Alcohols, Phenols and Ethers — Class 12 MCQs with Answers

Alcohols, Phenols and Ethers — important questions & MCQs with answers (Class 12 Chemistry)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Functional group of alcohol:

   A−OHB−COOHC−CHOD−NH₂
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   Answer: −OH

   Explanation: The functional group -OH defines an alcohol; -COOH marks a carboxylic acid, -CHO an aldehyde, and -NH₂ an amine, so only -OH fits an alcohol here.

2. Q2 of 6Easy

   General formula of ether:

   AR−NH₂BR−OHCR−COOHDR−O−R'
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   Answer: R−O−R'

   Explanation: An ether has an oxygen bonded to two carbon groups, R-O-R′, unlike R-OH (alcohol) or R-COOH (acid) — the single ether oxygen carries no H, which is the giveaway.

3. Q3 of 6Medium

   Phenol is more acidic than alcohol because:

   AIt's volatileBIt's smallerCPhenoxide ion is resonance stabilisedDNo reason
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   Answer: Phenoxide ion is resonance stabilised

   Explanation: Losing H⁺ from phenol gives phenoxide, whose negative charge delocalises onto the ring's ortho/para carbons by resonance; an alkoxide from a plain alcohol has nowhere to spread that charge, so phenol is far more acidic.

4. Q4 of 6Medium

   Williamson synthesis fails with tertiary alkyl halides because:

   ATertiary halides ionize too slowlyBTertiary R-X is unreactiveCSodium alkoxide is unstableDAlkoxide acts as base causing elimination (E2) giving alkene
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   Answer: Alkoxide acts as base causing elimination (E2) giving alkene

   Explanation: With 3° R-X, the bulky alkoxide preferentially abstracts a β-H (E2), giving alkene instead of ether.

5. Q5 of 6Hard

   Alcohols have higher BP than alkanes of similar mass because of:

   AAromatic ringBHigher massCLower massDHydrogen bonding
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   Answer: Hydrogen bonding

   Explanation: The -OH lets alcohol molecules hydrogen-bond to each other (O-H···O), an attraction alkanes of similar mass lack entirely (only weak van der Waals forces), so alcohols need far more energy to boil.

6. Q6 of 6Hard

   Williamson ether synthesis uses:

   ATwo alcoholsBAlcohol + acidCAlkoxide + alkyl halide (Sn2)DCarbonyl + amine
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   Answer: Alkoxide + alkyl halide (Sn2)

   Explanation: Williamson synthesis is alkoxide + alkyl halide via SN2, so it needs a primary (or methyl) halide for backside attack; a tertiary halide instead undergoes E2 with the alkoxide as base, giving alkene, not ether.

Alcohols, Phenols and Ethers notes: key concepts (Class 12 Chemistry)

Alcohols, Phenols and Ethers covers the preparation and reactions of the three –OH/–O– families, the resonance-based acidity of phenols, hydrogen bonding, distinguishing tests (Lucas), electrophilic substitution in phenol, and named reactions — Williamson ether synthesis, Kolbe's reaction and the Reimer–Tiemann reaction.

Classification and structure

Alcohols carry –OH on sp³ carbon (1°/2°/3°), phenols carry –OH on an aromatic ring, and ethers have R–O–R'; the C–O and O–H bonds are polar.

Preparation of alcohols

By acid hydration or hydroboration–oxidation of alkenes, reduction of aldehydes/ketones/acids, and addition of Grignard reagents to carbonyl compounds.

Preparation of phenol

Industrially from cumene (cumene hydroperoxide route, gives phenol + acetone), and from chlorobenzene (Dow process) or benzene diazonium salt.

Acidity of phenol

Phenol is more acidic than alcohols and water because the phenoxide ion is resonance-stabilised; electron-withdrawing groups (–NO₂) raise acidity, electron-donating lower it.

Reactions of alcohols with HX

Alcohols form haloalkanes with HX; the Lucas test (conc. HCl/ZnCl₂) distinguishes 3° (immediate turbidity) > 2° > 1° (no turbidity cold).

Dehydration of alcohols

Conc. H₂SO₄ at ~443 K removes water to give an alkene; ease of dehydration follows 3° > 2° > 1° and obeys Zaitsev's rule.

Oxidation of alcohols

1° alcohols oxidise to aldehydes then carboxylic acids; 2° alcohols to ketones; 3° alcohols resist oxidation (no α-H on the C–OH).

Esterification

Alcohols react with carboxylic acids in acid to give esters and water (a reversible reaction); the acid acts as catalyst and dehydrating agent.

Electrophilic substitution in phenol

The –OH group is activating and ortho/para directing, so phenol undergoes easy nitration, halogenation (Br₂ water → 2,4,6-tribromophenol) and sulphonation.

Williamson ether synthesis

A sodium alkoxide displaces halide from a primary alkyl halide by SN2 to give an ether; using a 3° halide instead causes elimination to an alkene.

Cleavage of ethers by HI

Ethers are cleaved by hot HI; the smaller/less hindered alkyl group leaves as an iodide, and with aryl ethers the aryl–O bond does not break.

Hydrogen bonding and boiling point

Intermolecular O–H···O hydrogen bonds give alcohols and phenols much higher boiling points than ethers or alkanes of similar molar mass.