Atropisomerism in biphenyl compounds MCQs With Answer

Atropisomerism in biphenyl compounds MCQs With Answer

Atropisomerism is a form of axial chirality arising from restricted rotation about a single bond, especially common in substituted biphenyl compounds. For B. Pharm students, understanding atropisomerism links stereochemistry to drug design, metabolic stability, stereoselective synthesis, and analytical separation. This introduction covers key concepts like rotational barrier, dihedral angle, ortho-substitution effects, configurational stability, and techniques such as chiral HPLC, CD spectroscopy, NMR and X-ray crystallography. Mastery of these topics helps predict when biphenyls exist as isolable atropisomers and how to analyze or synthesize them selectively. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is atropisomerism?

• Isomerism due to different connectivity between atoms
• Isomerism caused by restricted rotation around a single bond creating stable stereoisomers
• Isomerism due to cis-trans double bond arrangement
• Isomerism due to ring strain differences

Correct Answer: Isomerism caused by restricted rotation around a single bond creating stable stereoisomers

Q2. In biphenyl systems, which structural feature most commonly induces atropisomerism?

• Para-substitution on both rings
• Ortho-substitution that creates steric hindrance
• Meta-substitution causing electronic effects
• Aromatic ring size increase

Correct Answer: Ortho-substitution that creates steric hindrance

Q3. What term describes the chirality present along the bond axis in atropisomers?

• Planar chirality
• Point chirality
• Axial chirality
• Stereogenic center

Correct Answer: Axial chirality

Q4. Which experimental method directly gives absolute configuration of an atropisomeric biphenyl?

• 1H NMR spectroscopy
• Infrared spectroscopy
• X-ray crystallography
• Thin layer chromatography

Correct Answer: X-ray crystallography

Q5. Which analytical technique is most commonly used to separate atropisomeric enantiomers in the laboratory?

• Gas chromatography on achiral stationary phase
• Chiral HPLC
• Ultraviolet-visible spectrophotometry
• Gel electrophoresis

Correct Answer: Chiral HPLC

Q6. The energy barrier to rotation (ΔG‡) determines:

• The boiling point of a biphenyl compound
• Whether atropisomers interconvert at room temperature
• The UV absorption maximum
• The melting point only

Correct Answer: Whether atropisomers interconvert at room temperature

Q7. Roughly what magnitude of rotational barrier (ΔG‡) is often required for an atropisomer to be isolable at room temperature?

• Less than 5 kcal/mol
• Around 10 kcal/mol
• Approximately 20 kcal/mol or higher
• Over 100 kcal/mol

Correct Answer: Approximately 20 kcal/mol or higher

Q8. Which spectroscopic change is a sign of dynamic atropisomerism in NMR studies?

• Complete disappearance of all signals
• Line broadening and coalescence of signals with temperature change
• Sharp singlets at room temperature only
• Only changes in carbonyl peaks

Correct Answer: Line broadening and coalescence of signals with temperature change

Q9. How does introducing bulky ortho-substituents on biphenyl affect the dihedral angle between rings?

• It reduces the dihedral angle to zero
• It typically increases the twist (larger dihedral angle)
• It always makes the rings coplanar
• It converts the system into a non-aromatic structure

Correct Answer: It typically increases the twist (larger dihedral angle)

Q10. Which equation relates reaction rate constants to activation free energy useful in atropisomerism kinetics?

• Henderson-Hasselbalch equation
• Arrhenius equation
• Eyring equation (transition state theory)
• Beer-Lambert law

Correct Answer: Eyring equation (transition state theory)

Q11. What effect do electron-withdrawing ortho-substituents generally have on atropisomeric barrier compared to hydrogen?

• They always eliminate atropisomerism
• They can alter barrier via electronic and steric effects; effect depends on substituent
• They universally lower the barrier
• They convert axial chirality to point chirality

Correct Answer: They can alter barrier via electronic and steric effects; effect depends on substituent

Q12. Which technique can provide information on the sign and magnitude of optical activity of an atropisomer?

