Reactivity of alkenes:
Alkenes are more reactive than alkanes.
Alkenes have a double bond which consists of a strong sigma (δ) bond (bond energy = 83 Kcal/mol) and a weak pi (π )-bond (bond energy = 63 Kcal/mol). The density of π-electrons is high above and below the bond axis.
In other words π-electrons are away from the nuclei of the carbon atoms so hold of nuclei is less on them. Thus they are exposed to electrophilic attack. So an electrophile can easily attack and can break pi-bond by using less energy and therefore alkenes are more reactive.
Alkenes being unsaturated compounds, mostly shows addition reactions. Beside addition reactions they also show oxidation reactions. An oxidizing agent can remove their loosely bound π-electrons.
The reactivity order of aliphatic hydrocarbons is
Alkenes > Alkynes > Alkanes
It means that alkenes are most reactive in this series. Alkynes also have π-bonds but their π-bonds are comparatively stronger than that of alkenes due to shorter distance from carbons nuclei.