Conformational isomerism in n-butane MCQs With Answer

Conformational isomerism in n-butane MCQs With Answer

Conformational isomerism in n-butane is a core topic in organic chemistry and stereochemistry for B. Pharm students, covering anti and gauche conformations, Newman projections, torsional and steric strain, and energy diagrams. Understanding n-butane conformations helps explain rotational barriers, relative stability (anti vs gauche), and how molecular shape influences reactivity and drug conformation. These MCQs emphasize dihedral angles, energy differences (approx. 0.9 kcal/mol between anti and gauche), eclipsed vs staggered states, and experimental implications such as NMR and thermodynamics. Questions range from basic definitions to problem-solving and expected populations at physiological temperatures. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which dihedral angle corresponds to the anti conformation in n-butane?

• 0° (methyl-methyl eclipsed)
• 60° (gauche)
• 120° (partially eclipsed)
• 180° (methyl groups opposite)

Correct Answer: 180° (methyl groups opposite)

Q2. What is the typical energy difference between the anti and gauche conformations of n-butane?

• About 0.9 kcal/mol (anti lower)
• About 5.0 kcal/mol (gauche lower)
• About 10 kcal/mol (anti lower)
• Zero, they are identical

Correct Answer: About 0.9 kcal/mol (anti lower)

Q3. Which conformation of n-butane is the most stable?

• Fully eclipsed methyl-methyl
• Gauche
• Anti
• Partially eclipsed methyl-hydrogen

Correct Answer: Anti

Q4. In a Newman projection of n-butane looking down the C2–C3 bond, what does the anti conformation show?

• Front methyl directly eclipses back methyl
• Front methyl exactly opposite back methyl
• Hydrogens are all eclipsed
• Methyl groups separated by 60°

Correct Answer: Front methyl exactly opposite back methyl

Q5. What type of strain is primarily responsible for the higher energy of eclipsed conformations?

• Torsional strain from eclipsing bonds
• Hydrogen bonding strain
• Pi–pi stacking strain
• Hyperconjugation relief

Correct Answer: Torsional strain from eclipsing bonds

Q6. The gauche conformation in n-butane is characterized by a dihedral of approximately:

• 0°
• 60°
• 120°
• 180°

Correct Answer: 60°

Q7. Which statement about the gauche conformers of n-butane is true?

• They are identical and not chiral
• They form an enantiomeric pair but interconvert rapidly
• Gauche conformers are more stable than anti
• Gauche conformers have zero torsional strain

Correct Answer: They form an enantiomeric pair but interconvert rapidly

Q8. The energy barrier for rotation around the central C–C bond in n-butane is approximately:

• ~0.5 kcal/mol
• ~1 kcal/mol
• ~5 kcal/mol
• ~15 kcal/mol

Correct Answer: ~5 kcal/mol

Q9. Which technique is commonly used to study conformational populations of n-butane experimentally?

• Infrared spectroscopy only
• Mass spectrometry only
• Nuclear magnetic resonance (NMR) spectroscopy
• Flame photometry

Correct Answer: Nuclear magnetic resonance (NMR) spectroscopy

Q10. In an energy vs dihedral angle plot for n-butane, how many minima are observed between 0° and 360°?

• 1 minimum
• 2 minima
• 3 minima
• 4 minima

Correct Answer: 3 minima

Q11. Which conformer corresponds to a local maximum in the n-butane rotational energy profile?

• Anti (180°)
• Gauche (60°)
• Fully eclipsed methyl-methyl (0°)
• Staggered methyl-methyl (180°)

Correct Answer: Fully eclipsed methyl-methyl (0°)

Q12. Why is the anti conformer lower in energy than the gauche conformer?

• Antis have stronger hydrogen bonding
• Anti minimizes steric repulsion between methyl groups
• Gauche maximizes hyperconjugation
• Anti has more torsional strain

Correct Answer: Anti minimizes steric repulsion between methyl groups

Q13. What is the primary reason gauche conformers are still significantly populated at room temperature?

• Gauche is lower in energy than anti
• The energy difference (~0.9 kcal/mol) is small so both are populated
• Gauche is stabilized by hydrogen bonding
• Rotation is forbidden so both persist

Correct Answer: The energy difference (~0.9 kcal/mol) is small so both are populated

Q14. Which term describes interaction between two methyl groups in the gauche conformation?

• Gauche interaction (steric repulsion)
• 1,3-diaxial interaction
• Conjugation
• Resonance stabilization

Correct Answer: Gauche interaction (steric repulsion)

Q15. When constructing a Newman projection for n-butane, what do front and back circles represent?

• Front and back hydrogen atoms
• Front and back carbon atoms along the inspected bond
• Methyl groups only
• Electron density lobes

Correct Answer: Front and back carbon atoms along the inspected bond

Q16. Which conformation is observed when the dihedral angle is 0°?

