Ethene, the simplest alkene, was once called ethylene. It is a plant hormone that ripens fruit. It is also the raw material for polyethylene, the most common plastic in the world. Alkenes contain at least one carbon-carbon double bond. That double bond changes everything. It makes the molecule more reactive and more rigid. The general formula is CnH2n. Ethene is C2H4, propene is C3H6, butene is C4H8.

The double bond consists of a sigma bond and a pi bond. The pi bond is weaker and more exposed, which makes it vulnerable to attack by electrophiles. That is why alkenes undergo addition reactions, where atoms add across the double bond, converting it to a single bond. Hydrogenation adds hydrogen, turning an alkene into an alkane. Bromination adds bromine, decolorizing bromine water, which is a classic test for alkenes. Polymerization links many alkene units into a long chain, forming plastics.

Alkenes are not just industrial chemicals. They occur in nature. They are found in plant oils, insect pheromones, and some fragrances. They are also intermediates in many metabolic pathways. The double bond is a versatile functional group.

Alkenes can be linear, branched, or cyclic. They can have one double bond or several. They can be cis or trans, depending on the arrangement of groups around the double bond. That geometric isomerism affects physical properties and biological activity.