Question

Which of the following compounds could be protonated without destroying its aromaticity?

Short answer

Pyridine's lone-pair electrons are sp²electrons. They, therefore, are not part of the π cloud (only π electrons are in the π cloud), so they can be protonated without destroying the aromaticity of the pyridine ring.

Step-by-step solution

How we determine this

Pyridine's lone-pair electrons are sp²electrons. They, therefore, are not part of the π cloud (only π electrons are in the π cloud), so they can be protonated without destroying the aromaticity of the pyridine ring.
We know that pyrrole's lone-pair electrons are π electrons because they can be used to form a π bond in a resonance contributor. Because pyrrole's lone-pair electrons are one of the three pairs of π electrons in the π cloud, pyrrole's aromaticity is destroyed when it is protonated and, therefore, is no longer part of the π cloud.

Final answer

Pyridine's lone-pair electrons are sp²electrons. They, therefore, are not part of the π cloud (only π electrons are in the π cloud), so they can be protonated without destroying the aromaticity of the pyridine ring.