Questions: Electrophilic Aromatic Substitution (EAS)

Question 1

Nitration of aniline (PhNH₂) under mild conditions produces which major product(s)?

• A. Meta-nitroaniline, because the NH₂ group withdraws electrons from the ring
• B. A statistical mixture of ortho, meta, and para products in roughly equal amounts
• C. Predominantly ortho- and para-nitroaniline, because the NH₂ group donates electrons and directs to ortho/para positions
• D. No reaction, because aniline does not undergo electrophilic aromatic substitution

Answer: (C)

The –NH₂ group is a strongly electron-donating group (EDG) via resonance — its lone pair donates into the ring pi system, particularly stabilizing the arenium ion intermediates at ortho and para positions. EDGs activate the ring (faster reaction) and direct to ortho/para. Option A is wrong because –NH₂ is an electron donor, not a withdrawor. Option B is wrong because directing effects strongly favor certain positions.

Question 2

Meta-directing groups direct incoming electrophiles to the meta position because they stabilize the arenium ion intermediate formed at that position.

• T. True
• F. False

Answer: False

This is the key misconception about meta directors. Electron-withdrawing groups (EWGs) do NOT stabilize the meta arenium ion — they destabilize the ortho and para intermediates MORE severely. At ortho or para attack, one resonance structure of the arenium ion places a positive charge directly on the carbon bearing the EWG, compounding the electron deficiency. At meta attack, no such resonance contributor exists. The electrophile ends up at meta by avoidance of the worst-case structures, not by attraction.

Question 3

Explain why halogens (e.g., –Cl) are classified as ortho/para directors but deactivators of the benzene ring. Why does this seem contradictory, and how is it resolved?

Model answer: Halogens exert two opposing electronic effects. Inductively (through sigma bonds), they are strongly electron-withdrawing due to high electronegativity — this removes electron density from the ring, making it less reactive toward electrophiles (deactivation, slower rate). By resonance, lone pairs on the halogen can donate into the ring pi system, specifically stabilizing the ortho and para arenium ion intermediates (directing to ortho/para). The inductive effect is stronger and controls overall reactivity (deactivating); the resonance effect controls regioselectivity (ortho/para directing). These are two separate effects operating through different mechanisms, so they can point in different directions.

This question targets the halogen exception directly. Students often assume activating groups are always ortho/para and deactivating groups are always meta. Halogens break this pattern because inductive and resonance effects operate independently. Understanding this exception requires a mechanistic picture of the arenium ion intermediates.