Question

The compound that is most difficult to protonate is -

Answer 1

Phenol is most difficult to protonate

Explanation:

Protonaton takes place on oxygen atom due to lone pair of electrons

• In given compounds protonation takes place in $H_2O$ easily as lone pair of electrons are available for proton to attack easily.
• protonation takes place more easily in $CH_3OH, CH_3OCH_3$ due to $+I$ effect of methyl groups which increases electron density on oxygen and  thus makes proton to attack more easily.
• In the last compound, phenol, electron displacement takes place in benzene ring due to resonance. And thus it makes some partial positive charge on oxygen. Hence attacking of proton is most difficult.
• Therefore The compound that is most difficult to protonate is phenol.  

Answer 2

Answer:

answer 4 is correct because phenol is resonance stabilized