Stereospecific reactions MCQs With Answer

Stereospecific reactions MCQs With Answer

Understanding stereospecific reactions is essential for B. Pharm students studying drug action, synthesis and metabolism. This concise introduction covers how stereochemistry of reactants dictates product stereo-outcomes, contrasts stereospecific and stereoselective processes, and highlights key examples—SN2 inversion, syn/anti additions (hydrogenation, bromination), E2 anti-periplanar eliminations, epoxidation and enzyme-catalyzed transformations. Mastery of these concepts helps predict chirality changes during drug synthesis and biotransformation. Relevant keywords: stereospecific reactions, stereochemistry, SN2 inversion, syn addition, anti addition, E2 anti-periplanar, epoxidation, enantioselective processes, B. Pharm students. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What best defines a stereospecific reaction?

• A reaction that gives only one stereoisomer regardless of substrate stereochemistry
• A reaction where the stereochemistry of the product depends directly on the stereochemistry of the reactant
• A reaction that favors one stereoisomer over another but both can form
• A reaction that does not involve stereochemistry

Correct Answer: A reaction where the stereochemistry of the product depends directly on the stereochemistry of the reactant

Q2. How is stereoselective different from stereospecific?

• Stereoselective: substrate stereochemistry fully determines product; stereospecific: catalyst chooses enantiomer
• Stereoselective: one stereoisomer is preferred from an achiral or prochiral substrate; stereospecific: different stereoisomeric substrates give different products
• They are identical terms
• Stereoselective applies only to enzymatic reactions

Correct Answer: Stereoselective: one stereoisomer is preferred from an achiral or prochiral substrate; stereospecific: different stereoisomeric substrates give different products

Q3. Which substitution mechanism is classically stereospecific and gives inversion of configuration?

• SN1
• SN2
• E1
• Radical substitution

Correct Answer: SN2

Q4. What is the typical stereochemical outcome of an SN1 reaction at a chiral center?

• Complete inversion only
• Complete retention only
• Racemization (mixture of retention and inversion)
• Formation of a diastereomeric pair only

Correct Answer: Racemization (mixture of retention and inversion)

Q5. Which addition across a double bond is stereospecific syn addition example?

• Bromination (Br2) of alkenes
• Anti-dihydroxylation
• Hydrogenation (H2 with metal catalyst)
• Bimolecular nucleophilic substitution

Correct Answer: Hydrogenation (H2 with metal catalyst)

Q6. Bromination of an alkene proceeds by which stereochemical pathway?

• Syn addition giving cis-dibromide
• Free-radical chain yielding racemic mixture
• Anti addition via bromonium ion giving trans-vicinal dibromide
• Concerted pericyclic syn addition

Correct Answer: Anti addition via bromonium ion giving trans-vicinal dibromide

Q7. Hydroboration–oxidation of an alkene is stereospecific for which outcome?

• Markovnikov addition with anti stereochemistry
• Anti-Markovnikov syn addition of H and OH
• Radical-mediated racemization
• Carbocation rearrangement then racemic product

Correct Answer: Anti-Markovnikov syn addition of H and OH

Q8. Peracid (mCPBA) epoxidation of an alkene is stereospecific because:

• It proceeds through a free-radical that scrambles stereochemistry
• The oxygen adds concertedly to give an epoxide preserving relative stereochemistry (syn)
• It forms a carbocation intermediate that racemizes
• The reaction is enantioselective only with chiral catalysts

Correct Answer: The oxygen adds concertedly to give an epoxide preserving relative stereochemistry (syn)

Q9. Opening of an epoxide under basic (nucleophilic) conditions at a chiral center proceeds by which stereochemical course?

• SN1-like attack leading to racemization
• SN2 attack at the less hindered carbon causing inversion at attacked center
• Concerted syn addition preserving stereochemistry
• Radical ring-opening leading to retention

Correct Answer: SN2 attack at the less hindered carbon causing inversion at attacked center

Q10. Epoxide opening under acidic conditions typically occurs at which carbon and with what stereochemistry?

• Less substituted carbon with retention
• More substituted carbon via SN1-like pathway with backside attack and inversion
• More substituted carbon with nucleophilic attack that gives inversion at that center
• Both carbons equally with racemization

Correct Answer: More substituted carbon with nucleophilic attack that gives inversion at that center

Q11. E2 elimination stereochemistry is best described by which requirement?

• Syn-periplanar alignment of H and leaving group is always required
• Anti-periplanar alignment of H and leaving group is generally required for maximum rate and stereospecific outcome
• No stereochemical requirement; E2 is completely random
• The leaving group determines stereochemistry but conformation is irrelevant

Correct Answer: Anti-periplanar alignment of H and leaving group is generally required for maximum rate and stereospecific outcome

Q12. Which reaction sequence can give net retention of configuration at a stereocenter originally prone to SN2 displacement?

