transesterification

Transesterification is swapping one alcohol piece of an ester for a different alcohol. Instead of building an ester from an acid, you take an existing ester and exchange its -OR' group for a new -OR''. It is the reaction that turns vegetable oil into biodiesel, and it is how chemists relabel an ester without ever isolating the free acid.

The setup is an ester R-CO-OR' plus a different alcohol R''-OH, with an acid or base catalyst, giving a new ester R-CO-OR'' plus the displaced alcohol R'-OH. Mechanistically it is nucleophilic acyl substitution where the incoming nucleophile is an alcohol (or alkoxide) and the leaving group is the original alcohol part — the acyl group stays put while the two alcohol pieces trade places through the familiar tetrahedral intermediate. Like Fischer esterification, it is an equilibrium, so you drive it by using a large excess of the incoming alcohol or by removing the departing alcohol as it forms.

Biodiesel is the headline application: triglycerides (the esters in vegetable oil or animal fat) react with methanol under a base catalyst, and each fatty-acid chain is transferred from glycerol onto methanol, giving fatty-acid methyl esters (biodiesel) plus glycerol. Transesterification also appears in polyester manufacture and in many syntheses where it is more convenient to start from an available ester than from the acid. Because no acid chloride or harsh dehydration is needed, it is often a mild, practical way to install the exact ester you want.