Racemic modification MCQs With Answer

Introduction:

Racemic modification MCQs with Answer provide B.Pharm students a focused, exam-oriented study of racemization, chiral resolution, and stereochemical implications in drug development. This concise, keyword-rich introduction covers racemic mixtures, enantiomers, chirality, racemization mechanisms, resolution techniques (chemical, enzymatic, crystallization), analytical methods (polarimetry, chiral chromatography) and clinical significance of stereochemistry in pharmacokinetics and toxicity. Ideal for pharmaceutical chemistry revision and practical understanding, these MCQs reinforce concepts such as enantiomeric excess, dynamic kinetic resolution, chiral inversion and regulatory considerations for racemic drugs. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What defines a racemic mixture?

• Equal amounts of enantiomers having no net optical rotation
• Unequal amounts of diastereomers with net optical activity
• A mixture of achiral molecules that rotates plane-polarized light
• Only one enantiomer of a chiral drug

Correct Answer: Equal amounts of enantiomers having no net optical rotation

Q2. Which analytical technique is most commonly used to measure enantiomeric excess?

• Ultraviolet-visible spectroscopy
• Polarimetry
• Infrared spectroscopy
• Mass spectrometry

Correct Answer: Polarimetry

Q3. What is racemization?

• Conversion of an achiral compound into a chiral one
• Interconversion of two enantiomers resulting in a racemic mixture
• Separation of enantiomers by crystallization
• Formation of diastereomers from enantiomers

Correct Answer: Interconversion of two enantiomers resulting in a racemic mixture

Q4. Which mechanism commonly causes racemization at an sp3 stereocenter adjacent to a carbonyl?

• Nucleophilic substitution at sp2 center
• Formation of an achiral carbanion or enolate intermediate
• Pericyclic rearrangement without intermediate
• Radical recombination retaining configuration

Correct Answer: Formation of an achiral carbanion or enolate intermediate

Q5. Which of the following describes a racemic compound in crystallization?

• Crystals that contain equal numbers of two enantiomers in the same lattice
• Physical mixture of separate enantiomer crystals
• Single enantiomer crystals only
• Crystals composed of achiral molecules only

Correct Answer: Crystals that contain equal numbers of two enantiomers in the same lattice

Q6. What is a conglomerate in context of racemic crystallization?

• A single crystal containing both enantiomers randomly mixed
• An equimolar crystalline complex of enantiomers in same lattice sites
• A mechanical mixture of separate crystals each containing one enantiomer
• A solution behavior where enantiomers form dimers

Correct Answer: A mechanical mixture of separate crystals each containing one enantiomer

Q7. Which method separates enantiomers by forming diastereomeric salts?

• Enzymatic resolution
• Chiral chromatography
• Resolution by salt formation using a chiral acid or base
• Racemization

Correct Answer: Resolution by salt formation using a chiral acid or base

Q8. Which is an example of chiral chromatographic technique?

• Normal-phase silica gel without chiral selector
• Chiral stationary phase HPLC
• Size-exclusion chromatography
• Ion-exchange chromatography with achiral resin

Correct Answer: Chiral stationary phase HPLC

Q9. What does enantiomeric excess (ee) quantify?

• Total concentration of both enantiomers
• Difference in percentage between two enantiomers
• Molecular weight variation between enantiomers
• Number of chiral centers in a molecule

Correct Answer: Difference in percentage between two enantiomers

Q10. Which statement about enantiomers is true?

• They have identical interactions with all chiral biological receptors
• They have identical physical properties in achiral environments
• They always show different boiling points in achiral solvents
• They cannot be interconverted by chemical reaction

Correct Answer: They have identical physical properties in achiral environments

Q11. Which drug is a classic example of clinical stereoselectivity and racemization concerns?

• Paracetamol
• Ibuprofen
• Amoxicillin
• Metformin

Correct Answer: Ibuprofen

Q12. What is chiral inversion?

