Question

Electrophilic halogenation of phenol does not require catalyst because

• A. Phenol is electron rich
• B. Oxygen in phenol is more reactive
• C. Phenol is electron deficient as oxygen is the second is the second most electronegative element
• D. Phenol is a planar molecule

Answer 1

The Correct Option is A

To understand why electrophilic halogenation of phenol does not require a catalyst, we need to examine the electronic structure and reactivity of phenol.

1. Understanding Phenol's Structure:
Phenol consists of a hydroxyl group (-OH) attached to a benzene ring. The oxygen atom in the hydroxyl group has lone pairs that can interact with the aromatic ring.

2. Electron Density in Phenol:
The oxygen's lone pairs participate in resonance with the benzene ring, making the ring electron-rich. This occurs through two effects:
- Resonance effect: Oxygen's lone pairs delocalize into the ring
- Inductive effect: The oxygen withdraws some electron density, but the resonance effect dominates

3. Reactivity Towards Electrophiles:
The increased electron density makes the ortho and para positions highly reactive towards electrophiles like halogens. This explains why:
- No catalyst is needed for halogenation
- The reaction occurs readily at room temperature

4. Why Other Options Are Incorrect:
- While oxygen is electronegative, phenol is not electron-deficient overall
- The planarity of phenol facilitates resonance but doesn't directly explain the reactivity
- Oxygen's reactivity alone doesn't account for the aromatic ring's behavior

Final Answer:
Electrophilic halogenation of phenol does not require a catalyst because phenol is electron-rich due to resonance donation from the hydroxyl group into the aromatic ring.