Aryl bromides and chlorides can be prepared from arenes by electrophilic substitution. This reaction is carried out by treating the arene with chlorine or bromine in the presence of iron (III) chloride in the absence of light. Iron (III) chloride, that is, FeCl₃ is a Lewis acid, which generates the electrophile required to take the reaction forward. FeCl₃ forms a coordination compound with Cl₂, making the complex Cl⁺[FeCl₄^-]. The chloride ion gains a partial positive charge, acting as an electrophile, and attacking the ϖ bonds. The final products are o-arylhalide and p-arylhalide.