Nitrogen as a nucleophile amides

Nucleophilic attack only occurs upon heating the ammonium salt, resulting in overall dehydration of the salt. Consequently, it is usual to prepare amides by using a more favourable substrate than the carboxylic acid, one that is more reactive towards nucleophiles by virtue of possessing a better leaving group, and where salt formation does not hinder the reaction. 

Acyl halides and anhydrides are the most reactive class of carboxylic acid derivatives, and readily react with amines to give amides. It should be noted that in both cases the leaving group is a conjugate base that, upon protonation during the reaction, will become an 

Esters also react smoothly with amines, which is a useful reaction if the corresponding acyl halides or anhydrides are not easily available. The reaction proceeds through the anticipated tetrahedral anionic intermediate. There are two possible leaving groups 

The consequence of this is that an ester can react with ammonia to give an amide, but the reverse reaction does not occur simply treating amides with 

Note that, in all of these reaction mechanisms, a proton needs to be removed from the nitrogen nucleophile. Hence ammonia, primary amines, and 

---