• Circular dichroism (CD) spectroscopy
• Mass spectrometry
• Infrared spectrometry
• Polarimetry does not measure optical activity

Correct Answer: Circular dichroism (CD) spectroscopy

Q13. Dynamic kinetic resolution in atropisomeric systems involves:

• Separating enantiomers without racemization
• Simultaneous racemization and enantioselective transformation to drive high ee
• Only chromatographic separation at low temperature
• Racemization followed by achiral workup

Correct Answer: Simultaneous racemization and enantioselective transformation to drive high ee

Q14. Which coupling reaction is commonly used to synthesize biphenyl frameworks in medicinal chemistry?

• Wittig reaction
• Suzuki–Miyaura cross-coupling
• Friedel–Crafts alkylation
• Benedict reaction

Correct Answer: Suzuki–Miyaura cross-coupling

Q15. Which statement about racemization of atropisomers is true?

• Racemization cannot occur once a compound is synthesized
• Racemization is the interconversion between enantiomers via rotation about the chiral axis
• Racemization converts atropisomers into diastereomers only
• Racemization is unrelated to rotational energy barrier

Correct Answer: Racemization is the interconversion between enantiomers via rotation about the chiral axis

Q16. Which parameter derived from kinetic studies indicates how fast atropisomers interconvert?

• ΔHfus
• Rate constant (k) for racemization
• Boiling point
• Partition coefficient (Log P)

Correct Answer: Rate constant (k) for racemization

Q17. In the context of atropisomerism, what does configurational stability mean?

• The molecule cannot be synthesized
• The enantiomers do not interconvert under given conditions
• The molecule is achiral
• The molecule rapidly racemizes at all temperatures

Correct Answer: The enantiomers do not interconvert under given conditions

Q18. Which of the following structural changes is most likely to decrease the rotational barrier in a biphenyl?

• Removing bulky ortho-substituents
• Adding more ortho-tert-butyl groups
• Introducing sterically large biaryl substituents
• Forming intramolecular hydrogen bonds that lock conformation

Correct Answer: Removing bulky ortho-substituents

Q19. What is the consequence of atropisomerism for drug action?

• No consequence since enantiomers always behave identically
• Potential differences in receptor binding, ADME and toxicity between enantiomers
• Only affects color of the drug
• Prevents the drug from being orally bioavailable

Correct Answer: Potential differences in receptor binding, ADME and toxicity between enantiomers

Q20. Which computational method is commonly used to estimate rotational barriers in atropisomeric compounds?

• Density Functional Theory (DFT) calculations
• Empirical rule-of-thumb only
• Classical molecular docking alone
• Statistical thermodynamics without geometry

Correct Answer: Density Functional Theory (DFT) calculations

Q21. Which description best fits a biphenyl with two bulky ortho substituents that cannot rotate at room temperature?

• Constitutional isomer
• Atropisomeric biaryl with high rotational barrier
• Rapidly equilibrating conformer
• Non-aromatic saturated biphenyl

Correct Answer: Atropisomeric biaryl with high rotational barrier

Q22. What experimental observation would indicate that two atropisomers are enantiomers rather than diastereomers?

• They have different molecular formulas
• They have identical physical properties in achiral environment but opposite optical rotation
• They have different NMR spectra under achiral conditions
• They cannot be separated by chiral stationary phases

Correct Answer: They have identical physical properties in achiral environment but opposite optical rotation

Q23. Which synthetic strategy can be employed to preferentially form one atropisomer?

• Non-selective radical coupling
• Atroposelective catalysis using chiral ligands
• Heating to high temperatures to equilibrate enantiomers
• Using achiral solvents only

Correct Answer: Atroposelective catalysis using chiral ligands

Q24. Which factor does NOT generally influence the barrier to rotation in biphenyls?

• Steric hindrance from substituents
• Electronic conjugation between rings
• Solution viscosity at nanomolar concentration only
• Intramolecular hydrogen bonding that locks conformation

Correct Answer: Solution viscosity at nanomolar concentration only

Q25. What is the effect of temperature on the rate of atropisomer interconversion?

• Rate decreases with increasing temperature
• Rate is independent of temperature
• Rate increases with increasing temperature according to Arrhenius/Eyring relations
• Temperature only affects chirality sign, not rate

Correct Answer: Rate increases with increasing temperature according to Arrhenius/Eyring relations

Q26. Which experiment can be used to measure the activation free energy (ΔG‡) for racemization?

