Transesterification

from Wikipedia, the free encyclopedia

A transesterification is a chemical reaction in which an ester is converted to another. During transesterification, the alcohol residue of an ester is replaced by another alcohol residue. The reaction can be carried out acid-catalytically and with excesses of the free alcohol or by using the much more nucleophilic alcoholates . Acid-catalyzed this reaction is an equilibrium reaction and is therefore subject to the law of mass action .

Basics

A transesterification is the attack of an alcoholate ion as a nucleophile on an ester of the same or a different alcohol component of an ester. So at some point an equilibrium is established between the competing alcoholate ions. The reaction is also known as the Zemplén reaction, among others . In acid-catalyzed transesterification, the carbonyl oxygen atom is protonated, and the alcohol can attack the strongly positive carbonyl carbon atom with its oxygen atom. Due to the law of mass action, either large excesses of the alcohol component must be used or the alcohol formed must be removed, e.g. B. by distillation .

Acid catalyzed transesterification by alcoholysis: the alkoxy group OR 2 is the alkoxy group OR 3 replaced

It is also possible to use two different carboxylic acid esters , which then exchange their alcohol residues under acid catalysis. However, this approach is of little importance in practice, since you usually get mixtures of substances:

acid-catalyzed transesterification of two carboxylic acids

In the acid-catalyzed transesterification of two carboxylic acid esters, the alcohol radicals R 2 in blue and R 4 in purple are exchanged for one another.

mechanism

A transesterification (alcoholysis) can be catalyzed by both acids and bases. It is an equilibrium reaction . In acidic conditions, the reaction begins with protonation of the carbonyl oxygen atom, followed by an attack by the alcohol on the carbonyl carbon atom. In the alkaline, the alcohol is first deprotonated . Then the alcoholate anion attacks the positively polarized carbonyl carbon atom of the ester, see figure:

General mechanism of transesterification

In this example, methanolate is selected as the alcoholate, thus creating a methyl ester .

Technical importance

Example of the acid-catalyzed transesterification of a natural triglyceride (above) in fats and oils. The fatty acid residue marked blue is saturated, the green marked is single, the red marked triple unsaturated . In the equilibrium reaction, glycerine is split off and FAME (biodiesel, below) is formed, a mixture of fatty acid methyl esters, in the example three different ones.

Transesterification is used for the following technical products:

Application in the food industry

Transesterification is used in food chemistry for the production of:

used. The dried and deacidified fat is stirred for this at 80-100 ° C with sodium methoxide as a catalyst. A site- specific transesterification can be achieved with lipases .

Analytics

In gas chromatography in particular , less volatile esters such as fatty acid esters are often transesterified into more volatile methyl esters in order to make them accessible here.

Individual evidence

  1. ↑ Karl W. Rosenmund, Felix Zymalkowski, Eckart Güssow: About the use of aluminum alcoholates for transesterification and saponification reactions. In: Archives of Pharmacy. 286, 1953, pp. 324-330, doi : 10.1002 / ardp.19532860704 .
  2. ^ Kurt Peter C. Vollhardt, Neil Eric Schore, Katrin-M. Roy, Holger Butenschön: Organic Chemistry . 5th edition. Wiley-VCH, Weinheim 2011, ISBN 978-3-527-32754-6 , pp. 1006 ( limited preview in Google Book search).
  3. ^ Römpp - food chemistry. 9th edition, Thieme, Stuttgart 1995, ISBN 3-13-736601-1 .
  4. ↑ Hans-Dieter Belitz, Peter Schieberle, Werner Grosch: Textbook of food chemistry ⁵ . Springer, 2001, p. 645 ( limited preview in Google Book Search).

literature

  • Charles E. Mortimer: Chemistry - The basic knowledge of chemistry. Thieme Verlag, Stuttgart 2007, 9th revised edition, 766 pp.