Introduction: Resolution of racemic mixtures MCQs With Answer is an essential topic for B. Pharm students studying stereochemistry, chiral drugs and pharmaceutical analysis. This concise guide covers methods of resolution such as diastereomeric salt formation, chiral chromatography, enzymatic and kinetic resolution, preferential crystallization and dynamic kinetic resolution. It also emphasizes key concepts like enantiomeric excess (ee), optical rotation, resolving agents (e.g., tartaric acid, brucine), and analytical techniques like polarimetry and chiral HPLC. Understanding these principles is vital for drug development, regulatory compliance and quality control of enantiomerically pure APIs. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is the definition of a racemic mixture?
- A mixture containing equal amounts of two diastereomers
- A mixture containing unequal amounts of enantiomers
- A mixture containing equal amounts of enantiomers
- A single optically active enantiomer
Correct Answer: A mixture containing equal amounts of enantiomers
Q2. Which analytical technique directly measures optical rotation to assess enantiomeric purity?
- Mass spectrometry
- Polarimetry
- Infrared spectroscopy
- NMR spectroscopy
Correct Answer: Polarimetry
Q3. Enantiomeric excess (ee) is defined as:
- The absolute difference in moles between two diastereomers
- The percentage of the major enantiomer minus the minor enantiomer relative to total
- The ratio of racemate to pure enantiomer
- The sum of optical rotations of both enantiomers
Correct Answer: The percentage of the major enantiomer minus the minor enantiomer relative to total
Q4. Which resolving agent is commonly used to separate racemic amines by forming diastereomeric salts?
- Tartaric acid
- Sodium hydroxide
- Hydrogen peroxide
- Chloroform
Correct Answer: Tartaric acid
Q5. What is the main principle behind resolution by formation of diastereomeric salts?
- Conversion of enantiomers to achiral compounds
- Formation of diastereomers with different physical properties allowing separation
- Complete chemical racemization
- Conversion to identical stereochemical products
Correct Answer: Formation of diastereomers with different physical properties allowing separation
Q6. Which method uses enzymes to selectively react with one enantiomer of a racemate?
- Chiral chromatography
- Enzymatic resolution
- Mechanical separation
- Preferential crystallization
Correct Answer: Enzymatic resolution
Q7. In kinetic resolution, the theoretical maximum yield of the desired enantiomer without racemization is:
- 100%
- 50%
- 75%
- 25%
Correct Answer: 50%
Q8. Dynamic kinetic resolution (DKR) improves yield by combining resolution with:
- Chromatographic separation
- Continuous racemization of the undesired enantiomer
- Mechanical grinding
- Recrystallization without racemization
Correct Answer: Continuous racemization of the undesired enantiomer
Q9. Which chiral resolving agent is often used to separate racemic carboxylic acids?
- Brucine
- Sodium chloride
- Brønsted base
- (+)-Ephedrine or chiral amines
Correct Answer: (+)-Ephedrine or chiral amines
Q10. Preferential crystallization is most effective when:
- Both enantiomers form identical crystals
- A racemate forms a racemic compound with identical solubility
- The enantiomers have different solubilities and form conglomerates
- Only one enantiomer exists in solution
Correct Answer: The enantiomers have different solubilities and form conglomerates
Q11. Which chromatographic technique is commonly used for analytical and preparative resolution of enantiomers?
- Chiral HPLC
- Size-exclusion chromatography
- Ion-exchange chromatography for achiral separations
- Normal silica gel column without chiral selector
Correct Answer: Chiral HPLC
Q12. Which statement about diastereomeric salts is true?
- Diastereomeric salts are always racemic
- Diastereomeric salts are enantiomers of each other
- Diastereomeric salts have identical melting points
- Diastereomeric salts have different physical properties enabling separation
Correct Answer: Diastereomeric salts have different physical properties enabling separation
Q13. What is a common drawback of classical resolution by diastereomeric salt formation?
- It always gives 100% yield without further steps
- It often requires a stoichiometric amount of resolving agent and may need recycling
- It cannot separate chiral acids
- It never requires control of temperature
Correct Answer: It often requires a stoichiometric amount of resolving agent and may need recycling
Q14. Which chiral derivatizing agent is used to analyze enantiomeric composition by NMR?
- Chloroform
- Mosher’s reagent (MTPA chloride)
- Sodium sulfate
- Hydrochloric acid
Correct Answer: Mosher’s reagent (MTPA chloride)
Q15. Which process can convert a racemic mixture into a single enantiomer without forming diastereomers?
- Classical diastereomeric salt formation
- Asymmetric synthesis using chiral catalysts
- Recrystallization of racemate
- Mixing with an achiral solvent
Correct Answer: Asymmetric synthesis using chiral catalysts
Q16. Enantiomeric purity measured as ee% of a sample containing 80% R and 20% S is:
- 20% ee
- 100% ee
- 60% ee
- 80% ee
Correct Answer: 60% ee
Q17. Which reagent is historically used as a resolving agent for basic compounds like amino acids?
