The nucleophilic substitution mechanism describes how a nucleophile replaces a leaving group in a molecule. This process plays a crucial role in the reactivity of carboxylic acids.

In nucleophilic substitution, a nucleophile attacks an electrophilic carbon atom, which results in the departure of the leaving group. Typically, the nucleophile is either a negatively charged ion or a molecule that possesses a lone pair of electrons. The leaving group is commonly a halogen or a sulfonate group.

Carboxylic acids are particularly reactive in nucleophilic substitution reactions due to the presence of the carbonyl group. The carbonyl carbon is electrophilic, making it susceptible to attack by nucleophiles. Moreover, in reactions involving carboxylic acids, the leaving group is usually a hydroxyl group, which is an effective leaving group because it can stabilize the negative charge.

A prominent example of nucleophilic substitution in carboxylic acids is the reaction with amines to form amides. In this reaction, the amine acts as the nucleophile, targeting the carbonyl carbon of the carboxylic acid. The hydroxyl group then departs as a water molecule, resulting in the formation of an amide bond.

In summary, nucleophilic substitution is a vital mechanism in the reactivity of carboxylic acids, facilitating the formation of a diverse array of functional groups.