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 exhibits limited protonation due to the delocalization of the hydroxyl group's oxygen lone pair into the benzene ring via resonance. This resonance effect significantly reduces phenol's basicity relative to alcohols.

1. Resonance Effect:

In phenol, the lone pair on the hydroxyl oxygen (\( -OH \)) is delocalized into the aromatic system. This electron density transfer to the benzene ring results in resonance structures such as: \[ \text{C}_6\text{H}_5\text{O} \rightleftharpoons \text{C}_6\text{H}_5\text{O}^- \] The consequence of this delocalization is a diminished electron density on the oxygen atom, thereby decreasing its affinity for accepting a proton (\( \text{H}^+ \)).

2. Basicity Comparison:

Consequently, the oxygen in phenol is less basic than that in alcohols. In alcohols, the oxygen's lone pair is not delocalized into an aromatic ring and remains more accessible for proton acceptance, rendering alcohols more basic than phenol.

3. Protonation of Phenol:

The reduced electron density on phenol's oxygen atom makes it less prone to protonation. Conversely, alcohols, with their more available oxygen lone pairs, readily undergo protonation.

Conclusion:

Phenol resists protonation due to resonance-induced electron withdrawal from the oxygen into the benzene ring, which lowers its basicity compared to alcohols.