Question
Why are carboxylic acids more acidic than alcohols? Compare their pKa values and explain the role of resonance in stabilizing the carboxylate ion.
(CBSE 2024 Board Exam — also a favourite concept in JEE Main organic chemistry)
Solution — Step by Step
Both alcohols and carboxylic acids donate a proton from an –OH group. The question is: what happens to the negative charge after the proton leaves?
When an alcohol loses H⁺, we get an alkoxide ion (R–O⁻). The negative charge sits on a single oxygen atom — nowhere to go, nothing to stabilise it. The ion is unhappy, and an unhappy conjugate base means a weak acid.
Typical pKa of ethanol: ~16. That’s barely acidic.
When a carboxylic acid loses H⁺, we get the carboxylate ion (R–COO⁻). Here the negative charge is delocalised across both oxygen atoms through resonance:
The true structure is a hybrid — both C–O bonds are equivalent (bond length ~136 pm, between single and double bond). The charge is spread over two electronegative oxygens instead of one.
A more stable conjugate base means the acid ionises more readily — the equilibrium shifts right. This is the core logic: stability of conjugate base ∝ strength of acid.
Carboxylate ion (resonance stabilised) > Alkoxide ion (no resonance) → Carboxylic acid >> Alcohol in acidity.
Typical pKa of acetic acid: ~4.75. That’s a difference of more than 11 pKa units — carboxylic acids are roughly 10¹¹ times more acidic than alcohols.
| Compound | Conjugate Base | pKa |
|---|---|---|
| Ethanol (C₂H₅OH) | Ethoxide (C₂H₅O⁻) | ~16 |
| Acetic acid (CH₃COOH) | Acetate (CH₃COO⁻) | ~4.75 |
| Phenol (C₆H₅OH) | Phenoxide | ~10 |
Lower pKa = stronger acid. Acetic acid wins by a landslide.
Why This Works
The key principle here is that acids are strong when their conjugate bases are stable. Resonance is the most powerful stabilising force in organic chemistry — it distributes charge over multiple atoms, reducing electron-electron repulsion.
The carboxylate ion gets two resonance structures of equal energy. This isn’t just textbook theory — X-ray crystallography confirms that both C–O bonds in acetate are 136 pm (not 120 pm for C=O or 143 pm for C–O), proving the charge is genuinely delocalised.
Alcohols have no such luck. The oxygen in an alkoxide is sp³ hybridised with lone pairs in unhybridised orbitals that can’t overlap with the adjacent C–H bonds for any meaningful resonance. The charge stays put, the ion stays unstable, and the alcohol stays a weak acid.
Alternative Method — Inductive Effect Argument
You can also approach this from the perspective of the carbonyl group’s electron-withdrawing effect, without drawing resonance structures.
The C=O group adjacent to the –OH in a carboxylic acid is strongly electron-withdrawing (–I effect). It pulls electron density away from the O–H bond, weakening it and making proton release easier.
In ethanol, the adjacent –CH₂– group is weakly electron-donating (+I), which actually pushes electron density toward oxygen, making the O–H bond slightly stronger and harder to break.
In board exams and JEE Main short-answer questions, resonance is the expected explanation. Use the inductive effect as a supporting argument or when asked for a “second reason.” Examiners specifically look for the resonance hybrid structure drawn correctly.
Common Mistake
Drawing resonance incorrectly for the carboxylate ion.
Many students draw the two resonance structures with the negative charge on the same oxygen in both. The whole point is that charge moves: in structure 1, O⁻ is on the single-bond oxygen; in structure 2, O⁻ is on the other oxygen (the one that was double-bonded). If both structures look identical, you’ve drawn the same structure twice — not resonance.
Also: never put a full negative charge on carboxylate and also show a C=O double bond in the same structure. In each individual resonance contributor, one oxygen is negative (single bond) and the other has the double bond. The hybrid has partial negative on both.
Quick Recap
The acidity order comes down to one idea: where does the negative charge go after the proton leaves?
- Alcohol → charge stuck on one oxygen → unstable alkoxide → pKa ~16
- Carboxylic acid → charge spread over two oxygens via resonance → stable carboxylate → pKa ~5
Carboxylic acids are ~10¹¹ times more acidic than alcohols because their conjugate bases are resonance-stabilised.