Halogenation:

The meaning of stable aromatic ring is that aromatic compounds are less reactive than alkenes to electrophiles. For instance, an alkene will react with Br₂ while an aromatic ring will not. Hence, we have to activate the aromatic ring (that is make it a better nucleophile) or activate the Br₂ (that is make it a better electrophile) if we want a reaction to take place. We will describe how electron-donating substituents on an aromatic ring raise the nucleophilicity of the aromatic ring. We shall observe how a Br₂ molecule can be activated to make it a better electrophile. This can be completed through adding a Lewis acid like FeCl₃, FeBr₃, or AlCl₃ to the reaction medium. These compounds all consist of a central atom (iron or aluminum) that is strongly electrophilic and does not have a full valence shell of electrons. The result of it is, the central atom can accept a lone pair of electrons,  even  from  a  weakly  nucleophilic  atom  like  a  halogen.  In the instance displayed in figure bromine uses a lone pair of electrons to create a bond to the Fe atom in FeBr₃ and turns into positively charged. Bromine is now activated to behave like an electrophile and will react more simply with a nucleophile (the aromatic ring) through the normal mechanism for electrophilic substitution.

An aromatic ring can be chlorinated in an identical fashion, by using Cl₂   in the existence of FeCl₃.

Figure: Mechanism by which a Lewis acid activates bromine towards electrophilic substitution.