H Acid Anhydrides COOH Two molecules of acid combine with the loss of water to form the anhydride. Anhydrides are more reactive than acids. but less reactive than acid chlorides. A carboxylate ion is the leaving group in nucleophilic acyl substitution reactions. 8on no8R-RBo2a => Chapter 21 12Chapter 21 12 Acid Anhydrides • Two molecules of acid combine with the loss of water to form the anhydride. • Anhydrides are more reactive than acids, but less reactive than acid chlorides. • A carboxylate ion is the leaving group in nucleophilic acyl substitution reactions. R C O O H C R O H O R C O O C O R =>