Saponification

Under saponification ( lat. Saponification ) are understood in the narrow sense, the hydrolysis of an ester through the aqueous solution of a hydroxide such. B. by sodium hydroxide , or by special enzymes , the esterases . In contrast to acidic ester hydrolysis (the reverse reaction of esterification ), it is irreversible , since the carboxylic acid lacks the proton required for esterification . As a result the products of the reaction of alcohol and the salt of the acid (carboxylate ion) from which the ester component. In a broader sense, every hydrolysis is called saponification.

Mechanism of saponification of carboxylic acid esters

The saponification belongs to the addition-elimination mechanism .

The hydroxide ion ( 1 ) attacks the ester 2 nucleophilically . An orthocarboxylic acid monoester anion 3 is formed . The alcoholate ion 5 is split off with the formation of a carboxylic acid 4 . Finally: proton transfer from the carboxylic acid to the alcoholate ion with the formation of the carboxylate group in 6 and the alcohol 7 (irreversible saponification step). If the hydroxide ion ( 1 ) comes from the sodium hydroxide solution , a sodium cation is the counterion of 6 ; starting from potassium hydroxide solution , the potassium salt of the carboxylic acid would be formed analogously .

Subsequent addition of a stronger acid, e.g. B. dilute sulfuric acid , to 6 is formed by protonation the corresponding carboxylic acid. However, this protonation is not part of the actual saponification reaction.

use

Originally saponification only referred to the basic ester hydrolysis of triglycerides , e.g. B. animal fats or vegetable oils , with alkalis , originally soda , today mainly caustic soda or potassium hydroxide , in the soap mill . This creates the trihydric alcohol glycerine and the respective alkali salts of the fatty acids that are esterified in the fats . The salts of fatty acids are called soaps . Saponification with caustic soda provides curd soap (a sodium soap ), i.e. sodium salts of fatty acids (R ¹ to R ³ = long-chain alkyl group or alkenyl group of fatty acids):

The saponification with potassium hydroxide results in soft soap - a potassium soap , i.e. the potassium salts of fatty acids.

A chemical index in fat analysis is the saponification number .

The so-called unsaponifiable material , which mainly consists of sterols , can be obtained from the saponification batch by extraction with hydrocarbons .

Individual evidence

1. ↑ ^{a b c} Entry on saponification. In: Römpp Online . Georg Thieme Verlag, accessed on June 15, 2014.
2. ^ Siegfried Hauptmann : Organic Chemistry, 2nd revised edition, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 1985, p. 418, ISBN 3-342-00280-8 .
3. ^ Organikum , Wiley-VCH Verlag GmbH, 23rd edition, 2009, pp. 494-495, ISBN 978-3-527-32292-3 .