Aromatic Reactions: Halogenation

Benzene molecules react with halogens (such as Br2 or Cl2), catalyzed by aluminum halides (AlBr3 or AlCl3), to halogenate benzenes. In this reactions, AlBr3 (or AlCl3) helps catalyze the reaction to add a halogen group to the benzene molecule. The location of the addition depends on what functional group(s) is present on the benzene molecule prior to the reaction starting. If the specific position is occupied by another functional group, the reaction will not occur as there is not and available location for the addition to happen:

Halogenation Electron Withdrawing Group (EWG)

Halogenation Electron Donating Group (EDG)

Mechanism

The reaction mechanism for is depicted below:

In the first step, the halogen (Br2 in this example) reacts with the Lewis Acid (AlBr3 in this example), connecting the Br2 to the AlBr3 molecule.

In the second step, the pi electrons from the benzene double bond attack the positively charged bromine atom, breaking the bromine-bromine bond forming AlBr4.

In the third step, the AlBr4 pulls the hydrogen atom off the benzene molecule, sending the electrons from the hydrogen bond to the benzene ring, re-establishing aromaticity.

This reaction can also occur with iron(III) halide (ferric bromide, FeBr3, or ferric chloride, FeCl3).

Make sure you learn which groups are considered electron withdrawing and electron donating as professors like to trip students up on this topic. Additionally, check for open positions on the benzene ring; if the para position had been occupied in the mechanism example above, the reaction could proceeded on either ortho position.

Practice this reaction using our Reaction Solver!