This reaction works only for chlorine and bromine and is carried in the presence of a Lewis acid such as FeX3 (laboratory method). The role of the Lewis acid is to polarize the halogen-halogen bond, making the halogen molecule more electrophilic. Industrially, this is done by treating the aromatic compound with X2 in the presence of iron metal. When the halogen is pumped into the reaction vessel, it reacts with iron, generating FeX3 in catalytic amounts. The reaction mechanism can be represented as follows: