Optical isomerism MCQs With Answer are essential for B. Pharm students to master stereochemistry and understand its profound effects on drug activity and safety. This concise introduction covers chiral centers, enantiomers, diastereomers, racemic mixtures, optical rotation, and resolution techniques, highlighting impacts on pharmacodynamics, pharmacokinetics, toxicity, stereoselective synthesis, and regulatory issues. Practice emphasizes Cahn-Ingold-Prelog rules, Fischer and Newman projections, polarimetry, chiral chromatography, and interpretation of enantiomeric excess. Regular practice of these targeted MCQs improves interpretation of stereochemical data, aids formulation development, and sharpens clinical decision-making regarding chiral drugs. ‘Now let’s test your knowledge with 50 MCQs on this topic.’
Q1. Which feature defines a molecule as chiral?
- Presence of a plane of symmetry
- Superimposability on its mirror image
- Non-superimposable mirror images
- Conjugated double bonds
Correct Answer: Non-superimposable mirror images
Q2. A carbon atom bonded to four different substituents is called:
- Aromatic carbon
- Prochiral center
- Chiral center (stereogenic center)
- Planar carbon
Correct Answer: Chiral center (stereogenic center)
Q3. Enantiomers have identical physical properties except for:
- Melting point
- Optical rotation and interaction with plane-polarized light
- Boiling point
- Density
Correct Answer: Optical rotation and interaction with plane-polarized light
Q4. A racemic mixture contains:
- Only the R-enantiomer
- Equal amounts of both enantiomers
- Only the S-enantiomer
- Mixture of diastereomers
Correct Answer: Equal amounts of both enantiomers
Q5. Cahn-Ingold-Prelog priority rules are used to:
- Assign cis/trans configuration
- Determine absolute configuration (R/S) at stereocenters
- Predict reaction rates
- Calculate optical rotation
Correct Answer: Determine absolute configuration (R/S) at stereocenters
Q6. Which instrument measures optical rotation of a chiral compound?
- Mass spectrometer
- Polarimeter
- NMR spectrometer
- UV-Vis spectrophotometer
Correct Answer: Polarimeter
Q7. Enantiomeric excess (ee) is defined as:
- Sum of concentrations of both enantiomers
- Difference in percentage between two enantiomers
- Ratio of diastereomer concentrations
- Molecular weight difference between enantiomers
Correct Answer: Difference in percentage between two enantiomers
Q8. Which statement about diastereomers is correct?
- They are non-superimposable mirror images
- They have identical physical properties in all cases
- They are stereoisomers that are not mirror images and have different physical properties
- They rotate plane-polarized light in equal and opposite directions
Correct Answer: They are stereoisomers that are not mirror images and have different physical properties
Q9. A meso compound:
- Always shows optical activity
- Contains chiral centers but is achiral due to an internal plane of symmetry
- Has no stereogenic centers
- Is a racemic mixture
Correct Answer: Contains chiral centers but is achiral due to an internal plane of symmetry
Q10. Which technique separates enantiomers based on interaction with a chiral stationary phase?
- Normal-phase chromatography
- Chiral HPLC
- Gas chromatography without chiral column
- Size-exclusion chromatography
Correct Answer: Chiral HPLC
Q11. Optical rotation sign (+ or −) indicates:
- Absolute configuration R or S directly
- Direction of rotation of plane-polarized light
- Molecular weight
- Purity of compound only
Correct Answer: Direction of rotation of plane-polarized light
Q12. Thalidomide is a classic example illustrating:
- Atropisomerism
- Enantiomers with identical biological activity
- Differing biological activities and toxicities of enantiomers
- Geometric isomerism
Correct Answer: Differing biological activities and toxicities of enantiomers
Q13. Which of the following can be a source of chirality besides tetrahedral carbon?
- Planar symmetry only
- Axial chirality (e.g., biphenyls)
- Only sp2 carbons
- Only ionic centers
Correct Answer: Axial chirality (e.g., biphenyls)
Q14. In Fischer projection, horizontal bonds project:
- Toward the viewer
- Away from the viewer
- In the plane of the page
- As double bonds
Correct Answer: Toward the viewer
Q15. The optical purity of a sample with 80% R and 20% S is:
- 100% ee
- 60% ee
- 40% ee
- 80% ee
Correct Answer: 60% ee
Q16. Resolution by diastereomeric salt formation requires:
- Use of achiral resolving agents
- Formation of diastereomers using a chiral resolving agent
- Only chromatography
- Only enzymatic methods
Correct Answer: Formation of diastereomers using a chiral resolving agent
Q17. Which projection is most useful to visualize eclipsing and staggered conformations?
