Stability of cycloalkanes – Baeyer’s strain theory MCQs With Answer

Stability of cycloalkanes – Baeyer’s strain theory MCQs With Answer
Understanding the stability of cycloalkanes is essential for B.Pharm students studying organic chemistry and drug molecular design. Baeyer’s strain theory explains how deviation from the ideal tetrahedral angle (109.5°) creates angle strain; combined with torsional and steric strain, these factors determine ring stability. Key topics include ring strain energy, heats of combustion, conformational analysis (chair, boat, envelope), and experimental evidence that supports or limits Baeyer’s postulates. This concise, keyword-rich introduction prepares you to analyze cycloalkane behavior, predict reactivity, and apply concepts to medicinal chemistry. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary cause of strain in cycloalkanes according to Baeyer’s strain theory?

• Deviation of bond angles from 109.5°
• Torsional strain due to eclipsing hydrogens
• Steric interactions between substituents
• Electronic conjugation in the ring

Correct Answer: Deviation of bond angles from 109.5°

Q2. Which component of ring strain is due to eclipsing interactions between adjacent C–H bonds?

• Angle strain
• Torsional strain
• Steric strain
• Resonance stabilization

Correct Answer: Torsional strain

Q3. According to experimental heats of combustion, which cycloalkane has the highest ring strain per CH2 group?

• Cyclopentane
• Cyclohexane
• Cyclopropane
• Cyclooctane

Correct Answer: Cyclopropane

Q4. Which conformation of cyclohexane is the most stable and strain-free?

• Boat
• Chair
• Twist-boat
• Planar hexagon

Correct Answer: Chair

Q5. Baeyer’s strain theory was originally based on the idea that carbon atoms prefer which angle?

• 120°
• 90°
• 109.5°
• 180°

Correct Answer: 109.5°

Q6. Which ring size is nearly strain-free when it adopts the chair conformation?

• Cyclobutane
• Cyclopentane
• Cyclohexane
• Cycloheptane

Correct Answer: Cyclohexane

Q7. Which cycloalkane adopts an envelope conformation to reduce torsional strain?

• Cyclopropane
• Cyclobutane
• Cyclopentane
• Cyclohexane

Correct Answer: Cyclopentane

Q8. Baeyer’s strain theory fails to fully explain the low strain of which molecule?

• Cyclopropane
• Cyclobutane
• Cyclopentane
• Cyclohexane

Correct Answer: Cyclohexane

Q9. What experimental method is commonly used to estimate ring strain energy?

• NMR spectroscopy
• Heats of combustion
• Infrared spectroscopy
• Gas chromatography

Correct Answer: Heats of combustion

Q10. Which factor is NOT considered a part of ring strain?

• Angle strain
• Torsional strain
• Steric strain
• Hydrogen bonding

Correct Answer: Hydrogen bonding

Q11. Cyclobutane reduces torsional strain by adopting what kind of geometry?

• Completely planar square
• Puckered, partial envelope
• Perfect chair
• Linear chain

Correct Answer: Puckered, partial envelope

Q12. Which cycloalkane shows significant angle strain because its internal bond angles are forced near 60°?

• Cyclopropane
• Cyclobutane
• Cyclopentane
• Cyclohexane

Correct Answer: Cyclopropane

Q13. In substituted cyclohexanes, which position is generally preferred by bulky substituents to minimize 1,3-diaxial interactions?

• Axial
• Equatorial
• Planar
• Perpendicular

Correct Answer: Equatorial

Q14. Which conformation of cyclohexane has greater steric strain due to 1,3-diaxial interactions?

• Chair with bulky group equatorial
• Chair with bulky group axial
• Twist-boat with equatorial group
• Planar conformation

Correct Answer: Chair with bulky group axial

Q15. The strain energy of cyclohexane in its most stable conformation is closest to which value?

• ~27 kcal/mol
• ~6 kcal/mol
• ~0 kcal/mol
• ~50 kcal/mol

Correct Answer: ~0 kcal/mol

Q16. Which statement best describes the limitation of Baeyer’s original strain theory?

• It overemphasized torsional strain and ignored angles
• It could not predict the low strain of non-planar conformations like the cyclohexane chair
• It only applied to aromatic rings
• It required quantum mechanical calculations

Correct Answer: It could not predict the low strain of non-planar conformations like the cyclohexane chair

Q17. What term describes the total extra energy a cycloalkane has relative to a strain-free acyclic alkane?

• Resonance energy
• Strain energy
• Bond dissociation energy
• Activation energy

Correct Answer: Strain energy

Q18. Which of the following decreases when cyclohexane flips from one chair conformation to another?

• Relative positions of axial and equatorial substituents
• Overall molecular formula
• Number of carbon atoms
• Covalent bond types

Correct Answer: Relative positions of axial and equatorial substituents

Q19. Which of these cycloalkanes has appreciable torsional strain because of eclipsing hydrogens in a planar conformation?

