Acidity of carboxylic acids MCQs With Answer

Acidity of carboxylic acids MCQs With Answer is a focused resource for B. Pharm students studying organic and medicinal chemistry. This introduction explains how resonance stabilization, inductive effects, hybridization and solvent interactions determine the pKa and behavior of carboxylic acids in drug molecules. Understanding substituent effects (electron-withdrawing vs donating), ortho/para influences, dimerization, and comparative acidity versus phenols and alcohols is essential for drug design, formulation and pharmacokinetics. These concise, exam-oriented MCQs emphasize mechanisms and real-world examples to build problem-solving skills. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which statement best explains why carboxylic acids are more acidic than alcohols?

• Because the O-H bond in carboxylic acids is weaker than in alcohols
• Because the conjugate base of carboxylic acid is resonance-stabilized
• Because carboxylic acids have higher molecular weight
• Because carboxylic acids are generally solids at room temperature

Correct Answer: Because the conjugate base of carboxylic acid is resonance-stabilized

Q2. Typical pKa of acetic acid (CH3COOH) is closest to which value?

• pKa ≈ 1
• pKa ≈ 4.8
• pKa ≈ 7
• pKa ≈ 10

Correct Answer: pKa ≈ 4.8

Q3. Which factor increases the acidity of benzoic acid when a nitro group is placed at the para position?

• Resonance donation from nitro group to the ring
• Inductive and resonance electron-withdrawing stabilizing the carboxylate
• Increase in molecular size
• Steric hindrance preventing solvation

Correct Answer: Inductive and resonance electron-withdrawing stabilizing the carboxylate

Q4. Which of the following has the lowest (most acidic) pKa?

• Acetic acid (pKa ≈ 4.8)
• Benzoic acid (pKa ≈ 4.2)
• Trifluoroacetic acid (pKa ≈ -0.25)
• Phenol (pKa ≈ 10)

Correct Answer: Trifluoroacetic acid (pKa ≈ -0.25)

Q5. How does the inductive effect of an electron-withdrawing substituent affect carboxylic acid acidity?

• It destabilizes the conjugate base, reducing acidity
• It stabilizes the conjugate base by delocalizing negative charge, increasing acidity
• It converts the acid into an ester
• It has no effect on acidity

Correct Answer: It stabilizes the conjugate base by delocalizing negative charge, increasing acidity

Q6. Which statement about resonance in the carboxylate anion is correct?

• The negative charge is localized on one oxygen atom only
• The negative charge is delocalized equally over both oxygens
• The negative charge is delocalized onto the carbonyl carbon
• Resonance does not stabilize the carboxylate anion

Correct Answer: The negative charge is delocalized equally over both oxygens

Q7. Which substituent would most increase acidity of benzoic acid?

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

Correct Answer: Para-nitro (-NO2)

Q8. Which is the main reason trifluoroacetic acid is much stronger than acetic acid?

• Fluorine increases molecular weight
• Strong electron-withdrawing inductive effect of fluorines stabilizes conjugate base
• Fluorines donate electrons by resonance to stabilize the acid
• Trifluoroacetic acid forms stronger hydrogen bonds with water

Correct Answer: Strong electron-withdrawing inductive effect of fluorines stabilizes conjugate base

Q9. Intramolecular hydrogen bonding in o-hydroxybenzoic acid (salicylic acid) affects acidity how?

• Always increases acidity by stabilizing the conjugate base
• Decreases acidity because it stabilizes the acid more than the conjugate base
• Has no effect on acidity
• Converts the acid into an ester

Correct Answer: Decreases acidity because it stabilizes the acid more than the conjugate base

Q10. Which hybridization of the carbon bearing the acidic hydrogen makes it most acidic?

• sp3
• sp2
• sp
• sp3d

Correct Answer: sp

Q11. How does electron-donating substituent on aromatic carboxylic acid affect acidity?

• Increases acidity by stabilizing conjugate base
• Decreases acidity by destabilizing conjugate base
• Has no electronic effect, only steric
• Converts acid into a stronger acid

Correct Answer: Decreases acidity by destabilizing conjugate base

Q12. Which comparison is correct for acidity?