• Conversion of a chiral center into an achiral center permanently
• In vivo conversion of one enantiomer into its mirror image
• Laboratory separation of enantiomers by crystallization
• Formation of diastereomers from enantiomers

Correct Answer: In vivo conversion of one enantiomer into its mirror image

Q13. Which reagent class is commonly used as chiral auxiliaries in asymmetric synthesis?

• Achiral Lewis acids
• Chiral carboxylic acids or amines
• Strong oxidizing agents
• Common inorganic bases

Correct Answer: Chiral carboxylic acids or amines

Q14. Dynamic kinetic resolution achieves high yield of a single enantiomer by combining which two processes?

• Racemization and selective conversion of one enantiomer
• Crystallization and melting point depression
• Oxidation and reduction cycles only
• Filtration and drying under vacuum

Correct Answer: Racemization and selective conversion of one enantiomer

Q15. Which rule is used to assign R/S configuration at a stereocenter?

• Hückel rule
• CIP (Cahn–Ingold–Prelog) priority rules
• Nernst equation
• Markovnikov’s rule

Correct Answer: CIP (Cahn–Ingold–Prelog) priority rules

Q16. What is the Walden inversion?

• Inversion of configuration during SN2 substitution at a stereocenter
• Retention of configuration during radical substitution
• Conversion of a racemate into a meso compound
• Stereospecific formation of a new chiral center without inversion

Correct Answer: Inversion of configuration during SN2 substitution at a stereocenter

Q17. Which pharmacokinetic parameter can differ between enantiomers?

• Absorption, distribution, metabolism, and excretion
• Molecular weight
• Number of stereocenters
• Elemental composition

Correct Answer: Absorption, distribution, metabolism, and excretion

Q18. Which technique can provide stereochemical assignment by comparing optical rotation with literature?

• Polarimetry
• Gas chromatography without chiral column
• Thin-layer chromatography on achiral silica
• Infrared spectroscopy

Correct Answer: Polarimetry

Q19. Which enzymatic method is often used to resolve racemates?

• Catalytic hydrogenation with achiral catalyst
• Enantioselective hydrolysis using lipases or esterases
• Free radical polymerization
• Thermal racemization at high temperature

Correct Answer: Enantioselective hydrolysis using lipases or esterases

Q20. In chiral HPLC separation, what determines retention difference between enantiomers?

• Only molecular weight differences
• Non-specific van der Waals interactions only
• Differential transient interactions with chiral selector on stationary phase
• Eluent pH exclusively

Correct Answer: Differential transient interactions with chiral selector on stationary phase

Q21. Which term describes a compound with two stereocenters that is superposable on its mirror image?

• Enantiomer
• Meso compound
• Racemic mixture
• Conformer

Correct Answer: Meso compound

Q22. Which factor accelerates racemization of acidic α-hydrogens adjacent to carbonyl groups?

• Strongly acidic or basic conditions that form enolates or carbanions
• Presence of inert gases only
• Low temperatures and neutral pH
• Steric hindrance preventing deprotonation

Correct Answer: Strongly acidic or basic conditions that form enolates or carbanions

Q23. What is enantiospecific synthesis?

• Synthesis that produces an enantiomeric excess by random processes
• Synthesis where chirality of starting material determines stereochemistry of product
• Resolution by crystallization only
• Synthesis that yields racemates exclusively

Correct Answer: Synthesis where chirality of starting material determines stereochemistry of product

Q24. Which of the following is a regulatory concern specific to racemic drugs?

• Requirement to evaluate pharmacology and toxicity of each enantiomer separately
• Uniform approval without stereochemical data
• Inability to patent chiral drugs
• Mandatory conversion to meso forms

Correct Answer: Requirement to evaluate pharmacology and toxicity of each enantiomer separately

Q25. What is the primary goal of asymmetric synthesis in pharmaceuticals?

