Chapter 7: Alcohols, Phenols and Ethers Assertion-Reason Questions | chemistry Class 12 CBSE

Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 1

Logic Check

Assertion (A): Phenol is a stronger acid than ethanol. Reason (R): The phenoxide ion is stabilised by delocalisation of the negative charge over the benzene ring through resonance.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 2

Logic Check

Assertion (A): The boiling point of ethanol is higher than that of methoxymethane despite both having the same molecular mass. Reason (R): Ethanol forms intermolecular hydrogen bonds through the O–H group, which require more energy to break.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 3

Logic Check

Assertion (A): Tertiary alcohols are most easily dehydrated among primary, secondary, and tertiary alcohols. Reason (R): Tertiary carbocations are more stable than secondary and primary carbocations due to hyperconjugation and inductive effect of alkyl groups.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 4

Logic Check

Assertion (A): o-Nitrophenol is more volatile and steam-distillable compared to p-nitrophenol. Reason (R): o-Nitrophenol forms intramolecular hydrogen bonds, making it a discrete non-associated molecule with lower effective intermolecular forces.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 5

Logic Check

Assertion (A): Tertiary alcohols do not undergo oxidation with mild oxidising agents. Reason (R): Tertiary alcohols have no hydrogen atom attached to the carbon bearing the –OH group (no α-H on the carbinol carbon), so the C–H/O–H cleavage required for forming C=O cannot occur.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 6

Logic Check

Assertion (A): Williamson synthesis fails to give ethers when tertiary alkyl halides are used. Reason (R): Alkoxides are strong bases that cause E2 elimination with tertiary alkyl halides, giving alkenes instead of ethers.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 7

Logic Check

Assertion (A): Hydroboration-oxidation of propene gives propan-1-ol (anti-Markovnikov product). Reason (R): In hydroboration, boron attaches to the less substituted (more hydrogen-bearing) carbon of the double bond.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 8

Logic Check

Assertion (A): Phenol reacts with bromine water to give 2,4,6-tribromophenol without any Lewis acid catalyst. Reason (R): The –OH group in phenol is an electron-withdrawing group that deactivates the benzene ring toward electrophilic substitution.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 9

Logic Check

Assertion (A): The solubility of alcohols in water decreases with increase in the size of the alkyl group. Reason (R): Larger alkyl groups are hydrophobic and reduce the ability of the –OH group to form hydrogen bonds with water.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 10

Logic Check

Assertion (A): Methanol treatment with a patient suffering from methanol poisoning involves intravenous infusion of ethanol. Reason (R): Ethanol competes with methanol for the alcohol dehydrogenase enzyme, slowing methanol oxidation and allowing kidneys to excrete it.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 11

Logic Check

Assertion (A): The C–O bond angle in ethers is slightly greater than the tetrahedral angle. Reason (R): The two lone pairs on the oxygen of ethers cause greater repulsion than the alkyl groups, forcing the bond angle below tetrahedral.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 12

Logic Check

Assertion (A): In the cleavage of methoxybenzene (anisole) with HI, methyl iodide and phenol are formed — not iodobenzene and methanol. Reason (R): The Ar–O bond in anisole has partial double bond character due to resonance, making it stronger and less susceptible to nucleophilic cleavage than the O–CH₃ bond.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 13

Logic Check

Assertion (A): Phenoxide ion is more reactive than phenol toward electrophilic aromatic substitution. Reason (R): In phenoxide ion, oxygen carries a full negative charge, donating electrons more strongly into the ring via resonance than neutral –OH.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 14

Logic Check

Assertion (A): Boiling points of alcohols decrease with increase in branching of the carbon chain. Reason (R): Branching reduces the surface area of the molecule, decreasing van der Waals (London dispersion) forces between molecules.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 15

Logic Check

Assertion (A): Alcohols are weaker acids than water. Reason (R): Alkyl groups are electron-releasing and increase electron density on oxygen, strengthening the O–H bond and making proton donation more difficult.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 16

Logic Check

Assertion (A): p-Cresol (4-methylphenol) is less acidic than phenol. Reason (R): The methyl group is an electron-withdrawing group that decreases electron density at the para position, destabilising the phenoxide ion.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 17

Logic Check

Assertion (A): The addition of water to an unsymmetrical alkene in acid-catalysed hydration follows Markovnikov's rule. Reason (R): The more stable (more substituted) carbocation is formed preferentially, leading to –OH attachment at the more substituted carbon.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 18

Logic Check

Assertion (A): Ethers have boiling points similar to alkanes of comparable molecular mass. Reason (R): Ethers cannot form hydrogen bonds with each other because they lack an O–H bond, so only weak van der Waals forces operate between ether molecules.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 19

Logic Check

Assertion (A): LiAlH₄ can reduce carboxylic acids to primary alcohols. Reason (R): LiAlH₄ is a mild reducing agent suitable only for ketone reduction.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 20

Logic Check

Assertion (A): Glycerol (propane-1,2,3-triol) is classified as a trihydric alcohol. Reason (R): It contains three –OH groups in its structure.

