EAS General Mechanism — Mermaid Process Diagram — Notes

Reaction Pathway — Electrophilic Aromatic Substitution:

Key Points per Step:

Step	Description	Energy Comment
1	Catalyst activates the reagent to form $E^{+}$	Preparatory
2 (RDS)	$E^{+}$ attacks pi cloud → arenium ion	Slow; aromaticity lost; high activation energy
3	$H^{+}$ leaves from $sp^{3}$ carbon → rearomatization	Fast; thermodynamic driving force = regaining aromatic stability

Why substitution, not addition? The ring loses aromaticity upon addition of $E^{+}$ to form the arenium ion. Loss of $H^{+}$ (step 3) is thermodynamically driven by the large resonance energy of benzene (~36 kcal/mol). Addition would give a non-aromatic product (higher energy), so the system eliminates $H^{+}$ to restore aromaticity.

SMILES for arenium ion intermediate (from nitration): [H]C1=CC=CC(=[NH+])C1 — conceptual; arenium has $sp^{3}$ carbon bonded to E and H simultaneously.