• Carboxylic acids < phenols < alcohols (in acidity)
• Alcohols < phenols < carboxylic acids (in acidity)
• Phenols < carboxylic acids < alcohols (in acidity)
• All have similar acidity

Correct Answer: Alcohols < phenols < carboxylic acids (in acidity)

Q13. The pKa of benzoic acid is approximately:

• pKa ≈ 2.0
• pKa ≈ 4.2
• pKa ≈ 7.0
• pKa ≈ 10.0

Correct Answer: pKa ≈ 4.2

Q14. Which phenomenon often occurs between carboxylic acid molecules in nonpolar solvents and affects observed acidity?

• Dimerization via hydrogen bonding
• Polymerization to peptides
• Oxidation to ketones
• Covalent cross-linking

Correct Answer: Dimerization via hydrogen bonding

Q15. Which derivative is least acidic?

• Carboxylic acid
• Acyl chloride
• Carboxylate anion
• Carboxylic ester

Correct Answer: Carboxylic ester

Q16. How does solvation influence acidity of carboxylic acids in water?

• Solvation stabilizes the conjugate base, increasing acidity
• Solvation stabilizes only the acid, reducing acidity
• Water decreases acidity by hydrogen bonding to the acid
• Solvation has no effect on acidity

Correct Answer: Solvation stabilizes the conjugate base, increasing acidity

Q17. Which statement about pKa and Ka is correct?

• pKa = -log(Ka)
• Ka = -log(pKa)
• pKa and Ka are numerically equal
• pKa = 10 × Ka

Correct Answer: pKa = -log(Ka)

Q18. Which of these increases acidity of an α-hydrogen (next to carbonyl) in a carboxylic acid derivative?

• Electron-donating groups on alpha carbon
• Conjugation and adjacent electron-withdrawing groups
• Conversion to tertiary alcohol
• Hydrogenation of the carbonyl

Correct Answer: Conjugation and adjacent electron-withdrawing groups

Q19. Which aromatic position generally gives the strongest electron-withdrawing resonance effect to stabilize a carboxylate?

• Ortho position only
• Para position for resonance, meta mainly inductive
• Meta position for resonance, para mainly inductive
• Positions do not matter

Correct Answer: Para position for resonance, meta mainly inductive

Q20. The ortho effect in substituted benzoic acids often results from:

• Only resonance donation of ortho substituents
• Steric hindrance and altered solvation leading to increased acidity
• Conversion into phenols
• Formation of intramolecular salts

Correct Answer: Steric hindrance and altered solvation leading to increased acidity

Q21. Which technique is commonly used to determine pKa of a carboxylic acid in the lab?

• Melting point determination
• UV-visible spectroscopy without indicator
• Potentiometric titration (pH titration)
• Thin layer chromatography

Correct Answer: Potentiometric titration (pH titration)

Q22. How does converting a carboxylic acid to its sodium salt affect water solubility?

• Decreases solubility in water
• Increases solubility in water
• No change in solubility
• Makes it volatile

Correct Answer: Increases solubility in water

Q23. Which of the following groups will most decrease acidity of acetic acid if attached to the methyl group?

• Trifluoromethyl (-CF3)
• Nitro (-NO2)
• Methyl (-CH3)
• Chloro (-Cl)

Correct Answer: Methyl (-CH3)

Q24. For a carboxylic acid, what does a smaller pKa value indicate?

• Weaker acid
• Stronger acid
• Lower ionic strength
• Less electronegativity

Correct Answer: Stronger acid

Q25. Which conjugate base is most stabilized by resonance?

• Alkoxide (from alcohol)
• Carboxylate (from carboxylic acid)
• Alkyl carbanion
• Amide anion

Correct Answer: Carboxylate (from carboxylic acid)

Q26. Which halogen substituent generally exerts the strongest inductive electron-withdrawing effect to increase acidity?

