Figure 9.8. Synthesis of an Anhydride via Carboxylic Acid Condensation
Two molecules of carboxylic acid come together and lose a molecule of water in the formation of an
anhydride.Certain cyclic anhydrides can be formed simply by heating carboxylic acids, as shown in Figure 9.9.
The reaction is driven forward by the increased stability of the newly formed ring; as such, only
anhydrides with five- or six-membered rings are easily made. Just as with all anhydride formations,
the hydroxyl group of one –COOH acts as the nucleophile, attacking the carbonyl on the other –
COOH.
Figure 9.9. Intramolecular Anhydride Formation
Heat and the increased stability of the newly formed ring drive this intramolecular ring formation
reaction forward.