• Single SN2 displacement only
• SN1 displacement followed by racemization
• Two consecutive SN2 steps (double inversion)
• Radical substitution

Correct Answer: Two consecutive SN2 steps (double inversion)

Q13. The Mitsunobu reaction is stereospecific and typically gives which outcome at an alcohol stereocenter?

• Retention of configuration without inversion
• Racemization
• Inversion of configuration during substitution of the alcohol
• No substitution takes place

Correct Answer: Inversion of configuration during substitution of the alcohol

Q14. Sharpless asymmetric epoxidation is best classified as which type of process?

• Stereospecific (dependent on substrate stereochemistry only)
• Enantioselective catalysis producing one enantiomer from an achiral or prochiral allylic alcohol
• Non-selective radical oxidation
• SN2 substitution

Correct Answer: Enantioselective catalysis producing one enantiomer from an achiral or prochiral allylic alcohol

Q15. In catalytic asymmetric hydrogenation of a prochiral alkene, the reaction is described as:

• Enantioselective, producing one enantiomer preferentially
• Stereospecific, independent of catalyst chirality
• Racemizing the substrate
• Anti addition giving trans product only

Correct Answer: Enantioselective, producing one enantiomer preferentially

Q16. Diels–Alder cycloaddition is stereospecific because:

• It proceeds via free radicals
• The stereochemistry of the dienophile and diene is preserved in the cycloadduct through a concerted suprafacial mechanism
• It involves a carbocation intermediate that rearranges
• It always gives racemic mixtures

Correct Answer: The stereochemistry of the dienophile and diene is preserved in the cycloadduct through a concerted suprafacial mechanism

Q17. Which of the following examples demonstrates anti addition of OH and Br across an alkene?

• Hydroboration–oxidation
• Bromohydrin formation from Br2 in water
• Hydrogenation with Pd/C
• Metallation followed by protonation

Correct Answer: Bromohydrin formation from Br2 in water

Q18. Enzymatic reactions that convert one enantiomer of substrate to a single stereochemical product are best described as:

• Stereospecific and often enantioselective
• Non-specific and racemizing
• Only stereoselective but never stereospecific
• Irrelevant to pharmaceutical synthesis

Correct Answer: Stereospecific and often enantioselective

Q19. Which process is most likely to produce racemization at a stereocenter during reaction?

• SN2 displacement at that center
• SN1 via planar carbocation intermediate
• Stereospecific syn addition to a double bond adjacent to the center
• Concerted pericyclic transformation that preserves stereochemistry

Correct Answer: SN1 via planar carbocation intermediate

Q20. A syn addition across an alkyne by catalytic hydrogenation leads to which stereochemical alkene product?

• Trans-alkene (E)
• Cis-alkene (Z)
• A racemic mixture of E and Z
• No alkene formed, only alkane

Correct Answer: Cis-alkene (Z)

Q21. Which reagent combination gives stereospecific anti addition of two OH groups across an alkene?

• OsO4 (syn dihydroxylation)
• KMnO4 cold (syn dihydroxylation)
• Halohydrin formation followed by base-catalyzed substitution (gives vicinal diol after hydrolysis) with anti relationship
• Hydrogenation over Pd/C

Correct Answer: Halohydrin formation followed by base-catalyzed substitution (gives vicinal diol after hydrolysis) with anti relationship

Q22. Which statement about SN2′ (allylic) substitution is true regarding stereochemistry?

• SN2′ is non-stereospecific and gives random stereochemistry
• SN2′ can be stereospecific, leading to allylic shift and predictable stereochemical outcome
• SN2′ proceeds via a carbocation and racemizes
• SN2′ always leads to retention at the reacting center

Correct Answer: SN2′ can be stereospecific, leading to allylic shift and predictable stereochemical outcome

Q23. Which factor often determines whether an SN2 reaction at a stereocenter will be stereospecific in practice?

• Presence of a strong nucleophile and polar aprotic solvent favoring backside attack and inversion
• Use of a polar protic solvent that stabilizes the nucleophile
• High temperature causing racemization
• Light irradiation promoting homolysis

Correct Answer: Presence of a strong nucleophile and polar aprotic solvent favoring backside attack and inversion

Q24. Neighboring group participation (anchimeric assistance) during substitution can lead to which stereochemical outcome?

• Only inversion of configuration
• Retention of configuration via a bridged intermediate followed by attack giving net retention
• Complete racemization always
• No substitution occurs

Correct Answer: Retention of configuration via a bridged intermediate followed by attack giving net retention

Q25. Which reaction is stereospecific due to orbital alignment and stereoelectronic control (Woodward–Hoffmann rules)?