• Iodine (I)
• Bromine (Br)
• Chlorine (Cl)
• Fluorine (F)

Correct Answer: Fluorine (F)

Q27. In the Hammett equation, a positive sigma (σ) value for a substituent indicates:

• Electron-donating through resonance
• Electron-withdrawing effect increasing acidity
• No electronic effect
• Steric hindrance only

Correct Answer: Electron-withdrawing effect increasing acidity

Q28. Which statement about carboxylate anion geometry is true?

• The carboxylate carbon is tetrahedral
• The carboxylate group is planar with sp2 carbon and equivalent C–O bonds
• The two C–O bonds are very different in length
• The anion adopts a puckered nonplanar shape

Correct Answer: The carboxylate group is planar with sp2 carbon and equivalent C–O bonds

Q29. Which reagent would convert a carboxylic acid into a less acidic methyl ester?

• Sodium hydroxide (NaOH)
• Methanol with acid catalyst (H+ / MeOH)
• Hydrogenation (H2/Pd)
• Diazomethane only increases acidity

Correct Answer: Methanol with acid catalyst (H+ / MeOH)

Q30. Which is true about acidity comparison: benzoic acid vs p-nitrobenzoic acid?

• Benzoic acid is more acidic than p-nitrobenzoic acid
• p-Nitrobenzoic acid is more acidic than benzoic acid
• Both have identical acidity
• Acidity depends only on solvent

Correct Answer: p-Nitrobenzoic acid is more acidic than benzoic acid

Q31. Which experimental observation supports resonance stabilization of carboxylate anion?

• Different C–O bond lengths in carboxylate
• Equalization of C–O bond lengths in carboxylate
• Carboxylate is nonpolar
• Carboxylate has sp3 hybridization at carbon

Correct Answer: Equalization of C–O bond lengths in carboxylate

Q32. Which process will shift the acid–base equilibrium of a weak carboxylic acid toward deprotonation?

• Lowering the pH below the acid pKa
• Raising the pH above the acid pKa
• Adding excess neutral solvent
• Increasing temperature only

Correct Answer: Raising the pH above the acid pKa

Q33. Which is a common pharmaceutical implication of carboxylic acid acidity?

• Acidic drugs cannot form salts for formulation
• pKa influences absorption and ionization state at physiological pH
• Acidity always prevents membrane permeation
• Carboxylic acids are not metabolized in the body

Correct Answer: pKa influences absorption and ionization state at physiological pH

Q34. Which best describes the effect of conjugation between a carboxyl group and C=C double bond?

• Conjugation always decreases acidity by destabilizing the conjugate base
• Conjugation can delocalize charge and potentially increase acidity
• Conjugation converts acid into an alcohol
• Conjugation removes resonance stabilization

Correct Answer: Conjugation can delocalize charge and potentially increase acidity

Q35. Which is the major reason electron-donating groups (EDGs) decrease acidity when attached to aromatic carboxylic acids?

• EDGs stabilize the conjugate base
• EDGs destabilize the conjugate base by donating electron density
• EDGs increase solubility in water
• EDGs remove the carboxyl group

Correct Answer: EDGs destabilize the conjugate base by donating electron density

Q36. Which of the following acids is expected to be most acidic in aqueous solution?

• p-Anisic acid (p-methoxybenzoic acid)
• p-Toluic acid (p-methylbenzoic acid)
• p-Nitrobenzoic acid
• p-Aminobenzoic acid

Correct Answer: p-Nitrobenzoic acid

Q37. Which statement about decarboxylation of carboxylic acids is generally true?

• Simple aliphatic carboxylic acids decarboxylate easily at room temperature
• β-Keto acids decarboxylate more readily due to stabilization of transition state
• Carboxylic acids never undergo decarboxylation
• Decarboxylation increases acidity

Correct Answer: β-Keto acids decarboxylate more readily due to stabilization of transition state

Q38. Which of the following will most increase acidity of acetic acid in the gas phase relative to solution?

