What is the primary reagent used for the esterification of alcohols?

The primary reagent used for the esterification of alcohols is a carboxylic acid. In this reaction, a carboxylic acid reacts with an alcohol to form an ester and water. This process is known as a condensation reaction because it involves the elimination of a small molecule—in this case, water—when the ester bond is formed.

The significance of the carboxylic acid as the primary reagent lies in its functional group, which contains the carbonyl (C=O) and hydroxyl (–OH) groups. This structure facilitates the nucleophilic attack by the alcohol's hydroxyl group on the carbonyl carbon of the carboxylic acid, leading to the creation of the ester linkage.

While alcohol plays a critical role in esterification as one of the reactants, it does not act as the primary reagent; rather, it is paired with the carboxylic acid in the reaction. Other options like ethyl chloride and alkenes do not participate in the typical esterification process, making the carboxylic acid the unequivocal primary reagent needed to synthesize esters from alcohols.