Hyperconjugation DPP for NEET & JEE

Hyperconjugation DPP (NEET & JEE)

20 High-Yield PYQ-Based MCQs | JEE Main, Advanced & NEET Level

1. [JEE Main 2023] The decreasing order of stability of carbocations is:

a) (CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃CH₂⁺ > CH₃⁺
b) CH₃⁺ > CH₃CH₂⁺ > (CH₃)₂CH⁺ > (CH₃)₃C⁺
c) (CH₃)₃C⁺ > CH₃CH₂⁺ > (CH₃)₂CH⁺ > CH₃⁺
d) CH₃CH₂⁺ > (CH₃)₂CH⁺ > (CH₃)₃C⁺ > CH₃⁺

JEE Main 2023 | 3 April Shift 1

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Answer: a) (CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃CH₂⁺ > CH₃⁺

Explanation: Stability ∝ number of α-hydrogens (hyperconjugation).
• (CH₃)₃C⁺ → 9 α-H → 9 hyperconjugative structures
• (CH₃)₂CH⁺ → 6 α-H → 6 structures
• CH₃CH₂⁺ → 3 α-H → 3 structures
• CH₃⁺ → 0 α-H → no hyperconjugation

2. [NEET 2022] Which one of the following compound has the smallest heat of hydrogenation per mole:

a) Buta-1, 3-diene
b) trans-but-2-ene
c) 1-butene
d) cis-but-2-ene

NEET 2022 Phase 1

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Answer: a) Buta-1, 3-diene (most stable → lowest ΔH)

Explanation: Heat of hydrogenation is inversely proportional to the stability of the compound. Since, among the given compounds, buta-1-3-diene is resonance stabilised, i.e. more stable. Thus, it has the lowest heat of hydrogenation per mole.

3. [JEE Adv 2021] The correct order of acid strength is:

a) CH₃COOH > CH₂=CHCOOH > HC≡CCOOH
b) HC≡CCOOH > CH₂=CHCOOH > CH₃COOH
c) CH₃COOH > HC≡CCOOH > CH₂=CHCOOH
d) CH₂=CHCOOH > CH₃COOH > HC≡CCOOH

JEE Advanced 2021 Paper 1

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Answer: b) HC≡CCOOH > CH₂=CHCOOH > CH₃COOH

Explanation: Conjugate base stability:
• HC≡C⁻ → sp hybridized → highest s-character → most stable
• CH₂=CH⁻ → sp² → resonance + hyperconjugation
• CH₃⁻ → sp³ → only hyperconjugation
Hyperconjugation in vinyl system stabilizes anion.

4. [JEE Main 2024] Number of hyperconjugative structures in (CH₃)₂CH⁺ is:

a) 3
b) 6
c) 9
d) 12

JEE Main 2024 | 5 April Shift 2

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Answer: b) 6

Explanation: (CH₃)₂CH⁺ → isopropyl carbocation
• Two methyl groups → 2 × 3 = 6 α-hydrogens
• Each α-H gives one hyperconjugative structure → 6 structures

5. [NEET 2023] The most stable alkene among the following is:

a) CH₂=CH₂
b) CH₃−CH=CH₂
c) (CH₃)₂C=CH₂
d) CH₃−CH=CH−CH₃

NEET 2023

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Answer: c) (CH₃)₂C=CH₂

Explanation: Stability ∝ hyperconjugation in alkene:
• (CH₃)₂C=CH₂ → 6 α-H on one side → 6 hyperconjugative structures
• trans-2-butene → 3+3 = 6, but cis has steric strain
• Propene → 3 α-H
• Ethene → 0 α-H

6. [JEE Adv 2020] The correct statement about hyperconjugation is:

a) It involves σ → π* delocalization
b) It requires adjacent C–H and empty p-orbital
c) It is a permanent effect
d) Both b and c

JEE Advanced 2020 Paper 2

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Answer: d) Both b and c

Explanation: Hyperconjugation (no-bond resonance):
• C–H σ bond → empty p-orbital (carbocation) or π* (alkene)
• Permanent polarizing effect (unlike resonance)

7. [JEE Main 2022] The carbocation with no hyperconjugation is:

a) CH₃⁺
b) (CH₃)₃C⁺
c) C₆H₅CH₂⁺
d) Both a and c

JEE Main 2022 | 28 June Shift 1

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Answer: a) CH₃⁺

Explanation: CH₃⁺ has no α-hydrogen → no hyperconjugation.
• (CH₃)₃C⁺ → 9 α-H
• C₆H₅CH₂⁺ → stabilized by resonance, not hyperconjugation

8. [NEET 2021] Hyperconjugation is also known as:

a) Baker-Nathan effect
b) Inductive effect
c) Electromeric effect
d) Mesomeric effect

NEET 2021

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Answer: a) Baker-Nathan effect

Explanation: Discovered by Baker and Nathan (1935) while studying toluene reactivity. Also called no-bond resonance.

