Acidity of aromatic acids MCQs With Answer

Mastering the acidity of aromatic acids is essential for B. Pharm students preparing for exams and pharmaceutical applications. This Student-friendly guide, Acidity of aromatic acids MCQs With Answer, clarifies how resonance, inductive effect, hydrogen bonding and ortho/para/meta substitution alter acidity and pKa in benzoic acids, phenols and substituted aromatic carboxylic acids. Topics include electron-withdrawing/donating groups, conjugate-base stabilization, Hammett constants, solvent effects and intramolecular hydrogen bonding. Clear explanations paired with targeted practice improve conceptual understanding and problem-solving for medicinal chemistry and analysis. All MCQs include answers to aid rapid revision and exam readiness. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which factor most stabilizes the conjugate base of a substituted benzoic acid, increasing acidity?

• Electron-donating resonance (+R) from a para substituent
• Electron-withdrawing inductive effect (-I) from a para nitro group
• Hyperconjugation from an alkyl substituent
• Steric hindrance preventing solvation

Correct Answer: Electron-withdrawing inductive effect (-I) from a para nitro group

Q2. Which compound is the strongest acid?

• Benzoic acid
• p-Nitrobenzoic acid
• p-Methoxybenzoic acid
• Benzyl alcohol

Correct Answer: p-Nitrobenzoic acid

Q3. Compared to aliphatic acetic acid (pKa ~4.76), benzoic acid (pKa ~4.20) is:

• Less acidic due to aromatic ring donating electrons
• More acidic due to resonance stabilization of benzoate anion
• About the same because aromaticity cancels effects
• Less acidic because conjugate base is less stable

Correct Answer: More acidic due to resonance stabilization of benzoate anion

Q4. Which substituent at the para position increases acidity of benzoic acid the most?

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

Correct Answer: -NO2 (nitro)

Q5. Why does a para-nitro group increase benzoic acid acidity?

• By donating electrons through resonance to the ring
• By withdrawing electron density through resonance and induction to stabilize the conjugate base
• By forming intramolecular hydrogen bonds with the carboxyl proton
• By increasing steric hindrance and preventing solvation

Correct Answer: By withdrawing electron density through resonance and induction to stabilize the conjugate base

Q6. Which statement correctly compares phenol and benzoic acid acidity?

• Phenol is more acidic than benzoic acid because of aromatic stabilization
• Benzoic acid is more acidic than phenol because carboxylate is better stabilized than phenoxide
• Both have identical acidity because both are aromatic
• Phenol is more acidic because -OH forms stronger hydrogen bonds

Correct Answer: Benzoic acid is more acidic than phenol because carboxylate is better stabilized than phenoxide

Q7. Which effect dominates when a fluorine substituent is present at the para position of benzoic acid?

• Strong resonance donation (+R)
• Strong inductive withdrawal (-I) with weak resonance donation
• Only steric effects
• No electronic effect

Correct Answer: Strong inductive withdrawal (-I) with weak resonance donation

Q8. Which benzoic acid is expected to have the lowest pKa?

• p-Methoxybenzoic acid
• m-Nitrobenzoic acid
• p-Nitrobenzoic acid
• o-Methylbenzoic acid

Correct Answer: p-Nitrobenzoic acid

Q9. The Hammett sigma constant is used to quantify:

• Steric hindrance of substituents
• Electronic effects of substituents on reaction rates and equilibria
• Solubility of aromatic acids in water
• The acidity of carboxylic acids in nonpolar solvents only

Correct Answer: Electronic effects of substituents on reaction rates and equilibria

Q10. Which positional isomer of nitrobenzoic acid is generally most acidic and why?

• Ortho, due to resonance donation
• Meta, due to strongest resonance stabilization of conjugate base
• Para, due to both resonance and inductive stabilization of conjugate base
• All are equal because nitro is equally withdrawing

Correct Answer: Para, due to both resonance and inductive stabilization of conjugate base

Q11. Intramolecular hydrogen bonding in o-substituted benzoic acids commonly causes:

• Significant increase in acidity by stabilizing conjugate base
• Decrease in acidity by stabilizing the neutral acid or reducing solvation of conjugate base
• No change in acidity
• Conversion to non-acidic species

Correct Answer: Decrease in acidity by stabilizing the neutral acid or reducing solvation of conjugate base

Q12. Which mechanism explains why electron-donating groups decrease acidity of aromatic acids?

