Electrophilic Aromatic Substitution (EAS) reactions involve the substitution of hydrogen on an aromatic ring with an electrophile.
Bromination is a specific type of EAS where bromine (Br2) is the electrophile used.
Comparison with Alkene Additions
Alkenes: Electrophilic addition reactions involve the addition of reagents like HCl to alkenes. The electrophilic hydrogen ion attaches to one carbon of the double bond, forming a carbocation intermediate.
Aromatic Rings: Less reactive than alkenes, requiring a catalyst for reactions like bromination.
Example: Br2 reacts instantly with alkenes but needs a catalyst (FeBr3) to react with benzene.
Mechanism of Bromination
Catalyst Activation:
FeBr3 catalyst polarizes Br2, forming a species that acts like Br+ (electrophile).
This species reacts with the benzene ring to form a nonaroamtic carbocation intermediate.
Formation of Carbocation Intermediate:
The intermediate is resonance-stabilized with three resonance structures.
Despite resonance stabilization, it is less stable than the original benzene ring.
Substitution vs. Addition:
Instead of the bromine adding to form a new bond, the carbocation loses a proton (H+), allowing the aromatic ring to reform.
The result is bromobenzene, where Br substitutes for H.
Energetics
Reaction with benzene is slower compared to alkenes due to the stability of the aromatic ring.
EAS reactions are endergonic with significant activation energy due to the intermediate formation.
Loss of aromatic stability in an addition would make the reaction endergonic overall, but substitution preserves aromatic stability, making it favorable.
Energy Diagram
The energy diagram shows the process as exergonic when proceeding through substitution.
Aromatic substitution retains the ring's stability, whereas addition would not.
Reaction Summary
Overall, bromination results in the substitution of H+ by Br+ in the aromatic ring.
The catalyst, FeBr3, is regenerated at the end of the reaction.
Practical Application
Understanding the mechanism of bromination and other EAS reactions is crucial for predicting product outcomes in organic synthesis.
Problem Example
Problem 16-1: Monobromination of toluene results in a mixture of three bromotoluene products. Identify and name them.
Conclusion
Electrophilic bromination of benzene involves key steps of catalyst activation and formation of a stable aromatic product through substitution, retaining aromatic stability.