Definition

Treatment of an aromatic such as benzene with iodine (I2) and copper (II) leads to the formation of the iodinated benzene by electrophilic aromatic substitution.

Structural reaction scheme for Iodination [I2/catalyst], diagram 1 of 3
Figure 1. Iodination [I2/catalyst]

Explanation

Since iodine is unreactive toward aromatic rings, oxidizing agents such as CuCl2 are used as a catalyst. As well as in any electrophilic aromatic substitution the catalyst is activated first:


And then benzene makes a nucleophilic attack to electrophilic iodine to form sigma complex, or arenium ion, which is resonance stabilized. This cationic intermediate is temporary charged-delocalized which is but with subsequent fast deprotonation regenerates the (now substituted) aromatic ring.

Structural reaction scheme for Iodination [I2/catalyst], diagram 2 of 3
Figure 2. Iodination [I2/catalyst]
Structural reaction scheme for Iodination [I2/catalyst], diagram 3 of 3
Figure 3. Iodination [I2/catalyst]