3.6. CARBOXYLIC ACIDS AND DERIVATIVES

What are carboxylic acids and their properties

• Functional Group: -COOH.

• Properties:

  • Weak acids; do not fully dissociate in water.

  • Equilibrium lies to the left, with fewer H⁺ ions in solution.

  • pH < 7.

How can primary alcohols and aldehydes be oxidised to form carboxylic acids?

• Reagents:

  • Acidified potassium dichromate (K₂Cr₂O₇) or acidified potassium manganate (KMnO₄).

• Conditions:

  • Reflux the alcohol or aldehyde with oxidising agent.

• Reaction:

  1. Primary Alcohol → Aldehyde → Carboxylic Acid: CH₃CH₂OH + [O] → CH₃CHO + [O] → CH₃COOH.

  2. Aldehyde → Carboxylic Acid: CH₃CHO + [O] → CH₃COOH.

• Colour Changes:

  • K₂Cr₂O₇: Orange dichromate ions (Cr₂O₇²⁻) → Green Cr³⁺ ions.

  • KMnO₄: Purple manganate ions (MnO₄⁻) → Colourless Mn²⁺ ions.

How are nitriles hydrolysed to form carboxylic acids?

• Reagents:

  • Dilute acid or dilute alkali, followed by acidification.

• Reaction Mechanism:

  1. Dilute Acid:

     • Hydrolysis forms carboxylic acid and ammonium salt: R–CN + 2H₂O + H⁺ → R–COOH + NH₄⁺.

  2. Dilute Alkali + Acidification:

     • Hydrolysis forms sodium carboxylate and ammonia: R–CN + NaOH → R–COONa + NH₃.

     • Acidification converts carboxylate to carboxylic acid: R–COONa + H⁺ → R–COOH + Na⁺.

• Key Note:

  • Converts the -CN group at the chain end into a -COOH group.

How are esters hydrolysed to form carboxylic acids?

• Reagents:

  • Dilute acid or dilute alkali, followed by acidification.

• Conditions:

  • Heat during hydrolysis.

• Reaction Mechanism:

  1. Dilute Acid Hydrolysis (Reversible):

     • Forms alcohol and carboxylic acid: RCOOR' + H₂O ⇌ RCOOH + R'OH.

  2. Dilute Alkali Hydrolysis (Irreversible):

     • Forms alcohol and sodium carboxylate salt: RCOOR' + NaOH → RCOONa + R'OH.

     • Acidification converts carboxylate to carboxylic acid: RCOONa + H⁺ → RCOOH + Na⁺.

What are the key reactions of carboxylic acids?

• Reactions Include:

  1. Redox: With reactive metals to form hydrogen gas.

  2. Neutralisation: With alkalis to form salt and water.

  3. Acid-Base: With carbonates to produce CO₂ gas.

  4. Esterification: With alcohols to form esters.

  5. Reduction: Using LiAlH₄ to form primary alcohols.

How are esters produced via esterification?

• Reaction Overview:

  • Esters are formed from the condensation reaction between a carboxylic acid and an alcohol.

  • Requires concentrated H₂SO₄ as a catalyst.

• Reagents & Conditions:

  • Carboxylic acid + alcohol, heated under reflux with concentrated H₂SO₄.

• Example Reaction: CH₃COOH + C₃H₇OH ⇌ CH₃COOC₃H₇ + H₂O (Ethanoic acid reacts with propanol to form propyl ethanoate and water).

• Key Notes:

  • The ester’s name: First part comes from the alcohol; second part comes from the carboxylic acid.

  • Esters are known for their sweet, fruity aromas.

What happens during the hydrolysis of esters using dilute acid?

• Reaction Overview:

  • Esters are hydrolysed to reform the alcohol and carboxylic acid.

  • Dilute acid acts as a catalyst and hydrolysis is reversible, establishing an equilibrium.

• Reagents & Conditions:

  • Dilute acid (e.g., H₂SO₄) and heat.

• Example Reaction: CH₃COOC₂H₅ + H₂O ⇌ CH₃COOH + C₂H₅OH (Ethyl ethanoate hydrolyses to form ethanoic acid and ethanol).

• Key Notes:

  • Reversible reaction; equilibrium mixture contains ester, alcohol, carboxylic acid, and water.

What happens during the hydrolysis of esters using dilute alkali?

• Reaction Overview:

  • Esters are fully hydrolysed, producing a carboxylate salt and alcohol.

  • The reaction is irreversible.

• Reagents & Conditions:

  • Dilute alkali (e.g., NaOH) and heat.

• Reaction Mechanism:

  1. Ester reacts with alkali to produce a sodium carboxylate salt and alcohol. CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH.

  2. Acidification converts the carboxylate salt into carboxylic acid. CH₃COONa + HCl → CH₃COOH + NaCl.

• Key Notes:

  • Irreversible reaction; ester is completely broken down.