MP Board Class 12 Chemistry: Alcohols Phenols & Ethe…

MP Board Class 12 Chemistry: Alcohols, Phenols & Ethers Important Questions 2027 — Alcohols, Phenols and Ethers is one of the most important chapters in Class 12 Organic Chemistry, carrying 8–10 marks in the MP Board 2027 exam. This chapter covers the structure, nomenclature, preparation methods, physical properties, and chemical reactions of alcohols, phenols, and ethers. Mastering this chapter is essential because it forms the foundation for understanding Aldehydes, Ketones, Carboxylic Acids, and Biomolecules. This guide provides the most important questions with detailed answers, reaction mechanisms, and exam tips to help you score full marks.

📊 1. Chapter Overview & Mark Distribution

Alcohols, Phenols and Ethers is Chapter 7 of Class 12 NCERT Chemistry (Part II). In the MP Board exam, this chapter contributes approximately 8–10 marks, making it the third most important Organic Chemistry chapter after Aldehydes/Ketones and Biomolecules.

❓ 2. Very Short Answer Type Questions (1 Mark)

Q1. What is the general formula of monohydric alcohols?

Answer: C_nH_2n+1OH or R—OH, where R is an alkyl group.

Q2. What is the IUPAC name of CH₃CH(OH)CH₃?

Answer: Propan-2-ol (or Isopropyl alcohol).

Q3. Give the structure of picric acid.

Answer: 2,4,6-Trinitrophenol — C₆H₂(NO₂)₃OH.

Q4. What is the product when ethanol is heated with excess conc. H₂SO₄ at 170°C?

Answer: Ethene (C₂H₄) is formed by dehydration of ethanol.

Q5. Write the formula of Lucas reagent.

Answer: Lucas reagent is a mixture of anhydrous ZnCl₂ and concentrated HCl.

Q6. Which reagent is used for the distinction between primary, secondary and tertiary alcohols?

Answer: Lucas reagent (anhydrous ZnCl₂ + conc. HCl). Primary alcohols do not react at room temperature, secondary react in 5–10 minutes, tertiary react immediately.

Q7. What is the IUPAC name of anisole?

Answer: Methoxybenzene (C₆H₅OCH₃).

Q8. Why is phenol more acidic than ethanol?

Answer: In phenol, the phenoxide ion is stabilized by resonance with the benzene ring, while in ethanol the ethoxide ion has no such resonance stabilization. The phenoxide ion is more stable than ethoxide ion, making phenol more acidic.

📝 3. Short Answer Type Questions (2–3 Marks)

Q9. Explain the preparation of ethanol from glucose by fermentation. (3 marks)

Answer: Ethanol is commercially prepared by fermentation of glucose (from molasses, sugarcane juice, or grains). The process involves the following steps:

• Glucose (C₆H₁₂O₆) is converted to ethanol in the presence of the enzyme zymase, present in yeast.
• Chemical reaction: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
• The fermentation is carried out at 30–35°C in the absence of air (anaerobic conditions).
• The resulting solution contains 8–10% ethanol, which is then concentrated by fractional distillation to obtain 95% ethanol (rectified spirit).
• Absolute alcohol (99.5% ethanol) is obtained by removing the remaining water using azeotropic distillation with benzene.

Q10. How can you distinguish between phenol and ethanol? Give chemical tests. (3 marks)

Answer: Phenol can be distinguished from ethanol using the following tests:

Q11. Explain the mechanism of dehydration of ethanol to ethene. (3 marks)

Answer: Dehydration of ethanol to ethene is an E1 elimination reaction carried out in the presence of conc. H₂SO₄ at 170°C.

Step 1 — Protonation: Ethanol is protonated by H₂SO₄ to form ethyl oxonium ion (CH₃CH₂OH₂⁺).

Step 2 — Formation of carbocation: The ethyl oxonium ion loses a water molecule to form ethyl carbocation (CH₃CH₂⁺).

Step 3 — Elimination of proton: The carbocation loses a proton (H⁺) from the adjacent carbon to form ethene (CH₂=CH₂). The H⁺ is regained by HSO₄⁻ to reform H₂SO₄.

Note: At 140°C, ether (diethyl ether) is formed instead of ethene through a different mechanism (SN2).