• Anti (staggered)
• Gauche (staggered)
• Fully eclipsed methyl-methyl
• Partially staggered

Correct Answer: Fully eclipsed methyl-methyl

Q17. Which effect lowers the energy of staggered conformations relative to eclipsed ones?

• Increase in bond length
• Relief of torsional strain through staggered arrangement
• Increase in steric crowding
• Formation of new sigma bonds

Correct Answer: Relief of torsional strain through staggered arrangement

Q18. How many unique staggered conformations exist for n-butane when considering a full 360° rotation?

• 1
• 2 (two distinct staggered types: anti and gauche)
• 3
• 4

Correct Answer: 2 (two distinct staggered types: anti and gauche)

Q19. Which conformer corresponds to dihedral angles of ±60° and ±300°?

• Anti
• Gauche
• Fully eclipsed
• Planar

Correct Answer: Gauche

Q20. In terms of kinetics, why do gauche and anti interconvert readily at room temperature?

• Because the barrier (~5 kcal/mol) is easily surmounted at room temperature
• Because the barrier is extremely high (>20 kcal/mol)
• They do not interconvert; rotation is frozen
• Due to irreversible bond cleavage

Correct Answer: Because the barrier (~5 kcal/mol) is easily surmounted at room temperature

Q21. Which computational method is commonly used to calculate conformational energies of n-butane?

• Density Functional Theory (DFT)
• X-ray crystallography
• Gas chromatography
• Thin-layer chromatography

Correct Answer: Density Functional Theory (DFT)

Q22. Which orbital interaction contributes to stability of staggered conformations?

• Hyperconjugation between C–H sigma and adjacent sigma* orbitals
• Pi–pi overlap
• Meso interaction
• Ionic bonding

Correct Answer: Hyperconjugation between C–H sigma and adjacent sigma* orbitals

Q23. For n-butane, the energy of the fully eclipsed methyl-methyl conformation relative to anti is approximately:

• 0.9 kcal/mol
• ~5 kcal/mol
• ~15 kcal/mol
• ~20 kcal/mol

Correct Answer: ~5 kcal/mol

Q24. How does temperature affect the population of conformers for n-butane?

• Higher temperature increases population of higher-energy conformers
• Temperature has no effect
• Higher temperature favors only anti
• Lower temperature increases higher-energy conformers

Correct Answer: Higher temperature increases population of higher-energy conformers

Q25. Which is a stereochemical implication of butane conformations relevant to drug design?

• Conformation never affects binding affinity
• Molecular conformation can alter receptor binding and pharmacokinetics
• Conformations are irrelevant in small molecules
• Only chirality matters, not conformation

Correct Answer: Molecular conformation can alter receptor binding and pharmacokinetics

Q26. In a simple Boltzmann distribution at room temperature, which conformer of n-butane is predicted to be most populated?

• Fully eclipsed
• Anti
• Gauche exclusively
• All conformers equally populated

Correct Answer: Anti

Q27. Which conformer pair are enantiomers in n-butane?

• Anti conformers
• Gauche left- and right-handed forms
• Both eclipsed conformers
• No conformer pairs are enantiomeric

Correct Answer: Gauche left- and right-handed forms

Q28. Which factor does NOT significantly influence the anti vs gauche energy difference in n-butane?

• Steric repulsion between methyl groups
• Torsional strain
• Hyperconjugative stabilization
• Pi conjugation with an aromatic ring

Correct Answer: Pi conjugation with an aromatic ring

Q29. When analyzing butane conformations, what is a Newman projection most useful for?

• Showing 3D bond lengths only
• Visualizing relative orientations of substituents about a bond
• Predicting boiling points directly
• Measuring bond energies experimentally

Correct Answer: Visualizing relative orientations of substituents about a bond

Q30. Which educational concept explains why staggered conformations are stabilized by hyperconjugation?

• Overlap of filled C–H sigma orbitals with adjacent sigma* orbitals
• Formation of pi bonds
• Hydrogen bonding networks
• Dipole–dipole repulsion

Correct Answer: Overlap of filled C–H sigma orbitals with adjacent sigma* orbitals

Q31. Which of the following best describes the term ‘torsional strain’?

• Repulsion due to nonbonded atoms across space
• Energy required to rotate about a bond due to eclipsing interactions
• Attraction between opposite charges
• Stabilization from resonance

Correct Answer: Energy required to rotate about a bond due to eclipsing interactions

Q32. How many distinct eclipsed conformations are present in a full rotation of n-butane?

• 1
• 2
• 3
• 4

Correct Answer: 3

Q33. Which experimental observable in NMR can be influenced by conformational averaging in n-butane derivatives?