- Fischer projection
- Newman projection
- Haworth projection
- Chair projection
Correct Answer: Newman projection
Q18. Epimers are diastereomers that differ at:
- All stereogenic centers
- Exactly one stereogenic center
- No stereogenic centers
- Only in conformation
Correct Answer: Exactly one stereogenic center
Q19. Which of the following statements about enantiomers is FALSE?
- They have identical solubilities in achiral solvents
- They rotate plane-polarized light equally but in opposite directions
- They have identical interactions with a chiral biological receptor
- They have identical melting points
Correct Answer: They have identical interactions with a chiral biological receptor
Q20. Atropisomerism arises due to:
- Rapid free rotation around single bonds
- Restricted rotation about a bond leading to isolable stereoisomers
- Formation of meso compounds
- Conjugation with a carbonyl group
Correct Answer: Restricted rotation about a bond leading to isolable stereoisomers
Q21. Optical rotation [α]D depends on:
- Concentration, path length, solvent, temperature, and wavelength
- Only molecular weight
- Only density of sample
- Only pH
Correct Answer: Concentration, path length, solvent, temperature, and wavelength
Q22. The R/S descriptor is assigned after:
- Measuring optical rotation
- Applying priority rules and orienting lowest priority away
- Calculating molecular weight
- Running an HPLC
Correct Answer: Applying priority rules and orienting lowest priority away
Q23. Which method uses enzymes to preferentially react one enantiomer over the other?
- Chemical racemization
- Enzymatic resolution
- Planar chromatography
- Recrystallization from achiral solvents
Correct Answer: Enzymatic resolution
Q24. A compound with two stereogenic centers can have a maximum of how many stereoisomers?
- 2
- 4
- 6
- 8
Correct Answer: 4
Q25. Chiral chromatography separates enantiomers by:
- Mass differences
- Differential interactions with a chiral selector
- Boiling point differences
- UV absorbance differences only
Correct Answer: Differential interactions with a chiral selector
Q26. An enantiomeric pair will have which relation of specific rotation values?
- Equal magnitude, same sign
- Equal magnitude, opposite sign
- Different magnitudes, same sign
- Different magnitudes, opposite sign
Correct Answer: Equal magnitude, opposite sign
Q27. Which reagent is commonly used for stereoselective reduction leading to chiral alcohols?
- Sodium borohydride only
- Catalytic hydrogenation with a chiral catalyst
- Hydrochloric acid
- Pyridine
Correct Answer: Catalytic hydrogenation with a chiral catalyst
Q28. Optical activity can be destroyed by:
- Racemization
- Purification
- Cooling
- Dilution only
Correct Answer: Racemization
Q29. A molecule with no stereogenic center but that is chiral due to restricted rotation exhibits:
- Meso behaviour
- Atropisomerism
- Constitutional isomerism
- Achirality
Correct Answer: Atropisomerism
Q30. Diastereomeric excess is to diastereomers as enantiomeric excess is to:
- Constitutional isomers
- Enantiomers
- Conformers
- Resonance structures
Correct Answer: Enantiomers
Q31. In a resolution by fractional crystallization, separation is based on:
- Different solubilities of enantiomers in achiral solvent
- Different solubilities of diastereomeric salts or derivatives
- Optical rotation measurement
- NMR shift differences only
Correct Answer: Different solubilities of diastereomeric salts or derivatives
Q32. Which of the following is an example of planar chirality?
- Cyclohexane chair conformer
- Substituted paracyclophane
- Simple benzene
- Ethane
Correct Answer: Substituted paracyclophane
Q33. Which technique can determine absolute configuration via anomalous X-ray scattering?
- Polarimetry
- X-ray crystallography with heavy atom
- IR spectroscopy
- Thin layer chromatography
Correct Answer: X-ray crystallography with heavy atom
Q34. Which of these drugs is known to have enantiomers with distinct activities?
- Ibuprofen
- Water
- Sodium chloride
- Glucose (racemate)
Correct Answer: Ibuprofen
Q35. The process of converting a racemate into a single enantiomer by chemical means is called:
- Racemization
- Resolution
- Isomerization
- Polymerization
Correct Answer: Resolution
Q36. Which statement is true for enantiomers interacting with chiral receptors?