• They destabilize the neutral acid by hyperconjugation
• They destabilize the conjugate base by increasing electron density on the anion
• They increase hydrogen bonding with solvent
• They convert the acid into a base

Correct Answer: They destabilize the conjugate base by increasing electron density on the anion

Q13. Which of the following increases acidity of phenol most effectively?

• Para-methoxy substitution
• Para-nitro substitution
• Para-methyl substitution
• Para-amino substitution

Correct Answer: Para-nitro substitution

Q14. The conjugate base of phenol (phenoxide) is stabilized mainly by:

• Inductive effect only
• Resonance delocalization of the negative charge into the aromatic ring
• Hyperconjugation from the ring hydrogens
• Ionic bonding with solvent

Correct Answer: Resonance delocalization of the negative charge into the aromatic ring

Q15. Which pair has the correct acidity order (most to least acidic)?

• p-Nitrobenzoic acid > benzoic acid > p-Methoxybenzoic acid
• p-Methoxybenzoic acid > benzoic acid > p-Nitrobenzoic acid
• Benzoic acid > p-Nitrobenzoic acid > p-Methoxybenzoic acid
• p-Methoxybenzoic acid > p-Nitrobenzoic acid > benzoic acid

Correct Answer: p-Nitrobenzoic acid > benzoic acid > p-Methoxybenzoic acid

Q16. Which substituent at ortho position is likely to increase benzoic acid acidity despite being electron-donating?

• Ortho-methoxy solely due to +R effect
• Ortho-alkyl due to steric hindrance causing ortho effect
• Ortho-nitro due to -I and -R effects
• Ortho-amino due to strong donation

Correct Answer: Ortho-nitro due to -I and -R effects

Q17. Which statement best describes the ortho effect in benzoic acids?

• Ortho substituents always decrease acidity
• Steric and electronic effects at ortho can unpredictably alter acidity, often increasing it irrespective of electronic nature
• Ortho substituents have no influence compared to para
• Only resonance effects matter at ortho position

Correct Answer: Steric and electronic effects at ortho can unpredictably alter acidity, often increasing it irrespective of electronic nature

Q18. Which solvent effect tends to increase observed acidity of aromatic acids in water?

• Poor solvation of conjugate base
• Strong solvation and hydrogen bonding stabilizing the conjugate base
• Nonpolar solvent decreasing ionization
• Solvent acidity independent effects

Correct Answer: Strong solvation and hydrogen bonding stabilizing the conjugate base

Q19. Which compound has the higher acidity: p-nitrophenol or m-nitrophenol, and why?

• m-Nitrophenol, due to stronger resonance stabilization
• p-Nitrophenol, because para position allows resonance withdrawal stabilizing phenoxide
• They are identical in acidity
• m-Nitrophenol, due to intramolecular hydrogen bonding increasing acidity

Correct Answer: p-Nitrophenol, because para position allows resonance withdrawal stabilizing phenoxide

Q20. Which measurement directly quantifies acid strength in solution?

• Boiling point
• pKa value
• Optical rotation
• Refractive index

Correct Answer: pKa value

Q21. Which substituent combination will most increase acidity of benzoic acid?

• Para-methoxy and meta-methyl
• Para-nitro and ortho-nitro
• Para-amino and para-hydroxy
• Meta-methoxy and ortho-methyl

Correct Answer: Para-nitro and ortho-nitro

Q22. How does resonance (+R) of a substituent like -OCH3 at para position affect benzoic acid acidity?

• It withdraws electron density and increases acidity
• It donates electrons to ring, decreasing acidity by destabilizing conjugate base
• It has no effect because resonance cannot act through ring
• It converts the acid into an ester

Correct Answer: It donates electrons to ring, decreasing acidity by destabilizing conjugate base

Q23. Why is p-chlorobenzoic acid more acidic than p-methylbenzoic acid?

• Chlorine is electron-donating, increasing acidity
• Chlorine has weak -I (inductive) effect while methyl is electron-donating by hyperconjugation
• Methyl strongly withdraws electrons
• Chlorine increases steric hindrance enhancing acidity

Correct Answer: Chlorine has weak -I (inductive) effect while methyl is electron-donating by hyperconjugation

Q24. Which aromatic acid is least acidic?

