SolutionsAromatic Electrophilic Substitution
1. Which is the electrophile?
In the following electrophilic aromatic substitution (SEAr) reactions, indicate which is formally the electrophilic species.
In the fourth reaction the primary propyl carbocation rearranges to the more stable secondary isopropyl cation before attacking the ring: that is why isopropylbenzene, and not propylbenzene, is obtained.
2. Why a catalyst?
What role do FeBr3 in the bromination and H2SO4 in the nitration of benzene play?
Both turn a weakly electrophilic reagent into an electrophile able to attack the π cloud of benzene: FeBr3, a Lewis acid, polarizes Br2 and activates it; H2SO4 protonates HNO3, which loses water and forms the nitronium ion, NO2+.
3. Benzenonium
Draw the resonance forms of the benzenonium ion formed in the chlorination of benzene. At which positions relative to the attacked carbon does the positive charge lie?
There are three resonance forms, with the positive charge at the ortho (two) and para positions relative to the sp3 carbon bearing Cl and H. It never lies at meta.