Esterfication using a carboxylic acid and an alcohol requires an acid catalyst. In this experiment, the acid form
of an ion-exchange resin is used. This resin, in the form of small beads, is a cross-linked polystyrene that bears
sulfonic acid groups on some of the phenyl groups. Essentially it is an immobilized form of p-toluenesulfonic acid,
an organic-substituted sulfuric acid. This catalyst has the distinct advantage that at the end of the reaction it can be
removed simply by filtration. Immobilized catalysts of this type are becoming more and more common in organic
synthesis.