• p-Nitrobenzoic acid
• Phenol
• Benzoic acid
• p-Anisic acid (p-methoxybenzoic acid)

Correct Answer: p-Anisic acid (p-methoxybenzoic acid)

Q25. In the Hammett equation, a positive rho (ρ) value indicates:

• Reaction is accelerated by electron-donating groups
• Reaction is sensitive to electron-withdrawing groups
• No substituent effect
• Only steric effects matter

Correct Answer: Reaction is sensitive to electron-withdrawing groups

Q26. Which effect predominates in m-nitrobenzoic acid increasing acidity?

• Resonance (-R) effect only
• Inductive (-I) effect primarily, as resonance contribution to carboxyl group is limited at meta
• Resonance donation (+R) increasing electron density
• Steric hindrance increasing solvation

Correct Answer: Inductive (-I) effect primarily, as resonance contribution to carboxyl group is limited at meta

Q27. How does multiple nitro substitution (e.g., 3,5-dinitrobenzoic acid) affect acidity?

• Decreases acidity by donating electrons
• Greatly increases acidity by strong cumulative -I and -R stabilization of conjugate base
• No significant change compared to mono-nitro
• Converts acid to base

Correct Answer: Greatly increases acidity by strong cumulative -I and -R stabilization of conjugate base

Q28. Which is a correct reason why phenoxide ion is less stabilized than benzoate ion?

• Phenoxide cannot delocalize charge into ring
• Negative charge in phenoxide is localized mainly on oxygen without as much resonance stabilization compared to carboxylate delocalization over two oxygens
• Benzoate has higher basicity
• Phenoxide forms stronger hydrogen bonds with solvent

Correct Answer: Negative charge in phenoxide is localized mainly on oxygen without as much resonance stabilization compared to carboxylate delocalization over two oxygens

Q29. Which experimental technique is commonly used to determine pKa of aromatic acids?

• Infrared spectroscopy
• Titration with a strong base and monitoring pH
• Mass spectrometry
• TLC (thin-layer chromatography)

Correct Answer: Titration with a strong base and monitoring pH

Q30. Which statement about intramolecular hydrogen bonding in o-hydroxybenzoic acid (salicylic acid) is true?

• It always decreases acidity compared to benzoic acid by stabilizing conjugate base
• Intramolecular H-bonding in the neutral molecule can reduce solvation of the conjugate base and affect observed acidity
• It has no effect on acidity
• It prevents ionization completely

Correct Answer: Intramolecular H-bonding in the neutral molecule can reduce solvation of the conjugate base and affect observed acidity

Q31. Ranking acidity of substituted benzoic acids: which is correct from strongest to weakest?

• p-NO2 > p-Cl > p-CH3 > p-OCH3
• p-OCH3 > p-CH3 > p-Cl > p-NO2
• p-CH3 > p-OCH3 > p-Cl > p-NO2
• All have same acidity

Correct Answer: p-NO2 > p-Cl > p-CH3 > p-OCH3

Q32. Which of the following increases acidity of an aromatic carboxylic acid in polar protic solvent?

• Poor solvation of conjugate base
• Strong hydrogen-bonding interactions that stabilize the conjugate base
• Nonpolar substituents only
• Decreased dielectric constant

Correct Answer: Strong hydrogen-bonding interactions that stabilize the conjugate base

Q33. Which substituent shows both -I and +R effects, making its net effect position-dependent?

• -NO2
• -OCH3
• -CF3
• -CH3

Correct Answer: -OCH3

Q34. For aromatic acids, stabilization of the conjugate base by resonance is most effective when the substituent is at which position?

• Ortho and para positions
• Meta position only
• Only ortho position
• Substituent position never matters

Correct Answer: Ortho and para positions

Q35. Which factor can make an ortho-substituted benzoic acid more acidic relative to para-substituted analog?

• Intramolecular hydrogen bonding stabilizing the conjugate base exclusively
• Steric hindrance reducing resonance between ring and carboxyl group
• Conjugation enhancement at ortho only
• Ortho position has no electronic interactions

Correct Answer: Steric hindrance reducing resonance between ring and carboxyl group

Q36. Which aromatic acid has a conjugate base where negative charge is delocalized over two oxygens?

• Phenol
• Benzoic acid
• Aniline
• Toluene

Correct Answer: Benzoic acid

Q37. Which substituent would most likely decrease acidity of benzoic acid?

• Para-nitro
• Para-fluoro
• Para-methoxy
• Meta-nitro

Correct Answer: Para-methoxy

Q38. Which of the following best describes mesomeric (resonance) effect on aromatic acidity?

