Electrophilic Aromatic Substitution (EAS)

Introduction to EAS

Concept: Interaction of pi bonds with electrophiles, specific to aromatic systems.
Difference from Addition Reactions:
- Aromatic systems prioritize maintaining aromaticity for stability.
- Instead of an addition product, EAS results in a substitution where an electrophile replaces a hydrogen atom.

Example: Benzene interacting with an electrophile.
Steps:
1. A pi bond in benzene interacts with an electrophile, leaving a carbocation (arenium ion intermediate).
2. Arenium ion has three resonance structures with delocalized positive charge and pi electron density.
3. Difference from Addition:
  - In EAS, the molecule (A-) extracts a proton, restoring aromaticity.
  - Forms a new pi bond, maintaining the aromatic ring's stability.
Thermodynamics:
- Rate-determining step: Breaking aromaticity to form the arenium ion is endothermic and energetically unfavorable.
- Restoring aromaticity by extracting a proton is energetically favorable.

Example: Bromination of benzene.
Catalyst Requirement:
- Needed to lower activation energy as benzene does not readily interact with bromine alone.
- Catalyst Used: Lewis acid catalyst (iron tribromide).
  - Iron acts as an electron acceptor (Lewis acid).
  - Forms a catalytic complex with bromine, allowing interaction with benzene.
Mechanism:
1. Pi bond in benzene interacts with bromine in the catalytic complex.
2. Forms arenium ion intermediate.
3. Bromide (Br-) extracts a proton, restores aromaticity, and forms HBr byproduct.
4. Catalyst is regenerated, maintaining its role in the reaction.
Note on Chlorination: Mechanism identical to bromination, using chlorine and iron trichloride.

Generalized Mechanism Recap: Reflects the details in specific reactions like halogenation.
Importance of Catalyst: Essential for facilitating the reaction by lowering energy barriers.

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