Optical isomerism and biological action MCQs With Answer

Optical isomerism and biological action MCQs With Answer

Understanding optical isomerism is essential for B. Pharm students because chirality and enantiomers profoundly influence drug pharmacodynamics, pharmacokinetics, safety, and efficacy. This concise collection focuses on stereochemistry concepts—chiral centers, enantiomers, diastereomers, racemates, specific rotation, resolution methods, and enantioselective metabolism—linked directly to biological action in drugs such as thalidomide and ibuprofen. Each MCQ is designed to reinforce practical understanding of how stereochemistry alters receptor binding, toxicity, and therapeutic outcome, and to prepare you for exam and clinical reasoning in pharmacy practice. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which statement best defines optical isomerism?

• The presence of atoms with similar electronegativities
• Isomerism resulting from different connectivity of atoms
• Isomerism due to molecules that are non-superimposable mirror images
• Isomerism caused by differences in molecular weight

Correct Answer: Isomerism due to molecules that are non-superimposable mirror images

Q2. What is a chiral center in an organic molecule?

• A carbon atom bonded to two identical substituents
• A carbon atom bonded to four different substituents
• A carbon atom with a double bond
• A carbon atom in an aromatic ring

Correct Answer: A carbon atom bonded to four different substituents

Q3. Enantiomers are best described as:

• Geometric isomers that differ in cis/trans orientation
• Mirror-image stereoisomers that rotate plane-polarized light in equal and opposite directions
• Stereoisomers with different connectivity of atoms
• Compounds that cannot be separated by any method

Correct Answer: Mirror-image stereoisomers that rotate plane-polarized light in equal and opposite directions

Q4. Which instrument is commonly used to measure optical rotation?

• Mass spectrometer
• Nuclear magnetic resonance (NMR)
• Polarimeter
• Infrared spectrometer

Correct Answer: Polarimeter

Q5. Specific rotation [α] is defined as:

• Total rotation observed divided by chemical shift
• Observed rotation divided by concentration and path length, corrected to standard conditions
• Observed rotation multiplied by molecular weight
• Rotation measured only at zero concentration

Correct Answer: Observed rotation divided by concentration and path length, corrected to standard conditions

Q6. A racemic mixture contains:

• Only the R enantiomer
• Only the S enantiomer
• A 1:1 mixture of two enantiomers with no net optical rotation
• An unequal mixture with net optical activity

Correct Answer: A 1:1 mixture of two enantiomers with no net optical rotation

Q7. Diastereomers differ from enantiomers by:

• Being non-mirror-image stereoisomers with different physical properties
• Being mirror images with identical properties
• Having the same configuration at all stereocenters
• Always rotating plane-polarized light in opposite directions

Correct Answer: Being non-mirror-image stereoisomers with different physical properties

Q8. Which of the following drugs is a classic example where enantiomers have different biological effects?

• Paracetamol
• Thalidomide
• Amoxicillin
• Metformin

Correct Answer: Thalidomide

Q9. Chiral inversion refers to:

• The interconversion of R and S enantiomers in vivo
• The permanent removal of chirality
• The synthesis of racemates only in the lab
• Optical rotation measurement error

Correct Answer: The interconversion of R and S enantiomers in vivo

Q10. Which technique is most commonly used for enantiomeric separation in analytical labs?

• Normal-phase TLC
• Chiral high-performance liquid chromatography (chiral HPLC)
• Gas chromatography with non-chiral stationary phase
• Ultraviolet–visible spectroscopy

Correct Answer: Chiral high-performance liquid chromatography (chiral HPLC)

Q11. The R/S nomenclature assigns configuration based on:

• Alphabetical order of substituents only
• Priority rules based on atomic number and spatial arrangement
• Optical rotation magnitude
• Molecular weight differences

Correct Answer: Priority rules based on atomic number and spatial arrangement

Q12. Which statement about enantiomers and receptors is true?

