The most common reaction to prepare an aryl chloride or bromide is by electrophilic aromatic substitution. In this reaction, molecular Cl2 or Br2 is used to replace an aryl hydrogen atom. The reaction usually needs a Lewis acid catalyst. AlCl3 or FeCl3 is used with Cl2. FeBr3 or Fe (which generates FeBr3) is used with Br2. Substituents already present on the aryl ring control which hydrogen(s) are replaced (Section 9.7A).