Orgo chem lecture 1-27

Learning Objectives

• Objective 13a: Understand electrophilic aromatic substitution (EAS) for nitration of benzene.

• Objective 13b: Provide arrow pushing mechanism for generating a strong electrophile in EAS.

• Objective 13c: Explain how catalysts are used in EAS reactions.

Electrophilic Aromatic Substitution (EAS)

• EAS is a process where an electrophile replaces a hydrogen atom in an aromatic ring (like benzene).

• Benzene acts as a nucleophile due to its aromatic stability.

  • Aromaticity: Refers to the stability of benzene, making it less reactive.

  • Strong electrophiles are necessary to overcome this stability.

Nitration of Benzene (Learning Objective 13a)

• Starting Materials:

  • Benzene (C6H6) acting as a nucleophile.

  • Nitric Acid (HNO3) is used in the reaction.

  • Catalyst: Sulfuric Acid (H2SO4).

• Process:

  • Electrophile Generation:

    • H2SO4 protonates HNO3 to generate the nitronium ion (NO2+), a strong electrophile.

  • Reaction Mechanism:

    1. Benzene attacks the nitronium ion.

    2. A C-N bond forms; a proton is lost, reforming the aromatic ring.

    3. Byproducts include water and regenerated catalyst H2SO4.

• Overall Reaction:

  • Benzene + HNO3 (with H2SO4) → Nitrobenzene + H2O + H2SO4

Mechanism Details (Learning Objective 13b)

• Draw the mechanism showing:

  • Formation of Electrophile:

    HNO3 + H2SO4 → NO2+ + HSO4-

  • Nucleophilic Attack: Benzene double bond attacks nitronium ion.

  • Deprotonation: Electrons from the C-H bond return to restore aromaticity.

  • Regenerate catalyst: H2SO4 remains unchanged.

Key Characteristics of Electrophiles

• Electrophiles must be strong enough to react with stable aromatic rings.

• Nitration creates nitro groups (NO2) on the benzene ring.

Catalyst Regeneration

• The catalyst (H2SO4) is returned to its original state after the reaction, demonstrating its role in facilitating reaction without being consumed.

Sulfonation (Learning Objective 13b)

• Reagents: H2SO4 and SO3

  • SO3 is a strong electrophile; requires heat to promote reaction.

  • Sulfonation Mechanism:

    1. SO3 reacts with benzene (similar nucleophilic attack).

    2. Regenerate H2SO4 with byproduct formation.

Halogenation (Learning Objective 13c)

• Candidates: Bromine, Chlorine (Iodine requires strong electrophile formulations).

• Reagents: Bromine (Br2) in presence of Lewis acid (FeBr3) or AlCl3 as catalysts.

• Reaction Mechanism:

  1. Generation of bromonium ion (Br+) via catalyst interaction.

  2. Nucleophilic attack on benzene ring.

  3. Products include brominated benzene and HBr, with catalyst regeneration.

Key Takeaways

• EAS involves generating a strong electrophile, a nucleophile attack, and regeneration of catalysts.

• Different electrophiles (NO2+, SO3, Br+) result in different substituents on benzene (NO2, SO3H, Br).

• Always track charge and byproducts/client components in reactions.