Why are alkenes classified as unsaturated hydrocarbons?

Alkenes are classified as unsaturated hydrocarbons primarily because they contain one or more double bonds between carbon atoms. This double bond means that alkenes have fewer hydrogen atoms than saturated hydrocarbons, like alkanes, which contain only single bonds. The presence of a double bond makes it possible for alkenes to react with more hydrogen atoms during hydrogenation, which is a characteristic of unsaturated compounds.

This property distinguishes alkenes from alkanes, as alkanes have no double or triple bonds and are thus fully saturated with hydrogen atoms. The concept of saturation in organic chemistry refers to the maximum number of hydrogen atoms that can bond to the carbon skeleton, and the existence of double bonds in alkenes indicates that they can bond with additional atoms, confirming their classification as unsaturated hydrocarbons.