Explain why carboxylic acids are more acidic than phenols and alcohols. How do electron-withdrawing and electron-donating groups affect their acidity? Arrange CH₃COOH, ClCH₂COOH, Cl₂CHCOOH and HCOOH in increasing order of acidity.

1. 1 · Define acid strength. RCOOH + H₂O ⇌ H₃O⁺ + RCOO⁻; the larger the Ka (smaller pKa), the stronger the acid. Strength depends on the stability of the conjugate base.
2. 2 · Explain the carboxylate ion. RCOO⁻ has two equivalent resonance structures; the negative charge is shared by two electronegative O atoms.
3. 3 · Draw the resonance. Draw R-C(=O)-O⁻ ↔ R-C(-O⁻)=O with a double-headed arrow, then the hybrid with half a negative charge on each O.
4. 4 · Compare with phenol and alcohol. Phenoxide structures are not equivalent and put charge on carbon; an alkoxide has no resonance. So RCOOH > phenol > alcohol.
5. 5 · Give the substituent effects. Electron-withdrawing groups (Cl, F, NO₂, CN) stabilise RCOO⁻ and raise acidity; electron-donating groups (alkyl) lower it.
6. 6 · Arrange. CH₃COOH (4.76) < HCOOH (3.75) < ClCH₂COOH (2.87) < Cl₂CHCOOH (1.29).
7. 7 · Link to real life. Baking soda fizzes with vinegar: acetic acid is strong enough to free CO₂ from NaHCO₃.
8. 8 · Close in one line. So acidity follows the stability of the conjugate base.

Scoring tip: Always link acidity to the stability of the anion, and name the effect (−I or +I) for each group you compare.