Stereoselective reactions MCQs With Answer

Stereoselective reactions MCQs With Answer are essential for B. Pharm students to master stereochemistry, asymmetric synthesis, and drug activity relationships. This focused introduction covers enantioselectivity, diastereoselectivity, stereospecific vs stereoselective processes, chiral catalysts, and common mechanistic models (Felkin–Anh, Cram) used in pharmaceutical synthesis. Understanding how reagents, catalysts, and substrates control stereochemical outcomes helps predict drug stereoisomer formation, optimize asymmetric hydrogenation, epoxidation, and reductions, and avoid racemization. These MCQs emphasize real-world examples, mechanisms, and stereochemical reasoning to strengthen exam and practical skills. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which term describes a reaction that produces one stereoisomer preferentially over others?

• Stereoselective reaction
• Stereospecific reaction
• Regioselective reaction
• Constitutional isomerization

Correct Answer: Stereoselective reaction

Q2. A reaction that converts a single stereoisomer of substrate into a single stereoisomer of product is called:

• Stereoselective
• Stereospecific
• Enantioselective
• Regioselective

Correct Answer: Stereospecific

Q3. Which model best predicts nucleophilic addition to a non-chelating aldehyde giving the major stereoisomer?

• Cornforth model
• Felkin–Anh model
• Cram chelation model
• Zimmerman–Traxler model

Correct Answer: Felkin–Anh model

Q4. In asymmetric hydrogenation using a chiral catalyst, the selectivity that produces predominantly one enantiomer is called:

• Diastereoselectivity
• Enantioselectivity
• Regioselectivity
• Stereospecificity

Correct Answer: Enantioselectivity

Q5. Which reagent is commonly used for stereoselective reduction of ketones to alcohols with high enantioselectivity in the CBS reduction?

• Sodium borohydride
• Corey–Bakshi–Shibata (CBS) catalyst with borane
• Lithium aluminum hydride
• Hydrogen gas with palladium

Correct Answer: Corey–Bakshi–Shibata (CBS) catalyst with borane

Q6. What term describes preference for formation of one diastereomer over another?

• Enantioselectivity
• Regioselectivity
• Diastereoselectivity
• Atropisomerism

Correct Answer: Diastereoselectivity

Q7. Which of the following best explains neighboring group participation influencing stereochemical outcome?

• Formation of achiral intermediate
• Internal nucleophile stabilizes a carbocation leading to stereospecific substitution
• External base abstracts proton randomly
• Radical chain propagation

Correct Answer: Internal nucleophile stabilizes a carbocation leading to stereospecific substitution

Q8. In SN2 reactions at a stereogenic center, what is the stereochemical outcome?

• Retention of configuration
• Inversion of configuration
• Racemization
• Formation of meso compound

Correct Answer: Inversion of configuration

Q9. Which stereochemical phenomenon refers to conversion of one enantiomer into the other, reducing optical purity?

• Resolution
• Racemization
• Epimerization
• Isomerization

Correct Answer: Racemization

Q10. What does a high enantiomeric excess (ee) indicate?

• Equal amounts of enantiomers
• Predominance of one enantiomer over the other
• Only diastereomers present
• Complete racemate

Correct Answer: Predominance of one enantiomer over the other

Q11. Which catalytic system is well-known for asymmetric epoxidation of allylic alcohols?

• Sharpless epoxidation
• Wacker oxidation
• Birch reduction
• Barton decarboxylation

Correct Answer: Sharpless epoxidation

Q12. Which term describes an achiral molecule with stereocenters that is superimposable on its mirror image?

• Enantiomer
• Meso compound
• Diastereomer
• Atropisomer

Correct Answer: Meso compound

Q13. In dihydroxylation of alkenes using OsO4 with chiral ligands, the selectivity achieved is an example of:

• Regioselectivity
• Enantioselective oxidation
• Polymerization
• Radical addition

Correct Answer: Enantioselective oxidation

Q14. Cram’s rule is used to predict stereochemical outcome in which reaction type?

• Nucleophilic addition to carbonyls
• Electrophilic aromatic substitution
• Free radical halogenation
• Pericyclic reactions

Correct Answer: Nucleophilic addition to carbonyls

Q15. What is the primary difference between stereospecific and stereoselective reactions?

• Stereospecific gives one product from any substrate; stereoselective depends solely on temperature
• Stereospecific outcome depends on substrate stereochemistry; stereoselective favors one stereoisomer but not exclusively
• They are synonymous terms
• Stereoselective always gives racemate

Correct Answer: Stereospecific outcome depends on substrate stereochemistry; stereoselective favors one stereoisomer but not exclusively

Q16. Which technique is commonly used to separate enantiomers of chiral drugs?

