Reactions of chiral molecules MCQs With Answer

Reactions of chiral molecules MCQs With Answer

This concise introduction covers key concepts for B.Pharm students studying reactions of chiral molecules. It highlights stereochemistry, enantiomers, diastereomers, racemization, stereospecific and stereoselective reactions, asymmetric synthesis, chiral catalysts, chiral auxiliaries, and analytical resolution techniques. Emphasis is placed on reaction mechanisms (SN1, SN2, addition, elimination, oxidation, reduction) and how chirality influences reaction outcome and drug activity. These targeted, exam-oriented MCQs will strengthen understanding of reaction pathways, stereochemical outcomes, and pharmaceutical implications. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. In an SN2 reaction at a chiral center, what stereochemical outcome is typically observed?

• Retention of configuration
• Complete inversion of configuration
• Racemization producing equal enantiomers
• No change in stereochemistry

Correct Answer: Complete inversion of configuration

Q2. Which process leads to racemization of a chiral center?

• Concerted backside attack
• Formation of a planar carbocation intermediate
• Concerted proton transfer without intermediate
• Formation of a chiral transition state only

Correct Answer: Formation of a planar carbocation intermediate

Q3. A stereoselective reaction is one that:

• Produces only one constitutional isomer
• Produces one stereoisomer preferentially from an achiral substrate
• Requires chiral reagents to proceed
• Always proceeds via a chiral intermediate

Correct Answer: Produces one stereoisomer preferentially from an achiral substrate

Q4. Which term describes a reaction where the stereochemistry of the product is directly determined by the stereochemistry of the starting material?

• Stereoselective
• Stereospecific
• Regioselective
• Chemoselective

Correct Answer: Stereospecific

Q5. During catalytic asymmetric hydrogenation, enantioselectivity is typically achieved by:

• Using high pressure hydrogen only
• Using a chiral metal-ligand catalyst
• Running the reaction at very low temperature only
• Adding a racemic mixture of reagents

Correct Answer: Using a chiral metal-ligand catalyst

Q6. Sharpless asymmetric epoxidation is especially useful for converting which functional group into an epoxide with high enantioselectivity?

• Allylic alcohols
• Terminal alkynes
• Carboxylic acids
• Benzylic halides

Correct Answer: Allylic alcohols

Q7. In the CBS reduction, the chiral reagent controls the formation of which product type?

• Primary amines
• Secondary alcohols from prochiral ketones
• Carboxylic acids from aldehydes
• Epoxides from alkenes

Correct Answer: Secondary alcohols from prochiral ketones

Q8. Which of the following best describes enantiomeric excess (ee)?

• The ratio of diastereomers in a mixture
• The percentage difference between two enantiomers
• The total yield of a chiral product
• The optical rotation of a racemic mixture

Correct Answer: The percentage difference between two enantiomers

Q9. A chiral auxiliary is used in asymmetric synthesis to:

• Direct regioselectivity only
• Temporarily induce chirality and control stereochemistry
• Increase reaction temperature tolerance
• Eliminate the need for purification

Correct Answer: Temporarily induce chirality and control stereochemistry

Q10. Resolution of enantiomers by diastereomeric salt formation requires:

• An achiral resolving agent
• A chiral resolving agent to form separable diastereomeric salts
• High temperature crystallization only
• Complete racemization before separation

Correct Answer: A chiral resolving agent to form separable diastereomeric salts

Q11. Which statement about SN1 reactions at a stereogenic center is true?

• SN1 always gives complete inversion
• SN1 typically gives racemization due to planar carbocation
• SN1 proceeds with retention exclusively
• SN1 cannot occur at tertiary centers

Correct Answer: SN1 typically gives racemization due to planar carbocation

Q12. In nucleophilic addition to a prochiral carbonyl forming a new stereocenter, which factor most influences enantioselectivity?

• Solvent polarity only
• Chiral catalyst or reagent
• High concentration of nucleophile only
• Temperature above 200 °C

Correct Answer: Chiral catalyst or reagent

Q13. Which reagent pair is commonly used for enantioselective reduction of ketones (e.g., via oxazaborolidine catalyst)?