• Cyclohexane in chair form
• Cyclopropane (planar)
• Cyclohexane in twist-boat
• Cyclohexane in planar hexagon

Correct Answer: Cyclopropane (planar)

Q20. The heat of combustion per CH2 group is used to compare strain because it normalizes for what?

• Different numbers of hydrogens only
• Ring size by number of CH2 units
• Electronic polarity of substituents
• Temperature differences in experiments

Correct Answer: Ring size by number of CH2 units

Q21. Which cycloalkane is most likely to have significant pyramidalization and bent bonds leading to orbital overlap changes?

• Cyclohexane
• Cyclopentane
• Cyclobutane
• Cyclopropane

Correct Answer: Cyclopropane

Q22. Which phenomenon allows cyclohexane to avoid angle strain despite a six-membered ring?

• Planarization of the ring
• Puckering into a chair conformation that maintains near-tetrahedral angles
• Formation of double bonds
• Hydrogen bonding within the ring

Correct Answer: Puckering into a chair conformation that maintains near-tetrahedral angles

Q23. In conformational analysis, which cyclohexane conformation is higher in energy due to steric crowding and eclipsing?

• Chair
• Boat
• Twist-boat (slightly lower than boat)
• Tetrahedral

Correct Answer: Boat

Q24. Which technique can provide information about axial vs equatorial populations of substituents in solution?

• IR spectroscopy
• NMR spectroscopy (coupling constants and shifts)
• Mass spectrometry
• UV–Vis spectroscopy

Correct Answer: NMR spectroscopy (coupling constants and shifts)

Q25. In relation to Baeyer’s theory, what experimental observation challenged the idea that small rings must be planar?

• Cyclopropane being aromatic
• Cyclobutane and cyclopentane adopting puckered conformations to reduce torsional strain
• Cyclohexane being planar
• All rings showing identical heats of combustion

Correct Answer: Cyclobutane and cyclopentane adopting puckered conformations to reduce torsional strain

Q26. Which cycloalkane generally shows the lowest heat of combustion per CH2 group, indicating minimal ring strain?

• Cyclopropane
• Cyclobutane
• Cyclopentane
• Cyclohexane

Correct Answer: Cyclohexane

Q27. What is 1,3-diaxial interaction?

• Interaction between two equatorial substituents
• Steric interactions between an axial substituent and axial hydrogens on carbon-3 and carbon-5 of cyclohexane
• Hydrogen bonding across the ring
• π–π stacking in aromatic rings

Correct Answer: Steric interactions between an axial substituent and axial hydrogens on carbon-3 and carbon-5 of cyclohexane

Q28. Which structural feature of cyclopropane causes its C–C bonds to have increased s-character and higher reactivity?

• Planar trigonal carbons
• Severe ring angle compression forcing bonds to bend (“banana bonds”)
• Delocalized π system
• Perfect tetrahedral geometry

Correct Answer: Severe ring angle compression forcing bonds to bend (“banana bonds”)

Q29. Which cycloalkane is best described as having both angle and torsional strain but lesser overall strain than cyclopropane?

• Cyclobutane
• Cyclohexane
• Cycloheptane
• Cyclooctane

Correct Answer: Cyclobutane

Q30. Which term explains energy differences between conformers of substituted cyclohexanes?

• Conformational energy due to axial vs equatorial positions
• Resonance energy exclusively
• Only electronic delocalization
• Isotopic substitution energy

Correct Answer: Conformational energy due to axial vs equatorial positions

Q31. Which ring would Baeyer predict to be highly strained purely on angle considerations?

• Cyclodecane
• Cyclopropane
• Cyclohexane
• Cyclopentane

Correct Answer: Cyclopropane

Q32. Which effect contributes most to the stability of substituted cyclohexanes when bulky groups are equatorial?

• Increased torsional strain
• Reduced steric and 1,3-diaxial interactions
• Increased angle strain
• Enhanced conjugation

Correct Answer: Reduced steric and 1,3-diaxial interactions

Q33. Which conformation of cyclohexane is least populated at room temperature due to its high energy?

• Chair (most stable)
• Boat (high energy)
• Twist-boat (moderate energy)
• Equatorial-only

Correct Answer: Boat (high energy)

Q34. How does substitution at the ring affect strain energy generally?

• Substitution always decreases strain energy
• Bulky substitution can increase steric and torsional strain, altering conformational preferences
• Substitution only affects electronic properties, not strain
• All substitutions make the ring planar

Correct Answer: Bulky substitution can increase steric and torsional strain, altering conformational preferences

Q35. Which cycloalkane typically shows significant puckering to relieve eclipsing interactions?

