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Carboxylic-acid strength and affecting factors: NEET MCQs with answers

50 questions on this topic of Organic Compounds Containing Oxygen (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Given below are two statements: Statement I: Carboxylate ion (RCOO⁻) is more stabilized by resonance than phenoxide ion (C₆H₅O⁻). Statement II: In carboxylate ion, the negative charge is delocalized over two highly electronegative oxygen atoms, whereas in phenoxide ion, it is delocalized over one oxygen atom and less electronegative carbon atoms. In light of the above statements, choose the most appropriate answer from the options given below:

  1. Both Statement I and Statement II are correct and Statement II is the correct explanation for Statement I
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
Show answer and explanation

Answer: (A) Both Statement I and Statement II are correct and Statement II is the correct explanation for Statement I

Even though phenoxide ion has more resonating structures, the negative charge in phenoxide resides predominantly on carbon atoms which are less electronegative than oxygen. In the carboxylate anion, the negative charge is equally shared between two identical, highly electronegative oxygen atoms, providing much greater resonance stabilization.

medium

2. Consider the following substituted benzoic acids: (I) 4-Aminobenzoic acid (II) 4-Cyanobenzoic acid (III) Benzoic acid (IV) 4-Formylbenzoic acid The correct sequence of increasing acidic strength is:

  1. (I) < (IV) < (III) < (II)
  2. (I) < (III) < (IV) < (II)
  3. (III) < (I) < (IV) < (II)
  4. (I) < (III) < (II) < (IV)
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Answer: (B) (I) < (III) < (IV) < (II)

The -NH₂ group is strongly electron-donating via resonance (+R > -I), greatly reducing acidity compared to benzoic acid. The -CHO and -CN groups are both strongly electron-withdrawing (-M/-R and -I), enhancing acidity. The cyano group (-CN) has a stronger overall electron-withdrawing effect than the formyl group (-CHO), making 4-cyanobenzoic acid more acidic than 4-formylbenzoic acid. Hence, the increasing order is (I) < (III) < (IV) < (II).

hard

3. Among chlorobutanoic acids, the strongest acid is:

  1. 4-Chlorobutanoic acid
  2. Butanoic acid
  3. 2-Chlorobutanoic acid
  4. 3-Chlorobutanoic acid
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Answer: (C) 2-Chlorobutanoic acid

The -I effect of chlorine decreases with distance from the carboxyl group, so chlorine at C-2 (α position) increases acidity most.

NCERT Class 12

medium

4. Which of the following statements is INCORRECT regarding the ortho-effect in substituted benzoic acids?

  1. The ortho-effect is solely dependent on the electronegativity and -I power of the ortho substituent.
  2. Ortho-substituted benzoic acids are usually stronger than benzoic acid regardless of the electronic nature of the substituent.
  3. Steric hindrance from the ortho-substituent causes the -COOH group to rotate out of the plane of the benzene ring.
  4. Loss of coplanarity inhibits the destabilizing +R resonance from the phenyl ring into the carboxylic group.
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Answer: (A) The ortho-effect is solely dependent on the electronegativity and -I power of the ortho substituent.

The ortho-effect is primarily a steric phenomenon, not solely dependent on the substituent's electronegativity or inductive effect. Bulk at the ortho position forces the -COOH group out of the aromatic plane, preventing cross-conjugation from the ring to the carbonyl carbon, thereby maximizing resonance stabilization within the carboxylate group itself.

easy

5. Which of the following carboxylic acids is the weakest acid (has the lowest Ka value)?

  1. HCOOH
  2. CH₃COOH
  3. (CH₃)₂CHCOOH
  4. (CH₃)₃CCOOH
Show answer and explanation

Answer: (D) (CH₃)₃CCOOH

Alkyl groups exert an electron-donating inductive effect (+I effect). The tert-butyl group in (CH₃)₃CCOOH has the strongest +I effect among the choices, which destabilizes the resulting carboxylate conjugate base the most and significantly decreases acidic strength.

hard

6. Consider the following three isomeric chlorobenzoic acids: (X) 2-Chlorobenzoic acid (Y) 3-Chlorobenzoic acid (Z) 4-Chlorobenzoic acid Which of the following statements is correct regarding their acidity?

  1. Y is less acidic than unsubstituted benzoic acid because chlorine donates electrons by resonance.
  2. Z is more acidic than Y because the resonance effect of chlorine operates strongly at the para position.
  3. X is the most acidic because the ortho effect and proximity of the -I effect strongly stabilize the conjugate base.
  4. All three isomers have identical pKa values because halogen atoms only exert inductive effects.
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Answer: (C) X is the most acidic because the ortho effect and proximity of the -I effect strongly stabilize the conjugate base.

2-Chlorobenzoic acid (X) is by far the strongest acid due to the ortho effect combined with the strong -I effect of chlorine positioned closest to the carboxylate group. For 3-chlorobenzoic acid (Y), chlorine acts only through -I effect, making it stronger than benzoic acid. For 4-chlorobenzoic acid (Z), the -I effect is partly counterbalanced by +R donation, making it slightly weaker than Y (pKa: ortho ≈ 2.92, meta ≈ 3.83, para ≈ 3.99).

medium

7. Given below are two statements: Statement I: The first dissociation constant (Ka₁) of maleic acid is significantly greater than that of fumaric acid. Statement II: The monoanion of maleic acid is stabilized by intramolecular hydrogen bonding, whereas that of fumaric acid cannot form such a hydrogen bond. In the light of the above statements, choose the most appropriate answer from the options given below:

  1. Both Statement I and Statement II are correct and Statement II is the correct explanation of Statement I.
  2. Both Statement I and Statement II are correct but Statement II is not the correct explanation of Statement I.
  3. Statement I is correct but Statement II is incorrect.
  4. Statement I is incorrect but Statement II is correct.
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Answer: (A) Both Statement I and Statement II are correct and Statement II is the correct explanation of Statement I.

