Esterification Of Hydroxy Acids

The hydroxy acid molecule contains two functional groups, hydroxyl and carboxyl, that can react with each other. Therefore, hydroxy acids can undergo intramolecular esterification or intermolecular esterification.
1. Intramolecular esterification-formation of lactone
γ-hydroxy acid and δ-hydroxy acid form lactone under neutral or acidic conditions; under alkaline conditions, lactone can open to form carboxylate, and then form lactone after acidification.
2. Intermolecular esterification-generating lactide
Alpha-hydroxy acids are prone to intermolecular esterification when heated to form lactide
3. Aggregation
In addition to hydroxy acids that have a strong tendency to form five-membered and six-membered ring lactones, other hydroxy acids can obtain polyesters with high relative molecular weight under the action of suitable esterification catalysts and by continuously removing water during the reaction process. Catalysts are commonly used protonic acids or Lewis acids; if high temperatures are required, in order to avoid dehydration of the hydroxyl group, basic catalysts such as Sb₂O3 Zn(OAc)2 are commonly used