• CBS catalyst with BH3 or borane·THF
• LiAlH4 alone
• NaBH4 in methanol at room temperature
• H2 with palladium on carbon only

Correct Answer: CBS catalyst with BH3 or borane·THF

Q14. What is the stereochemical outcome when a nucleophile attacks a trigonal planar carbocation intermediate derived from a chiral center?

• Exclusive retention only
• Attack from either face leading to racemization or partial racemization
• Backside attack leading to inversion only
• No nucleophilic attack is possible

Correct Answer: Attack from either face leading to racemization or partial racemization

Q15. Enzymatic reactions on chiral substrates often display:

• No stereoselectivity due to enzyme flexibility
• High stereoselectivity, acting on one enantiomer preferentially
• Only racemization of substrate
• Conversion to achiral products exclusively

Correct Answer: High stereoselectivity, acting on one enantiomer preferentially

Q16. What happens to optical rotation when a racemic mixture is formed from a single enantiomer?

• Optical rotation doubles
• Optical rotation becomes zero
• Optical rotation becomes negative only
• Optical rotation remains unchanged

Correct Answer: Optical rotation becomes zero

Q17. During epimerization at an alpha-carbon to a carbonyl, the process often proceeds via:

• Radical intermediates exclusively
• Enolate formation followed by reprotonation
• Concerted pericyclic mechanism
• Direct SN2 substitution

Correct Answer: Enolate formation followed by reprotonation

Q18. Which analytical technique is commonly used to measure enantiomeric excess?

• Infrared spectroscopy (IR)
• Chiral HPLC or chiral GC
• Mass spectrometry without chiral derivatization
• UV-visible spectroscopy alone

Correct Answer: Chiral HPLC or chiral GC

Q19. A reaction is called dynamic kinetic resolution when:

• Racemization of substrate and enantioselective transformation occur simultaneously
• Only kinetic isotope effects are measured
• Resolution is achieved solely by crystallization
• Both enantiomers react at identical rates

Correct Answer: Racemization of substrate and enantioselective transformation occur simultaneously

Q20. Which of the following is a common chiral ligand used in asymmetric hydrogenation?

• BINAP (2,2′-bis(diphenylphosphino)-1,1′-binaphthyl)
• Triphenylphosphine only
• Pyridine as a single ligand
• Sodium chloride as a ligand

Correct Answer: BINAP (2,2′-bis(diphenylphosphino)-1,1′-binaphthyl)

Q21. Which mechanism best explains retention of configuration in certain substitution reactions at a chiral center?

• Concerted SN2 with inversion only
• Double inversion via neighboring group participation or intermediate formation
• Free radical racemization only
• No substitution occurs

Correct Answer: Double inversion via neighboring group participation or intermediate formation

Q22. Which of the following can cause racemization under basic conditions for α-chiral carbonyl compounds?

• Formation of a stabilized enolate
• Hydrogen bonding without deprotonation
• Low polarity solvents only
• Electrophilic aromatic substitution

Correct Answer: Formation of a stabilized enolate

Q23. In the context of drug action, why is control of stereochemistry important?

• Stereochemistry has no effect on pharmacokinetics
• Enantiomers can have different pharmacological activities and toxicities
• All enantiomers are metabolized identically
• Stereochemistry only affects color, not activity

Correct Answer: Enantiomers can have different pharmacological activities and toxicities

Q24. What is a common method to prepare enantiomerically enriched alcohols from prochiral ketones?

• Non-selective hydrogenation only
• Asymmetric reduction using chiral catalysts (e.g., CBS, Noyori catalysts)
• Heating ketones to 300 °C
• Oxidation followed by radical coupling

Correct Answer: Asymmetric reduction using chiral catalysts (e.g., CBS, Noyori catalysts)

Q25. Which describes a diastereoselective reaction?

• Reaction that gives equal enantiomers only
• Reaction that produces one diastereomer preferentially when multiple stereocenters are present
• Reaction that is independent of stereochemistry
• Reaction that always forms racemates

Correct Answer: Reaction that produces one diastereomer preferentially when multiple stereocenters are present

Q26. Which strategy uses a temporary covalent bond to control stereochemistry in subsequent reactions?