• Free-radical halogenation
• Pericyclic electrocyclic ring closure (conrotatory or disrotatory) where stereochemistry is preserved by orbital symmetry
• SN1 substitution
• Simple acid–base neutralization

Correct Answer: Pericyclic electrocyclic ring closure (conrotatory or disrotatory) where stereochemistry is preserved by orbital symmetry

Q26. In cyclohexane-based E2 eliminations, what conformation is required for a stereospecific anti elimination?

• Equatorial H and equatorial leaving group
• Axial hydrogen and axial leaving group in anti-periplanar relationship
• Any conformation works equally
• Only boat conformation permits elimination

Correct Answer: Axial hydrogen and axial leaving group in anti-periplanar relationship

Q27. Which reaction is typically NOT stereospecific?

• SN2 displacement at a chiral center
• Diels–Alder cycloaddition
• SN1 substitution at a chiral center
• Epoxidation of a cis-alkene with peracid

Correct Answer: SN1 substitution at a chiral center

Q28. A front-side nucleophilic attack results in which stereochemical outcome at the reaction center?

• Inversion of configuration
• Retention of configuration
• Complete racemization
• Formation of diastereomers only

Correct Answer: Retention of configuration

Q29. Which hydrogenation method typically yields the trans (E) alkene via anti addition?

• Catalytic hydrogenation with Pd/C (syn)
• Lindlar catalyst partial hydrogenation (gives cis)
• Dissolving metal reduction (e.g., Na/NH3) giving trans-alkene
• Hydroboration–oxidation

Correct Answer: Dissolving metal reduction (e.g., Na/NH3) giving trans-alkene

Q30. How would you experimentally determine if a substitution reaction is stereospecific?

• Compare product ratios from racemic starting material only
• Perform reaction on a single enantiomeric substrate and observe whether the product’s stereochemistry correlates predictably with the substrate
• Only measure reaction rate constants
• Record melting point of product

Correct Answer: Perform reaction on a single enantiomeric substrate and observe whether the product’s stereochemistry correlates predictably with the substrate

Q31. Which statement about stereospecificity in medicinal chemistry is true?

• Stereospecific reactions are irrelevant to drug design
• Stereospecific reactions help control stereochemistry in drug synthesis, affecting activity and metabolism
• Stereospecific reactions always make drugs less active
• Stereospecific reactions always racemize chiral drugs

Correct Answer: Stereospecific reactions help control stereochemistry in drug synthesis, affecting activity and metabolism

Q32. In allylic substitution, what distinguishes an SN2′ pathway stereochemically?

• Attack at the allylic position with concomitant migration, often giving an overall stereospecific rearranged product
• Formation of a free radical intermediate
• Carbocation formation with no stereochemical control
• No change in connectivity

Correct Answer: Attack at the allylic position with concomitant migration, often giving an overall stereospecific rearranged product

Q33. Which reaction mechanism is most likely to maintain relative stereochemistry of substituents during ring closure?

• Stepwise radical cyclization with planar intermediates
• Concerted pericyclic cyclization (e.g., Diels–Alder) that is stereospecific
• SN1 ring closure always giving racemic product
• Random bimolecular collisions

Correct Answer: Concerted pericyclic cyclization (e.g., Diels–Alder) that is stereospecific

Q34. Which of these is a stereospecific outcome observed in carbohydrate glycosylation when neighboring group participation occurs?

• Racemization of the anomeric center
• Formation of a bridged intermediate leading to predominant retention or formation of a defined anomer
• Only SN2-like inversion without neighboring effects
• No reaction occurs

Correct Answer: Formation of a bridged intermediate leading to predominant retention or formation of a defined anomer

Q35. What stereochemical outcome is expected when a chiral secondary alcohol undergoes oxidation to a ketone?

• Retention of chirality at the oxidized center
• Loss of chirality at that center because the carbon becomes achiral
• Inversion of configuration at that center
• Racemization of adjacent stereocenters only

Correct Answer: Loss of chirality at that center because the carbon becomes achiral

Q36. Which reagent set gives syn dihydroxylation of alkenes stereospecifically?

• Br2 followed by HO−
• O3 then reductive workup
• OsO4 or KMnO4 (cold) producing syn diol
• HBr addition

Correct Answer: OsO4 or KMnO4 (cold) producing syn diol

Q37. The Walden inversion describes which phenomenon?

• Retention of configuration during a radical reaction
• Inversion of configuration at a stereocenter during SN2 substitution
• Racemization through carbocation formation
• Concerted pericyclic retention

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

Q38. Which solvent conditions favor SN2 stereospecific inversion over SN1 racemization?

• Polar protic solvents and weak nucleophiles
• Polar aprotic solvents and strong nucleophiles
• High dielectric constant solvents that stabilize carbocations
• Neat conditions with excess acid

Correct Answer: Polar aprotic solvents and strong nucleophiles

Q39. In the context of stereospecific reactions, what is enantioconvergent catalysis?