• Both enantiomers always have identical receptor affinity
• Only racemates bind to receptors
• Enantiomers can show different affinity and intrinsic activity at chiral biological targets
• Receptors are achiral, so enantiomers behave the same

Correct Answer: Enantiomers can show different affinity and intrinsic activity at chiral biological targets

Q13. The term “enantioselective metabolism” refers to:

• Equal metabolism of both enantiomers
• Preferential metabolism of one enantiomer over the other
• Metabolism that does not involve enzymes
• Metabolism only in the gastrointestinal tract

Correct Answer: Preferential metabolism of one enantiomer over the other

Q14. Which parameter directly indicates optical purity of a sample?

• Specific rotation compared with pure enantiomer
• Molecular weight
• Refractive index only
• Boiling point

Correct Answer: Specific rotation compared with pure enantiomer

Q15. Eudismic ratio in chiral pharmacology refers to:

• The ratio of toxic to therapeutic dose
• The potency ratio between two enantiomers (more active vs less active)
• The ratio of racemate to pure enantiomer in dosage form
• The enantiomeric excess required for approval

Correct Answer: The potency ratio between two enantiomers (more active vs less active)

Q16. Which method is a classical approach for resolving racemates into enantiomers?

• Direct crystallization of the racemate without chiral agents
• Formation of diastereomeric salts using a chiral resolving agent
• Heating racemate to induce inversion
• Using UV light only

Correct Answer: Formation of diastereomeric salts using a chiral resolving agent

Q17. Which of the following best explains why one enantiomer may be more active than the other?

• Different molecular weights of enantiomers
• Different three-dimensional fit to a chiral receptor or enzyme
• The enantiomer with higher boiling point is more active
• Optical rotation determines activity directly

Correct Answer: Different three-dimensional fit to a chiral receptor or enzyme

Q18. Which drug exhibits chiral switching, where a single enantiomer is marketed after racemate therapy?

• Ibuprofen (Dexibuprofen as single enantiomer)
• Paracetamol
• Amoxicillin
• Ranitidine

Correct Answer: Ibuprofen (Dexibuprofen as single enantiomer)

Q19. In polarimetry, a positive rotation (+) indicates:

• Rotation counterclockwise (levorotatory)
• Rotation clockwise (dextrorotatory)
• No rotation
• Optical rotation cannot be positive

Correct Answer: Rotation clockwise (dextrorotatory)

Q20. Which property usually differs between enantiomers?

• Melting point
• Optical rotation and interaction with other chiral substances
• Connectivity of atoms
• Number of atoms

Correct Answer: Optical rotation and interaction with other chiral substances

Q21. Which statement about diastereomers is correct?

• Diastereomers always have identical physical properties
• Diastereomers are mirror images
• Diastereomers have different physical and chemical properties and are not mirror images
• Diastereomers cannot be separated

Correct Answer: Diastereomers have different physical and chemical properties and are not mirror images

Q22. Optical antipode refers to:

• A compound that does not rotate plane-polarized light
• One of two enantiomers which are mirror images and rotate light in opposite directions
• A racemate with no biological activity
• A diastereomeric impurity

Correct Answer: One of two enantiomers which are mirror images and rotate light in opposite directions

Q23. Which of these is true about racemic drugs in therapy?

• Racemates always have better safety profiles than single enantiomers
• One enantiomer may be responsible for therapeutic effect while the other contributes to adverse effects
• Both enantiomers are pharmacologically inactive
• Racemates never undergo chiral inversion in vivo

Correct Answer: One enantiomer may be responsible for therapeutic effect while the other contributes to adverse effects

Q24. Which process describes preferential interaction of one enantiomer by an enzyme leading to different metabolic rates?

• Enantiomeric inversion
• Chiral discrimination
• Racemization at room temperature
• Isotopic labeling

Correct Answer: Chiral discrimination

Q25. Thalidomide tragedy highlighted the importance of:

• Only studying racemates
• Understanding stereochemistry and teratogenic effects of enantiomers
• Ignoring enantiomeric differences in drugs
• Metabolic stability without stereochemical considerations

Correct Answer: Understanding stereochemistry and teratogenic effects of enantiomers

Q26. Which term denotes the excess of one enantiomer over the other in a mixture?

