Esterification
An esterification (also known as ester formation ) is a chemical reaction in which a so-called ester is formed from an organic or inorganic oxo acid and an alcohol . In the classic sense, an esterification is the reaction of a carboxylic acid with an alcohol to form a carboxylic acid ester when water is split off. However, carboxylic acid esters can also be prepared by reacting alcohols and, for example, carboxylic acid chlorides or carboxylic acid anhydrides . The condensation of other oxo acids such as phosphoric acid , sulfuric acid or sulfonic acids with alcohols also lead to esters and can therefore also be regarded as esterifications.
Overview reaction
The reaction is an equilibrium and condensation reaction in which an alcohol or a phenol reacts with a carboxylic acid (or an inorganic acid) to form an ester and water.
The mechanism shown here in color is valid for primary and secondary alcohols, but not for tertiary alcohols. In the case of tertiary alcohols, the OH group of the alcohol is split off in the course of the reaction and the resulting carbenium ion is added to the carboxylic acid. The oxygen in the carboxylic acid remains in the ester.
Mechanism of the acid-catalyzed esterification of carboxylic acids (Fischer esterification)
Emil Fischer recognized that even small amounts of sulfuric acid greatly increase the ester yield and that large amounts of acid are not necessary, as was common at the time.
The first step of the esterification is the protonation of the carboxylic acid to a mesomeric stabilized cation .
The cation formed by the protonation is nucleophilically attacked by a lone pair of electrons from the hydroxyl group of the alcohol . The hydrogen atom of the original alkanol forms an intramolecular hydrogen bond to an oxygen atom of the carboxy group and ultimately forms a bond with this.
The oxonium ion formed in the last step above first splits off water to form a cation which, after deprotonation, gives the ester.
This mechanism could be proven by isotope labeling of the oxygen atom in alcohol.
The rapidly established equilibrium of ester formation requires an elevated temperature or the addition of acids. Sulfuric acid is often used for acid catalysis because it binds a certain amount of water due to its hydration capacity, so that the water is removed from the reaction equilibrium (the reaction equilibrium is shifted in favor of the ester). It is also possible to use p -toluenesulfonic acid or benzene. Hafnium (IV) salts (including hafnium dichloride oxide) have proven to be particularly effective catalysts (one hafnium (IV) salt molecule can bind 7 water molecules).
Further esterification reactions
There are a number of other ways to convert alcohols and acids or acid derivatives to esters, such. B. the reaction between alcohol and acid chloride ( Schotten-Baumann method ), alcohol and acid anhydride , the Steglich esterification or the Mitsunobu reaction . The transesterification allows the alcoholic or acid component of an existing ester to be exchanged.
Ester cleavage
The reverse reaction of the esterification is the acidic ester hydrolysis (see hydrolysis ). The basic or alkaline ester hydrolysis follows a different reaction mechanism than the acidic and is therefore explicitly shown under saponification .
Manufacturing process
Many chemical products are made in a chemical reaction and then distilled. In the production of an ester, both processes are combined. This process is called reactive distillation . During the reaction, one or more products (in the case of esterification: water) can be removed by evaporation. The chemical equilibrium is constantly re-established, and a greater yield of the ester can thereby be achieved. The process has the advantage that two work steps can be combined. The outlay on equipment is low and the process is inexpensive.
Individual evidence
- ↑ Théophile-Jules Pelouze, Amédée Gélis: Mémoire sur l'acide butyrique . In: Annales de Chimie et de Physique . 3 ème Série IX, 1844, p. 434–456, here p. 453 ( limited preview in Google book search).
- ↑ Emil Fischer , Arthur Speier: Representation of the Esters . In: Chemical Reports . 28, 1895, pp. 3252-3258. doi : 10.1002 / cber.189502803176 .
- ↑ Ivan Ernest: Binding, Structure and Reaction Mechanisms in Organic Chemistry . Springer, Vienna / New York 1972, ISBN 3-211-81060-9 , pp. 73-74.
- ^ Walter Beyer: Textbook of organic chemistry . tape 24 , 2004, pp. 272 .
- ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 1539.
- ↑ Chemistry and Life Science: reactive distillation opens up new perspectives .
