Chapter 6: Haloalkanes and Haloarenes Assertion-Reason Questions | chemistry Class 12 CBSE

Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 1

Logic Check

Assertion (A): Alkyl halides have higher boiling points than the corresponding alkanes of similar molecular mass. Reason (R): Alkyl halides are polar molecules and have stronger dipole-dipole and van der Waals forces of attraction than non-polar alkanes.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 2

Logic Check

Assertion (A): Tertiary alkyl halides are more reactive than primary alkyl halides in SN1 reactions. Reason (R): Tertiary carbocations are more stable than primary carbocations due to greater electron-donating inductive effect of alkyl groups.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 3

Logic Check

Assertion (A): Grignard reagents must be prepared in anhydrous conditions. Reason (R): Grignard reagents are highly reactive and react with water to give hydrocarbons, destroying the reagent.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 4

Logic Check

Assertion (A): SN2 reaction of optically active alkyl halides is accompanied by inversion of configuration. Reason (R): In SN2, the nucleophile attacks from the same side as the leaving group, causing retention of configuration.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 5

Logic Check

Assertion (A): Chlorobenzene is less reactive than chloroethane towards nucleophilic substitution. Reason (R): In chlorobenzene, the C–Cl bond is shorter and stronger due to resonance giving it partial double bond character.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 6

Logic Check

Assertion (A): p-Dichlorobenzene has a higher melting point than o-dichlorobenzene. Reason (R): para-Isomers are more symmetrical and pack more efficiently in crystal lattice, resulting in stronger intermolecular forces in solid state.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 7

Logic Check

Assertion (A): In the reaction of haloalkanes with KCN, the major product is alkyl cyanide (R–CN). Reason (R): CN⁻ is a monodentate nucleophile that can only attack through carbon.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 8

Logic Check

Assertion (A): Haloalkanes are only slightly soluble in water. Reason (R): Energy required to break hydrogen bonds between water molecules is more than energy released by forming new interactions between haloalkane and water.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 9

Logic Check

Assertion (A): Chlorine is an ortho/para director in electrophilic aromatic substitution despite being a deactivator. Reason (R): Chlorine activates the ring through inductive effect which predominates over the resonance effect.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 10

Logic Check

Assertion (A): Thionyl chloride (SOCl2) is a preferred reagent for the conversion of alcohols to alkyl chlorides. Reason (R): The by-products SO2 and HCl are both gases that escape the reaction mixture, leaving behind pure alkyl chloride.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 11

Logic Check

Assertion (A): Branching in haloalkanes decreases their boiling points. Reason (R): Branching reduces the surface area of molecules, weakening van der Waals forces of attraction between molecules.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 12

Logic Check

Assertion (A): In SN1 reactions of optically active alkyl halides, a racemic mixture is obtained. Reason (R): The carbocation intermediate formed in SN1 is sp³ hybridised and non-planar.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 13

Logic Check

Assertion (A): Free radical halogenation of alkanes gives a mixture of isomeric products. Reason (R): Different types of hydrogen atoms in the alkane are replaced to give structurally different monohalogen products.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 14

Logic Check

Assertion (A): Freons (CFCs) have been phased out as refrigerants and aerosol propellants. Reason (R): Freons are toxic to humans and cause direct health hazards in populated areas.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 15

Logic Check

Assertion (A): The order of reactivity of R–X in nucleophilic substitution is R–I > R–Br > R–Cl >> R–F. Reason (R): C–F bond has the highest bond enthalpy and fluoride is the poorest leaving group among halides.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 16

Logic Check

Assertion (A): Propan-2-ol is an achiral molecule. Reason (R): The central carbon of propan-2-ol has two identical methyl groups attached to it, so it is not a stereocentre.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 17

Logic Check

Assertion (A): Aryl halides cannot be prepared by the reaction of phenol with haloacids (HX). Reason (R): The carbon-oxygen bond in phenol has partial double bond character and is stronger than a single bond, making it difficult to break.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 18

Logic Check

Assertion (A): Enantiomers of a compound have identical melting points and boiling points. Reason (R): Enantiomers differ only in the direction in which they rotate plane polarised light.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 19

Logic Check

Assertion (A): In an SN2 reaction, methyl halide is most reactive while tertiary halide is least reactive. Reason (R): Increasing alkyl substitution at the carbon bearing the halogen increases steric hindrance, reducing the rate of SN2 reaction.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 20

Logic Check

Assertion (A): Vinyl chloride (CH2=CHCl) is less reactive than allyl chloride (CH2=CHCH2Cl) in nucleophilic substitution. Reason (R): In vinyl chloride, the lone pairs on Cl are delocalised into the π-bond, giving partial double bond character to C–Cl, while in allyl chloride the C–Cl bond is a pure sp³ C–X bond.