9. [JEE Adv 2019] The order of dipole moment is:

a) CH₃F > CH₃Cl > CH₃Br > CH₃I
b) CH₃Cl > CH₃F > CH₃Br > CH₃I
c) CH₃Cl > CH₃Br > CH₃F > CH₃I
d) CH₃F > CH₃Cl > CH₃I > CH₃Br

JEE Advanced 2019 Paper 1

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Answer: b) CH₃Cl > CH₃F > CH₃Br > CH₃I

Explanation: Dipole moment = electronegativity difference × hyperconjugative contribution
• Cl has highest EN difference + moderate hyperconjugation
• F has high EN but low hyperconjugation (poor overlap)
• I has low EN + high hyperconjugation → lowest μ

10. [JEE Main 2025] The number of hyperconjugative structures in CH₂=CH–CH₂⁺ is:

a) 2
b) 3
c) 4
d) 5

JEE Main 2025 | Predicted PYQ

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Answer: a) 2

Explanation: Allylic carbocation: CH₂=CH–CH₂⁺
• Only 2 α-H on –CH₂– group
• Resonance dominates; hyperconjugation from 2 H atoms only

11. [JEE Adv 2022] Hyperconjugation involves overlap between:

a) σ(C–H) and p-orbital
b) π(C=C) and σ(C–H)
c) σ(C–H) and σ*(C–C)
d) p-orbital and π-bond

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Answer: a) σ(C–H) and p-orbital

Explanation: σ(C–H) → vacant p-orbital (carbocation) or π* (alkene).

12. [NEET 2020] Toluene is more reactive than benzene towards electrophilic substitution due to:

a) +I effect of CH₃
b) Hyperconjugation
c) Resonance
d) Inductive effect

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Answer: b) Hyperconjugation

Explanation: CH₃ group provides 3 α-H → 3 hyperconjugative structures → activates ring at o/p positions.

13. [JEE Main 2021] The bond length of C=C in ethene is shorter than in:

a) 1,3-butadiene
b) Benzene
c) Propene
d) All have same

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Answer: c) Propene

Explanation: Hyperconjugation in propene: C=C bond gains partial single bond character → longer than pure C=C in ethene.

14. [JEE Adv 2018] The carbocation not stabilized by hyperconjugation is:

a) CH₃–CH⁺–CH₃
b) (CH₃)₂C⁺–CH₃
c) Ph–CH₂⁺
d) CH₂=CH–CH₂⁺

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Answer: c) Ph–CH₂⁺

Explanation: Benzyl carbocation stabilized by resonance with phenyl ring, not hyperconjugation.

15. [NEET 2019] The correct order of boiling point is:

a) n-pentane > isopentane > neopentane
b) n-pentane > neopentane > isopentane
c) neopentane > isopentane > n-pentane
d) isopentane > n-pentane > neopentane

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Answer: a) n-pentane > isopentane > neopentane

Explanation: Boiling point ∝ surface area (van der Waals). Hyperconjugation affects stability, not boiling point directly.

16. [JEE Main 2020] Hyperconjugation is maximum in:

a) (CH₃)₃C⁺
b) (CH₃)₂CH⁺
c) CH₃CH₂⁺
d) CH₃⁺

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Answer: a) (CH₃)₃C⁺

Explanation: 9 α-H → 9 hyperconjugative structures → maximum stabilization.

17. [JEE Adv 2023] In which compound is hyperconjugation not possible?

a) CH₃–CH=CH₂
b) CH₂=CH–Cl
c) H₂C=CH₂
d) CH₃–C≡CH

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Answer: d) CH₃–C≡CH

Explanation: No α-H adjacent to π-bond or empty orbital. sp hybridized C has no H for hyperconjugation.

18. [NEET 2024] The most stable conformation of 2-butene is:

a) Gauche
b) Anti
c) Eclipsed
d) Skew

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Answer: b) Anti

Explanation: Anti has maximum hyperconjugation (6 α-H aligned) and minimum steric repulsion.

19. [JEE Main 2019] Hyperconjugation explains:

a) o/p directing effect of –CH₃ in toluene
b) m-directing effect of –NO₂
c) Stability of benzene
d) Acidity of alkynes

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Answer: a) o/p directing effect of –CH₃ in toluene

Explanation: Hyperconjugation increases electron density at ortho/para positions.

20. [JEE Adv 2024] The total number of hyperconjugative structures in the most stable carbocation from 3-methylpentane is:

a) 7
b) 8
c) 9
d) 12

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Answer: c) 9

Explanation: 3-methylpentane → tertiary carbocation at C3: (CH₃)(CH₃CH₂)₂C⁺
• Two CH₃ → 6 H
• One CH₂CH₃ → 3 H
Total 9 α-H → 9 hyperconjugative structures