Chloroarenes (aryl chlorides) and bromoarenes (aryl bromides) can be easily prepared by direct chlorination or bromination of benzene or other aromatic hydrocarbons. These reactions are carried out in dark, at ordinary temperatures (310–320 K) in the presence of Lewis acid catalyst such as anhydrous ferric or aluminium halides (FeCl₃, FeBr₃, AlCl₃). These reactions are called electrophilic substitution of arenes. The Lewis acid acts as a catalyst and its function is to carry the halogen to the aromatic hydrocarbon and is also called halogen carrier. In actual practice, iron filings in the presence of chlorine or bromine are commonly used. The halogens react with iron filings to form corresponding Lewis acids.

Which of the following cannot be produced by direct halogenation of benzene?

Iodobenzene

The correct answer is option 3. Iodobenzene.

Iodobenzene cannot be obtained by direct iodination because the reaction is reversible and HI is a reductant. However, if strong oxidants like HNO₃ or HgO are used, the reaction is possible because they oxidize HI into I₂.

The reaction for the direct iodination of benzene is as follows:

This reaction is reversible, meaning that the iodobenzene can react with HI to form benzene. Additionally, HI is a reductant, meaning that it can be reduced to hydrogen gas. This means that if iodobenzene is formed, it can be easily reduced back to benzene.

However, if strong oxidants like HNO₃ or HgO are used, they can oxidize HI into I₂. This makes the reaction irreversible and prevents the iodobenzene from being reduced back to benzene.

Therefore, the only way to obtain iodobenzene by direct iodination is to use strong oxidants.