CBSE Class 12 Chemistry: Carboxylic Acids — Complete Notes 2026

Structure and Acidity of Carboxylic Acids

• Carboxyl group −COOH: carbonyl + hydroxyl; electron withdrawal from O−H bond makes H more acidic than alcohol
• Resonance in carboxylate anion: charge delocalised equally over both oxygens; carboxylate more stable than phenoxide in some aspects
• pKa of acetic acid ≈ 4.74; stronger acid than phenol (10) and water (15.7) but weaker than HCl (−7)

Preparation of Carboxylic Acids

• Oxidation of 1° alcohols or aldehydes: K₂Cr₂O₇/H₂SO₄ or KMnO₄
• Hydrolysis of nitriles: RCN + H₂O (H⁺ or OH⁻) → RCOOH; carbon chain extended by one
• Grignard + CO₂: RMgX + CO₂ → RCOOMgX → RCOOH (acid workup); increases chain length by one carbon

Reactions of Carboxylic Acids

• Esterification: RCOOH + R'OH ⇌ RCOOR' + H₂O (H⁺ catalyst, reversible; Fischer esterification)
• Acid chloride formation: RCOOH + SOCl₂ → RCOCl + SO₂ + HCl; more reactive derivative
• Decarboxylation: RCOOH → RH + CO₂; heating with soda lime (NaOH + CaO); used to reduce chain length

Acidic Strength Comparison

• Electron-withdrawing groups (−Cl, −F, −NO₂) increase acidity by stabilising carboxylate anion
• Trichloroacetic acid (CCl₃COOH) pKa = 0.66 much stronger than acetic acid (pKa = 4.74)
• Electron-donating groups (−CH₃) decrease acidity: propanoic acid slightly weaker than acetic acid

Derivatives of Carboxylic Acids

• Esters (RCOOR'): fruity smell; formed by esterification; hydrolysis gives acid + alcohol
• Amides (RCONH₂): formed from acid + NH₃; mild acid character; nylon peptide bond is amide linkage
• Anhydrides (RCO−O−OCR): from acid + acid chloride; highly reactive acylating agents; acetic anhydride → aspirin

Industrial Importance

• Acetic acid (vinegar): food preservation; acetate fibres; aspirin synthesis
• Citric acid: found in citrus fruits; used in food (E330), beverages, cleaning
• Amino acids contain −COOH and −NH₂; peptide bond between amino acids = amide bond; fundamental to proteins

CBSE Board Focus

• Carboxylic acids: 4–5 marks; preparation from Grignard + CO₂, esterification mechanism
• Acidity order problems: rank acids given structural formulas; explain using inductive/resonance effects
• Decarboxylation: reagent (NaOH + CaO), product, practical importance