The nucleophilic addition to the carboxyl carbon of the carboxylic acids is followed by elimination of the leaving group leading to a substitution product. Since the substitution takes place at the acyl carbon atom, it is also known as nucleophilic acyl substitution.
X = OH in case of the carboxylic acids.
Derivatives of Carboxylic Acids
The less reactive acid (acyl) derivatives can be prepared from the more reactive ones. The order of reactivity of various carboxylic acid derivatives is as follows:
Acid chloride > Acid anhydride > Ester > Amide
Acid chlorides are the most reactive ones whereas the amides are the least reactive.