Acidic Nature of Phenols - UNSOLVED PRACTICE SET
Chapter: Alcohols Phenols and Ethers | Topic: Acidic Nature of Phenols
ACIDIC NATURE OF PHENOLS - UNSOLVED PRACTICE SET
Topic: Acidic Nature of Phenols
Multiple Choice Questions
Q1. The pKa value of phenol is approximately:
- 4.75
- 10.0
- 15.9
- 16.0
Q2. Phenol is more acidic than ethanol because:
- Phenol has more hydrogen atoms
- The phenoxide ion is stabilized by resonance
- The phenyl group is electron-donating
- Phenol has a higher molecular mass
Q3. Which of the following is the most acidic?
- Phenol
- p-nitrophenol
- p-methylphenol
- p-methoxyphenol
Q4. The acidic strength of substituted phenols increases when:
- Electron-donating groups are present at ortho/para positions
- Electron-withdrawing groups are present at ortho/para positions
- Electron-withdrawing groups are present at meta position only
- No substituents are present
Q5. Phenol reacts with NaOH but not with NaHCO₃ because:
- Phenol is a strong acid
- Phenol is a weak acid (weaker than carbonic acid)
- Phenol is a strong base
- Phenol is insoluble in water
Q6. The acidity of phenol is affected by:
- Inductive effect only
- Resonance effect only
- Both inductive and resonance effects
- Neither inductive nor resonance effects
Short Answer Questions
Q7. Explain why phenol is more acidic than ethanol using resonance structures of phenoxide ion.
Q8. Arrange the following in order of increasing acidic strength: phenol, p-nitrophenol, p-cresol, p-methoxyphenol. Give reason.
Q9. Why does the presence of electron-withdrawing groups (like -NO₂) at ortho or para position increase the acidic strength of phenol?
Q10. Why does the presence of electron-donating groups (like -CH₃) at ortho or para position decrease the acidic strength of phenol?
Q11. Phenol does not react with NaHCO₃ but acetic acid does. Explain this difference in terms of relative acid strengths.
Q12. Your teacher asks you to compare the acidity of phenol and cyclohexanol. Which is more acidic and why?
Long Answer Questions
Q13. (a) Explain the acidic nature of phenol. Why is phenol more acidic than alcohols but less acidic than carboxylic acids?
(b) Draw resonance structures of phenoxide ion and explain how they stabilize the conjugate base.
(c) Compare the acidic strength of phenol, ethanol, and water. Give reasons.
Q14. (a) Explain the effect of substituents on the acidic strength of phenol.
(b) Arrange the following in order of increasing acidic strength and explain:
Phenol, o-nitrophenol, m-nitrophenol, p-nitrophenol, 2,4-dinitrophenol, picric acid
(c) Why is picric acid (2,4,6-trinitrophenol) a strong acid despite being a phenol derivative?
Q15. (a) Explain why phenol is more acidic than cyclohexanol using resonance and inductive effects.
(b) How does the acidity of phenol change when:
(i) A -Cl group is introduced at ortho position?
(ii) A -OCH₃ group is introduced at para position?
(iii) A -NO₂ group is introduced at meta position?
Numerical / Application-Based Problems
Q16. The pKa values of some phenol derivatives are given below:
| Compound | pKa |
|---|---|
| Phenol | 10.0 |
| p-Methylphenol (p-cresol) | 10.3 |
| p-Methoxyphenol | 10.2 |
| p-Chlorophenol | 9.4 |
| p-Nitrophenol | 7.1 |
| 2,4-Dinitrophenol | 4.0 |
| Picric acid (2,4,6-trinitrophenol) | 0.4 |
(a) Calculate the Ka values for phenol and picric acid.
(b) How many times more acidic is picric acid than phenol? (Calculate the ratio of their Ka values.)
(c) Plot a graph of pKa vs. number of nitro groups for phenol, p-nitrophenol, 2,4-dinitrophenol, and picric acid.
(d) What trend do you observe? Explain the trend in terms of resonance stabilization.
(e) A student claims that p-methoxyphenol should be more acidic than phenol because oxygen is electronegative. Is this correct? Use the data to evaluate the claim.
Q17. The following equilibrium constants are given:
Phenol + H₂O ⇌ Phenoxide + H₃O⁺; Ka = 1.0 × 10⁻¹⁰
H₂CO₃ + H₂O ⇌ HCO₃⁻ + H₃O⁺; Ka₁ = 4.3 × 10⁻⁷
CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺; Ka = 1.8 × 10⁻⁵
(a) Calculate the pH of 0.1 M phenol solution.
(b) Calculate the pH of 0.1 M acetic acid solution.
(c) Compare the two pH values. What does this tell you about the relative acid strengths?
(d) Will phenol react with sodium carbonate (Na₂CO₃)? Use the equilibrium constants to justify your answer.
(e) A student mixes phenol with sodium bicarbonate (NaHCO₃). No bubbles are observed. Another student mixes acetic acid with NaHCO₃ and observes vigorous bubbling. Explain the difference using acid-base equilibria.
Q18. In a school laboratory, students compare the acidity of different phenol derivatives.
(a) A student adds NaOH to phenol, p-nitrophenol, and p-cresol. All three dissolve. How would you distinguish between them using pH paper?
(b) Another student prepares a buffer solution using phenol and sodium phenoxide. If the pH of the buffer is 10.0 and the concentration of phenol is 0.1 M, calculate the concentration of sodium phenoxide required. (pKa of phenol = 10.0)
(c) A third student tests the effect of substituents on acidity by comparing o-nitrophenol and p-nitrophenol. o-Nitrophenol has a lower boiling point and is more volatile than p-nitrophenol. It also has a slightly lower pKa. Explain the lower pKa of o-nitrophenol. (Hint: Consider intramolecular hydrogen bonding.)
(d) The students learn that picric acid is used as an explosive and as a yellow dye. Why is picric acid explosive despite being an acid? (Consider the stability of the trinitrophenoxide ion and the energy released upon decomposition.)
(e) The teacher asks: "If you were designing a drug that needs to be absorbed in the stomach (pH ≈ 2), would you choose a phenol derivative or a carboxylic acid derivative? Why?" Give your answer with reasoning.