Acidity of phenols MCQs With Answer

Acidity of phenols MCQs With Answer is an essential revision resource for B. Pharm students preparing for organic chemistry and medicinal chemistry exams. This focused set covers pKa values, resonance stabilization of phenoxide ions, effects of electron-withdrawing and electron-donating substituents, positional (ortho/meta/para) influences, intramolecular hydrogen bonding, and practical implications for drug design and synthesis. Each MCQ is designed to build conceptual depth and application skills for pharmaceutical contexts, including reagent choices and spectral methods for acidity measurement. Clear explanations and repeated practice improve recall and problem-solving. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which statement correctly compares the acidity of phenol and ethanol?

• Phenol is more acidic than ethanol
• Phenol is less acidic than ethanol
• Both have equal acidity
• Neither is acidic

Correct Answer: Phenol is more acidic than ethanol

Q2. What is the approximate pKa value of unsubstituted phenol?

• About 5
• About 7
• About 10
• About 16

Correct Answer: About 10

Q3. Which factor most stabilizes the phenoxide ion after deprotonation?

• Inductive effect only
• Resonance delocalization of the negative charge
• Hydrophobic interactions
• Van der Waals forces

Correct Answer: Resonance delocalization of the negative charge

Q4. How does a para electron-withdrawing group (e.g., -NO2) affect phenol acidity?

• Decreases acidity
• Increases acidity
• No effect on acidity
• Makes phenol basic

Correct Answer: Increases acidity

Q5. Which substituted phenol is expected to be the most acidic?

• p-Nitrophenol
• p-Methoxyphenol
• p-Cresol
• p-Anisole

Correct Answer: p-Nitrophenol

Q6. Intramolecular hydrogen bonding in o-nitrophenol has which net effect on acidity compared to p-nitrophenol?

• It increases acidity compared to para isomer
• It decreases acidity compared to para isomer
• It makes both isomers equally acidic
• It converts phenol to a base

Correct Answer: It decreases acidity compared to para isomer

Q7. Which of the following is most acidic?

• Phenol
• p-Nitrophenol
• p-Methoxyphenol
• p-Cresol

Correct Answer: p-Nitrophenol

Q8. For maximum increase in acidity by a nitro group, which position is generally best?

• Ortho
• Meta
• Para
• Equatorial

Correct Answer: Para

Q9. Which common laboratory reagent will deprotonate phenol to give a soluble phenoxide salt?

• Sodium hydroxide (NaOH)
• Sodium chloride (NaCl)
• Hydrochloric acid (HCl)
• Water

Correct Answer: Sodium hydroxide (NaOH)

Q10. Which reagent will quantitatively deprotonate phenol to form the corresponding phenoxide under anhydrous conditions?

• Sodium hydride (NaH)
• Sodium chloride (NaCl)
• Acetic acid
• Water

Correct Answer: Sodium hydride (NaH)

Q11. How does the acidity of phenol compare to that of benzoic acid?

• Phenol is more acidic than benzoic acid
• Phenol is less acidic than benzoic acid
• Both have identical acidity
• Phenol is basic compared to benzoic acid

Correct Answer: Phenol is less acidic than benzoic acid

Q12. Which description correctly explains stabilization of the phenoxide ion?

• Negative charge localized only on oxygen without delocalization
• Negative charge delocalized over oxygen and ortho/para carbons
• Negative charge stabilized by hydrogen bonding to benzene ring
• Negative charge converted to positive by resonance

Correct Answer: Negative charge delocalized over oxygen and ortho/para carbons

Q13. What is the general effect of an electron-donating group (EDG) on phenol acidity?

• EDG increases acidity
• EDG decreases acidity
• EDG has no effect
• EDG converts phenol into a base

Correct Answer: EDG decreases acidity

Q14. Between p-chlorophenol and p-methoxyphenol, which is more acidic?

• p-Chlorophenol
• p-Methoxyphenol
• Both equal
• Neither acidic

Correct Answer: p-Chlorophenol

Q15. Which compound has the highest pKa (least acidic) among the following?

• p-Cresol (p-CH3)
• Phenol
• p-Chlorophenol
• p-Nitrophenol

Correct Answer: p-Cresol (p-CH3)

Q16. In resonance structures of phenoxide, the negative charge primarily appears on which atoms?

• Only on the para carbon
• On oxygen and the ortho/para carbons
• Only on the ortho carbons
• Only on hydrogen atoms

Correct Answer: On oxygen and the ortho/para carbons

Q17. Which substituent is the strongest electron-withdrawing group by resonance and strongly increases phenol acidity?

