Transesterification

Transesterification is a chemical reaction in which the alkoxy group of an ester is exchanged with the alkoxy group of an alcohol, producing a different ester and a different alcohol. The reaction is widely used to convert esters such as triglycerides from fats and oils into fatty acid esters (for example, methyl or ethyl esters), which serve as biofuels, solvents, and intermediates in fragrance and polymer synthesis.

The process can be catalyzed by acids, bases, or enzymes. Acid-catalyzed transesterification proceeds via protonation of the carbonyl, nucleophilic attack by the alcohol, and formation of a tetrahedral intermediate. Base-catalyzed transesterification involves nucleophilic attack by an alkoxide on the carbonyl, followed by transfer of an alkoxy group. Enzymatic transesterification uses lipases and is valued for milder conditions and high selectivity, often in solvent-free or aqueous-organic systems.

In biodiesel production, triglycerides react with methanol or ethanol to form fatty acid methyl esters (FAME) or fatty acid ethyl esters (FAEE) and glycerol. The reaction is reversible and equilibrium-controlled; using excess alcohol and removing the product alcohol drives the reaction toward esters. Saponification can occur if significant free fatty acids are present with basic catalysts, forming soaps that hinder the process.

Beyond biofuels, transesterification is used to synthesize specialty esters, recycle polymers by rearranging ester linkages, and modify polyesters in materials chemistry. Key factors include the choice of catalyst, the alcohol used, temperature, reactant ratios, moisture, and the presence of free fatty acids or water.