Why do only unsaturated hydrocarbons undergo addition reactions? Why do only unsaturated hydrocarbons undergo addition reactions? For example, ethene or ethyne will undergo an addition reaction with chlorine, whereas ethane will not. 

Furthermore, why does benzene not undergo addition reactions, even though it is not saturated?
To what would you add given a saturated hydrocarbon? You must substitute, abstract, or displace.

Benzene is aromatic &mdash; different pattern of reactivity.  It photochlorinates.  It UV reacts with maleic anhydride, then adds another mole thermally.  Palladium catalysis will swap appropriate substituents.