- (-)-Tartaric acid
- Conc. sulfuric acid
- Sodium borohydride
- Acetone
Correct Answer: (-)-Tartaric acid
Q18. Which resolution method relies on different rates of reaction of enantiomers with a chiral catalyst?
- Enantiomeric crystallization
- Kinetic resolution
- Simple extraction
- Sublimation
Correct Answer: Kinetic resolution
Q19. In chiral HPLC, what provides enantioselectivity?
- Use of achiral mobile phase only
- Chiral stationary phase or chiral selector in mobile phase
- High temperature alone
- UV detector wavelength
Correct Answer: Chiral stationary phase or chiral selector in mobile phase
Q20. Which term describes a solid composed of equal amounts of both enantiomeric crystals in a racemic compound?
- Conglomerate
- Racemic compound
- Enantiopure crystal
- Stereoisomeric gel
Correct Answer: Racemic compound
Q21. What is preferential crystallization also called when applied to conglomerates?
- Seeding crystallization
- Distillation
- Chromatographic elution
- Enzymatic hydrolysis
Correct Answer: Seeding crystallization
Q22. Which enzyme class is commonly used for kinetic resolution of esters and alcohols?
- Oxidoreductases
- Carboxylesterases and lipases
- Lyases
- Nucleases
Correct Answer: Carboxylesterases and lipases
Q23. A racemate that crystallizes as a physical mixture of enantiomerically pure crystals is called:
- Racemic compound
- Conglomerate
- Mesocompound
- Polymorph
Correct Answer: Conglomerate
Q24. In resolution by formation of diastereomeric salts, which property difference is most commonly exploited?
- Identical reactivity
- Different solubilities and melting points
- Same refractive index
- Equal optical rotations
Correct Answer: Different solubilities and melting points
Q25. Which technique can separate enantiomers based on transient formation of diastereomeric complexes with a chiral selector?
- Ion-exchange without chiral selector
- Capillary electrophoresis with chiral additives
- Simple distillation
- Thin-layer chromatography with achiral silica
Correct Answer: Capillary electrophoresis with chiral additives
Q26. Which phenomenon describes conversion of a single enantiomer into its mirror image under certain conditions?
- Epimerization
- Racemization
- Methylation
- Hydrogenation
Correct Answer: Racemization
Q27. Which method is often used industrially for large-scale resolution to avoid stoichiometric resolving agents?
- Batch diastereomeric salt separation without recycling
- Asymmetric catalysis and chiral synthesis
- Manual mechanical separation of crystals
- Thin-layer chromatography
Correct Answer: Asymmetric catalysis and chiral synthesis
Q28. When calculating ee from observed optical rotation, what additional information is required?
- Molar mass only
- Specific rotation of pure enantiomer
- Boiling point of racemate
- Concentration of resolving agent only
Correct Answer: Specific rotation of pure enantiomer
Q29. Which resolving agent is well-known for resolving racemic alkaloids historically?
- Brucine
- Sodium bicarbonate
- Acetaldehyde
- Ethyl acetate
Correct Answer: Brucine
Q30. What is the advantage of using chiral chromatography for resolution?
- It cannot provide preparative amounts
- High selectivity and applicability to many compound classes
- It always uses toxic reagents
- Requires racemization first
Correct Answer: High selectivity and applicability to many compound classes
Q31. Which statement describes catalytic kinetic resolution?
- A stoichiometric chiral reagent is consumed
- A chiral catalyst selectively transforms one enantiomer faster, allowing separation
- Both enantiomers react at identical rates
- It relies solely on crystallization
Correct Answer: A chiral catalyst selectively transforms one enantiomer faster, allowing separation
Q32. Which parameter indicates the efficiency of a chiral stationary phase in chromatography?
- Retention time only
- Resolution (Rs) between enantiomer peaks
- Detector sensitivity only
- Solvent flash point
Correct Answer: Resolution (Rs) between enantiomer peaks
Q33. Which approach can convert a racemate to a single enantiomer using light or chemical agents to selectively destroy one enantiomer?
- Chiral chromatography
- Chiral photolysis or enantioselective degradation
- Preferential crystallization
- Simple filtration
Correct Answer: Chiral photolysis or enantioselective degradation
Q34. Which is true about a meso compound in terms of optical activity?
- A meso compound is optically active
- A meso compound is achiral despite stereocenters and is optically inactive
- Meso compounds are always racemic mixtures
- Meso compounds cannot be resolved
Correct Answer: A meso compound is achiral despite stereocenters and is optically inactive
Q35. For resolution of racemic alcohols by esterification with a chiral acid, the resulting diastereomeric esters differ mainly in:
- Their chemical formula
- Their physical properties such as solubility and melting point
- Their elemental composition
- They remain identical in all properties
Correct Answer: Their physical properties such as solubility and melting point
Q36. Which of the following is an example of a chiral pool strategy rather than resolution?
