Guerbet condensation

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The Guerbet condensation - also Guerbet reaction - is a name reaction from the field of organic chemistry , which was first published in 1899 by the French chemist Marcel Guerbet (1861-1938). In the Guerbet condensation, primary or secondary alcohols are reacted at high temperatures and high pressure in the presence of alkali metal hydroxides or alkoxides via aldehydes and aldol products to form longer-chain alcohols, also called Guerbet alcohols in this context.

Overview

The Guerbet condensation can be divided into the formation of the higher valued aldehyde and the subsequent Cannizzaro reaction.

In the first part of the reaction, a primary alcohol (alternatively: secondary alcohol ) is converted to an aldehyde , among other things via an aldol condensation .

R = H or alkyl radical

In the second part of the reaction, the aldehyde formed is converted in a Cannizzaro reaction into a Guerbet alcohol, which is now of higher value than the alcohol that was used as the starting material .

Formation of Guerbet alcohol (R = H or alkyl radical )

In addition to the Guerbet alcohol, the identically substituted carboxylate is formed in equal parts .

Reaction mechanism

The reaction mechanism consists of the formation of the aldehyde and the subsequent formation of the Guerbet alcohol.

Formation of the aldehyde

A suggestion for the reaction mechanism of the formation of the aldehyde:

R = H or alkyl radical

First a primary alcohol (alternatively: secondary alcohol ) is deprotonated. As a result, a carbonyl group is formed at this point and the hydrogen atom of the hydroxyl group is split off as a hydride ion . This hydride ion deprotonates the resulting aldehyde 1 in the α-position, a hydrogen molecule is split off and aldehyde 2 is formed, which is deprotonated in the α-position . Further mesomeric boundary structures (hint: keto-enol tautomerism ) were omitted for the purpose of a better overview. An aldol condensation of this ion 2 with the aldehyde 1 produces the alkenal (α, β-unsaturated aldehyde) 3 . The addition of a hydride ion in the α position creates a negative charge on the β carbon atom of the alkenal 3 . A proton is then added at this point. This is how aldehyde 6 is formed . This also creates aldehyde 5 , which is identical to aldehyde 1 and can also undergo an aldol condensation with anion 2 .

Formation of Guerbet alcohol

Finally, alcohol 7 (Guerbet alcohol) is synthesized in a Cannizzaro reaction by reaction of two molecules of the aldehyde 6 with a hydroxide ion with elimination of a carboxylate ion (gray) :

One equivalent of aldehyde 6 is reduced to alcohol 7 , one equivalent is oxidized . (R = H or alkyl radical )

Individual evidence

  1. ^ M. Guerbet: Action de l'alcool amylique de fermentation sur son dérivé sodé . In: Comptes rendus de l'Académie des sciences . tape 128 , 1899, pp. 511-513 ( digitized on Gallica ).
  2. ^ Robert Miller, George Bennett: Producing 2-Ethylhexanol by the Guerbet Reaction . In: Industrial & Engineering Chemistry . tape 53 , no. 1 , January 1, 1961, p. 33-36 , doi : 10.1021 / ie50613a027 .
  3. Ch. Weizmann, E. Bergmann, M. Sulzbacher: Further Observations on the Guerbet Reaction . In: The Journal of Organic Chemistry . tape 15 , no. 1 , January 1, 1950, p. 54-57 , doi : 10.1021 / jo01147a010 .
  4. Ramesh Varadaraj, Jan Bock, Paul Valint, Stephen Zushma, Robert Thomas: Fundamental interfacial properties of alkyl-branched sulfate and ethoxy sulfate surfactants derived from Guerbet alcohols. 1. Surface and instantaneous interfacial tensions . In: The Journal of Physical Chemistry . tape 95 , no. 4 , February 1, 1991, p. 1671-1676 , doi : 10.1021 / j100157a033 .
  5. a b c d Z. Wang (Ed.): Comprehensive Organic Name Reactions and Reagents, 3 Volume Set , John Wiley & Sons, Hoboken, New Jersey, 2009, ISBN 978-0-471-70450-8 , p. 1298 .
  6. a b c Z. Wang (Ed.): Comprehensive Organic Name Reactions and Reagents, 3 Volume Set , John Wiley & Sons, Hoboken, New Jersey, 2009, ISBN 978-0-471-70450-8 , p. 1299.
  7. a b S. Veibel, JI Nielsen: On the mechanism of the Guerbet reaction . In: Tetrahedron . tape 23 , no. 4 , 1967, p. 1723-1733 , doi : 10.1016 / S0040-4020 (01) 82571-0 .