What is the term for the chemically active part of a member of a series of organic molecules?

The term for the chemically active part of a series of organic molecules is known as a functional group. Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Each functional group imparts particular properties to the organic compounds they are part of, influencing everything from polarity and solubility to reactivity and the overall behavior of the molecule in chemical reactions.

For example, a hydroxyl group (-OH) is a specific type of functional group that determines properties associated with alcohols, such as their solubility in water and their ability to participate in hydrogen bonding. The term radical group does not specifically refer to chemically active sites, while an alkyl group pertains to saturated hydrocarbon chains and does not inherently define reactivity. Hence, functional group is the most encompassing term that accurately describes the chemically reactive part of organic molecules.