aromatic halogenation

Bromine water will instantly decolorize when you drop in an alkene — the double bond grabs the bromine eagerly. Drop bromine onto plain benzene, though, and nothing happens; the aromatic ring is too stable and unreactive to bother. To get a halogen onto a benzene ring you have to make the halogen hungrier, and that is exactly what aromatic halogenation does: it installs a chlorine or bromine onto the ring using a helper catalyst.

The trick is a Lewis-acid catalyst, usually a metal halide like FeBr3 (for bromination) or AlCl3 / FeCl3 (for chlorination). The catalyst grips one end of the Br-Br (or Cl-Cl) molecule and pulls electron density away, polarizing it so strongly that the other end becomes an effective Br(+) electrophile — strong enough that even benzene's lazy pi cloud will attack it. From there the standard EAS two-step runs: the ring bonds to the bromine to make an arenium ion, then a base (the FeBr4(-) that formed) removes the proton to restore aromaticity. The products are the halobenzene and HBr, and the catalyst is regenerated.

Aromatic halogenation is the standard way to put a single chlorine or bromine directly onto a ring, giving products like chlorobenzene or bromobenzene that are launching points for many further reactions. Fluorination is too violent and iodination too sluggish to run this way directly (those usually go through diazonium salts instead). Note one subtlety: a halogen already on the ring is a deactivating but ortho/para-directing group, so a second halogenation is slower and lands mostly ortho and para to the first.