• Normal-phase chromatography without chiral selector
• Chiral chromatography using chiral stationary phase
• Distillation under reduced pressure
• Recrystallization of racemate only

Correct Answer: Chiral chromatography using chiral stationary phase

Q17. In asymmetric dihydroxylation developed by Sharpless, which co-oxidant is typically used with OsO4?

• KMnO4
• NMO (N-methylmorpholine N-oxide)
• LiAlH4
• Peracetic acid

Correct Answer: NMO (N-methylmorpholine N-oxide)

Q18. Which factor often governs reagent-controlled stereoselectivity?

• Chirality of the substrate only
• Chirality of catalyst or reagent directing approach to substrate
• Solvent viscosity exclusively
• Atmospheric pressure

Correct Answer: Chirality of catalyst or reagent directing approach to substrate

Q19. What outcome does a stereospecific syn addition to an alkene produce on a cis-alkene?

• Trans di-substituted product
• Cis addition leading to syn stereochemistry preserved
• Racemization
• Epimerization at remote center

Correct Answer: Cis addition leading to syn stereochemistry preserved

Q20. Which is a common chiral ligand used in asymmetric hydrogenation (e.g., BINAP)?

• EDTA
• BINAP
• PPh3 only
• Boc protecting group

Correct Answer: BINAP

Q21. An enantiomerically pure drug differs from its enantiomer primarily in:

• Number of atoms
• Three-dimensional arrangement affecting biological activity
• Molecular formula
• Presence of different functional groups

Correct Answer: Three-dimensional arrangement affecting biological activity

Q22. What is the meaning of diastereomeric excess (de)?

• Difference in amounts of two enantiomers
• Difference in amounts of two diastereomers
• Sum of enantiomer percentages
• Optical rotation measured at 0°C

Correct Answer: Difference in amounts of two diastereomers

Q23. Which mechanism often leads to racemization at a stereocenter adjacent to a carbonyl?

• Formation of enolate intermediate
• Concerted pericyclic shift
• SN2 inversion
• Photochemical isomerization only

Correct Answer: Formation of enolate intermediate

Q24. What stereochemical outcome is expected in a concerted cycloaddition (e.g., Diels–Alder) regarding stereochemistry of substituents?

• Random scrambling of stereocenters
• Retention of relative stereochemistry (stereospecific)
• Complete racemization
• Formation of only achiral products

Correct Answer: Retention of relative stereochemistry (stereospecific)

Q25. Which term describes two stereoisomers that are not mirror images?

• Enantiomers
• Diastereomers
• Conformers
• Identical compounds

Correct Answer: Diastereomers

Q26. In the presence of a chiral auxiliary, addition to a prochiral center is often directed to give:

• Racemic mixture
• One diastereomer predominantly
• Only constitutional isomers
• Uncontrolled polymerization

Correct Answer: One diastereomer predominantly

Q27. Which synthetic strategy uses naturally occurring chiral building blocks to control stereochemistry?

• Asymmetric catalysis
• Chiral pool synthesis
• Racemization
• Photochemical racemate formation

Correct Answer: Chiral pool synthesis

Q28. The Sharpless asymmetric epoxidation requires which functional group adjacent to the alkene to achieve high enantioselectivity?

• Allylic alcohol
• Tertiary amine
• Aromatic ring only
• Alkyne

Correct Answer: Allylic alcohol

Q29. Which is true about kinetic resolution in enantiomer separation?

• Both enantiomers react at the same rate
• One enantiomer reacts faster, allowing separation
• It converts enantiomers into identical products
• It always yields 100% isolated enantiomer

Correct Answer: One enantiomer reacts faster, allowing separation

Q30. Which method directly measures enantiomeric excess (ee)?

• IR spectroscopy
• Polarimetry or chiral chromatography
• Simple TLC with achiral solvent
• Elemental analysis

Correct Answer: Polarimetry or chiral chromatography

Q31. In the Felkin–Anh model, which factor minimizes in the preferred transition state?

• Steric interaction between nucleophile and smallest group
• Antiperiplanar alignment of orbitals
• Steric interaction between nucleophile and largest substituent
• Maximization of hyperconjugation

Correct Answer: Steric interaction between nucleophile and largest substituent

Q32. Which reaction is typically stereospecific and leads to retention of configuration via a cyclic intermediate?

• SN2 substitution
• Neighboring group participation through a bromonium ion opening
• Radical halogenation
• Acid-catalyzed hydration

Correct Answer: Neighboring group participation through a bromonium ion opening

Q33. A chiral catalyst that accelerates formation of one enantiomer is exhibiting:

• Enantioselective catalysis
• Regioselective degradation
• Photochemical racemization
• Isotopic labeling

Correct Answer: Enantioselective catalysis

Q34. Which of the following best defines epimerization?

