCH 3 ) or an aryl halide (e.g. chlorobenzene C 6 H 5 Cl) reacts with metallic magnesium in the presence of solvents . The Grignard reaction , which is carried out with the help of these reagents, is an important name reaction in organic chemistry .

education

This reaction takes place on the surface of the metal. The magnesium atom inserts itself into the carbon - halogen bond. In the following, the halogen bromine is used, although one of the halogens chlorine or iodine can also be used. In the first step in the synthesis of the organic magnesium compound, which is analogous to the Sandmeyer reaction , an electron is transferred from the metal to the alkyl or aryl halide, with a radical anion being formed. Because of the weak carbon-halogen bond, the radical anion decomposes to form an organyl radical R • and the halide. The last step is the Grignard connection:

Formation of the Grignard connection

This reaction is carried out in nucleophilic solvents which have no active hydrogen, such as anhydrous diethyl ether or higher ethers (dibutyl ether, anisole , tetrahydrofuran ). Each carry two free electron pairs of these ether molecules are complex to the magnesium bound and provide the necessary stability octet ago, what can be the example of tetrahydrofuran, presented here in the literature as follows:

Grignard compound in solvent (tetrahydrofuran)

Solutions of Grignard reagents can contain numerous other species that are related by labile equilibria.

Schlenk balance

The Schlenk equilibrium, which was named after Wilhelm Schlenk , describes the molecular composition of the Grignard compound as a function of the solvent. Depending on the solvent that stabilizes the Grignard compound in a complex , different structures of the Grignard compounds are formed.

properties

The Grignard compound has the general formula RMgX , where X stands for the halogens chlorine, bromine or iodine. The rate of reaction falls from iodide to chloride. A better yield is obtained with chloride than with bromide or iodide. Alkyl or aryl fluorides normally do not react to form a Grignard compound. Very reactive compounds such as iodides can be subject to side reactions, e.g. B. the Wurtz coupling .

In the Grignard compound, the magnesium carries a partial positive charge and the adjacent carbon atom a partial negative charge. This polarization is so strong that z. B. a nucleophilic attack on a positively polarized carbon atom can take place. The strength of the polarity can be expressed by the ionic character, which is 35%. Thus it behaves similarly to a carbanion and can therefore also be represented as a resonance formula, which has an ionic bond with charge separation.

Grignard compounds react with substances which have active hydrogen, such as. B. water , alcohols , phenols , carboxylic acids , thiol or amine groups. This is because Grignard compounds are strongly basic. In the figure below, the Grignard compound is hydrolyzed to the corresponding alkane and metal halide hydroxide (mixed salt).

Hydrolysis of the Grignard compound

Use of the Grignard compounds

The Grignard reaction is a name reaction in organic chemistry, which was named after its discoverer Victor Grignard . This organometallic chemical reaction in which the alkyl or aryl magnesium halides (Grignard compound) act as a nucleophile to electrophilic groups such as. B. carbonyl groups react, is used to build carbon-carbon single bonds.

Overview of the Grignard reaction

The importance of the Grignard compounds is that they are very good nucleophiles . They react to form a new carbon-carbon bond with electrophiles such as ketones , aldehydes , esters and nitriles . Grignard compounds react with carbon dioxide to form magnesium salts of carboxylic acids . Grignard compounds react analogously with carbon disulfide . The hydrolysis of the reaction products with carbon dioxide yields carboxylic acids, R-CO 2 H, or dithiocarboxylic acids, R-CS 2 H. In the reaction of Grignard compounds with elemental selenium , substances of the RSeMgX type are formed through an insertion reaction ; Their hydrolysis yields selenols with the exclusion of oxygen ; in the presence of air, oxygen is formed through oxidation of the selenols, diselenides . A method to quantitatively determine CH-acidic compounds in a sample with Grignard reagents is the Zerewitinow reaction .

Technical importance

In the past, an electrochemical variant of the Grignard reaction, i.e. the reactions of Grignard compounds, was important in the production of tetraethyl lead , an organic lead compound added to gasoline to increase the knock resistance . The Grignard reaction is widely used in the synthesis of drugs and other fine chemicals.

See also

literature

  • Heinz GO Becker, Werner Berger, Günter Domschke: Organikum . 22nd edition. Wiley-VCH, Weinheim 2004, ISBN 3-527-31148-3 .

Individual evidence

  1. ↑ Reinhard Brückner : reaction mechanisms. 3rd corrected edition. Spektrum Akademischer Verlag, 2007, ISBN 978-3-8274-1579-0 , p. 774.
  2. ↑ Heinz GO Becker, Werner Berger, Günter Domschke, Egon Fanghänel , Jürgen Faust, Mechthild Fischer, Fritjof Gentz, Karl Gewald, Reiner Gluch, Roland Mayer , Klaus Müller, Dietrich Pavel, Hermann Schmidt, Karl Schollberg, Klaus Schwetlick, Erika Seiler, Günter Zeppenfeld: Organikum. 19th edition. Barth, 1993, ISBN 3-335-00343-8 , p. 564.
  3. ^ NN Greenwood, A. Earnshaw: Chemistry of the elements. 1988, ISBN 3-527-26169-9 , pp. 162-169.
  4. ↑ K. Schwetlick: Organikum . 23rd edition. Wiley-VCH Verlag, Weinheim 2009, ISBN 978-3-527-32292-3 , pp. 563-572 .
  5. ↑ KPC Vollhardt, NE Schore: Organic Chemistry . Ed .: H. Butenschön. 4th edition. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2005, ISBN 3-527-31380-X , p. 348-349 .
  6. ^ V. Grignard: Sur quelques nouvelles combinaisons organométalliques du magnèsium et leur application à des synthèses d'alcools et d'hydrocarbures . In: CR Hebd. Séances Acad. Sci., Ser. C . tape 130 , 1900, pp. 1322–1324 ( digitized version in Gallica - French; German about some new organometallic compounds of magnesium and their application to the synthesis of alcohols and hydrocarbons ).