Q12. What are the products when ethanol reacts with (a) Na (b) PCl₅ (c) CH₃COOH (in presence of conc. H₂SO₄)? (2 marks)

• (a) Na: 2C₂H₅OH + 2Na → 2C₂H₅ONa (Sodium ethoxide) + H₂
• (b) PCl₅: C₂H₅OH + PCl₅ → C₂H₅Cl (Chloroethane) + POCl₃ + HCl
• (c) CH₃COOH (Esterification): C₂H₅OH + CH₃COOH ⇌ CH₃COOC₂H₅ (Ethyl acetate) + H₂O

✍️ 4. Long Answer Type Questions (4–5 Marks)

Q13. Explain in detail the preparation of phenol from cumene (cumene process). Write all chemical equations. (5 marks)

Answer: The cumene process (also called Hock process) is the most important industrial method for the preparation of phenol. Steps are:

• Step 1 — Alkylation of benzene: Benzene is alkylated with propylene in the presence of H₃PO₄ to form isopropylbenzene (cumene). C₆H₆ + CH₃CH=CH₂ → C₆H₅CH(CH₃)₂
• Step 2 — Oxidation: Cumene is oxidized with O₂ to form cumene hydroperoxide. C₆H₅CH(CH₃)₂ + O₂ → C₆H₅C(CH₃)₂OOH
• Step 3 — Acid cleavage: Cumene hydroperoxide is treated with dilute H₂SO₄ to give phenol and acetone. C₆H₅C(CH₃)₂OOH → C₆H₅OH + (CH₃)₂CO

Advantages: (i) Phenol and acetone (a valuable byproduct) are obtained simultaneously. (ii) The process is cost-effective and widely used industrially.

Q14. Describe the electrophilic substitution reactions of phenol. Explain with mechanisms and examples. (5 marks)

Answer: Phenol undergoes electrophilic substitution reactions readily because the —OH group activates the benzene ring by +R effect (resonance). The ortho and para positions become electron-rich and are attacked by electrophiles.

1. Nitration:

• Dilute HNO₃ at 298K gives a mixture of o-nitrophenol (major) and p-nitrophenol (minor).
• With conc. HNO₃ + conc. H₂SO₄, 2,4,6-trinitrophenol (picric acid) is formed.

2. Halogenation (Bromination):

• Bromine water at room temperature gives 2,4,6-tribromophenol (white precipitate).
• This reaction is used as a test for phenol.

3. Kolbe’s Reaction (Carboxylation):

• Sodium phenoxide is treated with CO₂ at 400K under 4–7 atm, followed by acidification to give salicylic acid (o-hydroxybenzoic acid).
• C₆H₅ONa + CO₂ → C₆H₄(OH)COONa → C₆H₄(OH)COOH (salicylic acid)

4. Reimer-Tiemann Reaction:

• Phenol is treated with CHCl₃ in the presence of NaOH at 340K, followed by acidification to give salicylaldehyde (o-hydroxybenzaldehyde).

Q15. Explain the Williamson synthesis of ethers. Give its mechanism and limitations. (4 marks)

Answer: Williamson synthesis is the most important method for preparing both symmetrical and unsymmetrical ethers. It involves the reaction of sodium alkoxide (or phenoxide) with an alkyl halide.

• General reaction: R—ONa + R’—X → R—O—R’ + NaX (SN2 mechanism)
• For symmetrical ethers: CH₃ONa + CH₃Br → CH₃OCH₃ + NaBr
• For unsymmetrical ethers: C₂H₅ONa + CH₃I → C₂H₅OCH₃ + NaI

Limitation: Tertiary alkyl halides cannot be used because they undergo elimination (E2) instead of substitution (SN2), forming alkenes instead of ethers.

Rule for unsymmetrical ethers: The alkyl halide should be primary and the alkoxide can be primary, secondary, or tertiary. This gives the best yield.

⚗️ 5. Name Reactions & Mechanisms

This section contains all the important name reactions from this chapter that are frequently asked in the MP Board exam. Memorize each one with reaction conditions.

📋 6. Previous Year Questions (PYQs)

Here are the actual questions asked in previous MP Board exams from this chapter. Practicing these gives you a clear idea of the question pattern.

