Why do simple alkenes undergo addition reaction, instead of substitution reaction?

Question

Isaac Aronson · Accepted Answer

Because alkenes are a more unstable structure than alkanes, and it takes more energy to break off bonds that are already there than to simply form new ones.

Alkenes are unsaturated molecules because at least one double bond exists between the carbon atoms, preventing them from having all the hydrogen that would otherwise be present in them. Although this structure is stable, it is not the most stable, so addition reactions occur when new substances or elements, such as fluorine, are added. When something is added to without anything being lost or altered, it's called an addition reaction. For instance, #C_2H_4 + F_2 -> C_2H_4F_2# As you can see, the mathematical addition to create 1,1-difluoroethane appears to be fairly straightforward. If you visualize the Lewis structures, you can see how they actually react. The double bond in the middle opens up like a gate to let the fluorine in, leaving behind an alkane-like structure made up of single bonds. They do not act as a substitute because breaking existing bonds takes more energy than just creating new ones.

Skylar Alexander · Answer

undefined Simple alkenes undergo addition reactions because the π bond in alkenes allows the incoming reactant to add across the double bond, forming new sigma bonds and breaking the π bond. Substitution reactions would involve replacing one atom or group with another, which is less favorable due to the stability of the π bond in alkenes.