Chiral molecules MCQs With Answer

Introduction: Chiral molecules MCQs With Answer are essential for B. Pharm students studying stereochemistry and drug action. Understanding chirality, enantiomers, diastereomers, R/S configuration, optical activity, and chiral resolution helps explain pharmacodynamics, pharmacokinetics, and therapeutic selectivity. This concise, informative set of MCQs with answers focuses on core concepts—Cahn–Ingold–Prelog rules, meso compounds, racemization, enantiomeric excess, polarimetry, asymmetric synthesis, and real drug examples—so you can strengthen exam readiness and practical understanding. Practice these targeted questions to master stereochemical reasoning and application in pharmaceutical contexts. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What defines a chiral molecule?

• A molecule that has a plane of symmetry
• A molecule that is identical to its mirror image
• A molecule that is non-superimposable on its mirror image
• A molecule with all atoms in the same plane

Correct Answer: A molecule that is non-superimposable on its mirror image

Q2. Which center is most commonly responsible for chirality in organic molecules?

• Carbon with four identical substituents
• Carbon with four different substituents
• Carbon with a double bond
• Carbon with three substituents and a lone pair

Correct Answer: Carbon with four different substituents

Q3. What is an enantiomer?

• One of two stereoisomers that are mirror images but not superimposable
• Two stereoisomers that are identical
• A conformer that rapidly interconverts
• A geometric isomer around a double bond

Correct Answer: One of two stereoisomers that are mirror images but not superimposable

Q4. What does the R/S nomenclature describe?

• Optical rotation direction in a polarimeter
• Absolute configuration of a stereocenter based on priority rules
• Relative configuration between two stereocenters
• Rate of racemization

Correct Answer: Absolute configuration of a stereocenter based on priority rules

Q5. Which rule set is used to assign priorities for R/S configuration?

• Le Bel–Van ’t Hoff rules
• Cahn–Ingold–Prelog rules
• Zaitsev’s rule
• Markovnikov’s rule

Correct Answer: Cahn–Ingold–Prelog rules

Q6. What is a meso compound?

• A compound with optically active enantiomers only
• An achiral compound that contains stereocenters due to an internal plane of symmetry
• A racemic mixture of enantiomers
• A conformational isomer

Correct Answer: An achiral compound that contains stereocenters due to an internal plane of symmetry

Q7. Which instrument measures optical rotation?

• Mass spectrometer
• Polarimeter
• NMR spectrometer
• IR spectrophotometer

Correct Answer: Polarimeter

Q8. What is racemic mixture?

• A mixture containing only the R-enantiomer
• A mixture of equal amounts of two enantiomers with no net optical rotation
• A single pure stereoisomer
• A mixture of diastereomers only

Correct Answer: A mixture of equal amounts of two enantiomers with no net optical rotation

Q9. How does enantiomeric excess (ee) relate to a racemic mixture?

• ee = 0% for a racemic mixture
• ee = 100% for a racemic mixture
• ee = 50% for a racemic mixture
• ee cannot be defined for racemic mixtures

Correct Answer: ee = 0% for a racemic mixture

Q10. Which property is identical for enantiomers in an achiral environment?

• Direction of optical rotation
• Specific rotation sign
• Melting point and boiling point
• Biological activity

Correct Answer: Melting point and boiling point

Q11. Which technique separates enantiomers based on interactions with a chiral stationary phase?

• Normal-phase TLC
• Chiral HPLC
• Gas chromatography with achiral column
• Distillation

Correct Answer: Chiral HPLC

Q12. What is atropisomerism?

• Chirality arising from a stereogenic carbon only
• Axial chirality due to restricted rotation about a bond
• Planar chirality from a double bond
• Optical activity caused by isotopes

Correct Answer: Axial chirality due to restricted rotation about a bond

Q13. Which drug famously had enantiomers with different biological effects leading to tragedy?

• Aspirin
• Thalidomide
• Paracetamol
• Ibuprofen

Correct Answer: Thalidomide

Q14. What does optical purity refer to?

• Purity with respect to chemical impurities only
• Ratio of enantiomers expressed by optical rotation relative to a pure enantiomer
• Percentage of diastereomers in a mixture
• Concentration of achiral impurities

Correct Answer: Ratio of enantiomers expressed by optical rotation relative to a pure enantiomer

Q15. Which statement about diastereomers is true?

• They are non-mirror-image stereoisomers and have different physical properties
• They are mirror images and always have identical properties
• They are conformers that interconvert rapidly
• They must have the same melting points

Correct Answer: They are non-mirror-image stereoisomers and have different physical properties

Q16. What is a prochiral center?

• A center that cannot become chiral
• An sp2 carbon that can become stereogenic upon substitution
• A carbon with four different substituents already
• A stereocenter in a meso compound

Correct Answer: An sp2 carbon that can become stereogenic upon substitution

Q17. In Cahn–Ingold–Prelog rules, which factor determines priority?

