The notation "C=O" represents a carbonyl group, which consists of a carbon atom double-bonded to an oxygen atom. This functional group is a key feature in many organic compounds, including aldehydes, ketones, and carboxylic acids. The carbonyl group plays a crucial role in determining the chemical properties and reactivity of these compounds. In organic chemistry, the presence of a carbonyl group often influences the behavior of molecules in reactions. For example, carbonyl compounds can undergo nucleophilic addition, where other molecules attack the carbon atom. This reactivity makes carbonyl groups important in the synthesis of various pharmaceuticals and biomolecules.