Here’s the mechanism of electrophilic aromatic substitution (EAS) explained without using any equations:
Step 1: Formation of the Electrophile
- The reaction begins with the creation of a strong positively charged species called an electrophile.
- This electrophile is necessary because it can attack the electron-rich aromatic ring.
Step 2: Attack on the Aromatic Ring
- The aromatic ring has a cloud of electrons that is very attractive to the electrophile.
- The ring temporarily donates electrons to the electrophile, forming a new bond.
- This results in a positively charged intermediate called a sigma complex or arenium ion.
- During this step, the aromatic stability is temporarily lost, which makes this the slowest and most important step of the reaction.
Step 3: Restoration of Aromaticity
- To regain stability, the intermediate loses a hydrogen atom from the carbon that bonded to the electrophile.
- This step restores the aromatic character of the ring.
- The final result is the aromatic compound with the new group attached.
Key Points
- Aromatic rings undergo substitution rather than addition to maintain their stability.
- The attack of the electrophile is the rate-determining step.
- The type of groups already on the ring can influence how fast the reaction occurs and where the new group attaches.