Question

Why is phenoxide ion a stronger acid than alkoxide ion?

Answer 1

The phenoxide ion is a WEAKER base than alkoxide ion, because the phenoxide ion is resonance stabilized, and requires less solvation.

Explanation:

The `pK_a` of phenol, `C_6H_5OH` `=` `9.95` in water. This makes phenol a MUCH stronger acid than comparable secondary alcohols, the which, if soluble in water, would undergo no acid base behaviour.

The charge density of the phenoxide anion is much less than the comparable aliphatic alkoxide, because the charge is delocalized around the ring to some extent. On the other hand, the charge on alkoxide is localized to the oxygen centre. Phenoxide, thus causes less solvent ORDER upon deprotonation, and its acidity is entropically favoured. This is thus an entropy rather than a enthalpy phenomenon.