• Conversion between enantiomers
• Change of configuration at one stereocenter in a diastereomer pair
• Pericyclic rearrangement to a new skeleton
• Stereospecific hydrogenation

Correct Answer: Change of configuration at one stereocenter in a diastereomer pair

Q35. Asymmetric allylation using chiral reagents aims to create new stereocenters with control. Which reagent class is commonly employed?

• Chiral boron reagents
• Unsubstituted Grignard only
• Neutral salts with no chirality
• Peroxides exclusively

Correct Answer: Chiral boron reagents

Q36. Which descriptor specifies absolute configuration at a stereocenter?

• cis/trans only
• R or S
• alpha or beta only
• ortho/meta/para

Correct Answer: R or S

Q37. Asymmetric induction by a chiral center already present in the molecule is termed:

• Reagent control
• Substrate control
• Regioselective control
• Photochemical control

Correct Answer: Substrate control

Q38. Which of the following is a commonly used stoichiometric chiral auxiliary?

• Camphorsultam (Evans auxiliary)
• Sodium chloride
• Acetic acid only
• Ethylene glycol

Correct Answer: Camphorsultam (Evans auxiliary)

Q39. In catalytic asymmetric hydrogenation of prochiral olefins, turnover number (TON) relates to:

• Amount of catalyst relative to substrate
• Number of substrate molecules converted per catalyst molecule
• Only the temperature used
• Color of the catalyst

Correct Answer: Number of substrate molecules converted per catalyst molecule

Q40. Which experimental condition often decreases stereoselectivity by promoting equilibration of stereoisomers?

• Low temperature
• Mild, nonracemizing conditions
• High temperature with acidic or basic catalysts promoting epimerization
• Use of chiral catalyst

Correct Answer: High temperature with acidic or basic catalysts promoting epimerization

Q41. The Zimmerman–Traxler model is used to rationalize stereochemistry in which reaction?

• Aldol reactions
• Nitration of aromatics
• Free radical bromination
• Acid-catalyzed dehydration

Correct Answer: Aldol reactions

Q42. Which stereochemical outcome is favored when a nucleophile attacks an sp2 carbon in a chairlike transition state minimizing 1,3-diaxial interactions?

• Attack giving axial substituents only
• Attack leading to equatorial substituents when possible
• Random stereochemistry
• Only racemic mixtures

Correct Answer: Attack leading to equatorial substituents when possible

Q43. Which process increases enantiomeric purity by selectively reacting one enantiomer faster under catalytic conditions?

• Dynamic kinetic resolution
• Simple recrystallization without catalyst
• Thermal racemization only
• Classical resolution via diastereomeric salts without racemization

Correct Answer: Dynamic kinetic resolution

Q44. What is atropisomerism?

• Stereoisomerism due to restricted rotation about a single bond leading to isolable conformers
• Isomerism only in alkanes
• Optical isomerism of inorganic salts
• Syn/anti stereochemistry in alkenes

Correct Answer: Stereoisomerism due to restricted rotation about a single bond leading to isolable conformers

Q45. In a stereoselective epoxide opening under acidic conditions, the nucleophile attacks:

• The less substituted carbon always
• The more substituted carbon due to carbocation-like character
• Random carbon equally
• Only oxygen atom

Correct Answer: The more substituted carbon due to carbocation-like character

Q46. Which analytical method can distinguish enantiomers by forming diastereomeric derivatives?

• Chiral derivatization followed by achiral NMR or HPLC
• Elemental analysis
• UV–Vis without chiral reagent
• Simple melting point determination

Correct Answer: Chiral derivatization followed by achiral NMR or HPLC

Q47. A reaction where both enantiomers of a racemate are converted into different products by a chiral catalyst is called:

• Enantioconvergent catalysis
• Catalytic racemization only
• Nonselective catalysis
• Stoichiometric resolution

Correct Answer: Enantioconvergent catalysis

Q48. Which of the following is true about chiral centers created via asymmetric synthesis?

• They always racemize instantly
• They can be created with high enantiomeric excess using chiral catalysts or auxiliaries
• They cannot be analyzed by chromatography
• They are irrelevant to drug safety

Correct Answer: They can be created with high enantiomeric excess using chiral catalysts or auxiliaries

Q49. In carbohydrate chemistry, selective formation of one anomer over another during glycosylation is an example of:

• Regioselectivity only
• Anomeric stereoselectivity
• Complete racemization
• Polymerization

Correct Answer: Anomeric stereoselectivity

Q50. Which general approach is often used in drug development to avoid unwanted stereoisomer-related side effects?

• Develop racemic mixtures only
• Design and produce single-enantiomer drugs using asymmetric synthesis or resolution
• Ignore stereochemistry entirely
• Use only achiral drugs irrespective of activity

Correct Answer: Design and produce single-enantiomer drugs using asymmetric synthesis or resolution

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