In maleic acid (cis-isomer), the two carboxyl groups are adjacent to each other. Upon the loss of the first proton, the resulting mono-carboxylate ion forms a stable seven-membered ring via intramolecular hydrogen bonding with the neighboring un-ionized -COOH group, facilitating loss of the first proton and making Ka₁ (maleic) > Ka₁ (fumaric).

medium

8. Consider 4-methoxybenzoic acid and 3-methoxybenzoic acid. Which of the following statements correctly explains their relative acidic strengths compared to benzoic acid?

  1. 3-Methoxybenzoic acid is stronger than benzoic acid, whereas 4-methoxybenzoic acid is weaker than benzoic acid.
  2. Both are stronger acids than benzoic acid due to the -I effect of the methoxy group.
  3. Both are weaker acids than benzoic acid due to the strong +R effect of the methoxy group.
  4. 4-Methoxybenzoic acid is stronger than 3-methoxybenzoic acid due to resonance stabilization.
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Answer: (A) 3-Methoxybenzoic acid is stronger than benzoic acid, whereas 4-methoxybenzoic acid is weaker than benzoic acid.

At the meta position, the -OCH₃ group cannot exert its electron-donating resonance (+R) effect and operates only via an electron-withdrawing inductive (-I) effect, making 3-methoxybenzoic acid stronger than benzoic acid. At the para position, the strong +R effect dominates over the -I effect, releasing electron density and destabilizing the carboxylate anion, making 4-methoxybenzoic acid weaker than benzoic acid.

easy

9. Which of the following carboxylic acids has the highest acidic strength?

  1. Trichloroacetic acid (CCl₃COOH)
  2. Trifluoroacetic acid (CF₃COOH)
  3. Tribromoacetic acid (CBr₃COOH)
  4. Triiodoacetic acid (CI₃COOH)
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Answer: (B) Trifluoroacetic acid (CF₃COOH)

The acidic strength of carboxylic acids increases with the electron-withdrawing inductive effect (-I effect) of the substituents. Since fluorine has the highest electronegativity (F > Cl > Br > I), CF₃COOH stabilizes the carboxylate anion most effectively through a powerful -I effect, making it the strongest acid in the series.

easy

10. Which of the following carboxylic acids has the smallest value of pKa?

  1. CH₂ClCOOH
  2. CHCl₂COOH
  3. CH₃COOH
  4. CCl₃COOH
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Answer: (D) CCl₃COOH

Trichloroacetic acid (CCl₃COOH) has three strongly electron-withdrawing chlorine atoms exerting a powerful -I effect. This stabilizes the trichloroacetate conjugate base significantly, making it the strongest acid with the lowest pKa value.

medium

11. The pKa values of four organic acids are given below: Acid W: pKa = 0.65 Acid X: pKa = 2.87 Acid Y: pKa = 4.19 Acid Z: pKa = 4.76 Which of the following correctly pairs these acids with their likely structures?

  1. W: CH₃COOH, X: C₆H₅COOH, Y: CH₂ClCOOH, Z: CCl₃COOH
  2. W: CCl₃COOH, X: CH₂ClCOOH, Y: C₆H₅COOH, Z: CH₃COOH
  3. W: CH₂ClCOOH, X: CCl₃COOH, Y: CH₃COOH, Z: C₆H₅COOH
  4. W: CCl₃COOH, X: C₆H₅COOH, Y: CH₂ClCOOH, Z: CH₃COOH
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Answer: (B) W: CCl₃COOH, X: CH₂ClCOOH, Y: C₆H₅COOH, Z: CH₃COOH

Lower pKa indicates greater acid strength. CCl₃COOH has three strong -I chlorine atoms, giving it the lowest pKa (0.65). Monochloroacetic acid has pKa ≈ 2.87. Benzoic acid (Ka ≈ 6.5 × 10⁻⁵) has pKa ≈ 4.19, while acetic acid (Ka ≈ 1.74 × 10⁻⁵) has pKa ≈ 4.76.

easy

12. Given below are two statements: Statement I: Carboxylic acids are distinctly more acidic than alcohols and phenols. Statement II: The carboxylate ion (RCOO⁻) is stabilized by two equivalent resonance structures in which the negative charge resides entirely on electronegative oxygen atoms. In the light of the above statements, choose the most appropriate answer from the options given below:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
Show answer and explanation

Answer: (A) Both Statement I and Statement II are correct

Both statements are correct and directly from NCERT. Carboxylic acids are stronger acids than alcohols and phenols because their conjugate base (carboxylate anion) is stabilized by two equivalent resonance structures with delocalized negative charge over two highly electronegative oxygen atoms, unlike the phenoxide ion where delocalization involves less electronegative carbon atoms.

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