18.8: Electrophilic Aromatic Substitution: Overview
In an electrophilic aromatic substitution reaction, an electrophile substitutes for a hydrogen of an aromatic compound.
Many functional groups can be added to aromatic compounds by these reactions. All electrophilic aromatic substitution reactions occur via a two-step mechanism. In the first step, the π system of the aromatic ring reacts with an electrophile, forming an arenium ion, which is resonance-stabilized. It is often referred to as a sigma complex because the electrophile forms a sigma bond with the aromatic ring.
In the second step, deprotonation of the arenium ion restores aromaticity and gives the substituted product.
The free energy diagram shows that the first step is relatively slow and endergonic because the ring loses its aromatic stability. This step is therefore the rate-determining step because of its higher free energy of activation. The second step is fast and exergonic because it restores the stability enhancing aromaticity. It has a lower free energy of activation. The overall electrophilic aromatic substitutions are exergonic reactions.