Alkenes are a class of unsaturated hydrocarbons characterized by at least one double bond. The presence of the double bond increases the reactivity of these compounds, if compared to alkanes, allowing for an increased number of reactions.

Alkanes are classified as acyclic and cyclic. If the compound is formed by a chain of carbons with double bonds, then it is considered acyclic. If instead the compound has a cyclic structure made of carbons with at least a double bond in it, it is considered cyclic.

One peculiar property of alkenes is the formation of geometric isomers. As the double bond doesn't allow any rotation, the substituents can be placed above or below it's plane. If both substituents are on the same side of the plane, the isomer is called cis. If one substituent is above and the other below the plane, it is called trans.