What is the functional group of alkenes?

• A. C=C
• B. C-C
• C. C≡C
• D. C-O

Answer: (A)

The key idea is that the functional group is the part of a molecule that drives its characteristic reactions. For alkenes, that feature is the carbon–carbon double bond, written as C=C. This double bond marks the molecule as unsaturated and is the site where reactions like addition occur, giving products by breaking the bond and adding other atoms across it. This single feature distinguishes alkenes from alkanes, which only have carbon–carbon single bonds (C–C), and from alkynes, which have triple bonds (C≡C). Other functional groups involve elements like oxygen (for example, C–O found in alcohols and ethers), which are not present as the defining feature of simple alkenes. So the C=C double bond is the defining functional group of alkenes.