• -NO2 (nitro)
• -OCH3 (methoxy)
• -CH3 (methyl)
• -NH2 (amino)

Correct Answer: -NO2 (nitro)

Q18. A positive Hammett sigma (σ) value for a substituent generally indicates what effect on phenol acidity?

• It is electron donating and decreases acidity
• It is electron withdrawing and increases acidity
• It has no electronic effect
• It increases basicity of the ring

Correct Answer: It is electron withdrawing and increases acidity

Q19. How does steric hindrance from bulky ortho substituents typically affect phenol acidity?

• Increases acidity by better resonance
• Decreases acidity by preventing solvation of the phenoxide
• Has no effect on acidity
• Converts phenol to an ether

Correct Answer: Decreases acidity by preventing solvation of the phenoxide

Q20. Why is anisole (methoxybenzene) not acidic like phenol?

• Because methoxy oxygen protonates easily
• Methoxy group donates electrons by resonance, reducing acidity
• Anisole is gaseous and cannot be acidic
• Because anisole has no aromatic ring

Correct Answer: Methoxy group donates electrons by resonance, reducing acidity

Q21. Among para-halogenated phenols (p-F, p-Cl, p-Br, p-I), which typically shows the highest acidity?

• p-Fluorophenol
• p-Iodophenol
• p-Bromophenol
• p-Chlorophenol

Correct Answer: p-Fluorophenol

Q22. When phenol is treated with NaOH, what is the primary product?

• Sodium phenoxide (phenoxide salt)
• Phenyl chloride
• Benzene
• Phenyl ester

Correct Answer: Sodium phenoxide (phenoxide salt)

Q23. How does increasing solvent polarity generally affect measured acidity of phenols?

• Decreases acidity by destabilizing ions
• Increases acidity by stabilizing the phenoxide ion
• No change in acidity
• Causes polymerization

Correct Answer: Increases acidity by stabilizing the phenoxide ion

Q24. Compared to a para nitro group, a meta nitro group affects phenol acidity how?

• Meta nitro increases acidity more due to resonance
• Para nitro increases acidity more because it stabilizes by resonance
• Meta has the same resonance effect as para
• Neither has any effect

Correct Answer: Para nitro increases acidity more because it stabilizes by resonance

Q25. During deprotonation of phenol, which bond is broken?

• O–H bond (hydroxyl hydrogen removed)
• C–H bond at ortho carbon
• C–O bond
• Ring C–C bond

Correct Answer: O–H bond (hydroxyl hydrogen removed)

Q26. Resonance forms of phenoxide show negative charge at which specific ring positions?

• Ortho and para positions
• Only meta positions
• Only the ipso carbon
• All ring hydrogens

Correct Answer: Ortho and para positions

Q27. What is the approximate pKa of p-nitrophenol?

• About 3
• About 7.2
• About 10
• About 12

Correct Answer: About 7.2

Q28. Which reagent selectively deprotonates phenols under mild aqueous conditions but typically not aliphatic alcohols?

• Sodium bicarbonate (NaHCO3)
• Sodium hydroxide (NaOH)
• Sodium chloride (NaCl)
• Hydrochloric acid (HCl)

Correct Answer: Sodium hydroxide (NaOH)

Q29. Between resonance and inductive effects, which predominantly stabilizes the phenoxide ion?

• Inductive effect is dominant
• Resonance delocalization is dominant
• Hydrophobic interactions are dominant
• None of the above

Correct Answer: Resonance delocalization is dominant

Q30. Which compound is the most acidic among the following?

• Phenol
• 2,4-Dinitrophenol
• p-Methoxyphenol
• p-Cresol

Correct Answer: 2,4-Dinitrophenol

Q31. How does catechol (1,2-dihydroxybenzene) acidity compare to phenol?

• Catechol is more acidic than phenol
• Catechol is less acidic than phenol
• Both have equal acidity
• Catechol is not acidic

Correct Answer: Catechol is more acidic than phenol

Q32. What effect do 2,6-dimethyl groups have on phenol acidity relative to unsubstituted phenol?

• They increase acidity by resonance
• They decrease acidity due to steric hindrance and poor solvation
• They convert phenol into an ether
• They have no effect

Correct Answer: They decrease acidity due to steric hindrance and poor solvation

Q33. Which classical qualitative test gives a color change for phenols due to complex formation?

• Ferric chloride test
• Tollens’ test
• Bromine water test
• Sodium bicarbonate test

Correct Answer: Ferric chloride test

Q34. Deprotonation of phenol by a base is best described by which acid–base theory?

• Lewis acid-base theory
• Brønsted–Lowry proton transfer
• Hard–soft acid–base theory
• Baldwin’s rules

Correct Answer: Brønsted–Lowry proton transfer

Q35. The –OH group on phenol is generally classified as which type of directing group in electrophilic aromatic substitution?

