What functional group defines carboxylic acids?

Carboxylic acids are specifically defined by the presence of the carboxyl group, which is denoted as -COOH. This functional group consists of a carbon atom that is double-bonded to an oxygen atom (carbonyl) and also single-bonded to a hydroxyl group (-OH). The unique structure of the carboxyl group imparts distinct properties to carboxylic acids, including their acidic behavior, which is the result of the ability to donate a proton (H+) from the hydroxyl portion of the group.

The carboxyl group is integral to the classification of these compounds as carboxylic acids, distinguishing them from other functional groups, such as aldehydes, hydroxyl groups, or amino groups. Each of those functional groups imparts different chemical properties and reactivities to the compounds they characterize, but none can define carboxylic acids as effectively as the carboxyl group does. This makes the identification of the carboxyl group crucial in organic chemistry, particularly when discussing the reactivity and classification of organic compounds.