• Increase racemization during manufacturing
• Produce predominantly one enantiomer with desired biological activity
• Make molecules achiral
• Convert enantiomers into meso forms

Correct Answer: Produce predominantly one enantiomer with desired biological activity

Q26. Which process describes spontaneous resolution?

• Enantiomers reacting to form racemic compounds exclusively
• Selective formation of one enantiomer via enzyme catalysis
• Crystallization from a racemic solution leading to separate enantiomer crystals without chiral additives
• Chemical inversion using strong acids

Correct Answer: Crystallization from a racemic solution leading to separate enantiomer crystals without chiral additives

Q27. Which analytical method can differentiate enantiomers using a chiral derivatizing agent?

• Achiral NMR without any additives
• NMR with chiral shift reagents or chiral derivatizing agents
• Mass spectrometry without derivatization
• Refractive index measurement only

Correct Answer: NMR with chiral shift reagents or chiral derivatizing agents

Q28. Which phenomenon explains why two enantiomers can have different pharmacodynamic profiles?

• They have different elemental compositions
• Different 3D interactions with chiral biological targets such as enzymes or receptors
• Difference in UV-visible absorption only
• Different isotopic distributions

Correct Answer: Different 3D interactions with chiral biological targets such as enzymes or receptors

Q29. Which of the following is true about diastereomers?

• They are mirror images of each other
• They have different physical and chemical properties
• They rotate plane-polarized light by equal magnitude in opposite directions
• They cannot be separated by conventional methods

Correct Answer: They have different physical and chemical properties

Q30. What is the effect of temperature on racemization rates generally?

• Racemization rate is independent of temperature
• Higher temperature typically increases racemization rate
• Lower temperature accelerates racemization
• Temperature only affects optical rotation sign

Correct Answer: Higher temperature typically increases racemization rate

Q31. Which chiral drug famously caused teratogenic effects due to enantiomer-specific activity?

• Aspirin
• Thalidomide
• Penicillin
• Ibuprofen

Correct Answer: Thalidomide

Q32. What is the principle of resolution by formation of diastereomeric derivatives?

• Convert enantiomers into chemically identical compounds
• React enantiomers with a chiral reagent to form diastereomers with different properties
• Use UV light to separate enantiomers
• Use isotopic labeling to track enantiomers

Correct Answer: React enantiomers with a chiral reagent to form diastereomers with different properties

Q33. Which term refers to the proportion of one enantiomer relative to the racemic mixture expressed as percentage?

• Enantiomeric purity
• Optical density
• Molar absorptivity
• Diastereomeric ratio

Correct Answer: Enantiomeric purity

Q34. Which catalytic approach is widely used for asymmetric hydrogenation to produce single enantiomers?

• Achiral platinum catalysts only
• Chiral transition-metal catalysts such as Rh or Ru with chiral ligands
• Radical initiators at high temperature
• Photochemical racemization

Correct Answer: Chiral transition-metal catalysts such as Rh or Ru with chiral ligands

Q35. Which descriptor indicates direction of rotation of plane-polarized light by an enantiomer?

• R/S configuration
• d or l (or + and −) optical rotation sign
• pH value
• Melting point

Correct Answer: d or l (or + and −) optical rotation sign

Q36. What is the purpose of using chiral auxiliaries?

• To racemize stereocenters intentionally
• To induce stereochemical bias during formation of new stereocenters and then be removed
• To reduce molecular weight of drug molecules
• To convert enantiomers into achiral compounds permanently

Correct Answer: To induce stereochemical bias during formation of new stereocenters and then be removed

Q37. Which of the following is true for racemic drugs when one enantiomer is active and the other is inactive or harmful?

• There is no regulatory interest in separating enantiomers
• Development of single-enantiomer drug may improve safety and efficacy
• Enantiomers always have identical toxicity profiles
• Racemic mixtures are always preferred for patentability

Correct Answer: Development of single-enantiomer drug may improve safety and efficacy

Q38. Which process can convert a racemic mixture into a single enantiomer without losing yield if coupled with racemization?

