Question

Why phenol does not undergo protonation readily? 

Hint:

The resonance of the lone pair on the oxygen in phenol with the benzene ring decreases its availability to accept a proton, making protonation less likely.

Answer 1

Phenol does not undergo protonation readily because the lone pair of electrons on the oxygen of the hydroxyl group is delocalized into the benzene ring through resonance. This resonance effect plays a crucial role in making phenol less basic compared to alcohols.

1. Resonance Effect:

In phenol, the lone pair of electrons on the oxygen atom of the hydroxyl group (\( -OH \)) is delocalized into the benzene ring. This means that the oxygen atom's electron density is partially transferred into the aromatic system, creating resonance structures like: \[ \text{C}_6\text{H}_5\text{O} \rightleftharpoons \text{C}_6\text{H}_5\text{O}^- \] The delocalization of the lone pair reduces the electron density on the oxygen atom, making it less available to accept a proton (\( \text{H}^+ \)).

2. Basicity Comparison:

Due to this delocalization, the oxygen in phenol is less basic than the oxygen in alcohols, where the lone pair on the oxygen atom is not involved in resonance with the benzene ring. In alcohols, the lone pair on the oxygen is more available to accept a proton, making alcohols more basic than phenol.

3. Protonation of Phenol:

The lower electron density on the oxygen in phenol makes it less likely to attract a proton (\( \text{H}^+ \)) and undergo protonation. In contrast, alcohols, where the lone pair on the oxygen is more readily available, can easily undergo protonation.

Conclusion:

Phenol does not undergo protonation readily because the electron density on the oxygen is reduced due to the resonance with the benzene ring, making the oxygen less basic than in alcohols.