Hydrogenation of Alkenes
Addition of X2 to Alkenes
One might assume that the reaction mechanism of the addition of halogens to alkenes is very similar to that of the addition of hydrogen. However, the mechanisms are completely different as shown by the stereochemistry of the addition products. Hydrogenation is a syn addition, whereas halogenation is an anti addition. Additionally, catalysts are required for hydrogenations, while halogenations with chlorine or bromine, for example, occur spontaneously.