Hydroxy

Phenols

The more stable the conjugate base, the more acidic the compound

Qualitative

Why is the phenoxide ion relatively stable?

The p-orbital of the oxygen atom overlaps with the \(pi\)-orbitals of the benzene ring to form a delocalized cloud. The negative charge on the oxygen atom of the phenoxide ion can thus be delocalized around the ring (p-\(pi\) interactions). This stabilizes the phenoxide ion and reduces the tendency of the phenoxide atom to attract protons. The phenoxide ion is said to be resonance stabilized. Thus, the equilibrium position (show equation) is comparatively more to the right, indicating that phenol is relatively stronger acid.

Why is ethoxide (conjugate base of ethanol) comparatively less stable?

The oxygen atom of the OH group is attached to an electron-donating alkyl group which increases the electron density on the oxygen atom in the ethoxide ion. This destabilizes the ion and makes it more ready to accept protons. Thus the equilibrium position (show equation) lies comparatively less to the right.

Effects of substituents on acid strength of phenol

Presence of electron withdrawing (donating) groups in the ring increases (decreases) the acid strength of phenol. The electron withdrawing (donating) group further disperses (intensifies) the electron density on the oxygen atom, thus further stabilizing (destabilizing) the phenoxide ion. Equilibrium position shifts comparatively more (less) to the right, thus XYZ is a stronger acid than phenol.