Which chemical bond type must be present for a hydrocarbon to be classified as an alkene?

For a hydrocarbon to be classified as an alkene, it must contain a double bond between carbon atoms. Alkenes are defined by the presence of at least one carbon-carbon double bond, which gives them unique properties and reactivity compared to other types of hydrocarbons. This double bond allows alkenes to undergo addition reactions, where various atoms or groups can attach to the carbon atoms involved in the double bond.

In contrast, hydrocarbons that contain only single bonds are classified as alkanes. Triple bonds, while they exist in alkynes, do not apply to alkenes. Cyclic bonds refer to connections in ring structures, which do not specifically define alkenes either. Thus, the identifying feature of alkenes is indeed the double bond, which is essential for distinguishing them from other hydrocarbon types.