When in ester is placed in a large excess of an alcohol along with presence of either an acid or a base there can be an exchange of alkoxy groups. The large excess of alcohol is used to drive the reaction forward. The most common method of transesterification is the reaction of the ester with an alcohol in the presence of an acid catalyst e.g.:

This reaction has the following mechanism:

Since both the reactants and the products are an ester and an alcohol, the reaction is reversible and the equilibrium constant is close to one.  Consequently, the Le Chatelier’s principle has to be exploited to drive the reaction to completion. The simplest way to do so is to use the alcohol as the solvent as well.

General reaction

Mechanism in basic conditions

Nucleophilic attack by an alkoxide

2) Leaving group removal