The reactions of this chapter have sometimes complex mechanisms and steps. All involve a nucleophilic attack on the $C^{\delta+}$ of the carboxyl group and a nucleophilic departure of $OH^-$
The acid chlorides $RCOCl$ are very reactive substances and replace advantageously the carboxylic acids in organic synthesis.
Acid anhydrides $R(CO)O(CO)R$ are reactive substances and advantageously replace the carboxylic acids in organic synthesis.
In a first step, an ammonium carboxylate is formed, which then heated leads to an amide. Amides are odorless and stable white solids.
For details, see chapter of → esters.