• Asymmetric catalysis only
• Use of a chiral auxiliary
• Radical chain transfer
• Photochemical racemization

Correct Answer: Use of a chiral auxiliary

Q27. Which of the following is TRUE about enantiomers?

• They have identical interactions with plane-polarized light but different melting points
• They are mirror images that are non-superimposable and have identical physical properties in achiral environments except optical rotation
• They always have different boiling points in achiral solvents
• They can be separated by ordinary column chromatography without modification

Correct Answer: They are mirror images that are non-superimposable and have identical physical properties in achiral environments except optical rotation

Q28. Which reagent is commonly used to convert secondary alcohols to ketones without affecting stereochemistry of remote centers?

• PCC (pyridinium chlorochromate)
• LiAlH4
• BH3·THF
• H2 with Pd/C

Correct Answer: PCC (pyridinium chlorochromate)

Q29. What is the main purpose of using chiral phase-transfer catalysts in synthesis?

• To racemize chiral centers rapidly
• To induce enantioselectivity in reactions of ionic species across phase boundaries
• To increase solvent polarity only
• To remove protecting groups selectively

Correct Answer: To induce enantioselectivity in reactions of ionic species across phase boundaries

Q30. In nucleophilic additions to carbonyls adjacent to stereocenters, what effect does a bulky substituent often have?

• It eliminates stereochemical preference
• It can direct approach of the nucleophile causing diastereoselectivity
• It always causes racemization
• It converts the carbonyl to an alkene

Correct Answer: It can direct approach of the nucleophile causing diastereoselectivity

Q31. Which technique can convert a racemic mixture into a single enantiomer without waste of material by combining racemization and selective reaction?

• Classical resolution by crystallization only
• Dynamic kinetic resolution (DKR)
• Simple distillation
• Acid–base extraction only

Correct Answer: Dynamic kinetic resolution (DKR)

Q32. A chiral center adjacent to an aromatic ring is less likely to racemize compared to one next to a carbonyl because:

• The aromatic ring stabilizes planar intermediates more
• There is less tendency to form stabilized carbanion or carbocation intermediates
• Aromatic systems always accelerate racemization
• All chiral centers racemize at the same rate

Correct Answer: There is less tendency to form stabilized carbanion or carbocation intermediates

Q33. In an enantioselective epoxidation using titanium-tartrate catalysts (Sharpless), the absolute configuration of the product is controlled by:

• The temperature only
• The choice of (+)- or (−)-diethyl tartrate
• The concentration of substrate only
• The solvent color

Correct Answer: The choice of (+)- or (−)-diethyl tartrate

Q34. Which describes kinetic resolution of enantiomers?

• Both enantiomers react at identical rates
• One enantiomer reacts faster, allowing separation based on reactivity
• It requires racemization during reaction
• It cannot yield enantiomerically enriched products

Correct Answer: One enantiomer reacts faster, allowing separation based on reactivity

Q35. Which outcome indicates a stereospecific reaction?

• Different stereochemical starting materials give the same stereochemical product
• The stereochemistry of the product depends on the stereochemistry of the reactant in a predictable way
• Product stereochemistry is unpredictable
• Reaction proceeds with complete racemization

Correct Answer: The stereochemistry of the product depends on the stereochemistry of the reactant in a predictable way

Q36. In Williamson ether synthesis at a chiral center, why is inversion observed for SN2 processes?

• Because of formation of a planar carbocation
• Because nucleophile attacks from backside displacing leaving group
• Because of radical recombination
• Because of retention via neighboring group

Correct Answer: Because nucleophile attacks from backside displacing leaving group

Q37. Which protective strategy can help prevent racemization of an α-chiral amine during peptide coupling?

• Use of carbodiimide coupling with racemization suppressors (e.g., HOBt or HOAt)
• High temperature coupling without additives
• Prolonged storage in basic solution
• Using only strong mineral acids

Correct Answer: Use of carbodiimide coupling with racemization suppressors (e.g., HOBt or HOAt)

Q38. Which statement is true about asymmetric induction by a chiral center in a substrate?

• It cannot influence newly formed stereocenters
• It can bias the formation of one diastereomer over another in nearby reactions
• It always leads to racemic products
• It eliminates the need for catalysts

Correct Answer: It can bias the formation of one diastereomer over another in nearby reactions

Q39. What is the role of a chiral Brønsted acid in enantioselective catalysis?

