Question

Explain why phenol is more acidic than ethyl alcohol.

Hint:

The phenoxide ion is resonance stabilised over the ring, while ethoxide keeps the charge on one oxygen and is destabilised by the +I ethyl group.

Answer 1

Step 1 (Acidity depends on anion stability): An -OH compound is more acidic when the anion left after losing H⁺ is more stable. So we compare the phenoxide ion (from phenol) with the ethoxide ion (from ethanol).
Step 2 (Phenoxide is resonance stabilised): In the phenoxide ion the oxygen lone pair and its negative charge are delocalised into the benzene ring through resonance, spreading the charge over the ortho and para carbons. This delocalisation strongly stabilises the phenoxide ion.
Step 3 (Ethoxide is not stabilised): In the ethoxide ion the negative charge stays localised on the single oxygen atom, with no ring to disperse it. Moreover the ethyl group is electron releasing (+I effect), which pushes electron density towards oxygen and destabilises the anion.
Step 4 (Bond polarity): In phenol the oxygen is attached to an sp² carbon of the ring, which is more electronegative and pulls electron density, making the O-H bond more polar and easier to break.
Conclusion: Because the phenoxide ion is much more stable (resonance) than the ethoxide ion, phenol releases H⁺ more readily and is therefore more acidic than ethyl alcohol.