What is a common disconnection strategy to form an amide from a carboxylic acid derivative and an amine?

• A. Reduce the carboxyl group to an alcohol before coupling.
• B. Convert the acid to an acid chloride (or anhydride) and couple with the amine.
• C. React the carboxylic acid directly with the amine without activation.
• D. Convert the acid to an ester and then perform aminolysis.

Answer: (B)

The key idea is activating the carboxyl group so the amine can attack efficiently. Carboxylic acids are not good electrophiles for direct amidation because the hydroxyl leaving group is poor, so a direct reaction with an amine is slow and low-yielding. Converting the acid into a more reactive derivative, such as an acid chloride or an anhydride, makes the carbonyl carbon highly electrophilic and provides a good leaving group (chloride or carboxylate) after nucleophilic attack by the amine. The amine then attacks the activated carbonyl, the tetrahedral intermediate collapses, and the amide forms cleanly. This strategy is widely used because it gives good yields across a broad range of substrates under relatively straightforward conditions.

Other options introduce extra steps or struggles: reducing the carboxyl to an alcohol changes the functional group entirely and isn’t a direct path to an amide; attempting to couple the unactivated acid with the amine is impractical due to poor reactivity; forming an ester and then doing aminolysis adds steps and can lead to transesterification or incomplete conversion. Activation to acid chloride or anhydride keeps the process efficient and reliable for amide formation.