• To racemize the substrate rapidly
• To provide a chiral environment and protonate substrates enantioselectively
• To act only as a solvent
• To oxidize the substrate

Correct Answer: To provide a chiral environment and protonate substrates enantioselectively

Q40. In allylic substitution reactions, which factor can lead to retention rather than inversion of stereochemistry?

• Direct SN2 at sp3 center only
• Formation of a symmetric π-allyl metal intermediate allowing both faces to react
• Radical chain propagation exclusively
• Lack of any metal catalyst

Correct Answer: Formation of a symmetric π-allyl metal intermediate allowing both faces to react

Q41. Which reagent/system is known for enantioselective epoxidation of allylic alcohols with high ee?

• t-Butyl hydroperoxide with Ti(OiPr)4 and diethyl tartrate (Sharpless)
• KMnO4 in water
• Ozone followed by reductive workup only
• H2/Pd-C in ethanol

Correct Answer: t-Butyl hydroperoxide with Ti(OiPr)4 and diethyl tartrate (Sharpless)

Q42. Which process describes inversion followed by retention resulting in net retention of configuration?

• Single SN2 only
• Double inversion via neighboring group participation
• Pure radical substitution
• Concerted pericyclic rearrangement that is racemic

Correct Answer: Double inversion via neighboring group participation

Q43. In organocatalysis for enantioselective transformations, chiral secondary amines often act by:

• Forming chiral iminium or enamine intermediates that control stereochemistry
• Only as bases without forming intermediates
• Generating free radicals exclusively
• Oxidizing substrates directly

Correct Answer: Forming chiral iminium or enamine intermediates that control stereochemistry

Q44. In asymmetric dihydroxylation (e.g., Sharpless AD), what is the typical oxidant used with osmium catalysts?

• KMnO4
• Osmium tetroxide with a stoichiometric reoxidant like NMO
• LiAlH4
• Hydrogen peroxide without catalyst

Correct Answer: Osmium tetroxide with a stoichiometric reoxidant like NMO

Q45. Why are diastereomers easier to separate than enantiomers by conventional chromatography?

• They have identical physical properties
• They have different physical properties (melting point, solubility, polarity)
• They are mirror images and interact identically with achiral media
• They always interconvert quickly

Correct Answer: They have different physical properties (melting point, solubility, polarity)

Q46. Which phenomenon describes the conversion of one stereoisomer into another with inversion at a single stereocenter without bond breaking at that center?

• Pericyclic rearrangement such as epimerization via enolization
• Photochemical dimerization only
• Simple SN2 substitution at remote site
• Electrophilic aromatic substitution

Correct Answer: Pericyclic rearrangement such as epimerization via enolization

Q47. In asymmetric allylation of aldehydes to form homoallylic alcohols, enantioselectivity is often achieved by:

• Using excess aldehyde only
• Using chiral allylmetal reagents or chiral catalysts (e.g., chiral boron reagents)
• High-temperature radical conditions
• Simple acid catalysis without chiral elements

Correct Answer: Using chiral allylmetal reagents or chiral catalysts (e.g., chiral boron reagents)

Q48. What is the effect of neighboring group participation on stereochemistry in substitution at a chiral center?

• It prevents any substitution from occurring
• It can lead to retention via internal return or cyclic intermediates
• It always causes racemization
• It leads to radical chain mechanisms only

Correct Answer: It can lead to retention via internal return or cyclic intermediates

Q49. Which type of reaction is typically stereospecific for addition to alkenes (syn vs anti addition)?

• Free radical halogenation is stereospecific
• Concerted syn addition like hydrogenation is syn-stereospecific
• SN1 substitution is syn-stereospecific
• Simple acid–base proton transfer is anti-stereospecific

Correct Answer: Concerted syn addition like hydrogenation is syn-stereospecific

Q50. Which approach minimizes racemization during formation of amide bonds in peptide synthesis?

• Use of strong bases without additives
• Use of coupling reagents with racemization suppressors and low temperatures
• High-temperature neat coupling
• Leaving carboxylic acid unactivated and heating

Correct Answer: Use of coupling reagents with racemization suppressors and low temperatures

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