Final answer:
Bromine (Br₂) reacts with alkenes through halogenation, which results in the anti-addition of bromine atoms across the double bond, leading to a dibrominated alkane.
Step-by-step explanation:
When bromine (Br₂) is added to an alkene, it reacts via a mechanism known as the halogenation reaction. This process adds bromine across the carbon-carbon double bond (C=C) of the alkene, resulting in a dibrominated alkane where each carbon of the former double bond now bears a bromine atom.
The halogenation process is typically anti-addition, meaning that the bromine atoms attach to opposite sides of the former double bond. This is in accordance with Markovnikov's rule, which predicts that in the addition of HX (a hydrogen halide) to an asymmetric alkene, the hydrogen (H) attaches to the carbon with the greater number of hydrogen atoms, and the halide (X) attaches to the carbon with fewer hydrogen atoms.
The reaction between an alkene and Br₂ is not only important for producing disubstituted alkyl halides but also serves as a test for the presence of alkenes. The reaction causes the disappearance of the brownish-red color of bromine, indicating the presence of an alkene.