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.
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.
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:
Benzene attacks the nitronium ion.
A C-N bond forms; a proton is lost, reforming the aromatic ring.
Byproducts include water and regenerated catalyst H2SO4.
Overall Reaction:
Benzene + HNO3 (with H2SO4) → Nitrobenzene + H2O + H2SO4
Draw the mechanism showing:
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.
Electrophiles must be strong enough to react with stable aromatic rings.
Nitration creates nitro groups (NO2) on the benzene ring.
The catalyst (H2SO4) is returned to its original state after the reaction, demonstrating its role in facilitating reaction without being consumed.
Reagents: H2SO4 and SO3
SO3 is a strong electrophile; requires heat to promote reaction.
Sulfonation Mechanism:
SO3 reacts with benzene (similar nucleophilic attack).
Regenerate H2SO4 with byproduct formation.
Candidates: Bromine, Chlorine (Iodine requires strong electrophile formulations).
Reagents: Bromine (Br2) in presence of Lewis acid (FeBr3) or AlCl3 as catalysts.
Reaction Mechanism:
Generation of bromonium ion (Br+) via catalyst interaction.
Nucleophilic attack on benzene ring.
Products include brominated benzene and HBr, with catalyst regeneration.
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.