• Cyclohexane chair
• Cyclopentane and cyclobutane
• Cyclopropane only
• Planar benzene

Correct Answer: Cyclopentane and cyclobutane

Q36. Why is cyclohexane chair favored over a planar hexagon model?

• Chair maximizes angle strain
• Chair minimizes both angle and torsional strain, approaching tetrahedral angles
• Planar hexagon has no torsional strain
• Chair increases conjugation

Correct Answer: Chair minimizes both angle and torsional strain, approaching tetrahedral angles

Q37. Which statement about cycloalkane reactivity is generally true?

• More strained rings are less reactive
• Ring strain often increases reactivity toward ring-opening reactions
• Cyclohexane reacts readily like alkenes
• Ring strain has no effect on reaction pathways

Correct Answer: Ring strain often increases reactivity toward ring-opening reactions

Q38. In drug design, why is understanding cycloalkane stability important?

• Stability affects conformational flexibility, which can alter binding and metabolism
• It only affects color of the compound
• It is irrelevant to pharmacokinetics
• Stability prevents all biological interactions

Correct Answer: Stability affects conformational flexibility, which can alter binding and metabolism

Q39. Which phenomenon describes conversion between two chair conformations of cyclohexane?

• Ring flip (chair-chair interconversion)
• Planarization
• Electrophilic substitution
• Radical rearrangement

Correct Answer: Ring flip (chair-chair interconversion)

Q40. Which ring size often experiences transannular strain (interactions across the ring) as an important factor?

• Small rings like cyclopropane
• Medium rings like cycloheptane and cyclooctane
• Cyclohexane only
• Aromatic six-membered rings

Correct Answer: Medium rings like cycloheptane and cyclooctane

Q41. How does Baeyer’s strain theory quantify the source of strain?

• By measuring dipole moments
• By attributing strain to deviations from ideal tetrahedral angles and estimating increased energy
• By counting hydrogen bonds
• By measuring color changes upon heating

Correct Answer: By attributing strain to deviations from ideal tetrahedral angles and estimating increased energy

Q42. Which is a direct experimental indicator that a cycloalkane is strained?

• Lower boiling point than expected
• Higher heat of combustion per CH2 unit compared to unstrained alkane reference
• Absence of NMR signals
• Enhanced UV absorption

Correct Answer: Higher heat of combustion per CH2 unit compared to unstrained alkane reference

Q43. Which conformer of cyclohexane reduces torsional strain better than the boat?

• Chair
• Planar hexagon
• Linear chain
• Axial-only form

Correct Answer: Chair

Q44. Which ring shows high ring strain but still participates in many organic reactions due to its high ring energy?

• Cyclohexane
• Cyclopropane
• Cycloheptane
• Cyclooctane

Correct Answer: Cyclopropane

Q45. What is the significance of ‘banana bonds’ in small rings?

• They denote perfect linear bonds
• They describe bent C–C bonds that relieve angle strain in small rings like cyclopropane
• They are a type of hydrogen bond
• They refer to aromatic π-bonding

Correct Answer: They describe bent C–C bonds that relieve angle strain in small rings like cyclopropane

Q46. Which structural change lowers torsional strain in cyclobutane compared to a hypothetical planar square?

• Becoming fully planar
• Puckering to a folded conformation
• Converting to cyclohexane
• Adding double bonds

Correct Answer: Puckering to a folded conformation

Q47. Which of the following best explains why cyclohexane is commonly used as a model for strain-free rings in textbooks?

• Because it is the smallest ring
• Because the chair conformation approximates ideal tetrahedral geometry, minimizing strain
• Because it is planar and aromatic
• Because it contains double bonds

Correct Answer: Because the chair conformation approximates ideal tetrahedral geometry, minimizing strain

Q48. In substituted cyclopentanes, what helps lower the overall strain compared to a fully planar five-membered ring?

• Maintaining a flat ring
• Adopting envelope or half-chair conformations to reduce eclipsing interactions
• Forming a six-membered ring instead
• Adding aromatic substituents

Correct Answer: Adopting envelope or half-chair conformations to reduce eclipsing interactions

Q49. Which observation supports the idea that both angle and torsional strain contribute to ring energy?

• Cyclohexane planar has the lowest energy
• Cyclobutane puckers to lower energy despite some angle strain remaining
• Cyclopropane always adopts a chair conformation
• All rings show identical heats of combustion

Correct Answer: Cyclobutane puckers to lower energy despite some angle strain remaining

Q50. For a B.Pharm student, why is mastery of cycloalkane strain and conformational analysis important?

• It only matters for inorganic chemistry
• It helps predict molecular shape, reactivity, binding to biological targets, and ADME properties
• It is irrelevant for medicinal chemistry
• It only impacts physical color and taste

Correct Answer: It helps predict molecular shape, reactivity, binding to biological targets, and ADME properties

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com