Conformational isomerism in ethane MCQs With Answer

Conformational isomerism in ethane MCQs With Answer is an essential topic for B. Pharm students studying stereochemistry and drug conformation. This clear, concise introduction explains ethane’s rotation about the C–C single bond, the energy difference between staggered and eclipsed conformations, the ~12 kJ/mol rotational barrier, and how Newman projections depict these forms. Understanding torsional strain, dihedral angles (0° and 60°), and the role of hyperconjugation helps predict molecular behavior relevant to drug design and stability. This keyword-rich primer prepares you for exam-style practice. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is conformational isomerism in ethane mainly caused by?

• Rotation about the C–C single bond
• Breaking of C–H bonds
• Change in molecular formula
• Isomerization of carbon atoms

Correct Answer: Rotation about the C–C single bond

Q2. What is the dihedral angle between adjacent C–H bonds in the most stable staggered conformation of ethane?

• 0°
• 30°
• 60°
• 120°

Correct Answer: 60°

Q3. Which conformation of ethane corresponds to the highest potential energy?

• Staggered
• Eclipsed
• Gauche
• Anti

Correct Answer: Eclipsed

Q4. Approximately what is the rotational energy barrier between staggered and eclipsed ethane?

• 0.5 kJ/mol
• 12 kJ/mol
• 100 kJ/mol
• 500 kJ/mol

Correct Answer: 12 kJ/mol

Q5. Which structural representation is most useful for visualizing ethane conformations along the C–C bond?

• Newman projection
• Fischer projection
• Haworth projection
• Ball-and-stick only

Correct Answer: Newman projection

Q6. What primary interaction is responsible for the torsional strain in eclipsed ethane?

• Repulsion between bonding electron pairs in adjacent C–H bonds
• Steric clash of bulky substituents
• Hydrogen bonding
• Ionic attraction

Correct Answer: Repulsion between bonding electron pairs in adjacent C–H bonds

Q7. How many staggered conformations occur during a full 360° rotation of ethane (counting distinct rotational positions)?

• 1
• 2
• 3
• 6

Correct Answer: 3

Q8. What is the dihedral angle at an eclipsed conformation of ethane?

• 180°
• 120°
• 60°
• 0°

Correct Answer: 0°

Q9. Which symmetry point group corresponds to the staggered ethane conformation?

• C2v
• D3d
• D3h
• C3v

Correct Answer: D3d

Q10. Which of the following best describes the potential energy profile for rotation about the C–C bond in ethane?

• Flat with no maxima or minima
• Single sharp peak only
• Sinusoidal with 120° periodicity and alternating minima and maxima
• Random fluctuations with no periodicity

Correct Answer: Sinusoidal with 120° periodicity and alternating minima and maxima

Q11. Which conformation is more populated at room temperature for ethane?

• Eclipsed conformation
• Staggered conformation
• Gauche conformation
• Cis conformation

Correct Answer: Staggered conformation

Q12. The term “torsional strain” in ethane is also commonly called:

• Angle strain
• Bond strain
• Eclipsing strain
• Van der Waals strain

Correct Answer: Eclipsing strain

Q13. Which phenomenon contributes to the stabilization of the staggered conformation of ethane besides reduced electron-electron repulsion?

• Hyperconjugation
• Ionic bonding
• Hydrogen bonding
• Resonance delocalization

Correct Answer: Hyperconjugation

Q14. What is the approximate energy difference between staggered and eclipsed ethane in kcal/mol?

• 0.1 kcal/mol
• 0.9 kcal/mol
• 2.9 kcal/mol
• 20 kcal/mol

Correct Answer: 2.9 kcal/mol

Q15. Which experimental technique can be used to measure rotational barriers in molecules (for larger barriers than ethane)?

• UV-Vis spectroscopy
• Variable-temperature NMR (VT-NMR)
• Infrared spectroscopy only
• Mass spectrometry

Correct Answer: Variable-temperature NMR (VT-NMR)

Q16. Are staggered and eclipsed conformations of ethane structural isomers or conformers?

• Structural isomers
• Conformers (conformational isomers)
• Enantiomers
• Cis-trans isomers

Correct Answer: Conformers (conformational isomers)

Q17. How many eclipsed conformations are there during one full rotation of ethane?

• 1
• 2
• 3
• 4

Correct Answer: 3

Q18. Which depiction shows the front carbon as a dot and the back carbon as a circle when visualizing ethane?

• Fischer projection
• Newman projection
• Sawhorse projection
• Chair conformation

Correct Answer: Newman projection

Q19. The rotation that converts one staggered form to another in ethane requires how many degrees of rotation?

