Acyl substitution is an MCAT favorite mechanism. The acyl substitution mechanism is the pathway between the carboxylic acid derivatives acyl chloride, acid anhydride, ester, carboxylic acid, and amide (from less to more stable; carboxylate anion is even more stable than amide). The general acyl substitution mechanism begins with a nucleophilic addition. When the nucleophile in nucleophilic acyl substitution is an amine, the product is an amide.