What is the primary feature of an alkene?

An alkene is characterized primarily by the presence of a double bond between carbon atoms. This double bond is a defining feature that differentiates alkenes from other types of hydrocarbons. The general formula for alkenes is CnH2n, indicating that for every n number of carbon atoms, there are twice as many hydrogen atoms minus two.

The double bond in alkenes not only influences their reactivity—allowing for various chemical reactions such as addition reactions—but also contributes to their geometric isomerism, which is not possible in alkanes that contain only single bonds. Alkenes can exhibit cis and trans isomers due to the restricted rotation around the double bond. This structural attribute is key to understanding many physical and chemical properties of alkenes.

The other features mentioned—such as triple bonds or only single bonds—relate to different classes of hydrocarbons. For example, hydrocarbons with triple bonds are classified as alkynes, while those with only single bonds are alkanes. Ring structures can be present in cyclic alkenes, but they are not a defining feature of alkenes overall. Thus, the presence of a double bond is indeed the primary feature that characterizes alkenes.