• 60°
• 90°
• 120°
• 180°

Correct Answer: 120°

Q20. Which statement about ethane rotation at room temperature is correct?

• Rotation is frozen and conformers are isolable
• Rotation is rapid, so distinct conformers are not isolable
• Ethane cannot rotate about the C–C bond
• Rotation causes permanent breaking of bonds

Correct Answer: Rotation is rapid, so distinct conformers are not isolable

Q21. In ethane, when the dihedral angle is 30°, the energy is:

• At a global minimum
• At a global maximum
• Intermediate between staggered and eclipsed
• Equal to the staggered energy

Correct Answer: Intermediate between staggered and eclipsed

Q22. What does the dihedral angle measure in ethane?

• Angle between C–H bond and C–C bond
• Angle between planes containing two bonds on adjacent carbons
• Bond length of C–C
• Bond order of C–H

Correct Answer: Angle between planes containing two bonds on adjacent carbons

Q23. Which theoretical method is commonly used to compute the rotational barrier of ethane accurately?

• Empirical pKa prediction
• Density Functional Theory (DFT)
• Simple molecular weight calculation
• Elementary stoichiometry

Correct Answer: Density Functional Theory (DFT)

Q24. Which of the following correctly compares electron repulsion and steric effects in ethane torsional strain?

• Steric effects dominate entirely
• Electron repulsion between bonding pairs is a major contributor
• Hydrogen bonding causes most strain
• Van der Waals forces are the only cause

Correct Answer: Electron repulsion between bonding pairs is a major contributor

Q25. Which of these is true about ethane staggered conformations?

• They are higher in energy than eclipsed
• They minimize torsional strain and are more stable
• They have maximum electron-electron repulsion
• They break a C–H bond

Correct Answer: They minimize torsional strain and are more stable

Q26. What periodicity does the ethane rotational energy curve show?

• 90° periodicity
• 120° periodicity
• 180° periodicity
• 360° periodicity only

Correct Answer: 120° periodicity

Q27. The energy maxima in ethane correspond to which dihedral arrangement of front and back C–H bonds?

• Perpendicular
• Anti-periplanar
• Eclipsed (overlapped)
• Orthogonal

Correct Answer: Eclipsed (overlapped)

Q28. In a Newman projection of staggered ethane, how are the six hydrogen atoms arranged?

• Three overlapping on top of three
• Evenly spaced at 60° intervals around the circle
• All on one side of the projection
• Clustered in pairs

Correct Answer: Evenly spaced at 60° intervals around the circle

Q29. Which effect is often cited along with torsional strain to explain staggered stabilization in ethane?

• Electrostatic attraction only
• Hyperconjugation between C–H σ bonds and C–C σ* orbitals
• π-π stacking
• Metal coordination

Correct Answer: Hyperconjugation between C–H σ bonds and C–C σ* orbitals

Q30. Are conformations of ethane stereoisomers that can be separated by simple physical methods at room temperature?

• Yes, they can be separated by filtration
• No, they interconvert rapidly and cannot be separated
• Yes, by distillation only
• Yes, by chromatography easily

Correct Answer: No, they interconvert rapidly and cannot be separated

Q31. Which phrase best describes conformational isomerism?

• Different connectivity of atoms
• Different spatial orientation due to rotation about single bonds without bond breaking
• Substitution reactions changing functional groups
• Formation of new covalent bonds

Correct Answer: Different spatial orientation due to rotation about single bonds without bond breaking

Q32. In drug design (B. Pharm relevance), why is understanding ethane-like conformational preferences important?

• It predicts melting point only
• It helps anticipate preferred orientations of drug fragments affecting binding and ADME
• It determines elemental composition
• It has no relevance to pharmacology

Correct Answer: It helps anticipate preferred orientations of drug fragments affecting binding and ADME

Q33. The energy difference between different conformations is most directly related to which of the following?

• Atomic number of hydrogen
• Interatomic interactions as a function of dihedral angle
• Molar mass of the molecule
• Color of the compound

Correct Answer: Interatomic interactions as a function of dihedral angle

Q34. Which conformation corresponds to a dihedral angle of 180° between two C–H bonds on opposite carbons in ethane?

• Staggered (but note dihedral of 180° occurs across non-adjacent Hs)
• Eclipsed
• Gauche
• Cis

Correct Answer: Staggered (but note dihedral of 180° occurs across non-adjacent Hs)

Q35. Which statement about Newman and sawhorse projections is true?

• Newman projects along a bond; sawhorse shows a tilted side view of the same bond
• Both are identical and indistinguishable
• Sawhorse is used only for rings
• Newman cannot show dihedral angles

Correct Answer: Newman projects along a bond; sawhorse shows a tilted side view of the same bond

Q36. Which interaction is minimized in the staggered conformation compared to the eclipsed conformation?

