Electrophilic aromatic substitutions are reactions in which an electrophile replaces one of the aromatic hydrogens.
These reactions allow the introduction of different functional groups onto aromatic rings.
In the first step of the reaction mechanism, the π system of the aromatic ring attacks the electrophile to form an arenium ion, which is resonance stabilized.
The arenium ion is also called a sigma complex because the electrophile forms a sigma bond with the aromatic ring.
In the second step, the arenium ion is deprotonated, restoring aromaticity and giving the substituted product.
As evident from the free energy diagram, the first step is endergonic because the ring loses its aromatic stability. This step has a higher free energy of activation and is slow. It is, therefore, the rate-determining step.
In contrast, the second step is exergonic because it restores aromatic stability to the system. It has lower free energy of activation and is fast.
Overall, electrophilic aromatic substitutions are exergonic reactions.
