Here we will try to understand 'why Iodination of Benzene is difficult?' by looking at the reaction mechanism, nature of the reactants and also try come up with few steps to overcome the challenge.
For a faster electrophilic aromatic substitution reaction, any one of the reagent Benzene or the halogen in the starting material has to be slightly more reactive than the other. For satisfying this condition, electron donating groups attached to the phenyl ring making it more nucleophilic are preferred over unsubstituted Benzene. Also, the electrophilicity of the halogen is increased by using a Lewis acid catalyst thereby making it more reactive. These changes in turn help to achieve transition state faster and stabilize it better.
For the halogens, the electronegativity and electrophilicity decrease from F to I in the periodic table. Fluorine is most electrophilic, and Iodine is least. Therefore, Fluorination is highly reactive, and Iodination is highly unreactive for electrophilic aromatic substitution reactions.
The exothermic rates of aromatic halogenation also decrease from Fluorine to Iodine. Fluorination reaction being highly exothermic and explosive, the reaction cannot be controlled resulting in polyfluorinated products. For Iodination, the reaction is endothermic with 12kJ/mol of energy absorbed. Therefore, it cannot be done using the conventional method using Lewis acid catalyst and requires strong oxidizing agents.
This is because, I2 adds to the Benzene reversibly generating HI. HI being a strong reducing agent regenerates I2 from aryl iodide giving back the aromatic hydrocarbon. However, in the presence of oxidizing agents such as HIO3, HI is converted back to Iodine thereby increasing the concentration of Iodine in the reaction mixture. According to Le-Chatelier’s principle, if the concentration of one of the reagents is increased then the equilibrium shifts in the forward direction to give aryl iodide as the desired product.
Another way to obtain aryl iodide is to remove HI as soon as it is formed in the reaction mixture, by forming salts. For example, when the Mercuric oxide is used, it converts Hydrogen Iodide to Mercuric Iodide that is then discarded.
As we know that for faster reactions, the nucleophilicity of the ring should be increased by adding electron donating groups or the electrophilicity of the electrophile that is halogen should be increased but unsubstituted Benzene is weakly nucleophilic. Also the Iodine is non-reactive and non-polar but its electrophilicity can be increased by forming a positive ion. These ions are more reactive than the complex of halogen with Lewis acid catalyst and react instantaneously with Benzene to give aryl iodide.
Oxidizing agents, when used, oxidize I2 to iodonium ion I+. The earliest reported reaction was using HNO3 or H2SO4 in the presence of Acetic acid and Iodine to give iodonium ion and was called Tronov-Novikov reaction.
Iodine can be easily oxidized using, nitric acid. HNO3 in the presence of an acid and Iodine generates Iodine cation that reacts with Benzene giving Iodobenzene, Nitrogen dioxide, and water. As HNO3 is consumed in the reaction, hence it is a reagent and not a catalyst.
A similar mechanism is seen for Cupric Chloride and Hydrogen peroxide for generation of Iodine cation.
Few examples of other oxidizing agents are Oleum, Cupric chloride, Silver sulfate, Sodium periodate, Periodic acid, Sodium hypochlorite, Iodic acid (HIO3), Nitric acid and Sulfuric acid, Sulfur trioxide, and Hydrogen peroxide. Sometimes Lewis acid catalyst is used along with the oxidizing agents.
The best reagent for Iodination is Iodine monochloride ICl, an interhalogen compound. Chlorine is more electronegative and pulls the electron cloud towards itself giving Iodine a (delta) positive charge. The Benzene ring can pick up this quickly than Iodine from the non-polar I2 bond thereby giving aryl iodide as the desired product.
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