Question

Step 2 of the iodination of benzene shows water acting as a base and removing a proton from the sigma complex. We did not consider the possibility of water acting as a nucleophile and attacking the carbocation, as in an electrophilic addition to an alkene. Draw the reaction that would occur if water reacted as a nucleophile and added to the carbocation. Explain why this type of addition is rarely observed.

Short answer

The reaction occurs when water is reacted as a nucleophile.

Step-by-step solution

Iodination of benzene

The iodination of benzene is an electrophilic aromatic substitution that occurs in two steps. The iodine acts as a nucleophile and attacks the benzene ring.Reaction with water gives the additional product.

Reaction when water acts as a nucleophile

In the second step of the iodination of benzene, water acts as a base and removes a proton from the sigma complex. The product formed by adding water to the sigma complex is not aromatic. It losts 151 kJ/mol of resonance-stabilization energy, and this addition reaction is not energetically favorable. So, the substitution prevails.

Reaction if water is reacted as a nucleophile