• Optical rotation index
• Enantiomeric excess (ee)
• Racemization coefficient
• Chiral impurity factor

Correct Answer: Enantiomeric excess (ee)

Q27. Which pharmaceutical technique can produce enantiomerically enriched compounds asymmetrically?

• Asymmetric synthesis using chiral catalysts
• Heating racemates to high temperature
• Using achiral reagents exclusively
• Simple recrystallization without chiral influence

Correct Answer: Asymmetric synthesis using chiral catalysts

Q28. Chiral centers necessary for optical activity are usually composed of which atom?

• Silicon with two identical substituents
• Carbon bonded to four different substituents
• Nitrogen with rapid inversion always making it chiral
• Phosphorus only in achiral form

Correct Answer: Carbon bonded to four different substituents

Q29. In clinical pharmacology, the concept “one enantiomer is an antagonist, the other an agonist” illustrates:

• Identical pharmacological profiles for enantiomers
• Enantiomer-dependent pharmacodynamics at chiral receptors
• That enantiomers are always inactive
• That racemates never show mixed pharmacology

Correct Answer: Enantiomer-dependent pharmacodynamics at chiral receptors

Q30. Which factor does NOT influence the extent of enantioselective metabolism?

• Expression level of stereoselective enzymes
• Presence of chiral transporters
• Patient genetic polymorphisms in metabolizing enzymes
• Color of the tablet coating

Correct Answer: Color of the tablet coating

Q31. Optical purity can be improved by which pharmaceutical process?

• Chiral resolution techniques like preparative chiral HPLC or crystallization of diastereomeric salts
• Mixing with additional racemate
• Heating to racemize intentionally
• Reducing molecular weight

Correct Answer: Chiral resolution techniques like preparative chiral HPLC or crystallization of diastereomeric salts

Q32. The specific rotation of a pure enantiomer is +25°. A sample shows +12.5° under same conditions. What is its enantiomeric excess (ee)?

• 25% ee
• 50% ee
• 75% ee
• 100% ee

Correct Answer: 50% ee

Q33. Chiral stationary phases in HPLC separate enantiomers mainly by:

• Different vapor pressures
• Transient diastereomeric interactions with the chiral selector
• Differences in atomic number
• Identical partition behavior for both enantiomers

Correct Answer: Transient diastereomeric interactions with the chiral selector

Q34. Which class of enzymes commonly shows stereoselectivity in drug metabolism?

• Cytochrome P450 enzymes
• Inert structural proteins
• Simple inorganic catalysts
• Elements like sodium and potassium

Correct Answer: Cytochrome P450 enzymes

Q35. If an enantiomer has therapeutic activity and its mirror image is toxic, the best pharmaceutical strategy is:

• Developing and marketing the pure therapeutic enantiomer
• Continuing to use the racemate without change
• Increasing the dose of the racemate
• Mixing with more impurities to dilute toxicity

Correct Answer: Developing and marketing the pure therapeutic enantiomer

Q36. Which phenomenon describes conversion of an enantiomer to its mirror image in solution under certain conditions?

• Enantiomeric enrichment
• Racemization
• Resolution
• Stereospecific synthesis

Correct Answer: Racemization

Q37. An “achiral” molecule is one that:

• Has at least one stereocenter
• Is superimposable on its mirror image
• Always rotates plane-polarized light
• Cannot interact with chiral receptors

Correct Answer: Is superimposable on its mirror image

Q38. Which example best illustrates the clinical relevance of enantiomer-specific dosing?

• Both enantiomers always have identical ADME profiles
• One enantiomer has a longer half-life and higher potency, requiring lower dosing
• Enantiomers do not influence dose selection
• Enantiomers are always eliminated unchanged

Correct Answer: One enantiomer has a longer half-life and higher potency, requiring lower dosing

Q39. Optical rotation depends on which of the following experimental parameters?

