Which statement best defines an unsaturated hydrocarbon?

• A. Hydrocarbon with one or more double or triple bonds
• B. Hydrocarbon with only single bonds
• C. Hydrocarbon with no carbon atoms
• D. Aromatic ring contains only single bonds

Answer: (A)

An unsaturated hydrocarbon is one that contains one or more carbon–carbon double or triple bonds. Those multiple bonds mean fewer hydrogens are attached to the carbon skeleton compared with a saturated hydrocarbon that has only single bonds. This makes unsaturated hydrocarbons reactive in addition reactions, where atoms can add across the double or triple bond. The other statements don’t fit: a hydrocarbon with only single bonds is saturated; a hydrocarbon with no carbon atoms isn’t a hydrocarbon; and an aromatic ring isn’t simply single bonds—benzene-like rings have delocalized pi bonds and are considered unsaturated as well.