Which functional group is characteristic of carboxylic acids?

Carboxylic acids are defined by the presence of the carboxyl group, which is represented as -COOH. This functional group consists of both a carbonyl group (C=O) and a hydroxyl group (-OH) attached to the same carbon atom. This unique structure gives carboxylic acids their distinct acidic properties. When dissolved in water, carboxylic acids can donate a proton (H+), leading to their classification as acids. The combination of the carbonyl and hydroxyl groups in the carboxyl group is essential for the characteristic behavior of these compounds, including their reactivity and solubility in water.

The other options refer to functional groups that are not specific to carboxylic acids. For instance, while the carbonyl group is present in many organic compounds, including aldehydes and ketones, it does not define carboxylic acids alone. The hydroxyl group is characteristic of alcohols and phenols. The alkyl group represents a saturated carbon chain and does not indicate the acidity or the defining features of carboxylic acids. Thus, the correct identification of the carboxyl group as the functional group characteristic of carboxylic acids is key to understanding their