• Resonance donation always increases acidity
• Resonance withdrawal (+R) stabilizes conjugate base and increases acidity
• Resonance donation (+R) can decrease acidity by increasing electron density on the conjugate base
• Resonance has no influence on acidity

Correct Answer: Resonance donation (+R) can decrease acidity by increasing electron density on the conjugate base

Q39. Which compound is expected to be more acidic: 4-chlorophenol or 4-nitrophenol?

• 4-Chlorophenol
• 4-Nitrophenol
• Both are equal
• Neither is acidic

Correct Answer: 4-Nitrophenol

Q40. What effect does a -CF3 group at para position have on benzoic acid acidity?

• Strong electron-withdrawing (-I) increasing acidity
• Strong electron-donating increasing acidity
• No effect because trifluoromethyl is neutral
• Decreases acidity by resonance donation

Correct Answer: Strong electron-withdrawing (-I) increasing acidity

Q41. Which aromatic acid would most readily ionize in water?

• p-Methoxybenzoic acid
• p-Nitrobenzoic acid
• p-Anisic acid
• p-Toluenecarboxylic acid

Correct Answer: p-Nitrobenzoic acid

Q42. Which description fits why electron-withdrawing groups increase acidity?

• They destabilize the conjugate base
• They stabilize the conjugate base by delocalizing or reducing negative charge density
• They increase molecular weight
• They enhance basicity of solvent

Correct Answer: They stabilize the conjugate base by delocalizing or reducing negative charge density

Q43. pKa of phenol (~10) is much higher than benzoic acid (~4.2). Main reason is:

• Phenoxide is more resonance-stabilized than benzoate
• Benzoate anion delocalizes negative charge over two oxygens while phenoxide delocalizes mainly onto ring carbon atoms
• Phenol forms stronger hydrogen bonds with water
• Phenol has higher molecular weight

Correct Answer: Benzoate anion delocalizes negative charge over two oxygens while phenoxide delocalizes mainly onto ring carbon atoms

Q44. Which process would decrease acidity of an aromatic carboxylic acid in a nonpolar solvent?

• Increased solvation of conjugate base
• Reduced solvation and lower dielectric constant favoring unionized form
• Addition of strong electron-withdrawing substituents
• Formation of salts

Correct Answer: Reduced solvation and lower dielectric constant favoring unionized form

Q45. Which analysis method correlates substituent effects with reaction rates or equilibria for aromatic systems?

• Hammett analysis
• NMR shielding only
• Boiling point elevation
• Melting point comparison

Correct Answer: Hammett analysis

Q46. Which is true about m-substituted benzoic acids compared to p-substituted in terms of resonance?

• Meta substituents strongly influence resonance stabilization of carboxylate
• Meta substituents cannot exert resonance effects on the carboxyl group, so inductive effects dominate
• Meta and para have identical resonance impacts
• Meta substituents always donate electrons by resonance

Correct Answer: Meta substituents cannot exert resonance effects on the carboxyl group, so inductive effects dominate

Q47. Which structural feature explains higher acidity of o-nitrobenzoic acid compared to benzoic acid?

• Only resonance donation from nitro group
• Combination of -I/-R effects and possible intramolecular interactions that stabilize conjugate base
• Nitro group donates electrons increasing acidity
• Steric bulk prevents ionization

Correct Answer: Combination of -I/-R effects and possible intramolecular interactions that stabilize conjugate base

Q48. For teaching B. Pharm students, why is understanding acidity of aromatic acids important?

• It has no pharmaceutical relevance
• Acidity influences drug ionization, absorption, solubility and formulation
• Only synthetic chemists need to know acidity
• It only affects color of compounds

Correct Answer: Acidity influences drug ionization, absorption, solubility and formulation

Q49. Which change would most increase pKa (make less acidic) of benzoic acid?

• Introduce a para-nitro group
• Introduce a para-methoxy group
• Introduce a para-fluoro group
• Add a second nitro at meta position

Correct Answer: Introduce a para-methoxy group

Q50. When comparing acidity of substituted aromatics, the best single approach to predict trends is to consider:

• Only steric effects
• Combined electronic effects (inductive and resonance), position (ortho/meta/para), and solvation
• Only molecular weight
• Only boiling point and melting point

Correct Answer: Combined electronic effects (inductive and resonance), position (ortho/meta/para), and solvation

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