• Presence of strong solvation by water
• Removal of solvation, showing intrinsic gas-phase acidity
• Conversion to methyl ester
• Hydrogen bonding with solvent molecules

Correct Answer: Removal of solvation, showing intrinsic gas-phase acidity

Q39. In a titration of a weak carboxylic acid with strong base, the half-equivalence point corresponds to:

• pH = 7 always
• pH = pKa of the acid
• Complete neutralization
• pH = pKb of conjugate base

Correct Answer: pH = pKa of the acid

Q40. Which structural feature of carboxylic acids contributes most to their ability to form hydrogen bonds?

• The carbon-carbon single bond
• The acidic O–H hydrogen and carbonyl oxygen acceptor
• The alkyl chain length only
• The aromatic ring in all acids

Correct Answer: The acidic O–H hydrogen and carbonyl oxygen acceptor

Q41. Which of these acids will be least likely to form a stable intramolecular hydrogen bond?

• o-Hydroxybenzoic acid (salicylic acid)
• m-Hydroxybenzoic acid
• o-Aminobenzoic acid
• o-Chlorobenzoic acid

Correct Answer: m-Hydroxybenzoic acid

Q42. Which reaction demonstrates the basicity of carboxylate anion in organic synthesis?

• Carboxylate acting as a nucleophile in esterification with alkyl halide
• Carboxylate acting as a reducing agent to alcohol
• Carboxylate undergoing electrophilic aromatic substitution
• Carboxylate spontaneously combusting

Correct Answer: Carboxylate acting as a nucleophile in esterification with alkyl halide

Q43. How does increasing temperature generally affect acidity (pKa) of carboxylic acids in solution?

• Always decreases pKa (more acidic)
• Always increases pKa (less acidic)
• Effect depends on enthalpy change of deprotonation
• Temperature has no effect

Correct Answer: Effect depends on enthalpy change of deprotonation

Q44. Which is true about hydrogen bonding in carboxylic acid dimers?

• They form a cyclic dimer stabilized by two hydrogen bonds
• They form ionic bonds only
• Dimers are stabilized by covalent bond formation between acids
• Dimers cannot form in gas phase

Correct Answer: They form a cyclic dimer stabilized by two hydrogen bonds

Q45. Which salt form is commonly used to increase bioavailability of acidic drugs?

• Free acid form only
• Sodium or potassium salt of the acid
• Neutral ester form only
• Metal oxide complex

Correct Answer: Sodium or potassium salt of the acid

Q46. Which carboxylic acid derivative typically has the highest acidity of the α-hydrogens?

• Simple ester
• β-Dicarbonyl compound (e.g., malonic ester)
• Amide
• Alkane

Correct Answer: β-Dicarbonyl compound (e.g., malonic ester)

Q47. Which functional group transformation reduces acidity most drastically?

• Conversion of carboxylic acid to carboxylate salt
• Conversion of carboxylic acid to ester
• Oxidation of carboxylic acid to carbon dioxide
• Formation of acid chloride

Correct Answer: Conversion of carboxylic acid to ester

Q48. For a medicinal chemist, why is knowledge of carboxylic acid pKa important?

• It determines color of the drug only
• It affects ionization, absorption, distribution and formulation strategies
• It is irrelevant to drug metabolism
• pKa only matters for basic drugs

Correct Answer: It affects ionization, absorption, distribution and formulation strategies

Q49. Which method best explains increased acidity in haloacetic acids compared to acetic acid?

• Inductive electron-withdrawing effect of halogen stabilizes conjugate base
• Halogens donate electrons to stabilize the acid
• Haloacetic acids are always dimerized
• Halogens convert the acid to an alcohol

Correct Answer: Inductive electron-withdrawing effect of halogen stabilizes conjugate base

Q50. Which of the following statements about carboxylic acids in drug design is FALSE?

• Carboxylic acids can form strong ionic interactions with biological targets
• Carboxylic acids always improve membrane permeability
• Carboxylic acids may require prodrug strategies to improve absorption
• Ionization state at physiological pH influences distribution and clearance

Correct Answer: Carboxylic acids always improve membrane permeability

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

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