Question
Arrange the following in order of decreasing acidity: phenol, p-nitrophenol, p-cresol, p-methoxyphenol, ethanol. Justify each ranking using electronic effects.
Solution — Step by Step
Acidity in O–H compounds depends on stability of the conjugate base (the alkoxide/phenoxide ion). Stable conjugate base ⇒ stronger acid.
Two effects matter:
- Resonance stabilization — phenoxides spread negative charge over the ring; alkoxides cannot.
- Substituent effects — electron-withdrawing groups (EWGs) stabilize the negative charge (more acidic); electron-donating groups (EDGs) destabilize it (less acidic).
Phenol (pKa ≈ 10) is much more acidic than ethanol (pKa ≈ 16) because the phenoxide ion is resonance-stabilized — the negative charge delocalizes onto three ortho/para ring positions.
- p-nitrophenol (pKa ≈ 7.2): The –NO group is a strong EWG (resonance + induction), pulling electron density away. Phenoxide is highly stabilized. Most acidic.
- Phenol (pKa ≈ 10): No substituent. Baseline.
- p-cresol (4-methylphenol, pKa ≈ 10.3): –CH is a weak EDG (hyperconjugation/induction). Phenoxide slightly destabilized. Slightly less acidic than phenol.
- p-methoxyphenol (pKa ≈ 10.2): –OCH is an EDG by resonance (lone pair on O donates into ring). Destabilizes phenoxide. Less acidic than phenol — but methoxy is also weakly EWG by induction, partially offsetting. Net effect: slightly less acidic than phenol.
From most acidic to least acidic:
Why This Works
Resonance is the dominant effect for the phenol/ethanol distinction — six orders of magnitude in comes from one extra resonance structure of phenoxide. Substituent effects on phenol are smaller perturbations (1–3 pKa units typically), but they let us rank within the phenol family.
The –NO group’s effect is dramatic because it can stabilize negative charge by both resonance (drawing it into a quinonoid form) and induction (the partial positive on N). Other EWGs (–CN, –CHO) work similarly.
Three reflexes for ranking phenol/alcohol acidity:
- Phenol > alcohol — almost always (resonance dominates).
- p-NO > p-COOH > p-X (halogen) > p-CH > p-OCH — the standard order.
- Ortho effects can override — ortho-nitrophenol is sometimes more acidic than p-nitrophenol due to internal H-bonding.
Alternative Method
Draw resonance structures of each phenoxide. Count the number of viable resonance forms and assess charge delocalization. p-Nitrophenoxide has an extra resonance form where negative charge sits on the nitro oxygen — heavily stabilized. Ethanol’s alkoxide has only the localized form on oxygen.
Students rank “alcohol > phenol” because they remember “more carbons = more electron donation.” Wrong direction — alcohols don’t have ring resonance, so they are always less acidic than phenols.
Final answer: p-nitrophenol > phenol > p-cresol ≈ p-methoxyphenol > ethanol.