The Uncatalyzed Direct Amide Formation Reaction - Mechanism Studies and the Key Role of Carboxylic Acid H-Bonding

Calorimetric studies of the mixing of a series of carboxylic acids and amines have been carried out to measure heat output, which has been compared with their ability to react to form carboxylate ammonium salts and amides. In order to identify which species (salt or H-bonded species) were formed, H-1 NMR studies were also carried out by mixing carboxylic acids and amines in [D-8] toluene and monitoring the resulting reactions. These experiments were also compared to DFT computational studies, from which the relative merits of different mechanistic schemes for direct amide formation could be assessed. A reaction mechanism involving zwitterionic intermediates could be eliminated on the basis of calculated energies in toluene, however, a neutral intermediate pathway, involving carboxylic acid dimerization by mutual hydrogen bonding was found to be accessible and may explain how the direct amide formation reaction occurs. Such a mechanism is not inconsistent with kinetic modelling of direct amide formation under different reactions conditions.