# Why is Br_2 an electrophile?

Because the $B r - B r$ bond is polarizable.
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 $I - B r$ 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 $I - B r$, which alkyl halides would you predict you would get?