Chapter 9: Hydrocarbons Assertion-Reason Questions | chemistry Class 11 CBSE

Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 1

Logic Check

Assertion (A): The staggered conformation of ethane is more stable than the eclipsed conformation. Reason (R): In the staggered conformation, the electron clouds of C-H bonds on adjacent carbons are maximally separated, minimising torsional strain.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 2

Logic Check

Assertion (A): Alkanes are called paraffins. Reason (R): Alkanes are highly reactive compounds that readily react with acids and bases under normal conditions.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 3

Logic Check

Assertion (A): Benzene undergoes electrophilic substitution reactions in preference to electrophilic addition reactions. Reason (R): Electrophilic substitution preserves the delocalised aromatic pi electron system, whereas addition would destroy aromaticity and the associated stabilisation energy.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 4

Logic Check

Assertion (A): In the addition of HBr to propene in the presence of peroxide, 1-bromopropane is the major product. Reason (R): In the peroxide effect, the secondary carbocation formed is more stable than the primary carbocation.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 5

Logic Check

Assertion (A): All C-C bond lengths in benzene are identical and equal to 139 pm. Reason (R): Benzene is a resonance hybrid of two Kekulé structures, and the six pi electrons are completely delocalised over all six carbon atoms.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 6

Logic Check

Assertion (A): Ethyne is more acidic than ethene and ethane. Reason (R): The C-H bonds in ethyne involve sp hybridised carbon atoms, which have greater s-character (50%) and hence greater electronegativity compared to sp2 or sp3 hybridised carbons.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 7

Logic Check

Assertion (A): The boiling point of n-pentane is higher than that of 2,2-dimethylpropane. Reason (R): n-Pentane has a linear shape with a larger surface area, allowing stronger van der Waals forces between molecules compared to the more spherical 2,2-dimethylpropane.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 8

Logic Check

Assertion (A): Geometrical (cis-trans) isomerism is exhibited by but-2-ene but not by 2-methylpropene. Reason (R): 2-Methylpropene has two identical groups (both are CH3) on one of the doubly bonded carbon atoms, so the condition for geometrical isomerism is not fulfilled.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 9

Logic Check

Assertion (A): Methane cannot be prepared by Kolbe's electrolytic method. Reason (R): In Kolbe's method, the formate radical generated from sodium formate decomposes to CO2 and H radical instead of coupling to give methane.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 10

Logic Check

Assertion (A): cis-but-2-ene has a higher boiling point than trans-but-2-ene. Reason (R): cis-but-2-ene is more polar (dipole moment 0.33 D) than trans-but-2-ene (dipole moment ≈ 0) due to the non-cancellation of bond dipoles in the cis arrangement.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 11

Logic Check

Assertion (A): Alkenes are more reactive than alkanes towards electrophilic reagents. Reason (R): Alkenes contain loosely held pi electrons that are easily available to electrophilic reagents, whereas alkanes have only strong sigma bonds with no pi electrons.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 12

Logic Check

Assertion (A): The Wurtz reaction is not preferred for the preparation of alkanes with an odd number of carbon atoms. Reason (R): Using two different alkyl halides in the Wurtz reaction gives a mixture of three different alkane products, making isolation of the desired product difficult.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 13

Logic Check

Assertion (A): Naphthalene (C10H8) is an aromatic compound. Reason (R): Naphthalene contains 10 pi electrons, satisfying Hückel's rule (4n+2 with n=2), and is planar with complete delocalisation of pi electrons.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 14

Logic Check

Assertion (A): The peroxide effect in HBr addition to alkenes is not observed with HCl. Reason (R): The H-Cl bond (430.5 kJ mol-1) is stronger than the H-Br bond (363.7 kJ mol-1) and cannot be homolytically cleaved by free radicals under the reaction conditions.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 15

Logic Check

Assertion (A): Alkanes are soluble in non-polar solvents like benzene and CCl4 but insoluble in water. Reason (R): Alkanes are polar molecules and polar substances dissolve in non-polar solvents according to the 'like dissolves like' principle.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 16

Logic Check

Assertion (A): Reduction of an internal alkyne with sodium in liquid ammonia gives a trans-alkene. Reason (R): The reduction proceeds by a stepwise radical-anion mechanism in which the two hydrogen atoms are added from opposite faces (anti addition), leading to the trans product.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 17

Logic Check

Assertion (A): Halogen substituents on benzene ring are ortho/para directors. Reason (R): Halogen atoms donate electron density to the ortho and para positions of the ring through the inductive effect, making them more reactive towards electrophiles.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 18

Logic Check

Assertion (A): Carbon-carbon triple bond in ethyne (C≡C) is shorter than the double bond in ethene (C=C). Reason (R): As the number of bonds between two carbons increases, more electron density is concentrated between the nuclei, pulling them closer and resulting in a shorter bond.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 19

Logic Check

Assertion (A): Benzene forms only one type of monosubstituted product when one hydrogen is replaced. Reason (R): All six hydrogen atoms in benzene are equivalent due to the symmetric, hexagonal planar structure with complete delocalisation of pi electrons.

