What is the main characteristic of ketones?

Ketones are characterized by the presence of a carbonyl group (C=O) that is located within the carbon chain, rather than at the end. This specific arrangement distinguishes ketones from other carbonyl-containing compounds, such as aldehydes, where the carbonyl group is located at the terminal end of the carbon chain. The positioning of the carbonyl group is crucial because it influences the chemical properties and reactivity of ketones, making them significant in various biochemical processes and organic reactions.

The other options describe functionalities that are not representative of ketones. A hydroxyl group refers to alcohols, while the presence of nitrogen groups is typical of amines or amides. Therefore, understanding the structural attribute of ketones is essential in identifying them among other organic compounds.