Here’s a explanation of the effect of resonance in aromatic reactions:
What Resonance Is
- Resonance in aromatic compounds is the delocalization of π-electrons over all the carbon atoms of the benzene ring.
- Instead of electrons being fixed in double or single bonds, they are spread evenly across the ring, giving extra stability.
Effect of Resonance on Reactions
- Stabilizes the Benzene Ring
- Resonance makes benzene less reactive toward addition reactions because adding something would break the stable electron delocalization.
- That’s why benzene prefers substitution reactions, which preserve aromaticity.
- Stabilizes Reaction Intermediates
- During electrophilic substitution, the intermediate sigma complex (arenium ion) is stabilized by resonance.
- The positive charge spreads over several carbon atoms, reducing instability and making the reaction possible.
- Determines the Position of Substitution
- Groups already attached to the ring can donate or withdraw electrons through resonance, affecting where new substituents attach (ortho, meta, or para positions).
- Electron-donating groups stabilize the intermediate more at ortho/para positions.
- Electron-withdrawing groups stabilize the intermediate more at meta positions.
Summary
- Resonance increases stability of the aromatic ring.
- It controls the type of reaction (favoring substitution over addition).
- It affects the rate and position of electrophilic aromatic substitution.