Transesterification

Transesterification is a chemical reaction in which an ester is converted to another. In the transesterification of the alcohol residue of the ester is replaced by a different alcohol group. The reaction can be carried out by acid catalysis and surpluses of the free alcohol or by using the much more nucleophilic alkoxides. Acid-catalyzed this reaction is an equilibrium reaction, and thus subject to the law of mass action.

Basics

In a transesterification is the attack of alkoxide anion as a nucleophile to an ester of the same or another alcohol component of the ester. So at some point, the adjustment of a balance between the competing alkoxide ions. The reaction is known, inter alia, as Zemplén reaction. In the acid-catalyzed transesterification, the carbonyl oxygen atom is protonated, the alcohol can attack the strong positivised carbonyl carbon atom with its oxygen atom. It must either be used with large excesses of the alcohol component, or the alcohol formed is removed, for example by distillation due to the law of mass action.

It can also be used, two different carboxylic acid esters, which can then exchange their alcohol groups with acid catalysis. This approach has in practice of little significance, since you get here usually mixtures.

In the acid-catalyzed transesterification of two carboxylic acid esters the alcohol radicals R2 are exchanged in blue and R4 in purple against each other.

Mechanism

A transesterification ( alcoholysis ) can be catalyzed both by acids and by bases. Is an equilibrium reaction. Under acidic conditions, the reaction with the protonation of the carbonyl oxygen atom, followed by attack of the alcohol to the carbonyl carbon atom starts. In basic medium, the alcohol is first deprotonated. Then attacks as alkoxide anion to the positively polarized carbonyl carbon atom of the ester, as shown:

In this example methanolate is selected as the alkoxide, thus formed a methyl ester.

Technical significance

The transesterification is used for the following industrial products:

• Production of biodiesel from triacylglycerols ( vegetable fats or oils) called by transesterification with methanol FAME (fatty acid methyl esters, such as rapeseed methyl ester ( RME) ) are generated. As a byproduct, among others, the trivalent alcohol glycerol. In Germany, biodiesel is produced on an industrial scale in transesterification.
• Production of polyethylene terephthalate from dimethyl terephthalate

Application in the food industry

The transesterification is in food chemistry to the preparation of:

• Margarine
• Sugar surfactants
• Monoglycerides of triglycerides
• Rapeseed oil methyl esters from rapeseed oil triglycerides

Be used. The dried and fat is deacidified it was stirred at 80-100 ° C with sodium methoxide as catalyst. With lipases, a site-specific transesterification can be achieved.

Analysis

Volatile esters such as fatty acid esters are transesterified often better in volatile methyl esters to make them accessible here, especially in the less gas chromatography.