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Aromatic Nitration - Mechanism

Aromatic Nitration - Mechanism. The combination of nitric acid and sulfuric acid produces NO 2 + (nitronium ion), a powerful electrophile. The reaction with benzene produces nitrobenzene. E +. Two Steps. Nu:. cationic intermediate – draw resonance structures. Aromatic Sulfonation.

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Aromatic Nitration - Mechanism

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  1. Aromatic Nitration - Mechanism The combination of nitric acid and sulfuric acid produces NO2+ (nitronium ion), a powerful electrophile. The reaction with benzene produces nitrobenzene. E+ Two Steps Nu: cationic intermediate – draw resonance structures

  2. Aromatic Sulfonation Add a sulfonic acid group (-SO3H) to the ring using sulfuric acid and SO3. With –CF3, the new group is added meta. What is the electrophile?

  3. Aromatic Sulfonation • Reaction with a mixture of sulfuric acid and SO3 gas (“fuming sulfuric acid”). • Electrophile is sulfur trioxide (SO3) or its conjugate acid. • Reaction occurs via cationic intermediate and is reversible.

  4. Review: EAS Reaction Conditions Halogenation (Cl2 or Br2): Benzene Nitration: Aromatic Sulfonation:

  5. Effect of Substituents But what if the benzene ring has a substituent? Q1 Where does the new group go? Q2 Is the reaction faster or slower than benzene? x

  6. Substituent Effects in Aromatic Rings Substituents can make a ring more reactive or less reactive: “activators” vs. “deactivators” Substituents also control orientation of the reaction:“ortho/para directors” vs. “meta directors” least reactive most reactive

  7. Substituent Effects Q. How do different substituents influence the rate of electrophilic aromatic substitution reactions? activate the ring = faster reaction deactivate ring = slower reaction Q. How do different substituents effect orientation of the substitution reaction? (ortho/para vs. meta substitution) Answer: Inductive EffectsResonance Effects

  8. Inductive vs. Resonance Effects Inductive EffectResonance Effect Based on: electronegativity conjugation Electrons Move: via sigma bonds pi bonds/lone pairs Distance: short-range may be long range Overall Effect: electron cloud pi bonds and is shifted lone pairs move by electronegativity by resonance Example:

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