• Atomic mass (higher atomic number gets higher priority)
• Number of atoms in the substituent regardless of atomic number
• Bond length
• Optical rotation magnitude

Correct Answer: Atomic mass (higher atomic number gets higher priority)

Q18. What is racemization?

• Conversion of an enantiomerically pure compound into a racemic mixture
• Separation of enantiomers
• Formation of a meso compound from achiral starting material
• Elimination reaction forming a double bond

Correct Answer: Conversion of an enantiomerically pure compound into a racemic mixture

Q19. How do enantiomers behave in a chiral biological environment?

• They always have identical pharmacological responses
• They may have different pharmacodynamics or pharmacokinetics
• They cannot bind to chiral receptors
• They convert into each other rapidly

Correct Answer: They may have different pharmacodynamics or pharmacokinetics

Q20. Which of the following is true about specific rotation [α]?

• It is independent of concentration and path length
• It depends on solvent, temperature, wavelength, concentration, and path length
• It is always positive for enantiomerically pure samples
• It measures mass spectrometric fragmentation

Correct Answer: It depends on solvent, temperature, wavelength, concentration, and path length

Q21. What is enantioselective synthesis?

• A synthesis producing a racemic product
• Synthesis that preferentially creates one enantiomer over the other
• Synthesis that only yields achiral compounds
• Synthesis that uses only achiral catalysts

Correct Answer: Synthesis that preferentially creates one enantiomer over the other

Q22. Which term describes the percentage difference between two enantiomers in a mixture?

• Optical density
• Enantiomeric excess (ee)
• Diastereomeric ratio
• Isomeric purity

Correct Answer: Enantiomeric excess (ee)

Q23. Which method can be used to determine enantiomeric excess?

• Infrared spectroscopy without chiral reagents
• Chiral chromatography or chiral NMR with shift reagents
• Simple boiling point measurement
• Elemental analysis

Correct Answer: Chiral chromatography or chiral NMR with shift reagents

Q24. What is the relationship between optical rotation and enantiomeric composition?

• Optical rotation is zero for a pure enantiomer
• Observed rotation is proportional to enantiomeric excess under fixed conditions
• Optical rotation is independent of enantiomeric purity
• Only racemic mixtures show rotation

Correct Answer: Observed rotation is proportional to enantiomeric excess under fixed conditions

Q25. Which of the following is a common chiral resolving agent?

• Racemic sodium chloride
• Di-p-toluoyl-L-tartaric acid (L-DTTA)
• Hexane
• Hydrochloric acid only

Correct Answer: Di-p-toluoyl-L-tartaric acid (L-DTTA)

Q26. What is the result when an achiral reagent reacts with a prochiral substrate in an achiral environment?

• Always produces a single enantiomer
• Produces enantiomers in equal amounts unless chiral induction occurs
• Produces only meso compounds
• Cannot form stereocenters

Correct Answer: Produces enantiomers in equal amounts unless chiral induction occurs

Q27. Which stereochemical descriptor applies to allenes with different substituents at each end?

• They cannot be chiral
• They can show axial chirality
• Only planar chirality applies
• They are always meso

Correct Answer: They can show axial chirality

Q28. Which of the following characteristics differentiates enantiomers in an achiral medium?

• Melting point
• Interaction with plane-polarized light
• Boiling point
• Mass

Correct Answer: Interaction with plane-polarized light

Q29. What is chiral pool synthesis?

• Synthesis using racemic mixtures exclusively
• Using naturally occurring chiral molecules as starting materials to build complex chiral targets
• Synthesis that avoids stereocenters entirely
• Using only achiral catalysts

Correct Answer: Using naturally occurring chiral molecules as starting materials to build complex chiral targets

Q30. Which reaction type is typically stereospecific?

• SN1 reactions
• Free radical halogenation
• SN2 reactions (inversion of configuration)
• Non-selective acid–base neutralization

Correct Answer: SN2 reactions (inversion of configuration)

Q31. What is the term for two stereoisomers that are not mirror images?

• Enantiomers
• Conformers
• Diastereomers
• Identical isomers

Correct Answer: Diastereomers

Q32. Which measurement directly gives the sign (+ or −) of optical rotation?

• Mass spectrometry
• Polarimetry
• NMR coupling constants
• Melting point determination

Correct Answer: Polarimetry

Q33. Which property often differs between diastereomers, aiding their separation?

• Specific rotation sign
• Solubility and melting point
• Exact same chemical shift in NMR always
• Identical chromatographic retention on chiral columns

Correct Answer: Solubility and melting point

Q34. How can one convert an achiral intermediate into a chiral product with high enantioselectivity?

• Use a racemic catalyst
• Use an achiral solvent only
• Use an enantioselective catalyst or chiral auxiliary
• Increase temperature only

Correct Answer: Use an enantioselective catalyst or chiral auxiliary

Q35. What does the term “optical antipode” mean?

