Questions: Directing Effects in Electrophilic Aromatic Substitution

Question 1

Nitrobenzene (C₆H₅NO₂) undergoes nitration with HNO₃/H₂SO₄. Where does the second nitro group attach predominantly, and why?

• A. Ortho, because the existing nitro group activates adjacent positions through induction
• B. Para, because para attack minimizes steric interactions between the two nitro groups
• C. Meta, because the σ-complex for ortho/para attack places positive charge adjacent to the electron-withdrawing nitro group, maximally destabilizing those intermediates
• D. Randomly distributed, because the existing substituent has minimal electronic effect at this distance

Answer: (C)

NO₂ is an electron-withdrawing group. When the electrophile attacks at ortho or para, one resonance structure of the σ-complex places positive charge on the carbon bearing the NO₂ group — an already electron-deficient carbon — maximally destabilizing that intermediate. Meta attack avoids placing positive charge on the substituted carbon, making meta the least-destabilized (not most-stabilized) pathway. EWGs do not activate the ring; they deactivate it and direct to meta.

Question 2

Aniline (C₆H₅NH₂) is treated with an electrophile. Which prediction is correct?

• A. Reaction is slow and produces mainly meta product, because nitrogen withdraws electrons inductively from the ring
• B. Reaction is fast and produces mainly ortho/para product, because the NH₂ lone pair stabilizes positive charge on the ring carbon bearing N via resonance in the σ-complex
• C. Reaction is fast but produces mainly meta product, because lone pairs are too tightly held by nitrogen to participate in ring stabilization
• D. Reaction rate equals unsubstituted benzene, because nitrogen's inductive withdrawal exactly cancels its resonance donation

Answer: (B)

NH₂ is an electron-donating group (EDG): its lone pair donates into the ring through resonance. When the electrophile attacks ortho or para, a resonance structure of the σ-complex places positive charge on the carbon bearing N — and the nitrogen lone pair directly stabilizes this charge. This lowers the activation energy for ortho/para attack, making aniline much more reactive than benzene and predominantly ortho/para-substituted. The inductive withdrawal is real but is outweighed by the stronger resonance donation.

Question 3

Chlorobenzene reacts more slowly than benzene in EAS AND directs incoming electrophiles predominantly to the meta position.

• T. True
• F. False

Answer: False

Only the first part is true: chlorine deactivates the ring inductively (pulls electron density via electronegativity), so chlorobenzene reacts more slowly than benzene. However, chlorine is an ortho/para director. In the σ-complex for ortho or para attack, a resonance structure places positive charge on the carbon bearing Cl — and chlorine's lone pairs can donate into the ring to stabilize this charge, even though chlorine also withdraws inductively. This resonance donation overrides the inductive effect for regiochemistry, making Cl an ortho/para director despite being deactivating.

Question 4

The regiochemical outcome of EAS — which position the electrophile attacks — is determined by the relative stability of the σ-complex intermediates at each position, not by the thermodynamic stability of the final substituted products.

• T. True
• F. False

Answer: True

EAS follows Hammond's postulate for endothermic steps: the transition state resembles the intermediate (σ-complex), so whichever intermediate is most stable has the lowest activation energy and forms the major product. The final aromatic product is the same energy regardless of regiochemistry (aromaticity is restored in all cases), so product stability is irrelevant. This is why mechanistic understanding of the σ-complex, not inspection of the product, is the right framework for predicting EAS regiochemistry.

Question 5

Electron-withdrawing groups direct EAS to the meta position. Explain why meta is favored — not because meta is especially stabilized, but because of what happens at the other positions.

Model answer: EWGs direct to meta because ortho and para attack are particularly destabilized, not because meta attack is specially favored. When the electrophile attacks ortho or para, one resonance structure of the σ-complex (arenium ion) places positive charge directly on the carbon bearing the EWG. An electron-withdrawing group cannot stabilize — and actively destabilizes — positive charge at that position. Meta attack never places the positive charge on the substituted carbon, so it avoids the worst-case destabilization. The meta product 'wins' by default: it is the least-bad option.

This framing — meta wins by being least destabilized rather than most stabilized — is the key insight. It explains why EWGs don't simply make all positions equally unfavorable; they specifically penalize ortho/para more than meta. Compare this with EDGs, which specifically stabilize ortho/para through resonance donation, actively favoring those positions.