Organic Acid StrengthJEE Main

Concept

Acidity is decided by the stability of the conjugate base. Electron-withdrawing groups (−I: Cl, NO₂, F) spread the anion's negative charge — acid stronger, pKa lower. Electron donors (+I: alkyl) do the opposite. Resonance is the heavyweight: carboxylate delocalises over two equivalent oxygens (pKa ~4-5), phenoxide only into the ring (~10), alkoxide nothing (~16).

Key formula

Derivation

Compare conjugate bases: CH₃COO⁻ has two equal C–O bonds (X-ray proven) — perfect two-oxygen delocalisation.

Each Cl on the α-carbon pulls density inductively: acetic 4.76 → mono 2.86 → di 1.29 → tri 0.65. The −I effect decays with distance: 2-chlorobutanoic ≫ 4-chlorobutanoic acid.

Scenarios to explore

• Organic Acid Strength — pKa ladder — inductive & resonance effects on acidity.

Real-world applications

• Buffer choice (pKa ≈ target pH).
• Drug ionisation & absorption (Henderson–Hasselbalch).
• Ranking questions across COOH/phenol/alcohol families — exam staple.

JEE exam tips

• Ortho-effect: ortho-substituted benzoic acids are stronger than expected (twisting kills ring donation).
• p-nitrophenol > phenol by ~800×: draw the quinonoid resonance structure.
• Acidity of C–H: terminal alkyne (~25) > NH₃ (~38) > alkene (~44) > alkane (~50).

Common mistakes

• Higher pKa = stronger (inverted — LOWER pKa is stronger).
• Forgetting distance-decay of the inductive effect.
• Comparing resonance vs induction wrongly: formic vs acetic is +I, chloroacetic is −I.

Exam traps to avoid

• F₃C is stronger −I than Cl₃C, but fluoroacetic vs chloroacetic single-substitution order can flip in solution data — exams stick to Cl series.
• Phenol is NOT stronger than carbonic acid (pKa₁ 6.35): CO₂ liberates phenol from phenoxide.