Phenol is more acidic than ethanol but less acidic than p-nitrophenol. Explain, with resonance structures where needed.

1. 1 · State the fact. Acid strength: p-nitrophenol (pKa 7.1) > phenol (10.0) > ethanol (15.9); a lower pKa means a stronger acid.
2. 2 · Phenol vs ethanol. In phenol, O is on an sp² carbon, so O-H is more polar. The phenoxide ion is stabilised by resonance; the ethoxide ion's charge stays on O, and the ethyl group's +I effect destabilises it.
3. 3 · Draw the resonance structures. Draw the five resonance structures of the phenoxide ion, with the negative charge on O and on the two ortho carbons and the para carbon.
4. 4 · Effect of -NO₂. At ortho or para, -NO₂ pulls the charge onto itself by −R and −I effects, so the ion is more stable and the acid stronger.
5. 5 · Effect of -CH₃. Electron-releasing -CH₃ destabilises the ion: cresols (pKa about 10.2) are weaker acids than phenol.
6. 6 · Close in one line. So anything that stabilises the anion makes the compound a stronger acid.

Scoring tip: Compare the ions (phenoxide vs ethoxide), not just the molecules; that is the key idea examiners look for.