1. The Role of the Hydroxyl Group:
* Electron-donating nature: The hydroxyl group is a strong electron-donating group. It increases electron density at the ortho and para positions of the benzene ring due to resonance.
* Directing effect: This increased electron density makes the ortho and para positions more susceptible to electrophilic attack.
2. The Nitration Mechanism:
* Electrophile: The nitronium ion (NO2+) acts as the electrophile in the reaction.
* Attack: The nitronium ion attacks the benzene ring at the ortho or para positions, where the electron density is higher.
* Intermediate: A carbocation intermediate is formed, which is stabilized by resonance.
* Deprotonation: A proton is removed from the carbocation, regenerating the aromatic ring and forming the nitrophenol product.
3. Why Not Meta?
The meta position is less electron-rich due to the deactivating effect of the positive charge on the oxygen of the hydroxyl group. This makes the meta position less susceptible to electrophilic attack.
4. Steric Hindrance:
Although the ortho position is favored due to electron density, it can be sterically hindered. The bulky nitro group may experience some steric repulsion from the hydroxyl group. This can contribute to the formation of para-nitrophenol as the major product in some cases.
In summary, the nitration of phenol produces two isomers, ortho and para, due to the directing effect of the hydroxyl group, which favors ortho and para positions for electrophilic attack. The meta position is less reactive due to the deactivating effect of the hydroxyl group.
