Carboxylic Acids vs Phenols – Acidity
Both carboxylic acids and phenols contain an acidic hydrogen attached to oxygen. However, carboxylic acids are considerably stronger acids than phenols.
C₆H₅OH ⇌ C₆H₅O⁻ + H⁺
Carboxylic acids >> Phenols
The strength of an acid depends largely on the stability of its conjugate base.
↓
More stable conjugate base
When a carboxylic acid loses H⁺, it forms a carboxylate ion. Its negative charge is stabilised very effectively by resonance.
Phenol forms a phenoxide ion, which is comparatively less stabilised.
The carboxylate ion has two equivalent resonance structures.
↔
R–C(–O⁻)=O
The negative charge is delocalised equally over the two oxygen atoms. Therefore, the carboxylate ion is highly stable.
→ High stability
→ High acidity of carboxylic acid
Phenol loses H⁺ to form the phenoxide ion. The negative charge can be delocalised into the benzene ring.
Although resonance occurs, some resonance structures place the negative charge on carbon atoms of the benzene ring.
These structures are less favourable because oxygen is more electronegative than carbon and stabilises negative charge better.
Carboxylate Ion
↓
Shared between TWO oxygen atoms
Both resonance structures are equivalent and highly stable.
Phenoxide Ion
↓
Delocalised over oxygen and carbon atoms
The resonance structures are not equivalent, and some place negative charge on carbon.
Therefore:
Carboxylic acid > Phenol in acidity
Electron-withdrawing groups increase acidity by stabilising the negative charge of the conjugate base.
↓
Stabilises conjugate base
↓
Acidity increases
Electron-releasing groups such as alkyl groups generally decrease the acidity of carboxylic acids by destabilising the carboxylate ion.
Electron-releasing group → Acidity ↓
| Property | Carboxylic Acid | Phenol |
|---|---|---|
| Acidic group | –COOH | –OH attached to benzene |
| Conjugate base | Carboxylate ion | Phenoxide ion |
| Resonance stabilisation | Very strong | Comparatively less effective |
| Negative charge | Delocalised over two O atoms | Delocalised into ring |
| Conjugate-base stability | Higher | Lower |
| Acidity | Higher | Lower |
The acid strength is inversely related to pKa.
Higher pKa → Weaker acid
Typical approximate values in water are:
Phenol: pKa ≈ 10
Q. Which is more acidic: carboxylic acid or phenol? Give reason.
Answer:
Carboxylic acids are more acidic than phenols because the carboxylate ion formed after loss of H⁺ is highly stabilised by resonance, with the negative charge delocalised over two oxygen atoms.
Q. Compare the acidity of carboxylic acids and phenols.
Answer:
- Carboxylic acids are stronger acids than phenols.
- Carboxylic acids form carboxylate ions in which the negative charge is equally delocalised over two oxygen atoms.
- The phenoxide ion is less effectively stabilised because some resonance structures place the negative charge on carbon atoms.
Q. Explain why carboxylic acids are stronger acids than phenols.
Answer:
Acid strength depends on the stability of the conjugate base formed after removal of H⁺.
The carboxylate ion is stabilised by two equivalent resonance structures. The negative charge is distributed over two oxygen atoms.
In contrast, the phenoxide ion has less effective resonance stabilisation because the negative charge is delocalised into the benzene ring and may reside on carbon atoms.
Carboxylic acid > Phenol
Q. Compare the acidity of carboxylic acids and phenols on the basis of resonance and inductive effects.
1. Ionisation
C₆H₅OH ⇌ C₆H₅O⁻ + H⁺
2. Resonance
The carboxylate ion has two equivalent resonance structures, with the negative charge shared between two oxygen atoms.
Phenoxide ion also shows resonance, but some contributing structures place the negative charge on carbon atoms.
3. Conjugate-Base Stability
The carboxylate ion is more stable than the phenoxide ion. Therefore, carboxylic acids lose H⁺ more readily.
4. Inductive Effect
Electron-withdrawing groups increase acidity by stabilising the conjugate base, whereas electron-releasing groups decrease acidity.
Carboxylic acids are much stronger acids than phenols.
Q. Give a detailed comparison of the acidity of carboxylic acids and phenols. Explain the role of resonance and inductive effects.
1. Acid Ionisation
2. Importance of Conjugate Base
A stronger acid forms a more stable conjugate base. Therefore, the relative stability of carboxylate and phenoxide ions determines their acidity.
3. Carboxylate Ion
The carboxylate ion is stabilised by resonance. The negative charge is equally distributed between two oxygen atoms.
↔
R–C(–O⁻)=O
4. Phenoxide Ion
Phenoxide ion is resonance stabilised, but the delocalisation extends into the benzene ring. Some contributing structures place negative charge on carbon atoms, which is less favourable than placing it on oxygen.
5. Inductive Effect
Electron-withdrawing groups increase acidity by stabilising the negative charge, while electron-releasing groups decrease acidity.
6. Final Comparison
Therefore:
Carboxylic acids >> Phenols
in acidity.
1. Which is more acidic?
✅ Answer
B) Carboxylic acid
2. The conjugate base of a carboxylic acid is:
✅ Answer
B) Carboxylate ion
3. The conjugate base of phenol is:
✅ Answer
A) Phenoxide ion
4. Acid strength mainly depends on:
✅ Answer
A
5. Carboxylate ion is stabilised by:
✅ Answer
A) Resonance
6. Negative charge in carboxylate ion is delocalised over:
✅ Answer
B) Two oxygen atoms
7. Which conjugate base is more stable?
✅ Answer
B) Carboxylate ion
8. Greater stability of conjugate base means:
✅ Answer
A) Stronger acid
9. Phenoxide ion is less stable mainly because:
✅ Answer
A
10. Electron-withdrawing groups generally:
✅ Answer
A) Increase acidity
11. Electron-releasing groups generally:
✅ Answer
B) Decrease acidity
12. The –COOH group is characteristic of:
✅ Answer
B
13. Phenol contains:
✅ Answer
A
14. Which ion has two equivalent major resonance forms?
✅ Answer
A) Carboxylate ion
15. Approximate pKa of a simple carboxylic acid is:
✅ Answer
A) 4–5
16. Approximate pKa of phenol is:
✅ Answer
C) 10
17. Lower pKa indicates:
✅ Answer
A) Stronger acid
18. Which conjugate base has negative charge mainly stabilised on oxygen atoms?
✅ Answer
A) Carboxylate ion
19. Correct acidity order is:
✅ Answer
B
20. The best explanation for greater acidity of carboxylic acids is:
✅ Answer
A) More stable carboxylate ion
🎯 Quick Revision
Phenol: C₆H₅OH
Carboxylic acid conjugate base: R–COO⁻
Phenol conjugate base: C₆H₅O⁻
Carboxylate ion: Negative charge delocalised over two oxygen atoms
Phenoxide ion: Negative charge delocalised into benzene ring
Conjugate-base stability: Carboxylate > Phenoxide
Acidity: Carboxylic acids >> Phenols
pKa: Carboxylic acids ≈ 4–5; Phenol ≈ 10