- Using (+)-limonene as a starting chiral building block
- Forming diastereomeric salts
- Enzymatic kinetic resolution of a racemate
- Chiral HPLC separation of enantiomers
Correct Answer: Using (+)-limonene as a starting chiral building block
Q37. What does a high enantioselectivity (E value) in an enzymatic kinetic resolution indicate?
- Low discrimination between enantiomers
- High discrimination between enantiomers by the enzyme
- That both enantiomers are consumed equally
- That catalyst is achiral
Correct Answer: High discrimination between enantiomers by the enzyme
Q38. Which process is most suitable to obtain gram-scale enantiopure compound when chiral catalysts are unavailable?
- Preparative chiral chromatography or classical resolution with recycling of resolving agent
- Ignoring chirality and using racemate
- Thin-layer chromatography
- UV spectroscopy
Correct Answer: Preparative chiral chromatography or classical resolution with recycling of resolving agent
Q39. What role does seeding play in preferential crystallization?
- It prevents crystallization
- It directs crystallization to form crystals of the seeded enantiomer preferentially
- It causes complete racemization
- It dissolves existing crystals
Correct Answer: It directs crystallization to form crystals of the seeded enantiomer preferentially
Q40. In practice, why is racemization coupled to resolution beneficial in DKR?
- It reduces overall enantiomeric purity
- It allows continuous conversion of the undesired enantiomer into the desired one, improving yield
- It increases the need for resolving agents
- It always lowers selectivity of catalysts
Correct Answer: It allows continuous conversion of the undesired enantiomer into the desired one, improving yield
Q41. Which measurement technique combined with chiral separation provides structural information and mass confirmation?
- Chiral HPLC coupled with mass spectrometry (LC-MS)
- Polarimetry without separation
- Visual inspection of crystals
- Simple refractometry
Correct Answer: Chiral HPLC coupled with mass spectrometry (LC-MS)
Q42. What is a major regulatory reason to control enantiomeric purity in pharmaceuticals?
- Enantiomers always have identical biological activity
- Different enantiomers may have different pharmacodynamics, pharmacokinetics or toxicity
- Regulators do not consider chirality important
- It only affects color of tablets
Correct Answer: Different enantiomers may have different pharmacodynamics, pharmacokinetics or toxicity
Q43. Which chiral resolving reagent is commonly used for resolving secondary alcohols into diastereomeric esters for separation?
- p-Toluenesulfonic acid
- Mosher’s acid (MTPA)
- Sodium chloride
- Acetone
Correct Answer: Mosher’s acid (MTPA)
Q44. What is the term for a racemic mixture in which enantiomers crystallize separately as pure crystals?
- Racemic compound
- Conglomerate
- Isomeric blend
- Homogeneous solid solution
Correct Answer: Conglomerate
Q45. During resolution, why might one perform salt formation followed by base treatment after separation?
- To permanently destroy both enantiomers
- To liberate the free base or free acid of the separated enantiomer from the salt
- To racemize the separated enantiomer
- To convert enantiomers into achiral compounds
Correct Answer: To liberate the free base or free acid of the separated enantiomer from the salt
Q46. Which factor is NOT typically considered when choosing a resolution method for an API?
- Scale and cost
- Environmental and regulatory impact
- Color of the laboratory walls
- Availability of chiral catalysts or resolving agents
Correct Answer: Color of the laboratory walls
Q47. Which technique can determine absolute configuration of an enantiomer unambiguously when combined with X-ray crystallography?
- Chiral chromatography alone
- Single-crystal X-ray diffraction with anomalous dispersion
- Polarimetry alone
- Simple melting point determination
Correct Answer: Single-crystal X-ray diffraction with anomalous dispersion
Q48. What is the typical first step in resolution by diastereomeric salt formation for a racemic amine?
- Direct distillation of racemate
- Treating the racemic amine with a chiral acid to form salts
- Immediate chiral HPLC without modification
- Heating to high temperature to decompose
Correct Answer: Treating the racemic amine with a chiral acid to form salts
Q49. Which modern approach can provide enantiopure products without resolution by relying on chiral catalysts and asymmetric induction?
- Classical resolution only
- Asymmetric catalysis (e.g., organocatalysis, metal-catalyzed asymmetric reactions)
- Random chemical treatment
- Untargeted photolysis
Correct Answer: Asymmetric catalysis (e.g., organocatalysis, metal-catalyzed asymmetric reactions)
Q50. When reporting enantiomeric purity for a pharmaceutical active ingredient, which two key values are commonly provided?
- Density and boiling point
- Enantiomeric excess (ee) and specific optical rotation [α]D
- Melting point and color
- pH and viscosity
Correct Answer: Enantiomeric excess (ee) and specific optical rotation [α]D