• Simple crystallization without further steps
• Dynamic kinetic resolution
• Dilution in achiral solvent
• Heating under inert atmosphere only

Correct Answer: Dynamic kinetic resolution

Q39. Which property is NOT typically identical for enantiomers in achiral conditions?

• Melting point
• Boiling point
• Specific rotation
• Density

Correct Answer: Specific rotation

Q40. Which stereochemical concept is illustrated by two stereoisomers that are not mirror images?

• Enantiomers
• Configurational isomers called diastereomers
• Constitutional isomers only
• Isotopomers

Correct Answer: Configurational isomers called diastereomers

Q41. Which experimental condition would you avoid to minimize racemization during storage of a chiral drug with labile stereocenter?

• Store at elevated temperatures and in presence of strong bases
• Store under controlled low temperature and inert atmosphere
• Protect from moisture and light as appropriate
• Use stabilizing excipients to prevent chemical degradation

Correct Answer: Store at elevated temperatures and in presence of strong bases

Q42. Which textbook technique helps determine absolute configuration experimentally?

• X-ray crystallography with anomalous dispersion
• UV-Vis spectroscopy alone
• Simple boiling point comparison
• Melting point alone

Correct Answer: X-ray crystallography with anomalous dispersion

Q43. In a racemic resolution by preferential crystallization, what is required?

• A conglomerate-forming racemate and seeding with desired enantiomer crystal
• Formation of a racemic compound only
• Presence of chiral catalysts in solution
• High-temperature melt-based separation

Correct Answer: A conglomerate-forming racemate and seeding with desired enantiomer crystal

Q44. Which computational concept helps predict enantioselectivity of a catalyst?

• Molecular mechanics and transition state modeling
• Simple empirical boiling point models
• Isotope pattern prediction only
• Kirchhoff’s law

Correct Answer: Molecular mechanics and transition state modeling

Q45. What is the significance of enantiomeric purity in dosage form development?

• It affects pharmacological potency, safety, and regulatory labeling
• It only affects color of tablet
• It is irrelevant to clinical outcomes
• It solely determines tablet hardness

Correct Answer: It affects pharmacological potency, safety, and regulatory labeling

Q46. Which laboratory practice can accidentally induce racemization during synthesis?

• Using strong bases and prolonged reaction times with labile stereocenters
• Carrying out reactions at low temperature with neutral reagents
• Using enantioselective catalysts properly
• Avoiding protic solvents when unnecessary

Correct Answer: Using strong bases and prolonged reaction times with labile stereocenters

Q47. What does the term specific rotation represent?

• Rotation of plane-polarized light per unit concentration and path length
• Total optical rotation regardless of concentration
• Rotation caused by achiral impurities
• Another name for refractive index

Correct Answer: Rotation of plane-polarized light per unit concentration and path length

Q48. Which approach is effective for preventing chiral inversion in vivo for susceptible drugs?

• Design prodrugs that undergo rapid racemization
• Modify structure to stabilize stereocenter against metabolic inversion
• Increase lipophilicity only
• Administer as racemic mixture to avoid monitoring

Correct Answer: Modify structure to stabilize stereocenter against metabolic inversion

Q49. Which term best describes two enantiomers that have different pharmacological activities but similar physicochemical properties?

• Stereoisomers with identical bioactivity
• Enantiomers with stereoselective pharmacodynamics
• Constitutional isomers with different molecular formulae
• Meso isomers with identical activity

Correct Answer: Enantiomers with stereoselective pharmacodynamics

Q50. Which experimental strategy can be used to monitor racemization kinetics?

• Chiral analytical methods such as chiral HPLC or chiral GC over time
• Measuring melting point at one time point only
• Observing color change qualitatively
• Measuring pH without chiral analysis

Correct Answer: Chiral analytical methods such as chiral HPLC or chiral GC over time

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