Hyperconjugation: Assertion & Reason Practice Set for NEET anf IIT-JEE
Directions: In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R). Mark the correct choice as:
- (A) Both A and R are true and R is the correct explanation of A.
- (B) Both A and R are true but R is NOT the correct explanation of A.
- (C) A is true but R is false.
- (D) A is false but R is true.
Assertion (A): Hyperconjugation is also known as "no-bond resonance".
Reason (R): In hyperconjugation, there is no real bond between the carbon and the hydrogen atoms in the contributing structures.
Answer: (A)
Assertion (A): Propene is more stable than ethene.
Reason (R): Propene has three alpha-hydrogen atoms which can undergo hyperconjugation, whereas ethene has none.
Answer: (A)
Assertion (A): The C—C single bond length in propene is shorter than the C—C bond length in ethane.
Reason (R): Due to hyperconjugation, the C—C single bond in propene acquires some double bond character.
Answer: (A)
Assertion (A): Tertiary butyl carbocation is more stable than secondary butyl carbocation.
Reason (R): Stability of carbocations increases with the increase in the number of hyperconjugative structures.
Answer: (A)
Assertion (A): Hyperconjugation involves the delocalization of σ (sigma) electrons of C—H bond.
Reason (R): Delocalization occurs into the empty p-orbital of an adjacent carbon atom or π-system.
Answer: (A)
Assertion (A): Toluene reacts faster than benzene in electrophilic substitution reactions.
Reason (R): The methyl group in toluene is electron-withdrawing due to the inductive effect.
Answer: (C) [Reason is false; Methyl is electron-donating via hyperconjugation/inductive effect]
Assertion (A): Trans-2-butene is more stable than cis-2-butene.
Reason (R): Trans-2-butene has more hyperconjugative structures than cis-2-butene.
Answer: (C) [Reason is false; both have the same number of alpha-H, but trans is stable due to less steric repulsion]
Assertion (A): Hyperconjugation is a permanent effect.
Reason (R): It operates in the ground state of the molecule and does not require an external reagent.
Answer: (A)
Assertion (A): The heat of hydrogenation of 2-methylpropene is less than that of 1-butene.
Reason (R): Greater the number of alkyl groups attached to the doubly bonded carbon atoms, greater is the stability of the alkene.
Answer: (A)
Assertion (A): Baker-Nathan effect is another name for hyperconjugation.
Reason (R): It was first observed by Baker and Nathan during the study of the reaction rates of alkyl-substituted benzyl bromides.
Answer: (A)
Assertion (A): The C—H bond is more effective for hyperconjugation than the C—D (Deuterium) bond.
Reason (R): The C—D bond is stronger than the C—H bond due to its lower zero-point energy, making it harder to overlap with adjacent orbitals.
Answer: (A)
Assertion (A): Alkyl groups act as ortho-para directing groups in electrophilic aromatic substitution.
Reason (R): Hyperconjugation increases the electron density specifically at the ortho and para positions of the benzene ring.
Answer: (A)
Assertion (A): The isopropyl radical is more stable than the ethyl radical.
Reason (R): The isopropyl radical has six alpha-hydrogen atoms available for hyperconjugation, whereas the ethyl radical has only three.
Answer: (A)
Assertion (A): In the case of CH3-CH=CH2, the carbon-carbon double bond has a slightly lower bond order than 2.
Reason (R): Hyperconjugation involves the partial shift of electrons from the C—H sigma bond into the pi-antibonding orbital (pi*) of the alkene.
Answer: (A)
Assertion (A): Hyperconjugation is observed in the (CH3)3C-CH2+ carbocation.
Reason (R): There are no alpha-hydrogen atoms present in the neopentyl carbocation to facilitate delocalization.
Answer: (D) [Assertion is false; neopentyl carbocation has zero alpha-hydrogens and cannot undergo hyperconjugation without rearrangement]