Question

Why is `Br_2` an electrophile?

Answer 1

Because the `Br-Br` bond is polarizable.

Explanation:

The halogen molecules have a large, polarizable electron cloud that (so we assume) can be distorted. `""^(delta+)X-X^(delta-)` representations are very common, and these explain the reaction of the halogens with electron rich olefin species.

Interhalogen species, say `I-Br` can also undergo electrophilic substitution. The iodine end of the molecule is more polarizable. The bromine is more electronegative. With an unsymmetrical alkene, say propylene, that reacts with 1 equiv `I-Br`, which alkyl halides would you predict you would get?