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Question -1 (3 Marks)
An organic compound ‘A’ with molecular formula C₄H₈O₂ undergoes acid hydrolysis to form two compounds ‘B’ and ‘C’. Oxidation of ‘C’ with acidified potassium permanganate also produces ‘B’. Sodium salt of ‘B’ on heating with soda-lime gives methane.
(a) Identify ‘A’, ‘B’ and ‘C’.
(b) Out of ‘B’ and ‘C’, which will have higher boiling point? Give reason.
Answer
- Infer functional class of A
- C₄H₈O₂ fits a saturated ester/acid formula.
- “Acid hydrolysis gives two compounds” → typical of ester hydrolysis (acid + alcohol).
- Use the soda-lime clue to find B
- “Sodium salt of B on heating with soda-lime gives methane.”
- Decarboxylation: R–COONa + NaOH/CaO → R–H + Na₂CO₃.
- To get methane (CH₄), R must be CH₃ → B = ethanoic (acetic) acid, CH₃COOH.
- Use the oxidation clue to find C
- “Oxidation of C with acidified KMnO₄ also produces B.”
- Primary alcohol that oxidizes to CH₃COOH is ethanol (C₂H₅OH) → C = ethanol.
- Identify A from hydrolysis products
- Ester that on acid hydrolysis gives CH₃COOH (B) + C₂H₅OH (C) is ethyl ethanoate (ethyl acetate), CH₃COOCH₂CH₃.
- Check formula: C₄H₈O₂
- Equations (for confirmation)
- Hydrolysis: CH₃COOCH₂CH₃ + H₂O/H⁺ → CH₃COOH (B) + C₂H₅OH (C)
- Oxidation: C₂H₅OH + [O] (KMnO₄/H⁺) → CH₃COOH (B)
- Soda-lime: CH₃COONa + NaOH/CaO → CH₄ + Na₂CO₃
(a) Answers
- A: Ethyl ethanoate (ethyl acetate), CH₃COOCH₂CH₃
- B: Ethanoic (acetic) acid, CH₃COOH
- C: Ethanol, C₂H₅OH
(b) Higher boiling point
- B (ethanoic acid) has a higher boiling point than C (ethanol) because carboxylic acids form stronger intermolecular hydrogen bonds and exist as dimers, and they also have higher molar mass → greater intermolecular attraction → higher b.p. (acetic acid ≈118 °C vs ethanol ≈78 °C).
Question-2 (3 Marks)
What are three common tests for carboxylic acids with reactions?
Three Common Tests for Carboxylic acids
1. Sodium Bicarbonate Test (Effervescence Test)
Principle: Carboxylic acids react with sodium bicarbonate (NaHCO₃) to liberate carbon dioxide gas, which produces effervescence.
Reaction:
Observation: Brisk effervescence of CO₂ gas. If the gas is passed through lime water, it turns milky due to formation of CaCO₃.
2. Esterification Test
Principle: Carboxylic acids react with alcohols in presence of concentrated sulfuric acid to form esters, which have a fruity smell.
Reaction:
Observation: Fruity odor confirms the presence of carboxylic acid.
3. Litmus Test
Principle: Carboxylic acids are acidic in nature, so they turn blue litmus paper red.
Reaction (acidic dissociation):
Observation: Blue litmus turns red.
