Question

Why is `Br_2` an electrophile?

Answer 1

Well, we don't have `Br^+` to start..........

Explanation:

We do have `Br_2`, and with suitable catalysis electron density is conceived to be polarized to give..........`stackrel(+delta)Br-stackrel(-delta)Br`.

Now here, electron density is denuded from one of the bromine atoms in the molecules. And it is precisely this end of the molecule with which the electron-rich, nucleophilic aryl reacts:

`C_6H_6 + stackrel(+delta)Br-stackrel(-delta)Brrarr [C_6H_6Br]^+ +Br^-`

`[C_6H_6Br]^+ + Br^(-) rarr C_6H_5Br + HBr`

Because a bromine electrophile has substituted an aryl-bound proton, this is precisely why we refer to this process as electrophilic aromatic substitution.