Reasoning Options

ByteNotes

Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 20

Logic Check

Assertion (A): Polynuclear aromatic hydrocarbons (PAHs) are carcinogenic. Reason (R): PAHs are metabolically activated in the body to form reactive epoxide intermediates that covalently bind to DNA, causing mutations.

Reasoning Options

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Chapter sections

Chapter 9: Hydrocarbons | Assertion-Reason

Assertion (A): The staggered conformation of ethane is more stable than the eclipsed conformation. Reason (R): In the staggered conformation, the electron clouds of C-H bonds on adjacent carbons are maximally separated, minimising torsional strain.

Assertion (A): Alkanes are called paraffins. Reason (R): Alkanes are highly reactive compounds that readily react with acids and bases under normal conditions.

Assertion (A): Benzene undergoes electrophilic substitution reactions in preference to electrophilic addition reactions. Reason (R): Electrophilic substitution preserves the delocalised aromatic pi electron system, whereas addition would destroy aromaticity and the associated stabilisation energy.

Assertion (A): In the addition of HBr to propene in the presence of peroxide, 1-bromopropane is the major product. Reason (R): In the peroxide effect, the secondary carbocation formed is more stable than the primary carbocation.

Assertion (A): All C-C bond lengths in benzene are identical and equal to 139 pm. Reason (R): Benzene is a resonance hybrid of two Kekulé structures, and the six pi electrons are completely delocalised over all six carbon atoms.

Assertion (A): Ethyne is more acidic than ethene and ethane. Reason (R): The C-H bonds in ethyne involve sp hybridised carbon atoms, which have greater s-character (50%) and hence greater electronegativity compared to sp2 or sp3 hybridised carbons.

Assertion (A): The boiling point of n-pentane is higher than that of 2,2-dimethylpropane. Reason (R): n-Pentane has a linear shape with a larger surface area, allowing stronger van der Waals forces between molecules compared to the more spherical 2,2-dimethylpropane.

Assertion (A): Geometrical (cis-trans) isomerism is exhibited by but-2-ene but not by 2-methylpropene. Reason (R): 2-Methylpropene has two identical groups (both are CH3) on one of the doubly bonded carbon atoms, so the condition for geometrical isomerism is not fulfilled.

Assertion (A): Methane cannot be prepared by Kolbe's electrolytic method. Reason (R): In Kolbe's method, the formate radical generated from sodium formate decomposes to CO2 and H radical instead of coupling to give methane.

Assertion (A): cis-but-2-ene has a higher boiling point than trans-but-2-ene. Reason (R): cis-but-2-ene is more polar (dipole moment 0.33 D) than trans-but-2-ene (dipole moment ≈ 0) due to the non-cancellation of bond dipoles in the cis arrangement.

Assertion (A): Alkenes are more reactive than alkanes towards electrophilic reagents. Reason (R): Alkenes contain loosely held pi electrons that are easily available to electrophilic reagents, whereas alkanes have only strong sigma bonds with no pi electrons.

Assertion (A): The Wurtz reaction is not preferred for the preparation of alkanes with an odd number of carbon atoms. Reason (R): Using two different alkyl halides in the Wurtz reaction gives a mixture of three different alkane products, making isolation of the desired product difficult.

Assertion (A): Naphthalene (C10H8) is an aromatic compound. Reason (R): Naphthalene contains 10 pi electrons, satisfying Hückel's rule (4n+2 with n=2), and is planar with complete delocalisation of pi electrons.

Assertion (A): The peroxide effect in HBr addition to alkenes is not observed with HCl. Reason (R): The H-Cl bond (430.5 kJ mol-1) is stronger than the H-Br bond (363.7 kJ mol-1) and cannot be homolytically cleaved by free radicals under the reaction conditions.

Assertion (A): Alkanes are soluble in non-polar solvents like benzene and CCl4 but insoluble in water. Reason (R): Alkanes are polar molecules and polar substances dissolve in non-polar solvents according to the 'like dissolves like' principle.

Assertion (A): Reduction of an internal alkyne with sodium in liquid ammonia gives a trans-alkene. Reason (R): The reduction proceeds by a stepwise radical-anion mechanism in which the two hydrogen atoms are added from opposite faces (anti addition), leading to the trans product.

Assertion (A): Halogen substituents on benzene ring are ortho/para directors. Reason (R): Halogen atoms donate electron density to the ortho and para positions of the ring through the inductive effect, making them more reactive towards electrophiles.

Assertion (A): Carbon-carbon triple bond in ethyne (C≡C) is shorter than the double bond in ethene (C=C). Reason (R): As the number of bonds between two carbons increases, more electron density is concentrated between the nuclei, pulling them closer and resulting in a shorter bond.

Assertion (A): Benzene forms only one type of monosubstituted product when one hydrogen is replaced. Reason (R): All six hydrogen atoms in benzene are equivalent due to the symmetric, hexagonal planar structure with complete delocalisation of pi electrons.

Assertion (A): Polynuclear aromatic hydrocarbons (PAHs) are carcinogenic. Reason (R): PAHs are metabolically activated in the body to form reactive epoxide intermediates that covalently bind to DNA, causing mutations.

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