Final answer:
Molecular halogens react with alkenes through an addition reaction where the weaker pi bond of the alkene is broken and turned into stronger sigma bonds, resulting in haloalkanes.
Step-by-step explanation:
Molecular halogens react with alkenes through an addition reaction where the pi bond (π bond) of the alkene is broken and replaced by two sigma bonds (σ bonds). This particular reaction is possible due to the alkene's reactive functional group, the double bond, which is weaker and thus, more reactive compared to the strong single bonds found in alkanes. Alkenes undergo this transformation by changing the hybridization of the carbon atoms in the double bond from sp² to sp³. This results in the halogens adding across the double bond, forming a dihaloalkane which has the same carbon skeleton as the original alkene. For example, bromine (Br₂) can be added to an alkene to form a dibromoalkane and this reaction can be visually observed as bromine's brownish-red color disappears upon reacting with the alkene.