• Identical stereoisomer
• Enantiomer that rotates plane-polarized light in the opposite direction
• Diastereomer with the same rotation
• Racemate decomposed into fragments

Correct Answer: Enantiomer that rotates plane-polarized light in the opposite direction

Q36. Which factor can cause faster racemization of a chiral center?

• Strong stabilization of the stereocenter
• Acidic or basic conditions facilitating inversion
• Extremely low temperatures only
• Presence of an internal plane of symmetry

Correct Answer: Acidic or basic conditions facilitating inversion

Q37. Which of the following is NOT a method of resolving enantiomers?

• Formation of diastereomeric salts and separation
• Chiral chromatography
• Use of achiral HPLC only
• Enzymatic kinetic resolution

Correct Answer: Use of achiral HPLC only

Q38. Which statement about chiral drugs is most accurate?

• Both enantiomers always have equal therapeutic effects
• One enantiomer may be more active or less toxic than the other
• Enantiomers are always metabolized identically
• Regulatory agencies never require enantiospecific data

Correct Answer: One enantiomer may be more active or less toxic than the other

Q39. In Fischer projection, horizontal lines represent bonds projecting in which direction?

• Toward the viewer (out of the plane)
• Away from the viewer (into the plane)
• Along the plane of the page
• No stereochemical information

Correct Answer: Toward the viewer (out of the plane)

Q40. What is the effect of a mirror plane in a molecule?

• It guarantees the molecule is chiral
• It makes the molecule achiral if the mirror maps the molecule onto itself
• It doubles the optical rotation
• It creates axial chirality

Correct Answer: It makes the molecule achiral if the mirror maps the molecule onto itself

Q41. Which type of chirality arises from a non-planar cyclic structure like substituted metallocenes?

• Point chirality only
• Planar chirality
• Optical isomerism due to isotopic substitution
• Meso chirality

Correct Answer: Planar chirality

Q42. What is the relationship between specific rotation and concentration?

• Specific rotation increases linearly with concentration for a single enantiomer under fixed conditions
• Specific rotation equals observed rotation divided by concentration and path length
• Specific rotation is independent and unrelated to observed rotation
• Specific rotation is inversely proportional to enantiomeric excess

Correct Answer: Specific rotation equals observed rotation divided by concentration and path length

Q43. What is a chiral auxiliary?

• An achiral reagent that speeds up reactions
• A chiral group temporarily attached to induce stereoselectivity during a synthesis
• A permanent chiral center that cannot be removed
• A type of chiral chromatography column

Correct Answer: A chiral group temporarily attached to induce stereoselectivity during a synthesis

Q44. Which statement about enantiomers and taste or smell is correct?

• Enantiomers always have identical smell and taste
• Enantiomers can smell or taste different due to chiral receptors
• Enantiomers are always odorless
• Only achiral molecules have distinctive smells

Correct Answer: Enantiomers can smell or taste different due to chiral receptors

Q45. Which class of reactions can be used for asymmetric induction by chiral catalysts?

• Non-selective radical polymerization only
• Enantioselective hydrogenation and epoxidation
• Simple acid–base neutralizations only
• Photochemical rearrangements exclusively

Correct Answer: Enantioselective hydrogenation and epoxidation

Q46. What does the term “optical inversion” refer to?

• Change in absolute configuration from R to S without racemization
• Switch in sign of optical rotation due to conversion to the opposite enantiomer
• Loss of molecular chirality only
• Conversion of diastereomers into enantiomers

Correct Answer: Switch in sign of optical rotation due to conversion to the opposite enantiomer

Q47. How does isotopic substitution (e.g., H to D) affect chirality?

• It cannot create stereogenic centers
• It can create stereogenic centers if substitution makes four groups different (chiral by isotopic substitution)
• It always produces meso compounds
• It has no effect on stereochemistry

Correct Answer: It can create stereogenic centers if substitution makes four groups different (chiral by isotopic substitution)

Q48. Which of these statements is true about chiral recognition in chromatography?

• Achiral stationary phases separate enantiomers well
• Chiral recognition depends on differential interactions like hydrogen bonding, π–π stacking, and steric fit
• All enantiomers elute at the same time on chiral columns
• Chiral recognition does not depend on temperature

Correct Answer: Chiral recognition depends on differential interactions like hydrogen bonding, π–π stacking, and steric fit

Q49. What is the maximum possible enantiomeric excess when one enantiomer is present at 90%?

• 10% ee
• 90% ee
• 80% ee
• 100% ee

Correct Answer: 80% ee

Q50. In pharmaceutical development, why is chiral purity important?

• Because enantiomers can differ in efficacy, toxicity, and metabolism
• It is only a cosmetic regulatory concern
• Chiral purity has no impact on drug safety
• It only affects the color of the drug

Correct Answer: Because enantiomers can differ in efficacy, toxicity, and metabolism

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