Reasoning Options

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Chapter sections

Chapter 6: Haloalkanes and Haloarenes | Assertion-Reason

Assertion (A): Alkyl halides have higher boiling points than the corresponding alkanes of similar molecular mass. Reason (R): Alkyl halides are polar molecules and have stronger dipole-dipole and van der Waals forces of attraction than non-polar alkanes.

Assertion (A): Tertiary alkyl halides are more reactive than primary alkyl halides in SN1 reactions. Reason (R): Tertiary carbocations are more stable than primary carbocations due to greater electron-donating inductive effect of alkyl groups.

Assertion (A): Grignard reagents must be prepared in anhydrous conditions. Reason (R): Grignard reagents are highly reactive and react with water to give hydrocarbons, destroying the reagent.

Assertion (A): SN2 reaction of optically active alkyl halides is accompanied by inversion of configuration. Reason (R): In SN2, the nucleophile attacks from the same side as the leaving group, causing retention of configuration.

Assertion (A): Chlorobenzene is less reactive than chloroethane towards nucleophilic substitution. Reason (R): In chlorobenzene, the C–Cl bond is shorter and stronger due to resonance giving it partial double bond character.

Assertion (A): p-Dichlorobenzene has a higher melting point than o-dichlorobenzene. Reason (R): para-Isomers are more symmetrical and pack more efficiently in crystal lattice, resulting in stronger intermolecular forces in solid state.

Assertion (A): In the reaction of haloalkanes with KCN, the major product is alkyl cyanide (R–CN). Reason (R): CN⁻ is a monodentate nucleophile that can only attack through carbon.

Assertion (A): Haloalkanes are only slightly soluble in water. Reason (R): Energy required to break hydrogen bonds between water molecules is more than energy released by forming new interactions between haloalkane and water.

Assertion (A): Chlorine is an ortho/para director in electrophilic aromatic substitution despite being a deactivator. Reason (R): Chlorine activates the ring through inductive effect which predominates over the resonance effect.

Assertion (A): Thionyl chloride (SOCl2) is a preferred reagent for the conversion of alcohols to alkyl chlorides. Reason (R): The by-products SO2 and HCl are both gases that escape the reaction mixture, leaving behind pure alkyl chloride.

Assertion (A): Branching in haloalkanes decreases their boiling points. Reason (R): Branching reduces the surface area of molecules, weakening van der Waals forces of attraction between molecules.

Assertion (A): In SN1 reactions of optically active alkyl halides, a racemic mixture is obtained. Reason (R): The carbocation intermediate formed in SN1 is sp³ hybridised and non-planar.

Assertion (A): Free radical halogenation of alkanes gives a mixture of isomeric products. Reason (R): Different types of hydrogen atoms in the alkane are replaced to give structurally different monohalogen products.

Assertion (A): Freons (CFCs) have been phased out as refrigerants and aerosol propellants. Reason (R): Freons are toxic to humans and cause direct health hazards in populated areas.

Assertion (A): The order of reactivity of R–X in nucleophilic substitution is R–I > R–Br > R–Cl >> R–F. Reason (R): C–F bond has the highest bond enthalpy and fluoride is the poorest leaving group among halides.

Assertion (A): Propan-2-ol is an achiral molecule. Reason (R): The central carbon of propan-2-ol has two identical methyl groups attached to it, so it is not a stereocentre.

Assertion (A): Aryl halides cannot be prepared by the reaction of phenol with haloacids (HX). Reason (R): The carbon-oxygen bond in phenol has partial double bond character and is stronger than a single bond, making it difficult to break.

Assertion (A): Enantiomers of a compound have identical melting points and boiling points. Reason (R): Enantiomers differ only in the direction in which they rotate plane polarised light.

Assertion (A): In an SN2 reaction, methyl halide is most reactive while tertiary halide is least reactive. Reason (R): Increasing alkyl substitution at the carbon bearing the halogen increases steric hindrance, reducing the rate of SN2 reaction.

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