Step 1 — Electrophile generation:

• Nitration: HNO3 + H2SO4 → NO2+ + HSO4- + H2O
• Halogenation: Cl2 + AlCl3 → Cl+ + AlCl4-
• Friedel-Crafts: RCl + AlCl3 → R+ + AlCl4-; RCOCl + AlCl3 → RCO+ + AlCl4-
• Sulfonation: SO3 (electrophilic S) from fuming H2SO4

Step 2 — Sigma complex formation (rate-determining): E+ attacks ring → cyclohexadienyl cation $\frac{arenium ion}{Wheland intermediate}$. Positive charge delocalized over 3 positions (ortho, para to attack site). Ring is NO LONGER aromatic at this stage.

Step 3 — Proton elimination: Base (often AlCl4-) removes H+ from the sp3 carbon, restoring aromaticity. This step is fast.

Why substitution over addition: Restoring aromaticity provides ~150 kJ/mol stabilization energy. Addition would permanently destroy aromaticity.