Carboxylic acids (RCOOH) are among the most acidic organic compounds you encounter in NEET — more acidic than phenols and far more acidic than alcohols. The high-frequency confusion: confusing the factors that increase or decrease acidity, particularly when electron-withdrawing groups (EWGs) and electron-donating groups (EDGs) are involved.
Why carboxylic acids are acidic. On ionisation, RCOOH → RCOO⁻ + H⁺. The carboxylate anion is stabilised by resonance delocalisation of the negative charge over two equivalent oxygen atoms. This equivalent resonance (both C–O bonds identical, bond order 1.5) is the key distinction from phenol, where the negative charge delocalises over the ring — less effectively.
pKa benchmark (NCERT Class 12 Chemistry Chapter 8, page 248): Acetic acid pKa ≈ 4.76. For comparison, phenol pKa ≈ 10, aliphatic alcohols pKa ≈ 16–18.
Substituent effects — the factor NEET tests most:
Watch-out for NEET: When ranking substituted acids, check (1) nature of substituent (EWG vs EDG), (2) number of substituents, (3) position relative to –COOH. The distractor that ignores resonance-vs-induction balance is a common trap in phenol acidity comparison questions that also appear in this topic context.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
The pKa of acetic acid in aqueous solution is approximately:
Answer: C. Acetic acid (CH₃COOH) has pKa ≈ 4.76 in aqueous solution, a standard NCERT value (Class 12 Chemistry Chapter 8, page 248).
Why A is wrong: A (pKa 0.7) corresponds to trichloroacetic acid (Cl₃CCOOH), not acetic acid. Confusing the highly-substituted halogenated acid with the parent acid.
Why B is wrong: B (pKa 10) corresponds to phenol, not a carboxylic acid. Mixing up the two functional group benchmarks.
Why D is wrong: D (pKa 16) corresponds to aliphatic alcohols. Carboxylic acids are far more acidic due to resonance stabilisation of the carboxylate anion.
Carboxylic acids are more acidic than phenols primarily because:
Answer: C. The carboxylate anion (RCOO⁻) has two equivalent resonance structures with equal charge distribution over both oxygens, providing superior stabilisation compared to phenoxide (where charge delocalises over the ring unevenly). NCERT Class 12 Chemistry Chapter 8.
Why A is wrong: A is wrong because O–H bond strength is not the primary determinant of acidity in solution — it is the stability of the conjugate base that matters.
Why B is wrong: B is wrong because molecular weight has no direct relationship with acid strength. Formic acid (MW 46) is stronger than acetic acid (MW 60).
Why D is wrong: D is wrong because the carbonyl group is electron-withdrawing (not donating), and while it contributes to acidity, the key reason is equivalent resonance stabilisation of the conjugate base.
Among formic acid, acetic acid, and propionic acid, the correct order of acid strength is:
Answer: D. Formic acid (pKa 3.75) > Acetic acid (pKa 4.76) > Propionic acid (pKa 4.87). Increasing alkyl chain length provides greater +I (electron-donating) effect, destabilising the carboxylate anion and reducing acidity.
Why A is wrong: A reverses the correct order. Larger alkyl groups donate more electron density (+I effect), which destabilises the conjugate base and makes the acid weaker, not stronger.
Why B is wrong: B is wrong because the +I effect of alkyl groups creates measurable pKa differences. They are not equal.
Why C is wrong: C incorrectly places acetic acid above formic acid. The H atom in HCOOH has no +I effect, so formic acid's conjugate base is less destabilised → formic acid is stronger.
Arrange the following in decreasing order of acid strength: (I) CH₃COOH, (II) ClCH₂COOH, (III) Cl₂CHCOOH, (IV) Cl₃CCOOH
Answer: A. More Cl atoms at the α-position provide greater –I (electron-withdrawing) effect, stabilising the carboxylate anion more effectively. pKa values: Cl₃CCOOH (0.65) < Cl₂CHCOOH (1.29) < ClCH₂COOH (2.86) < CH₃COOH (4.76). Lower pKa = stronger acid.
Why B is wrong: B gives the reverse order. Multiple EWGs increase acidity (lower pKa), not decrease it. This error comes from confusing electron-withdrawing with electron-donating effects on the conjugate base.
Why C is wrong: C incorrectly places monochloroacetic acid as strongest. Each additional Cl atom provides additional –I stabilisation of the anion — more substituents means stronger acid.
Why D is wrong: D scrambles the middle ordering. The –I effect is cumulative: each extra Cl lowers the pKa further (4.76 → 2.86 → 1.29 → 0.65), though the steps shrink.
Which of the following is the strongest acid?
Answer: B. Among the α-haloacetic acids, fluorine has the highest electronegativity (4.0 > 3.0 > 2.8) and therefore exerts the strongest –I effect at the same position. FCH₂COOH has the lowest pKa (≈2.59) and is the strongest acid.
Why A is wrong: A (propionic acid) has no EWG — only an ethyl group with a +I effect — making it the weakest acid in this set.
Why C is wrong: C (chloroacetic acid, pKa ≈ 2.86) is weaker than fluoroacetic acid because Cl is less electronegative than F, providing a weaker –I effect at the α-carbon.
Why D is wrong: D (bromoacetic acid, pKa ≈ 2.90) is weaker than both F and Cl substituted acids because Br has the lowest electronegativity among the three halogens listed.
p-Nitrobenzoic acid is a stronger acid than benzoic acid because the –NO₂ group:
Answer: A. The –NO₂ group is a strong EWG that operates via both inductive (–I) and resonance (–R) effects. At the para position, it effectively withdraws electron density from the carboxylate anion, stabilising it and increasing acidity.