- They bind identically to chiral receptors
- They can exhibit different binding affinity and pharmacological responses
- They have identical pharmacokinetics always
- They are metabolized the same way in all tissues
Correct Answer: They can exhibit different binding affinity and pharmacological responses
Q37. Optical purity can be determined experimentally by:
- Measuring melting point
- Polarimetry and comparison to pure enantiomer standard
- Measuring pH
- Measuring conductivity
Correct Answer: Polarimetry and comparison to pure enantiomer standard
Q38. Which reaction condition can lead to racemization of a chiral center adjacent to a carbonyl?
- Strong acid or base catalyzed enolization
- Purely neutral aqueous conditions at low temperature
- High vacuum
- Addition of inert gas
Correct Answer: Strong acid or base catalyzed enolization
Q39. Which description fits a chiral drug where only one enantiomer produces the therapeutic effect and the other is inactive?
- Enantiomeric equivalence
- Enantioselective pharmacology
- Racemate dominance
- Achiral pharmacology
Correct Answer: Enantioselective pharmacology
Q40. What is the impact of stereochemistry on drug metabolism?
- No impact; stereochemistry is irrelevant
- Different enantiomers can be metabolized at different rates and by different enzymes
- Enantiomers always metabolize into the same products at same rates
- Only affects drug absorption, not metabolism
Correct Answer: Different enantiomers can be metabolized at different rates and by different enzymes
Q41. Conversion of an optically active compound to an optically inactive mixture is called:
- Resolution
- Racemization
- Enantiomerization
- Isomerization without racemization
Correct Answer: Racemization
Q42. Which of the following is a practical advantage of developing a single enantiomer drug over a racemate?
- Reduced development cost always
- Potential for improved efficacy and reduced side effects
- Guaranteed extended patent life without studies
- Eliminates need for stereochemical analysis
Correct Answer: Potential for improved efficacy and reduced side effects
Q43. Chiral auxiliaries are used in synthesis to:
- Introduce chirality and control stereochemistry during reactions
- Increase reaction temperature only
- Remove chirality from molecules
- Convert enantiomers to meso compounds
Correct Answer: Introduce chirality and control stereochemistry during reactions
Q44. Which of the following is TRUE about meso tartaric acid?
- It is optically active
- It has an internal plane of symmetry and is optically inactive
- It exists as enantiomers only
- It is a racemic mixture by composition
Correct Answer: It has an internal plane of symmetry and is optically inactive
Q45. In chiral HPLC, retention time differences arise from:
- Only molecular weight differences
- Transient diastereomeric interactions with the stationary phase
- Differences in volatility
- Difference in color
Correct Answer: Transient diastereomeric interactions with the stationary phase
Q46. The Cahn-Ingold-Prelog priority is based primarily on:
- Atomic number of atoms directly attached to stereocenter
- Bond angles
- Boiling points
- Electronegativity only
Correct Answer: Atomic number of atoms directly attached to stereocenter
Q47. Which of the following conversions preserves stereochemistry (retention)?
- SN2 reaction at a stereocenter
- SN1 reaction with racemization
- Mitsunobu reaction typically inverts configuration
- Oxidation followed by non-stereospecific reduction
Correct Answer: SN1 reaction with racemization
Q48. Which analytical method provides information about enantiomeric composition without derivatization using chiral selectors?
- Achiral GC
- Chiral HPLC or chiral GC
- Simple TLC on silica without chiral additive
- UV-Vis absorption only
Correct Answer: Chiral HPLC or chiral GC
Q49. Which property allows two enantiomers to have different tastes or smells?
- Identical interactions with achiral receptors
- Differing interactions with chiral olfactory or taste receptors
- Difference in molecular weight
- Difference in color
Correct Answer: Differing interactions with chiral olfactory or taste receptors
Q50. Which pharmaceutical regulatory consideration is relevant to chiral drugs?
- No requirement to study stereochemistry
- Evaluation of each enantiomer’s pharmacology, toxicity, and pharmacokinetics may be required
- Only racemates are allowed
- Stereochemistry is only required for formulation aesthetics
Correct Answer: Evaluation of each enantiomer’s pharmacology, toxicity, and pharmacokinetics may be required