MP Board 2025 Chemistry Paper

• Q. Explain Kolbe’s reaction with chemical equation. (3 marks)
• Q. Write the IUPAC names of (a) CH₃CH(OH)CH₂CH₃ (b) C₆H₅OCH₃ (2 marks)
• Q. How can you distinguish between phenol and ethanol? Give any two chemical tests. (2 marks)

MP Board 2024 Chemistry Paper

• Q. Explain the mechanism of dehydration of ethanol to ethene. (3 marks)
• Q. Write Williamson synthesis of ethers. Give one example. (2 marks)
• Q. Why is phenol more acidic than ethanol? Explain. (2 marks)

MP Board 2023 Chemistry Paper

• Q. Explain Reimer-Tiemann reaction with chemical equation and conditions. (3 marks)
• Q. Distinguish between primary, secondary and tertiary alcohols using Lucas test. (3 marks)
• Q. What is the product when ethanol reacts with conc. H₂SO₄ at 413K? Write the reaction. (1 mark)

MP Board 2022 Chemistry Paper

• Q. Explain Kolbe’s reaction with mechanism. (4 marks)
• Q. Write the preparation of phenol from cumene. (3 marks)
• Q. Write two methods for the preparation of diethyl ether. (2 marks)
• Q. What happens when ethanol vapour is passed over Cu at 573K? (1 mark)

🧮 7. Numerical Problems & Conversions

Q16. How is phenol converted to the following compounds? Write chemical equations. (3 marks)

• (a) Phenol to salicylic acid: Kolbe’s reaction — Phenol + CO₂ (NaOH, 400K, 4–7 atm) → Sodium salicylate → H⁺ → Salicylic acid
• (b) Phenol to salicylaldehyde: Reimer-Tiemann reaction — Phenol + CHCl₃ (NaOH, 340K) → H⁺ → Salicylaldehyde
• (c) Phenol to picric acid: Phenol + conc. HNO₃ + conc. H₂SO₄ → 2,4,6-Trinitrophenol (Picric acid)

Q17. How will you convert ethanol to the following? (2 marks)

• (a) Ethanol to ethene: Conc. H₂SO₄, 443K (170°C) — Dehydration
• (b) Ethanol to diethyl ether: Conc. H₂SO₄, 413K (140°C) — Intermolecular dehydration
• (c) Ethanol to acetaldehyde: Cu, 573K — Dehydrogenation
• (d) Ethanol to ethyl acetate: CH₃COOH + conc. H₂SO₄ — Esterification

Q18. Arrange the following in increasing order of acidity. (2 marks)

(a) Ethanol, Phenol, Water, Acetic acid

Answer: Ethanol < Water < Phenol < Acetic acid

Reason: Ethanol is neutral (pKa ≈ 16), Water is weakly acidic (pKa ≈ 15.7), Phenol is more acidic due to resonance stabilization of phenoxide ion (pKa ≈ 10), Acetic acid is most acidic due to carboxyl group resonance (pKa ≈ 4.76).

⚠️ 8. Exam Tips & Common Mistakes

✅ Do’s — Must Follow

1. Draw mechanisms clearly: For every name reaction, show the stepwise mechanism with curved arrows, intermediates, and final products. MP Board examiners award marks for proper mechanistic understanding.
2. Write conditions carefully: Always mention temperature, catalyst, pressure, and time for each reaction. E.g., “Conc. H₂SO₄ at 170°C” (NOT just “heat”).
3. Use correct IUPAC names: Practice naming 10 compounds daily from this chapter. Propan-2-ol (not isopropyl alcohol), Methoxybenzene (not anisole).
4. Memorize distinguishing tests: FeCl₃ test for phenol, Lucas test for alcohol classification, Iodoform test for ethanol — these are 2-mark guaranteed questions.
5. Practice chemical conversions: Learn at least 10 conversion sequences (e.g., Benzene → Phenol → Salicylic acid → Aspirin).

❌ Don’ts — Common Mistakes

1. Confusing dehydration conditions: 170°C = ethene (intramolecular), 140°C = diethyl ether (intermolecular). Getting this wrong loses 3 marks.
2. Writing incomplete reactions: In Kolbe’s reaction, don’t forget to mention that acidification (H⁺) is required to obtain the final product from sodium salicylate.
3. Wrong acidity order: Students often write “Phenol > Acetic acid” — WRONG. Acetic acid (pKa 4.76) is far more acidic than phenol (pKa 10).
4. Forgetting about resonance in phenoxide: When explaining phenol’s acidity, always show at least 3 resonance structures of the phenoxide ion. This impresses examiners.
5. Ignoring byproducts: In Williamson synthesis, mention NaCl or KBr as byproduct. In esterification, mention water as byproduct. Full marks require complete balanced equations.
6. Using wrong reagent: For Lucas test, use “anhydrous ZnCl₂ + conc. HCl”. Writing just “HCl” without ZnCl₂ is incorrect and loses marks.

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