Bromination of benzene is a classic example of electrophilic aromatic substitution. This reaction showcases how aromatic compounds react with electrophiles, replacing a hydrogen atom with a bromine atom while maintaining the ring's aromaticity.
The mechanism involves a Lewis acid catalyst, forming a bromonium ion that attacks the benzene ring. This creates a resonance-stabilized carbocation intermediate before regenerating the aromatic system. Understanding this process is key to grasping aromatic reactivity and stability.
Electrophilic Aromatic Substitution: Bromination
Mechanism of benzene bromination
- Electrophilic aromatic substitution (EAS) reaction where an electrophile replaces a hydrogen on an aromatic ring
- Bromination of benzene specific example of EAS (halogenation)
- Bromine ($Br_2$) acts as the electrophile
- Lewis acid catalyst ($FeBr_3$) required for reaction to proceed
- Mechanism steps:
- $Br_2$ reacts with $FeBr_3$ forming an electrophilic bromonium ion ($Br^+$) and $FeBr_4^-$
- $Br^+$ electrophilically attacks benzene forming a resonance-stabilized carbocation intermediate (arenium ion)
- $FeBr_4^-$ removes a proton from the arenium ion regenerating the aromatic system and producing $HBr$ and $FeBr_3$
- The catalyst ($FeBr_3$) regenerated allowing the reaction to continue
- The electrophilic attack on the benzene ring is the rate-determining step of the reaction
Alkenes vs aromatic electrophilic reactions
- Electrophilic addition to alkenes:
- Alkenes more reactive than aromatic compounds due to higher electron density and lack of resonance stabilization
- Reaction typically exothermic with lower activation energy
- Proceeds through a carbocation intermediate not stabilized by resonance
- Electrophilic aromatic substitution:
- Aromatic compounds less reactive than alkenes due to resonance stabilization
- Reaction typically endothermic with higher activation energy
- Proceeds through a resonance-stabilized arenium ion intermediate
- Requires a strong electrophile and often a catalyst to overcome the aromatic stabilization energy
Substitution products in aromatic systems
- Aromatic compounds highly stable due to cyclic, planar structure and continuous overlapping p-orbitals
- Allows for delocalization of electrons resulting in resonance stabilization
- In EAS, aromatic system temporarily disrupted during formation of arenium ion intermediate
- Resonance stabilization of arenium ion lowers energy of this intermediate
- Final step of EAS involves restoration of aromaticity through loss of a proton
- Energetically favored step as it regenerates the stable aromatic system
- Addition products would result in permanent loss of aromaticity and resonance stabilization
- Energetically unfavorable compared to substitution product which maintains aromatic character of the compound
Substituent effects on EAS reactions
- Substituents on the aromatic ring can affect the reaction in various ways:
- Activating groups increase the reactivity of the ring towards electrophilic attack
- Deactivating groups decrease the reactivity of the ring towards electrophilic attack
- Substituents can also influence regioselectivity, directing the incoming electrophile to specific positions on the ring
- These effects are important in predicting the outcome and reaction kinetics of EAS reactions