• A process that converts a racemic mixture into a single enantiomeric product by different pathways for each enantiomer
• A process that produces racemic product from achiral substrate
• Another term for racemization
• A pericyclic reaction that gives both enantiomers equally

Correct Answer: A process that converts a racemic mixture into a single enantiomeric product by different pathways for each enantiomer

Q40. The stereochemical outcome of ozonolysis of an alkene is best described as:

• Stereospecific preservation of alkene stereochemistry in products
• Cleavage giving carbonyl products where original double-bond stereochemistry is lost due to bond cleavage
• Concerted syn addition of oxygen preserving stereochemistry
• Only racemization at adjacent stereocenters

Correct Answer: Cleavage giving carbonyl products where original double-bond stereochemistry is lost due to bond cleavage

Q41. Which reaction is stereospecific because it proceeds through a cyclic bromonium ion intermediate?

• Hydroboration–oxidation
• Bromination of alkenes (Br2) leading to anti addition
• SN2 displacement
• Free radical polymerization

Correct Answer: Bromination of alkenes (Br2) leading to anti addition

Q42. A chemical step that requires a specific conformation to proceed and thus yields a unique stereochemical product is described as:

• Stereospecific due to conformational control
• Completely random
• Always racemic
• Enantiomerization

Correct Answer: Stereospecific due to conformational control

Q43. In the Wittig reaction, the selectivity for E or Z alkene is usually described as:

• Strictly stereospecific in all cases
• Stereoselective, depending on nature of ylide and reaction conditions
• Always gives racemic mixture of alkenes
• Independent of the reagent structure

Correct Answer: Stereoselective, depending on nature of ylide and reaction conditions

Q44. Which process illustrates stereospecific retention of configuration via double inversion in nucleophilic substitution?

• Direct front-side SN2 attack only
• Neighboring group participation forming a cyclic intermediate followed by backside attack giving net retention
• SN1 ionization followed by nucleophilic capture
• Radical halogenation

Correct Answer: Neighboring group participation forming a cyclic intermediate followed by backside attack giving net retention

Q45. Which experimental observation indicates a stereospecific syn addition occurred across a double bond?

• Trans substitution pattern in products from a cis-alkene
• Formation of products with substituents on the same face when starting from a cis-alkene
• Racemization of newly formed stereocenters
• No relationship between starting alkene stereochemistry and product stereochemistry

Correct Answer: Formation of products with substituents on the same face when starting from a cis-alkene

Q46. Which of the following best explains why stereoelectronic effects influence stereospecific reactions?

• They change the temperature dependence only
• Proper orbital alignment (e.g., anti-periplanar) is required for bond-making/bond-breaking, making the reaction conformation-dependent and stereospecific
• They increase radical formation
• They always lead to racemization

Correct Answer: Proper orbital alignment (e.g., anti-periplanar) is required for bond-making/bond-breaking, making the reaction conformation-dependent and stereospecific

Q47. In drug synthesis, why are stereospecific reactions often preferred?

• They reduce the need for chiral resolution by producing predictable stereochemical products
• They always produce racemic mixtures
• They are cheaper but give impure products
• They eliminate the need for catalysts

Correct Answer: They reduce the need for chiral resolution by producing predictable stereochemical products

Q48. Which transformation is stereospecific when converting a cis-diene to a cyclohexene in a Diels–Alder reaction?

• Formation of trans stereochemistry irrespective of starting material
• Preservation of relative stereochemistry of substituents on the diene and dienophile in the cycloadduct
• Randomization of stereochemistry due to radicals
• Complete racemization of new stereocenters

Correct Answer: Preservation of relative stereochemistry of substituents on the diene and dienophile in the cycloadduct

Q49. Which factor can convert a potentially stereospecific substitution into a non-stereospecific process?

• Using a very strong nucleophile in a polar aprotic solvent
• Reaction conditions that favor carbocation formation (e.g., polar protic solvent, weak nucleophile), leading to SN1 and loss of stereospecificity
• Performing the reaction at low temperature
• Using a chiral catalyst that selects one face

Correct Answer: Reaction conditions that favor carbocation formation (e.g., polar protic solvent, weak nucleophile), leading to SN1 and loss of stereospecificity

Q50. Which approach is most effective to achieve stereospecific inversion in a laboratory synthesis of a chiral intermediate?

• Use SN1-prone conditions to form carbocation intermediate
• Use SN2 displacement with a good nucleophile under polar aprotic solvent conditions
• Subject compound to strong acid and heat to equilibrate stereocenters
• Expose the substrate to radicals to scramble configuration

Correct Answer: Use SN2 displacement with a good nucleophile under polar aprotic solvent conditions

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