• Monitoring optical rotation or enantiomeric excess decay over time and applying kinetic analysis
• Measuring melting point only
• Recording UV spectrum at a single time point
• Performing elemental analysis

Correct Answer: Monitoring optical rotation or enantiomeric excess decay over time and applying kinetic analysis

Q27. How do planarization tendencies (π-conjugation) influence biphenyl rotation?

• π-Conjugation always eliminates atropisomerism
• Tendency to planarize (conjugate) can lower barrier by favoring coplanarity, but steric effects may oppose it
• Planarization and steric effects are unrelated
• Conjugation always increases barrier regardless of substitution

Correct Answer: Tendency to planarize (conjugate) can lower barrier by favoring coplanarity, but steric effects may oppose it

Q28. Which term refers to isolable stereoisomers that arise solely from restricted bond rotation?

• Stereoisomers with stereogenic centers only
• Atropisomers
• Constitutional isomers
• Geometric isomers of alkenes

Correct Answer: Atropisomers

Q29. In chiral HPLC separation of atropisomers, what is the primary variable used to resolve enantiomers?

• Molecular weight differences
• Different interactions with a chiral stationary phase causing different retention times
• Differences in boiling points
• Reaction with the mobile phase to form diastereomers

Correct Answer: Different interactions with a chiral stationary phase causing different retention times

Q30. What is the likely effect of intramolecular hydrogen bonding that locks a biphenyl conformation?

• Decrease in rotational barrier
• Increase in conformational rigidity and potentially higher barrier
• No effect on rotation
• Breakdown of aromaticity

Correct Answer: Increase in conformational rigidity and potentially higher barrier

Q31. Which measurement directly indicates the presence of two non-interconverting atropisomers at room temperature?

• Single averaged NMR signal only
• Two distinct sets of NMR signals corresponding to each enantiomer without interconversion
• Mass spectral fragmentation patterns
• Identical retention times on chiral HPLC

Correct Answer: Two distinct sets of NMR signals corresponding to each enantiomer without interconversion

Q32. Why are ortho-methyl groups often used in model studies of biphenyl atropisomerism?

• They always make the compound planar
• They provide moderate steric hindrance to study effects on rotation
• They are strongly electron-withdrawing so they stop rotation
• They render the molecule fluorescent

Correct Answer: They provide moderate steric hindrance to study effects on rotation

Q33. Which of the following best describes classifying atropisomers as “class 3” in some literature?

• Rapidly interconverting conformers at room temperature
• Intermediate stability with moderate barriers
• High configurational stability with slow interconversion (practically isolable)
• Not related to rotational barriers

Correct Answer: High configurational stability with slow interconversion (practically isolable)

Q34. Which outcome indicates success in atroposelective synthesis?

• Formation of racemic mixture only
• Isolation of one atropisomer in high enantiomeric excess (ee)
• Destruction of axial chirality during workup
• Lowering melting point of product

Correct Answer: Isolation of one atropisomer in high enantiomeric excess (ee)

Q35. Which physical property is most likely different between two atropisomeric enantiomers measured in a chiral environment?

• Mass
• Optical rotation and interaction with chiral receptors
• Number of protons
• Exact elemental composition

Correct Answer: Optical rotation and interaction with chiral receptors

Q36. Which analytical approach can monitor racemization kinetics by following enantiomeric excess over time?

• Achiral TLC
• Chiral chromatographic analysis (e.g., chiral HPLC)
• Melting point determination
• IR peak intensity measurements only

Correct Answer: Chiral chromatographic analysis (e.g., chiral HPLC)

Q37. What is the effect of forming a biaryl bond via sterically hindered coupling conditions on atropisomer formation?

• It always yields only one atropisomer racemically
• It can produce atropisomers whose configuration depends on steric and catalytic control
• It prevents any axial chirality from arising
• It converts biphenyl into an alkane

Correct Answer: It can produce atropisomers whose configuration depends on steric and catalytic control

Q38. Which statement about dihedral angle in twisted biphenyl atropisomers is true?

• Dihedral angle is always 0° in atropisomers
• Dihedral angle measures twist between the two aromatic ring planes and affects conjugation and barrier
• Dihedral angle only applies to alkanes
• Dihedral angle cannot be measured experimentally

Correct Answer: Dihedral angle measures twist between the two aromatic ring planes and affects conjugation and barrier

Q39. What is one approach to determine whether a given biphenyl is configurationally stable at 25 °C?

