Aromaticity arises when six π-electrons delocalise around benzene’s planar ring, lowering its energy. Addition reactions disrupt this aromatic cloud, costing large stabilisation energy, so benzene usually chooses substitution instead.
Recall: substitution swaps a hydrogen yet preserves the stable π ring you just reviewed.
An electrophile is an electron-deficient species that accepts a π-electron pair from benzene during electrophilic aromatic substitution.
Lewis acids such as AlCl₃ or concentrated H₂SO₄ generate a stronger E⁺, powerful enough to attack the stable benzene ring.
Concentrated \( \mathrm{H_2SO_4} \) protonates \( \mathrm{HNO_3} \). This acid–base step forms \( \mathrm{H_2NO_3^+} \). Loss of water then yields the nitronium ion \( \mathrm{NO_2^+} \), ready for benzene nitration.
Follow each curved arrow to see mixed acids create the electrophile you must recognize—NO₂⁺.
The electrophile \( \text{NO}_2^+ \) grabs one π electron pair, giving a non-aromatic σ-complex. Resonance shifts the positive charge across three ring carbons, restoring partial stability. Sketch these three arenium ion forms to master their delocalization.
Goal: locate and label all three positively charged carbons.
A base, typically the bisulfate ion, removes the adjacent H⁺. The freed electron pair flows into the ring, regaining aromaticity and completing substitution to nitrobenzene.
Loss of H⁺ is the final move that lets the ring regain aromatic stability.
Halogenation, sulfonation, and Friedel-Crafts reactions follow the identical four-step electrophilic aromatic substitution script. First, the catalyst converts the reagent into a powerful electrophile. Then benzene attacks, the σ-complex resonates, and H⁺ departs, restoring aromaticity. Your task: pair each catalyst–reagent set with the electrophile it produces.
Match them: Br₂/FeBr₃, SO₃/H₂SO₄, RCl or RCOCl/AlCl₃ — which E⁺ forms?
Choose the correct sequence of electrophilic aromatic substitution (EAS) steps.
A) Electrophile generation → Sigma complex formation → Deprotonation
B) Sigma complex formation → Electrophile generation → Deprotonation
C) Electrophile generation → Deprotonation → Sigma complex formation
D) Deprotonation → Sigma complex formation → Electrophile generation
Tip: The sigma complex appears only after the ring donates π-electrons to the electrophile.