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Alcohols, Phenols and Ethers – Class 12 Chemistry notes, important questions and MCQs

NCERT Class 12 Chemistry, chapter 7. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Alcohols (R–OH), phenols (Ar–OH) and ethers (R–O–R) are oxygen-containing organic compounds with characteristic preparation methods and reactions.

Alcohols and phenols

Classified as 1°, 2°, 3°; Lucas test (ZnCl₂/HCl): 3° turbid at once, 2° in 5 min, 1° not at room temperature.
Oxidation: 1° → aldehyde → acid; 2° → ketone; 3° resist oxidation.
Phenol is more acidic than alcohols (phenoxide ion resonance-stabilised); electron-withdrawing groups (–NO₂) increase acidity.

Important reactions

Reimer–Tiemann: phenol + CHCl₃ + NaOH → salicylaldehyde; Kolbe: phenoxide + CO₂ → salicylic acid.
Williamson synthesis: R–ONa + R′X → R–O–R′ (ether).
Dehydration of ethanol with conc. H₂SO₄: 443 K gives ethene; 413 K gives diethyl ether.

Key terms

Lucas reagent: anhydrous ZnCl₂ in conc. HCl, distinguishes 1°, 2°, 3° alcohols
Williamson synthesis: preparation of ethers from alkoxide and alkyl halide
Hydroboration–oxidation: addition of B₂H₆ to an alkene followed by H₂O₂/OH⁻, giving an anti-Markovnikov alcohol

Formulas

Acidity: phenol > water > ethanol

Important questions with answers

Why do alcohols have higher boiling points than ethers of comparable mass?2 marks

Alcohols form intermolecular hydrogen bonds through –OH; ethers cannot, so more energy is needed to separate alcohol molecules.

Why is phenol more acidic than ethanol? Explain with the effect of a nitro group.3 marks

After losing H⁺, the phenoxide ion is stabilised by resonance with the benzene ring, while ethoxide is not (and the ethyl group pushes electrons in). An electron-withdrawing –NO₂ group at o- or p-positions further stabilises the phenoxide ion, so p-nitrophenol is more acidic than phenol.

Write the preparation of phenol from cumene with equations, and the reactions: Kolbe's reaction and Reimer–Tiemann reaction.5 marks

Cumene (isopropylbenzene) is oxidised by air to cumene hydroperoxide, which is hydrolysed with dilute acid to phenol and acetone: C₆H₅CH(CH₃)₂ + O₂ → C₆H₅C(CH₃)₂OOH → (H⁺/H₂O) → C₆H₅OH + CH₃COCH₃. Kolbe's reaction: sodium phenoxide + CO₂ (4–7 atm, 400 K), then acidify, gives salicylic acid (o-hydroxybenzoic acid). Reimer–Tiemann: phenol + CHCl₃ + aqueous NaOH, then acid, gives salicylaldehyde (o-hydroxybenzaldehyde).

Quick revision

3° alcohols react fastest with Lucas reagent.
Phenol is weakly acidic.
Williamson gives ethers.

Exam tips

Phenol gives a violet colour with neutral FeCl₃.
Acidic strength: phenol > water > ethanol, because the phenoxide ion is stabilised by resonance.

Practice MCQs

Lucas test distinguishes
1. aldehydes and ketones
2. 1°, 2°, 3° alcohols
3. phenols and ethers
4. acids and esters
Show answer
Answer: (b) 1°, 2°, 3° alcohols. Rate of turbidity.
Phenol with neutral FeCl₃ gives
1. red colour
2. white precipitate
3. violet colour
4. no change
Show answer
Answer: (c) violet colour. Violet complex.
Oxidation of a secondary alcohol gives
1. ketone
2. aldehyde
3. acid
4. ether
Show answer
Answer: (a) ketone. Ketone.
Which is the most acidic?
1. p-nitrophenol
2. phenol
3. ethanol
4. water
Show answer
Answer: (a) p-nitrophenol. –NO₂ stabilises the phenoxide.
Lucas test gives turbidity immediately with
1. methanol
2. 2-methylpropan-2-ol
3. propan-2-ol
4. ethanol
Show answer
Answer: (b) 2-methylpropan-2-ol. Tertiary alcohol.
Kolbe's reaction of sodium phenoxide with CO₂ gives
1. salicylaldehyde
2. benzoic acid
3. salicylic acid
4. picric acid
Show answer
Answer: (c) salicylic acid. Carboxylation.
Ethanol heated with conc. H₂SO₄ at 413 K mainly gives
1. ethene
2. ethanoic acid
3. ethanal
4. diethyl ether
Show answer
Answer: (d) diethyl ether. Intermolecular dehydration.
Phenol with bromine water gives
1. o-bromophenol only
2. p-bromophenol only
3. 2,4,6-tribromophenol (white ppt)
4. bromobenzene
Show answer
Answer: (c) 2,4,6-tribromophenol (white ppt). –OH strongly activates the ring.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.