• Predict by eyeballing the structure only
• Measure racemization half-life or monitor ee over time at 25 °C
• Only measure UV absorption
• Estimate molecular weight

Correct Answer: Measure racemization half-life or monitor ee over time at 25 °C

Q40. Which type of substituent placement tends to have the greatest steric impact on biphenyl rotation?

• Para-substituents
• Ortho-substituents adjacent to the biaryl bond
• Substituents on solvent molecules
• Substituents on distant heterocycles only

Correct Answer: Ortho-substituents adjacent to the biaryl bond

Q41. For an atropisomeric biphenyl used as a drug lead, why might a medicinal chemist prefer one enantiomer over the racemate?

• One enantiomer may have better potency, selectivity or safety profile
• Racemates are always cheaper and more effective
• Enantiomers have identical pharmacokinetics always
• Regulatory agencies prohibit enantiopure drugs

Correct Answer: One enantiomer may have better potency, selectivity or safety profile

Q42. Which experimental signature in CD spectroscopy supports the presence of axial chirality?

• No signal across UV region
• Distinct Cotton effects corresponding to chiral electronic transitions
• Only IR peaks present
• Broad unresolved peaks in mass spectrum

Correct Answer: Distinct Cotton effects corresponding to chiral electronic transitions

Q43. Which method can convert a racemic atropisomeric mixture into a single enantiomer by forming diastereomeric salts or complexes?

• Chiral resolution via diastereomeric salt formation
• Heating the racemate to high temperature only
• Chromatography on achiral silica gel
• Dilution with water only

Correct Answer: Chiral resolution via diastereomeric salt formation

Q44. What role can computational conformational scanning play in atropisomer studies?

• Predict rotational pathways, estimate barriers and preferred dihedral angles
• Replace all experimental characterization completely
• Only provide melting point predictions
• Increase the synthetic yield automatically

Correct Answer: Predict rotational pathways, estimate barriers and preferred dihedral angles

Q45. Which factor can complicate isolation of atropisomers during purification?

• High configurational stability
• Rapid interconversion (low barrier) leading to racemization during purification
• Presence of strong chromophores only
• High melting point

Correct Answer: Rapid interconversion (low barrier) leading to racemization during purification

Q46. Which synthetic modification often increases the rotational barrier and stabilizes atropisomers?

• Introducing small hydrogen substituents at ortho positions
• Adding bulky groups (e.g., tert‑butyl, isopropyl) at ortho positions
• Removing all substituents from both rings
• Hydrogenation to form cyclohexane rings

Correct Answer: Adding bulky groups (e.g., tert‑butyl, isopropyl) at ortho positions

Q47. Which characterization shows that two isolated atropisomers are mirror images and interconvert very slowly?

• They have different molecular formulas
• They are enantiomers with opposite optical rotation and identical achiral spectral data
• They display entirely different 1H NMR integrations in achiral solvent
• They cannot be crystallized

Correct Answer: They are enantiomers with opposite optical rotation and identical achiral spectral data

Q48. Which experimental approach can stabilize a preferred atropisomer during synthesis?

• Performing reactions at high temperatures to encourage equilibration
• Using chiral catalysts, auxiliaries or blocking groups during bond formation
• Leaving the molecule in air for prolonged periods
• Using strong acids to protonate aromatic rings exclusively

Correct Answer: Using chiral catalysts, auxiliaries or blocking groups during bond formation

Q49. How does solvent polarity sometimes affect atropisomer rotational barriers?

• Solvent polarity has no conceivable effect
• Polar solvents can stabilize transition states differently than nonpolar solvents, altering barriers modestly
• Only gaseous conditions affect barriers
• Solvent always destroys axial chirality

Correct Answer: Polar solvents can stabilize transition states differently than nonpolar solvents, altering barriers modestly

Q50. Which is a practical guideline for medicinal chemists working with biphenyl leads to assess atropisomer risk?

• Ignore ortho-substitution patterns during design
• Evaluate ortho-substitution sterics, compute/measure rotational barriers, and screen for enantiomeric stability and biological activity
• Assume all biphenyls are configurationally stable
• Always convert biphenyls to saturated analogs

Correct Answer: Evaluate ortho-substitution sterics, compute/measure rotational barriers, and screen for enantiomeric stability and biological activity

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