Hyperconjugation - UNSOLVED PRACTICE SET
Chapter: Organic Chemistry Basic Principles | Topic: Hyperconjugation
HYPERCONJUGATION - UNSOLVED PRACTICE SET
Topic: Hyperconjugation
Multiple Choice Questions
Q1. Hyperconjugation involves the delocalisation of:
- π electrons only
- σ electrons of an adjacent C–H or C–C bond into an empty p-orbital or π bond
- Lone pair electrons only
- All electrons equally
Q2. The number of hyperconjugative structures for ethyl cation (CH₃CH₂⁺) is:
- 3
- 6
- 9
- 0
Q3. Hyperconjugation is also known as:
- Resonance
- No-bond resonance
- Inductive effect
- Electromeric effect
Q4. The stability of carbocations increases in the order:
- 3° < 2° < 1° < CH₃⁺
- CH₃⁺ < 1° < 2° < 3°
- 2° < 1° < 3° < CH₃⁺
- 1° < CH₃⁺ < 2° < 3°
Q5. The greater the number of α-hydrogen atoms, the:
- Less stable the carbocation
- More stable the carbocation due to greater hyperconjugation
- No effect on stability
- More reactive the compound
Q6. Hyperconjugation explains:
- Why alkenes are more stable than alkanes
- The greater stability of more substituted alkenes
- Why benzene is aromatic
- Why ionic bonds are strong
Short Answer Questions
Q7. Define hyperconjugation. How does it differ from resonance?
Q8. Explain why propene is more stable than ethene using the concept of hyperconjugation. How many hyperconjugative structures does propene have?
Q9. Arrange the following carbocations in order of increasing stability and explain using hyperconjugation:
CH₃⁺, CH₃CH₂⁺, (CH₃)₂CH⁺, (CH₃)₃C⁺
Q10. What are α-hydrogen atoms? How does the number of α-hydrogen atoms affect the stability of alkenes?
Q11. Your teacher asks why 2-butene is more stable than 1-butene. Explain this using hyperconjugation. How many α-hydrogen atoms does each have?
Q12. How does hyperconjugation explain the relative stability of the following alkenes?
(a) CH₂=CH₂
(b) CH₃–CH=CH₂
(c) CH₃–CH=CH–CH₃ (cis)
(d) (CH₃)₂C=CH₂
Long Answer Questions
Q13. Discuss hyperconjugation in detail:
(a) Definition and concept of hyperconjugation (no-bond resonance)
(b) Conditions necessary for hyperconjugation
(c) Number of hyperconjugative structures and their relation to α-hydrogen atoms
(d) Hyperconjugation in carbocations, free radicals, and alkenes
(e) Difference between hyperconjugation and inductive effect
Q14. Explain how hyperconjugation accounts for:
(a) The stability order of carbocations: 3° > 2° > 1° > CH₃⁺
(b) The stability order of free radicals: 3° > 2° > 1° > CH₃·
(c) The stability order of alkenes: more substituted alkenes are more stable
(d) The preference for Markovnikov addition in alkenes
Draw relevant structures to illustrate each point.
Q15. Hyperconjugation has practical implications in chemistry and biology. Discuss:
(a) Why Saytzeff's rule (more substituted alkene is major product) in elimination reactions is explained by hyperconjugation
(b) How hyperconjugation affects the bond length of the C=C bond in propene compared to ethene
(c) The role of hyperconjugation in the stability of biomolecules like terpenes found in Indian spices (turmeric, cardamom)
(d) Why understanding hyperconjugation is essential for designing stable organic semiconductors
Numerical / Application-Based Problems
Q16. Consider the following carbocations:
A: CH₃⁺ (methyl cation)
B: CH₃CH₂⁺ (ethyl cation)
C: (CH₃)₂CH⁺ (isopropyl cation)
D: (CH₃)₃C⁺ (tert-butyl cation)
(a) Count the number of α-hydrogen atoms in each carbocation.
(b) Predict the number of hyperconjugative structures for each.
(c) Arrange them in order of decreasing stability and explain the trend.
Q17. The heat of hydrogenation values for the following alkenes are:
Ethene: -137 kJ/mol
Propene: -126 kJ/mol
cis-2-Butene: -120 kJ/mol
trans-2-Butene: -116 kJ/mol
2-Methylpropene: -117 kJ/mol
(a) Arrange these alkenes in order of increasing stability.
(b) Explain why trans-2-butene is more stable than cis-2-butene.
(c) Account for the trend in stability using hyperconjugation and the number of α-hydrogen atoms.
Q18. A research lab in Chennai is studying the elimination reaction of 2-bromobutane to form butenes.
(a) Write the two possible alkene products (1-butene and 2-butene).
(b) Using hyperconjugation, predict which product will be the major product according to Saytzeff's rule.
(c) Calculate the number of α-hydrogen atoms contributing to hyperconjugation in each product alkene and justify your prediction.