Which is NOT a characteristic of carboxylic acids?

Carboxylic acids are defined by the presence of a functional group that consists of both a carbonyl group (C=O) and a hydroxyl group (–OH) bonded to the same carbon atom, which gives rise to their unique properties.

The characteristic of containing a hydroxyl group is fundamental to carboxylic acids because this functional group, when combined with the carbonyl group, defines them structurally. Likewise, carboxylic acids indeed always have a carbonyl group, as this is integral to their identity.

In terms of hydrogen bonding, carboxylic acids are particularly capable of this due to their hydroxyl group. The ability to form hydrogen bonds significantly influences their boiling points and solubility in water.

The assertion that carboxylic acids always have a nitrogen atom attached is incorrect because nitrogen is not a requisite component of these compounds. Carboxylic acids can exist without any nitrogen atoms present. Thus, this lack of requirement for nitrogen is what distinguishes option C as the correct answer, as it does not align with the defining features of carboxylic acids.