• Chemical shift and coupling constants
• Isotopic mass only
• Chromatographic retention time only
• Elemental composition

Correct Answer: Chemical shift and coupling constants

Q34. If the dihedral angle is 120°, which general classification applies?

• Staggered (gauche-like)
• Staggered (anti-like)
• Partially eclipsed
• Fully eclipsed

Correct Answer: Partially eclipsed

Q35. Which of the following influences the rotational barrier most in n-butane?

• Strength of hydrogen bonds in the molecule
• Number and type of eclipsing interactions during rotation
• Presence of aromatic rings far away
• Ionization energy of carbons

Correct Answer: Number and type of eclipsing interactions during rotation

Q36. In medicinal chemistry context, why study conformational isomerism of small chains like n-butane?

• To ignore 3D shape when designing drugs
• To understand conformational preferences that affect binding and ADME
• Because they form strong ionic bonds with receptors
• Only to improve solubility predictions

Correct Answer: To understand conformational preferences that affect binding and ADME

Q37. Which statement about eclipsed methyl-hydrogen interactions in n-butane is correct?

• They are higher in energy than fully eclipsed methyl-methyl
• They are lower in energy than fully eclipsed methyl-methyl but still destabilizing
• They stabilize the molecule
• They do not exist in butane

Correct Answer: They are lower in energy than fully eclipsed methyl-methyl but still destabilizing

Q38. Which representation emphasizes backbone dihedral angles and spatial relationships in butane?

• Fischer projection
• Newman projection
• Hückel diagram
• Electron density map

Correct Answer: Newman projection

Q39. Which parameter would you calculate to estimate the fraction of molecules in gauche conformation at equilibrium?

• Partition coefficient only
• Boltzmann factor using ΔG between conformers
• pKa value
• Optical rotation

Correct Answer: Boltzmann factor using ΔG between conformers

Q40. If anti is 0.9 kcal/mol lower than gauche, approximate ratio of anti:gauche at room temperature is closest to:

• 1:10
• 5:1
• 1:1 (roughly equal)
• 100:1

Correct Answer: 5:1

Q41. Which conformer leads to maximum steric repulsion between methyl groups?

• Anti
• Gauche
• Fully eclipsed methyl-methyl
• Staggered anti only

Correct Answer: Fully eclipsed methyl-methyl

Q42. What is the effect of replacing a methyl group with a larger substituent on conformational preferences?

• No effect on conformational energy
• May increase steric interactions and change preferred conformation
• Always makes gauche preferred
• Eliminates rotational barriers

Correct Answer: May increase steric interactions and change preferred conformation

Q43. Which concept explains why staggered conformations have lower energy besides reduced torsional strain?

• Lone pair repulsion
• Hyperconjugation stabilization
• Increased bond angle strain
• Pi bond formation

Correct Answer: Hyperconjugation stabilization

Q44. Which observable change would indicate increased population of higher-energy conformers in a sample?

• Unchanged NMR signals
• Temperature-dependent changes in NMR chemical shifts or coupling
• Color change only
• Mass increase

Correct Answer: Temperature-dependent changes in NMR chemical shifts or coupling

Q45. When visualizing n-butane energy surface, which axis typically represents dihedral angle?

• X-axis
• Y-axis
• Energy axis
• None, dihedral angle isn’t plotted

Correct Answer: X-axis

Q46. What is the significance of the two equivalent gauche minima in butane?

• They are identical and noninterconvertible
• They are equivalent enantiomeric minima separated by rotation
• They indicate butane is chiral overall permanently
• They show that anti is a maximum

Correct Answer: They are equivalent enantiomeric minima separated by rotation

Q47. In drug molecules with n-butyl side chains, conformational flexibility primarily affects:

• Elemental composition
• 3D shape and entropic contribution to binding
• Only UV absorption
• Bond covalency

Correct Answer: 3D shape and entropic contribution to binding

Q48. For n-butane, which conformer has intermediate energy between anti and fully eclipsed?

• Another anti at 360°
• Partially eclipsed (e.g., 120°) or eclipsed methyl–H
• Completely planar conformation
• Gauche at 180°

Correct Answer: Partially eclipsed (e.g., 120°) or eclipsed methyl–H

Q49. Which of the following best summarizes conformational isomerism?

• Isomers differing by breaking bonds
• Isomers differing by rotation about single bonds without breaking bonds
• Compounds with different molecular formulas
• Isomers that are only image reflections

Correct Answer: Isomers differing by rotation about single bonds without breaking bonds

Q50. Why is it useful for B. Pharm students to master MCQs on conformational isomerism in n-butane?

• It has no relevance to pharmacy
• It builds understanding of molecular shape, energetics, and implications for drug design and analysis
• Only to pass a single exam question
• Because butane is the only alkane used in drugs

Correct Answer: It builds understanding of molecular shape, energetics, and implications for drug design and analysis

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