• Deactivating meta director
• Activating ortho/para director
• Non-directing
• Strongly withdrawing para director only

Correct Answer: Activating ortho/para director

Q36. Compared to phenol, p-aminophenol is expected to be:

• More acidic because –NH2 withdraws electrons
• Less acidic because –NH2 donates electrons by resonance
• Exactly the same acidity
• Not an aromatic compound

Correct Answer: Less acidic because –NH2 donates electrons by resonance

Q37. Which analytical technique is commonly used to determine the pKa of a phenol by observing absorbance changes between phenol and phenoxide?

• Infrared spectroscopy
• UV–Visible spectrophotometric titration
• Mass spectrometry
• X-ray crystallography

Correct Answer: UV–Visible spectrophotometric titration

Q38. Which para substituent would have the least electronic effect on phenol acidity?

• p-Methyl (p-CH3)
• p-Nitro (p-NO2)
• p-Methoxy (p-OCH3)
• p-Chloro (p-Cl)

Correct Answer: p-Methyl (p-CH3)

Q39. Why is phenol more acidic than cyclohexanol?

• Phenol has a higher molecular weight
• Phenoxide ion is resonance stabilized while cyclohexoxide is not
• Cyclohexanol is aromatic
• Phenol forms a stronger hydrogen bond with water

Correct Answer: Phenoxide ion is resonance stabilized while cyclohexoxide is not

Q40. Which ring positions bear the major resonance contributors for the phenoxide ion?

• Only the meta positions
• Ortho and para positions
• Only the ipso carbon
• The hydrogen atoms only

Correct Answer: Ortho and para positions

Q41. Which has the lowest pKa (strongest acid) among these?

• Phenol (pKa ~10)
• p-Nitrophenol (pKa ~7.2)
• 2,4-Dinitrophenol (pKa ~4)
• p-Cresol (pKa ~10.2)

Correct Answer: 2,4-Dinitrophenol (pKa ~4)

Q42. How does the reactivity of the phenoxide ion in electrophilic aromatic substitution compare to neutral phenol?

• Phenoxide is less activating than phenol
• Phenoxide is more activating than phenol
• Both have identical activation
• Phenoxide deactivates the ring completely

Correct Answer: Phenoxide is more activating than phenol

Q43. Which statement about pKa and acid strength is correct?

• Higher pKa indicates a stronger acid
• Lower pKa indicates a stronger acid
• pKa does not relate to acid strength
• pKa only applies to bases

Correct Answer: Lower pKa indicates a stronger acid

Q44. Which reagent is typically used to convert phenol into sodium phenoxide in aqueous media?

• Sodium hydroxide (NaOH)
• Pyridine
• Hydrochloric acid (HCl)
• Boron trifluoride (BF3)

Correct Answer: Sodium hydroxide (NaOH)

Q45. Which phenol is known to form a strong intramolecular hydrogen bond in the neutral form?

• o-Nitrophenol
• p-Nitrophenol
• m-Nitrophenol
• Phenol without substituents

Correct Answer: o-Nitrophenol

Q46. Between m-nitrophenol and p-nitrophenol, which is generally more acidic and why?

• m-Nitrophenol, because meta resonance is stronger
• p-Nitrophenol, because para position allows resonance stabilization
• Both equal because nitro effect is identical
• Neither is acidic

Correct Answer: p-Nitrophenol, because para position allows resonance stabilization

Q47. Which is the strongest acid among: phenol, benzoic acid, ethanol, acetic acid?

• Phenol
• Benzoic acid
• Ethanol
• Acetic acid

Correct Answer: Benzoic acid

Q48. Which synthetic reaction commonly uses phenoxide ions as nucleophiles?

• Williamson ether synthesis
• Friedel–Crafts acylation
• Bayer–Villiger oxidation
• Beckmann rearrangement

Correct Answer: Williamson ether synthesis

Q49. How does an ortho methoxy substituent typically influence phenol acidity compared to unsubstituted phenol?

• Always increases acidity due to strong withdrawal
• May decrease acidity via resonance donation but can also engage in hydrogen bonding effects
• Has no influence whatsoever
• Converts phenol to a peroxide

Correct Answer: May decrease acidity via resonance donation but can also engage in hydrogen bonding effects

Q50. Which principle explains why electron-withdrawing substituents increase phenol acidity?

• They destabilize the conjugate base
• They stabilize the conjugate base by delocalizing or inductively withdrawing electron density
• They increase steric hindrance only
• They convert phenol into a base

Correct Answer: They stabilize the conjugate base by delocalizing or inductively withdrawing electron density

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