Why is Phenol More Acidic than Ethanol?
Phenol (C₆H₅OH) is considerably more acidic than ethanol (C₂H₅OH). The main reason is the greater stability of the conjugate base formed after loss of H⁺.
Ethanol → H⁺ + Ethoxide ion
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More stable conjugate base
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Phenol is more acidic
Phenol
Phenol loses H⁺ to form the phenoxide ion.
Ethanol
Ethanol loses H⁺ to form the ethoxide ion.
Phenoxide Ion
In the phenoxide ion, the negative charge initially resides on oxygen. However, the lone pair/negative charge can be delocalised into the benzene ring through resonance.
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Negative charge delocalisation
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Ortho and para positions of the ring
Thus, the negative charge is not confined completely to oxygen. It is delocalised over the oxygen atom and the aromatic ring.
Ethoxide Ion
In ethoxide ion, the negative charge remains essentially localised on the oxygen atom. There is no benzene π-system available for resonance delocalisation.
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Negative charge mainly on O
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No resonance stabilisation
Therefore, ethoxide ion is less stable than phenoxide ion.
The ethyl group in ethanol has a +I (electron-releasing) effect.
Electron-releasing effect
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Negative charge becomes less stable
The electron-releasing alkyl group increases electron density around the oxygen atom and destabilises the ethoxide ion.
This further decreases the acidity of ethanol.
| Property | Phenol | Ethanol |
|---|---|---|
| Formula | C₆H₅OH | C₂H₅OH |
| Conjugate base | Phenoxide ion | Ethoxide ion |
| Resonance stabilisation | Present | Absent |
| Negative charge | Delocalised | Mainly localised on O |
| Conjugate-base stability | Higher | Lower |
| Acidity | Higher | Lower |
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Stability of Conjugate Base
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More Stable Conjugate Base
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Stronger Acid
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Phenol more acidic than Ethanol
Q. Why is phenol more acidic than ethanol?
Answer:
Phenol forms a resonance-stabilised phenoxide ion after loss of H⁺. The negative charge in phenoxide ion is delocalised over the oxygen atom and the aromatic ring.
Ethoxide ion does not have resonance stabilisation. Hence phenol is more acidic than ethanol.
Q. Explain the role of resonance in the greater acidity of phenol.
Answer:
- Phenol loses H⁺ to form the phenoxide ion.
- The negative charge of phenoxide ion is delocalised through resonance into the benzene ring.
- This delocalisation stabilises phenoxide ion and therefore makes phenol more acidic than ethanol.
Q. Compare the acidity of phenol and ethanol on the basis of stability of their conjugate bases.
Answer:
Phenol ionises as:
The phenoxide ion is resonance-stabilised because the negative charge can be delocalised over the aromatic ring.
Ethanol ionises as:
The ethoxide ion has no resonance stabilisation and the ethyl group also shows a +I effect, which destabilises the negative charge.
Therefore:
Phenol > Ethanol in acidity
Q. Explain with resonance why phenol is more acidic than ethanol.
Step 1 – Ionisation of Phenol
Step 2 – Resonance Stabilisation
The negative charge of the phenoxide ion is delocalised into the benzene ring through resonance. The resonance structures involve mainly the ortho and para positions.
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Negative charge delocalisation
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Resonance stabilisation
Step 3 – Ethanol
Ethoxide ion has no resonance stabilisation. Moreover, the ethyl group has a +I effect and destabilises the negative charge.
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Greater tendency of phenol to lose H⁺
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Phenol is more acidic than ethanol.
Q. Explain in detail why phenol is more acidic than ethanol using resonance stabilisation and inductive effects.
1. Ionisation of Phenol
Phenol produces the phenoxide ion after losing a proton.
2. Resonance Stabilisation
In phenoxide ion, the negative charge is delocalised from oxygen into the aromatic π-system. This produces several important resonance contributors involving the ortho and para positions.
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Delocalisation into benzene ring
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Resonance-stabilised phenoxide ion
3. Ionisation of Ethanol
The ethoxide ion has no aromatic π-system and therefore cannot delocalise its negative charge by resonance.
4. +I Effect of Ethyl Group
The ethyl group is electron-releasing. Its +I effect increases the electron density on the oxygen atom of ethoxide ion and makes the negative charge less stable.
5. Final Conclusion
Ethoxide ion → No resonance stabilisation + +I destabilisation
Phenoxide ion is more stable
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Phenol is more acidic than ethanol
1. Which is more acidic?
✅ Answer
B) Phenol
2. The conjugate base of phenol is:
✅ Answer
B) Phenoxide ion
3. Phenol is more acidic mainly because:
✅ Answer
A
4. The negative charge in phenoxide ion is:
✅ Answer
B) Delocalised
5. Ethoxide ion is less stable because:
✅ Answer
B
6. Resonance in phenoxide ion involves:
✅ Answer
A
7. The more stable conjugate base corresponds to:
✅ Answer
B
8. Which ion is more stable?
✅ Answer
B) Phenoxide
9. The ethyl group exhibits:
✅ Answer
A) +I effect
10. The +I effect of ethyl group:
✅ Answer
B
11. Phenol ionises to give:
✅ Answer
A
12. Ethanol ionises to give:
✅ Answer
B
13. Resonance stabilisation generally makes an ion:
✅ Answer
B
14. Phenoxide ion is stabilised by delocalisation of:
✅ Answer
B
15. Which statement is correct?
✅ Answer
B
16. The acidity of phenol is related directly to the stability of:
✅ Answer
A
17. Which ion lacks resonance stabilisation?
✅ Answer
B) Ethoxide
18. Phenol is more acidic than ethanol because:
✅ Answer
A
19. Greater conjugate-base stability means:
✅ Answer
A
20. Correct order of acidity is:
✅ Answer
B) Phenol > Ethanol
🎯 Quick Revision
Ethanol: C₂H₅OH
Phenol conjugate base: Phenoxide ion
Ethanol conjugate base: Ethoxide ion
Phenoxide: Resonance stabilised
Ethoxide: No resonance stabilisation
Ethyl group: +I effect
Final order: Phenol > Ethanol in acidity