Reasoning Options

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Chapter sections

Chapter 7: Alcohols, Phenols and Ethers | Assertion-Reason

Assertion (A): Phenol is a stronger acid than ethanol. Reason (R): The phenoxide ion is stabilised by delocalisation of the negative charge over the benzene ring through resonance.

Assertion (A): The boiling point of ethanol is higher than that of methoxymethane despite both having the same molecular mass. Reason (R): Ethanol forms intermolecular hydrogen bonds through the O–H group, which require more energy to break.

Assertion (A): Tertiary alcohols are most easily dehydrated among primary, secondary, and tertiary alcohols. Reason (R): Tertiary carbocations are more stable than secondary and primary carbocations due to hyperconjugation and inductive effect of alkyl groups.

Assertion (A): o-Nitrophenol is more volatile and steam-distillable compared to p-nitrophenol. Reason (R): o-Nitrophenol forms intramolecular hydrogen bonds, making it a discrete non-associated molecule with lower effective intermolecular forces.

Assertion (A): Tertiary alcohols do not undergo oxidation with mild oxidising agents. Reason (R): Tertiary alcohols have no hydrogen atom attached to the carbon bearing the –OH group (no α-H on the carbinol carbon), so the C–H/O–H cleavage required for forming C=O cannot occur.

Assertion (A): Williamson synthesis fails to give ethers when tertiary alkyl halides are used. Reason (R): Alkoxides are strong bases that cause E2 elimination with tertiary alkyl halides, giving alkenes instead of ethers.

Assertion (A): Hydroboration-oxidation of propene gives propan-1-ol (anti-Markovnikov product). Reason (R): In hydroboration, boron attaches to the less substituted (more hydrogen-bearing) carbon of the double bond.

Assertion (A): Phenol reacts with bromine water to give 2,4,6-tribromophenol without any Lewis acid catalyst. Reason (R): The –OH group in phenol is an electron-withdrawing group that deactivates the benzene ring toward electrophilic substitution.

Assertion (A): The solubility of alcohols in water decreases with increase in the size of the alkyl group. Reason (R): Larger alkyl groups are hydrophobic and reduce the ability of the –OH group to form hydrogen bonds with water.

Assertion (A): Methanol treatment with a patient suffering from methanol poisoning involves intravenous infusion of ethanol. Reason (R): Ethanol competes with methanol for the alcohol dehydrogenase enzyme, slowing methanol oxidation and allowing kidneys to excrete it.

Assertion (A): The C–O bond angle in ethers is slightly greater than the tetrahedral angle. Reason (R): The two lone pairs on the oxygen of ethers cause greater repulsion than the alkyl groups, forcing the bond angle below tetrahedral.

Assertion (A): Phenoxide ion is more reactive than phenol toward electrophilic aromatic substitution. Reason (R): In phenoxide ion, oxygen carries a full negative charge, donating electrons more strongly into the ring via resonance than neutral –OH.

Assertion (A): Boiling points of alcohols decrease with increase in branching of the carbon chain. Reason (R): Branching reduces the surface area of the molecule, decreasing van der Waals (London dispersion) forces between molecules.

Assertion (A): Alcohols are weaker acids than water. Reason (R): Alkyl groups are electron-releasing and increase electron density on oxygen, strengthening the O–H bond and making proton donation more difficult.

Assertion (A): p-Cresol (4-methylphenol) is less acidic than phenol. Reason (R): The methyl group is an electron-withdrawing group that decreases electron density at the para position, destabilising the phenoxide ion.

Assertion (A): The addition of water to an unsymmetrical alkene in acid-catalysed hydration follows Markovnikov's rule. Reason (R): The more stable (more substituted) carbocation is formed preferentially, leading to –OH attachment at the more substituted carbon.

Assertion (A): Ethers have boiling points similar to alkanes of comparable molecular mass. Reason (R): Ethers cannot form hydrogen bonds with each other because they lack an O–H bond, so only weak van der Waals forces operate between ether molecules.

Assertion (A): LiAlH₄ can reduce carboxylic acids to primary alcohols. Reason (R): LiAlH₄ is a mild reducing agent suitable only for ketone reduction.

Assertion (A): Glycerol (propane-1,2,3-triol) is classified as a trihydric alcohol. Reason (R): It contains three –OH groups in its structure.

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