10 min readConcept sheet1-mark MCQ or assertion-reason on acid strength; 2 or 3-mark 'arrange in order of acidity' or 'why is chloroethanoic acid stronger'; also a NEET and JEE favourite

Acidity of carboxylic acids

Carboxylic acids are stronger than phenols because the carboxylate ion has two equivalent resonance forms. Electron-withdrawing groups raise acidity; electron-donating groups lower it.

Ka and pKaRCOOH + H₂O ⇌ H₃O⁺ + RCOO⁻; Ka = [H₃O⁺][RCOO⁻]/[RCOOH]; pKa = −log Ka; smaller pKa means a stronger acid

Typical pKa valuesTextbook: benzoic acid 4.19, ethanoic acid 4.76, phenol about 10, ethanol about 16. Data-book value: methanoic acid 3.75

Test for -COOHRCOOH + NaHCO₃ → RCOONa + H₂O + CO₂: brisk effervescence; phenols do not give it

Strength scalepKa below 1: strong; 1 to 5: moderately strong; 5 to 15: weak; above 15: extremely weak

Reactions that show acidity

1
2RCOOH + 2Na → 2RCOONa + H₂
Active metals give hydrogen gas
2
RCOOH + NaOH → RCOONa + H₂O
Alkalis give salts and water
3★
RCOOH + NaHCO₃ → RCOONa + H₂O + CO₂
Even weak bases such as hydrogencarbonates and carbonates react, with brisk effervescence. Phenols do not, so this is the test for a carboxyl group
4
RCOOH + H₂O ⇌ H₃O⁺ + RCOO⁻
In water they ionise only partly. They are weaker than mineral acids such as HCl, but on the textbook's scale (pKa 1 to 5) most are moderately strong acids

★ key step

Why carboxylic acids are stronger than phenols

Carboxylate ion, RCOO⁻Two equivalent resonance structures. The negative charge is shared equally by two oxygen atoms, both highly electronegative. So the ion is very stable.

Phenoxide ion, C₆H₅O⁻Its resonance structures are not equivalent. In some of them, the negative charge sits on ring carbon atoms, which are less electronegative. So it is less stable.

The more stable the conjugate base, the stronger the acid: RCOOH (pKa about 4 to 5) > C₆H₅OH (about 10) > C₂H₅OH (about 16). An alkoxide ion has no resonance at all.

Effect of substituents (pKa in brackets: 4.19, 4.76, 3.41 and 4.46 are the textbook's; the others are standard data-book values)

Change	Effect on acidity	Example (stronger > weaker)
Electron-withdrawing group (EWG) near -COOH	Increases: it spreads the negative charge of RCOO⁻ (−I or −R effect)	ClCH₂COOH (2.87) > CH₃COOH (4.76)
Electron-donating group (EDG)	Decreases: it pushes more charge onto RCOO⁻ (+I effect)	HCOOH (3.75) > CH₃COOH (4.76) > CH₃CH₂COOH (4.88)
More halogen atoms	Increases	Cl₃CCOOH (0.65) > Cl₂CHCOOH (1.29) > ClCH₂COOH (2.87)
More electronegative halogen	Increases	FCH₂COOH (2.66) > ClCH₂COOH (2.87) > BrCH₂COOH (2.90)
EWG farther from -COOH	Effect falls with distance	CH₃CH₂CHClCOOH (2.86) > CH₃CHClCH₂COOH (4.05) > ClCH₂CH₂CH₂COOH (4.52)
Phenyl or vinyl joined directly to -COOH	Increases: an sp² carbon is more electronegative than sp³	C₆H₅COOH (4.19) > CH₃COOH (4.76)
EWG or EDG on the ring of benzoic acid	EWG increases, EDG decreases	4-O₂NC₆H₄COOH (3.41) > C₆H₅COOH (4.19) > 4-CH₃OC₆H₄COOH (4.46)

Orders to learn

Groups in increasing order of electron-withdrawing power, and so of the acidity of XCH₂COOH: Ph < I < Br < Cl < F < CN < NO₂ < CF₃.
CF₃COOH > CCl₃COOH > CHCl₂COOH > NO₂CH₂COOH > NCCH₂COOH > FCH₂COOH > ClCH₂COOH > BrCH₂COOH > HCOOH > ClCH₂CH₂COOH > C₆H₅COOH > C₆H₅CH₂COOH > CH₃COOH > CH₃CH₂COOH.
Acid strength: carboxylic acid > phenol > water > alcohol.

Worked example: arrange in order

Problem

Arrange in increasing order of acid strength: (i) CH₃COOH, ClCH₂COOH, FCH₂COOH, CH₃CH₂COOH (ii) 4-nitrobenzoic acid, 4-methylbenzoic acid, benzoic acid, 3,5-dinitrobenzoic acid.

Working

(i) An ethyl group pushes electrons (+I) more than a methyl group, so CH₃CH₂COOH is the weakest, then CH₃COOH.
(i) Halogens withdraw electrons (−I). F is more electronegative than Cl, so FCH₂COOH is the strongest.
(i) Order: CH₃CH₂COOH < CH₃COOH < ClCH₂COOH < FCH₂COOH.
(ii) -CH₃ pushes electrons into the ring, so 4-methylbenzoic acid is weaker than benzoic acid.
(ii) -NO₂ withdraws electrons; two -NO₂ groups withdraw more than one.
(ii) Order: 4-methylbenzoic acid < benzoic acid < 4-nitrobenzoic acid < 3,5-dinitrobenzoic acid.

Answer

(i) CH₃CH₂COOH < CH₃COOH < ClCH₂COOH < FCH₂COOH (ii) 4-methylbenzoic < benzoic < 4-nitrobenzoic < 3,5-dinitrobenzoic acid

In real life

Baking soda fizzes with vinegar or lemon juice because carboxylic acids push CO₂ out of NaHCO₃, the same fizz used as the lab test. Sodium benzoate, a salt of benzoic acid, preserves bottled sauces and squashes.

Exam angle

2 marks: “Why is chloroethanoic acid a stronger acid than ethanoic acid?” Cl withdraws electrons (−I effect) and spreads the negative charge of ClCH₂COO⁻, making it more stable than CH₃COO⁻. pKa: 2.87 against 4.76.

3 marks: “Why are carboxylic acids stronger acids than phenols? Give a chemical test that shows this difference.” The carboxylate ion has two equivalent resonance structures with the charge on two O atoms; the phenoxide ion's structures are not equivalent and put charge on C atoms. Test: NaHCO₃ gives CO₂ effervescence with RCOOH, not with phenol.

3 marks: “Arrange in decreasing order of acidity and give reasons: CF₃COOH, CH₃COOH, CCl₃COOH, HCOOH.” CF₃COOH > CCl₃COOH > HCOOH > CH₃COOH. Three F atoms withdraw electrons more than three Cl atoms (F more electronegative). HCOOH has no alkyl group, but CH₃ in CH₃COOH pushes electrons (+I).

Remember it

Pull electrons, push acidity up. Closer and more electronegative wins: F > Cl > Br, α > β > γ, three Cl > two > one.