Why B is wrong: B is wrong because –NO₂ is an electron-withdrawing group, not electron-donating. It withdraws electrons by both –I and –R mechanisms. Confusing –NO₂ with EDGs like –OH or –NH₂ is the error here.
Why C is wrong: C is wrong because molecular weight has no mechanistic relationship to acid strength. The relevant factor is electronic effect on conjugate base stability.
Why D is wrong: D is wrong because intramolecular H-bonding between –NO₂ and –COOH is not geometrically feasible at the para position, and even if it were, it would not explain increased acidity.
Consider 3-chlorobutanoic acid and 4-chlorobutanoic acid. Which statement correctly compares their acid strengths?
Answer: D. The –I effect operates through σ-bonds and falls off rapidly with the number of intervening carbons. At the β-position (3-chloro), Cl is closer to the –COOH group and exerts a stronger electron-withdrawing effect on the carboxylate anion than at the γ-position (4-chloro).
Why A is wrong: A reverses the distance effect. Greater distance from –COOH means weaker –I influence on the carboxylate anion, resulting in lower acidity — not higher.
Why B is wrong: B is wrong because although both have one Cl, the position matters critically. The –I effect is distance-dependent, so the number of carbons between Cl and –COOH determines how much stabilisation the anion receives.
Why C is wrong: C is wrong because Cl is an electron-withdrawing group (–I effect), not electron-donating. Both compounds are acidic; the question is which is more acidic.
Arrange the following in increasing order of acid strength: (I) Benzoic acid, (II) p-Methoxybenzoic acid, (III) p-Nitrobenzoic acid, (IV) p-Methylbenzoic acid
Answer: B. Step 1: Classify substituents. –OCH₃ is EDG (+R dominates over –I at para), –CH₃ is weak EDG (+I, +hyperconjugation), –NO₂ is strong EWG (–I, –R). Step 2: EDGs decrease acidity (destabilise carboxylate), EWGs increase it. Ordering by decreasing +electron-donation: –OCH₃ > –CH₃ > H > –NO₂. Therefore increasing acid strength: p-methoxy < p-methyl < benzoic < p-nitro, i.e., II < IV < I < III.
Why A is wrong: A reverses the entire order. –NO₂ is EWG (increases acidity), not EDG, and –OCH₃ is EDG (decreases acidity). This answer confuses which direction each substituent pushes the equilibrium.
Why C is wrong: C incorrectly places p-methyl as weakest. –OCH₃ has a stronger +R effect than –CH₃ (which operates only by +I and hyperconjugation), so p-methoxy is the weakest acid.
Why D is wrong: D incorrectly places unsubstituted benzoic acid as weakest. With no substituent, benzoic acid is intermediate — weaker than p-nitro but stronger than both EDG-substituted acids.
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Given
Four compounds to rank by acidity in aqueous solution:• p-Cresol: phenol with para –CH₃ (+I, EDG)• Phenol: pKa ≈ 10• p-Nitrophenol: phenol with para –NO₂ (EWG, –I and –R)• Acetic acid: pKa ≈ 4.76
Required
Decreasing order of acid strength (lowest pKa first).
Concept
Acid strength depends on conjugate base stability. For substituted phenols, EWG stabilises phenoxide (increases acidity), EDG destabilises it (decreases acidity). Carboxylic acids are inherently more acidic than phenols due to equivalent resonance.
Formula
pKa comparison: RCOOH (≈4–5) < substituted phenols with strong EWG (≈7–8) < phenol (≈10) < substituted phenols with EDG (>10).
Substitution
• Acetic acid: pKa = 4.76• p-Nitrophenol: pKa ≈ 7.15 (–NO₂ withdraws electrons, stabilises phenoxide)• Phenol: pKa = 10.0• p-Cresol: pKa ≈ 10.3 (–CH₃ donates electrons by +I, destabilises phenoxide slightly)
Calculation
Ranking by pKa (lower = stronger): 4.76 < 7.15 < 10.0 < 10.3
Final answer
Decreasing acid strength: Acetic acid > p-Nitrophenol > Phenol > p-Cresol
Common trap
Confusing the acidity ordering by assuming "all phenols are more acidic than carboxylic acids" — the opposite is true. Carboxylic acids have pKa 4–5 while even strongly EWG-substituted phenols rarely go below pKa 7. The resonance-vs-induction balance trap (from NEET pattern: phenol electrophilic substitution) applies: students sometimes rank p-nitrophenol above carboxylic acids because –NO₂ "sounds more powerful."
Similar NEET-style question
"Arrange in increasing order of acidity: (i) Ethanol, (ii) Phenol, (iii) p-Chlorophenol, (iv) Propanoic acid." (Answer: Ethanol < Phenol < p-Chlorophenol < Propanoic acid)
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Carboxylic acids (pKa ~4-5) more acidic than alcohols and phenols due to resonance stabilisation of carboxylate ion (delocalised over 2 O atoms). Electron-withdrawing groups increase acidity (Cl₃CCOOH << CCl₃COOH; pKa 0.7 vs CH₃COOH pKa 4.76).
-- NCERT Class 12 Chemistry, Ch. 8, p. 250Stronger acid than phenol due to more effective resonance over two equivalent oxygens. EWG substituents (Cl, NO2) increase acidity.
| Symbol | Quantity | SI Unit |
|---|---|---|
| pKa | -log Ka | - |
More in Alcohols, Phenols, Ethers and Carbonyl Compounds: 3 exam traps and mistakes · 1 formula · 3 question patterns from its other lessons.
1 question from NEET 2025. Answers verified against NTA official keys.
The correct order of decreasing acidity of the following aliphatic acids is
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