• Hydrogen bonding
• Torsional (eclipsing) interaction between adjacent C–H bonds
• π-bonding
• Metal-ligand coordination

Correct Answer: Torsional (eclipsing) interaction between adjacent C–H bonds

Q37. If a student draws the ethane potential energy curve, where will the lowest points be located?

• At 0°, 120°, 240° (eclipsed)
• At 60°, 180°, 300° (staggered)
• Only at 180°
• No minima exist

Correct Answer: At 60°, 180°, 300° (staggered)

Q38. How does increasing the size of substituents on ethane carbons generally affect the rotational barrier?

• Barrier decreases to zero
• Barrier increases due to added steric hindrance
• Barrier remains exactly the same
• Rotation becomes impossible always

Correct Answer: Barrier increases due to added steric hindrance

Q39. Which of the following best explains why ethane conformations interconvert at room temperature?

• Because the rotational barrier is low relative to thermal energy (kT)
• Because bonds are broken routinely
• Because of permanent ionization
• Because of polymerization

Correct Answer: Because the rotational barrier is low relative to thermal energy (kT)

Q40. The concept of “anti” and “gauche” is more relevant to which molecule rather than ethane?

• Methane
• Butane
• Hydrogen gas
• Ethene

Correct Answer: Butane

Q41. Which of the following is NOT a characteristic of conformational isomerism in ethane?

• Interconversion without bond breaking
• Dependence on rotation about single bond
• Different connectivity of atoms
• Presence of torsional strain in certain conformations

Correct Answer: Different connectivity of atoms

Q42. When teaching Newman projections for ethane, which element is customarily placed at the center front?

• Back carbon
• Front carbon (represented as a dot)
• Hydrogen atom
• Electron pair

Correct Answer: Front carbon (represented as a dot)

Q43. The modern interpretation of ethane’s staggered stability often includes which quantum-mechanical concept?

• Van der Waals attraction only
• Hyperconjugation and stabilization from overlap of σ and σ* orbitals
• Formation of π-bonds
• Radical formation

Correct Answer: Hyperconjugation and stabilization from overlap of σ and σ* orbitals

Q44. Which description applies to the conformational analysis of ethane?

• It is irrelevant to molecular energetics
• It quantifies energy changes as a function of torsional rotation
• It involves changing the molecular formula
• It only applies to ionic compounds

Correct Answer: It quantifies energy changes as a function of torsional rotation

Q45. In the context of B. Pharm, why might a pharmacist care about conformational preferences in small fragments like ethane?

• Because they determine crystal color
• Because fragment conformation can influence overall drug conformation, binding, and metabolism
• They are unrelated to pharmacology
• Only for theoretical chemistry, not practical use

Correct Answer: Because fragment conformation can influence overall drug conformation, binding, and metabolism

Q46. Which of the following is true regarding the C–C bond in ethane during rotation?

• The sigma bond disappears upon rotation
• The sigma bond remains intact while atoms rotate relative to each other
• The bond order changes from single to double
• The C–C bond becomes ionic

Correct Answer: The sigma bond remains intact while atoms rotate relative to each other

Q47. What is the immediate effect on potential energy as dihedral angle moves from 60° toward 0° in ethane?

• Energy decreases to a lower minimum
• Energy increases toward a maximum
• Energy stays constant
• Bond order increases dramatically

Correct Answer: Energy increases toward a maximum

Q48. Which statement best differentiates conformers from configurational isomers?

• Conformers require bond breaking to interconvert; configurational isomers do not
• Configurational isomers require bond breaking or breaking/changing stereocenters; conformers interconvert by rotation
• They are the same concept
• Only configurational isomers involve rotation about single bonds

Correct Answer: Configurational isomers require bond breaking or breaking/changing stereocenters; conformers interconvert by rotation

Q49. In ethane, when two C–H bonds are eclipsed, what is the approximate additional energy cost compared to the staggered arrangement per H–H eclipsing interaction?

• ~0.5 kJ/mol
• ~4 kJ/mol (contributing to total ~12 kJ/mol)
• ~50 kJ/mol
• ~100 kJ/mol

Correct Answer: ~4 kJ/mol (contributing to total ~12 kJ/mol)

Q50. Which practical skill in organic structural analysis helps visualize ethane conformations for exams and problem solving?

• Drawing resonance structures only
• Mastering Newman and sawhorse projections and calculating dihedral angles
• Only memorizing molecular formulas
• Ignoring stereochemistry completely

Correct Answer: Mastering Newman and sawhorse projections and calculating dihedral angles

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.

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