Skip to content
qorganica.es qorganica.comOrganic Chemistry made transparent Course map Español

SolutionsAromatic Halogenation

1. The chlorination diagram

Fill in the energy diagram of the chlorination of benzene with Cl2 and AlCl3, indicating whether the species are reactive intermediates (RI) or transition states (TS).

Blank energy diagram of an SEAr: reactants, two transition states, an intermediate and products, with the activation energy Ea and the enthalpy ΔH°
Chlorination diagram: benzene attacks the Cl–Cl–AlCl₃ complex (TS), the benzenonium ion and AlCl₄⁻ form (RI) and, after the second TS, chlorobenzene, HCl and AlCl₃ are obtained; the reaction releases 30 kcal/mol

2. Why no iodination?

Direct iodination of benzene with I2 is not possible even with a catalyst. Why? How is it achieved?

The reaction is endothermic (ΔH° ≈ +11 kcal/mol): the equilibrium lies on the side of the reactants. An oxidant such as HNO3 is used, which turns I2 into I+, a much more powerful electrophile, and removes the HI formed.

3. Fluorine

Why is benzene not fluorinated with F2?

The reaction is so exothermic (ΔH° ≈ −114 kcal/mol) that it is explosive and uncontrollable, and gives mixtures of degradation products.

← Back to the exercises