Question
Arrange methanol (CH₃OH), ethanol (C₂H₅OH), and phenol (C₆H₅OH) in increasing order of acid strength. Justify your answer.
Solution — Step by Step
Acid strength depends on how easily a compound donates a proton (H⁺) and how stable the resulting anion (conjugate base) is. More stable the anion → stronger the acid. We need to compare the stabilities of methoxide (CH₃O⁻), ethoxide (C₂H₅O⁻), and phenoxide (C₆H₅O⁻) ions.
Both methanol and ethanol lose a proton from their -OH group to form alkoxide ions. The methyl group (-CH₃) is electron-donating by the inductive effect. Ethanol has an ethyl group (-C₂H₅), which is more electron-donating than methyl (more carbon atoms, more electron density pushed toward oxygen).
More electron donation onto O⁻ → O⁻ is less stable (more negative charge density) → harder to remove the proton → weaker acid.
Therefore: methanol is more acidic than ethanol.
Phenol is dramatically more acidic than both alcohols because of resonance stabilisation of the phenoxide ion (C₆H₅O⁻). The negative charge on oxygen is delocalised into the benzene ring through resonance — the lone pair on O⁻ spreads across the ring’s pi system.
This resonance delocalisation makes phenoxide far more stable than any alkoxide. More stable conjugate base = much stronger acid.
Alcohols have no resonance stabilisation for their alkoxide ions.
Increasing order of acidity (weakest to strongest):
Or in terms of values: Ethanol () > Methanol () > Phenol ().
Phenol is about 10⁵ to 10⁶ times more acidic than a simple alcohol — resonance makes that enormous a difference.
Why This Works
Two different electronic effects are at play here:
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Inductive effect explains the methanol vs. ethanol difference — a larger alkyl group donates more electron density, destabilising the alkoxide anion.
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Resonance effect explains the alcohol vs. phenol difference — and resonance is a far stronger effect than induction. When two effects compete, resonance almost always dominates unless there are extraordinary steric or other constraints.
Alternative Method
We can also use values directly. A lower means a stronger acid:
- Ethanol:
- Methanol:
- Phenol:
This confirms the order: Ethanol < Methanol < Phenol in acidity. But for a theoretical question (the type CBSE and JEE actually ask), you need the resonance and inductive effect explanation — just writing values without reasoning won’t earn full marks.
This is one of the highest-frequency questions in CBSE Class 12 Alcohols, Phenols & Ethers. The exact comparison appears in board exams almost every year. Memorise: Phenol is far more acidic than alcohols due to resonance stabilisation of phenoxide ion.
Common Mistake
Students sometimes write the order as Methanol < Ethanol < Phenol, reversing methanol and ethanol. Remember: methanol has a smaller alkyl group (methyl), which donates less electron density than ethyl, so methanol’s conjugate base (methoxide) is slightly more stable → methanol is slightly more acidic than ethanol. Larger alkyl group = weaker acid among alcohols.