Here’s the explanation of the role of leaving groups in nucleophilic aromatic substitution (SNAr):
1. Quality of Leaving Group
- A good leaving group makes it easier for the nucleophile to replace it.
- Typical leaving groups: F > Cl > Br > I (unusual order compared to aliphatic substitution).
2. Why is Fluorine the Best Here?
- In SNAr, the key step is nucleophilic attack forming the Meisenheimer complex.
- Fluorine strongly withdraws electrons (highly electronegative), making the ring carbon more positive and easier to attack.
- That’s why Ar–F reacts fastest, even though F⁻ is usually a poor leaving group in other reactions.
3. Position Relative to Electron-Withdrawing Groups
- Leaving group must be ortho or para to strong EWGs (like –NO₂).
- This allows the negative charge in the intermediate to be stabilized, helping the leaving group go away.
In short:
- The leaving group controls how easily substitution happens.
- In SNAr: F > Cl > Br > I because fluorine activates the ring the most.
- Leaving group must be near EWGs to make the reaction fast.