• Wavelength of light, temperature, solvent, concentration, and path length
• Only the mass of the sample
• Only the pH of the solution
• Only the age of the sample

Correct Answer: Wavelength of light, temperature, solvent, concentration, and path length

Q40. Which describes an epimer?

• Stereoisomers differing in configuration at only one of several stereocenters
• Isomers differing by connectivity of atoms
• Enantiomers that are mirror images
• Compounds with identical stereochemistry at all centers

Correct Answer: Stereoisomers differing in configuration at only one of several stereocenters

Q41. Stereoselective drug-receptor interactions are mainly due to:

• Differences in atomic masses of enantiomers
• Three-dimensional complementarity between ligand and chiral binding site
• Changes in solvent polarity only
• Identical geometric fit for both enantiomers

Correct Answer: Three-dimensional complementarity between ligand and chiral binding site

Q42. Which regulatory consideration often applies to chiral drugs?

• Enantiomeric characterization and control of enantiomeric purity are required
• No information on stereochemistry is needed
• Only racemates are acceptable for approval
• Regulators forbid single enantiomer drugs

Correct Answer: Enantiomeric characterization and control of enantiomeric purity are required

Q43. Which of the following is NOT a common method to characterize stereochemistry of a drug?

• X-ray crystallography
• Nuclear magnetic resonance with chiral shift reagents
• Polarimetry
• Simple colorimetric test on paper without chiral reagents

Correct Answer: Simple colorimetric test on paper without chiral reagents

Q44. Chiral auxiliaries in synthesis are used to:

• Introduce chirality and control stereochemical outcome in subsequent reactions
• Remove chirality permanently
• Increase racemization rate intentionally
• Change atomic numbers of substituents

Correct Answer: Introduce chirality and control stereochemical outcome in subsequent reactions

Q45. Which term describes a mixture containing more of one enantiomer than the other but not pure?

• Racemate
• Enantiomeric excess mixture
• Achiral blend
• Isotopic mixture

Correct Answer: Enantiomeric excess mixture

Q46. The molecule ibuprofen has a stereocenter. Which statement is correct clinically?

• The marketed form is only the inactive R enantiomer
• Racemate is used, but R can be inverted to S in vivo, with S being mainly active
• Ibuprofen does not show any chiral behavior
• Only the R enantiomer is active and stable

Correct Answer: Racemate is used, but R can be inverted to S in vivo, with S being mainly active

Q47. Which factor increases the difficulty of separating enantiomers?

• Large differences in physical properties
• Lack of functional groups for derivatization or weak interaction with chiral selectors
• Presence of only one stereocenter
• High enantiomeric excess already

Correct Answer: Lack of functional groups for derivatization or weak interaction with chiral selectors

Q48. Which concept explains why a chiral drug may show different toxicity in different species?

• Species differences in stereoselective ADME and receptor chirality
• All species metabolize drugs identically
• Only humans are affected by stereochemistry
• Enantiomers are always equally toxic across species

Correct Answer: Species differences in stereoselective ADME and receptor chirality

Q49. In quality control of a chiral drug, which parameter is critical to report?

• Enantiomeric purity (ee) and specific rotation under defined conditions
• Only tablet hardness
• Only pH of unrelated solutions
• Color of the packaging

Correct Answer: Enantiomeric purity (ee) and specific rotation under defined conditions

Q50. Which best practice should pharmacists consider when dispensing chiral drugs?

• Assume racemate and single enantiomer have identical effects
• Be aware of enantiomer-specific dosing, interactions, and potential for chiral inversion
• Ignore stereochemistry because it is only a chemist’s concern
• Always prescribe higher doses to overcome stereoselectivity

Correct Answer: Be aware of enantiomer-specific dosing, interactions, and potential for chiral inversion

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