NaBH4 reduction MCQs With Answer

NaBH4 reduction MCQs With Answer

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NaBH4 reduction MCQs With Answer provide B. Pharm students with focused practice on sodium borohydride’s role in selective reductions. This introduction covers mechanism, chemoselectivity, solvent effects, scope and limitations of NaBH4 in reducing aldehydes, ketones, imines and related functional groups, plus comparisons with LiAlH4 and modified borohydride reagents used in pharmaceutical synthesis. Emphasis is placed on stereochemical outcomes, reaction conditions, workup, safety and practical tips for lab-scale and synthetic applications. These MCQs deepen conceptual understanding relevant to drug synthesis and quality control. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which functional group is most readily reduced by NaBH4 under mild conditions?

  • Aldehydes
  • Esters
  • Carboxylic acids
  • Nitriles

Correct Answer: Aldehydes

Q2. Compared to LiAlH4, NaBH4 is best described as:

  • More reactive and less selective
  • Less reactive and more selective
  • Equally reactive in all solvents
  • Unable to reduce carbonyls

Correct Answer: Less reactive and more selective

Q3. Which solvent is commonly used with NaBH4 for reductions of ketones and aldehydes?

  • Hexane
  • Methanol
  • Conc. sulfuric acid
  • Dimethyl sulfoxide (DMSO)

Correct Answer: Methanol

Q4. NaBH4 reacts with water to produce which gas, posing a safety consideration?

  • Nitrogen
  • Oxygen
  • Hydrogen
  • Carbon dioxide

Correct Answer: Hydrogen

Q5. Which reagent is preferred for reductive amination because of milder reactivity toward imines?

  • NaBH4
  • NaBH3CN
  • LiAlH4
  • BH3·THF

Correct Answer: NaBH3CN

Q6. The Luche reduction uses NaBH4 with which additive to favor 1,2-reduction of α,β‑unsaturated carbonyls?

  • Zinc chloride
  • Cerium(III) chloride
  • Sodium hydroxide
  • Pyridinium chlorochromate

Correct Answer: Cerium(III) chloride

Q7. NaBH4 typically does NOT reduce which functional group under standard conditions?

  • Aldehydes
  • Ketones
  • Esters
  • Imines

Correct Answer: Esters

Q8. Which property of NaBH4 explains its selectivity for carbonyls over esters?

  • Smaller hydride size
  • Lower hydride donor ability (milder reducing power)
  • High basicity
  • Strong nucleophilicity toward aromatic rings

Correct Answer: Lower hydride donor ability (milder reducing power)

Q9. How does steric hindrance affect NaBH4 reduction of ketones?

  • More hindered ketones are reduced faster
  • Steric hindrance has no effect
  • More hindered ketones are reduced more slowly
  • Steric hindrance converts reduction into oxidation

Correct Answer: More hindered ketones are reduced more slowly

Q10. In a conjugated enone, NaBH4 without additives generally gives which product?

  • 1,2‑reduction to allylic alcohol
  • 1,4‑conjugate (Michael) reduction to saturated ketone
  • No reaction
  • Epoxidation

Correct Answer: 1,2‑reduction to allylic alcohol

Q11. Which of the following increases the reactivity of NaBH4 toward less reactive carbonyls?

  • Cooling to 0 °C
  • Addition of protic solvents like methanol
  • Removing all solvent
  • Diluting with hexane

Correct Answer: Addition of protic solvents like methanol

Q12. NaBH4 is commonly used to reduce imines to which product?

  • Alcohols
  • Amines
  • Carboxylic acids
  • Ethers

Correct Answer: Amines

Q13. For protecting-group compatibility, NaBH4 is generally compatible with which protecting group?

  • TBS (tert-butyldimethylsilyl) ether
  • Boc‑protected amine in the presence of acid
  • Mitsunobu-protected alcohol
  • Nitrile protecting groups

Correct Answer: TBS (tert-butyldimethylsilyl) ether

Q14. What is the typical role of NaBH4 in pharmaceutical synthesis?

  • Oxidation of alcohols
  • Selective reduction of carbonyls to alcohols
  • Nitration of aromatic rings
  • Formation of Grignard reagents

Correct Answer: Selective reduction of carbonyls to alcohols

Q15. When NaBH4 is used in methanol, what side reaction may occur?

  • Formation of methyl borate and methoxide with H2 evolution
  • Nitration
  • Polymerization of solvent
  • Oxidation to borates without gas

Correct Answer: Formation of methyl borate and methoxide with H2 evolution

Q16. Which derivative of borohydride is specifically more selective for reductive amination of imines at acidic pH?

  • NaBH4
  • NaBH3CN
  • BH3·THF
  • LiBH4

Correct Answer: NaBH3CN

Q17. NaBH4 reduces which of the following better: cyclohexanone or benzophenone?

  • Cyclohexanone
  • Benzophenone
  • Both equally
  • Neither is reduced

Correct Answer: Cyclohexanone

Q18. Which additive can make NaBH4 capable of reducing esters under mild conditions?

  • Sodium chloride
  • Borane (BH3) or Lewis acids like TiCl4
  • Sodium bicarbonate
  • Water

Correct Answer: Borane (BH3) or Lewis acids like TiCl4

Q19. What is the stereochemical outcome commonly observed when NaBH4 reduces an acyclic ketone adjacent to a stereocenter under non-chelating conditions?

  • Chelation-controlled product
  • Felkin-Anh (non-chelation) approach product
  • Complete racemization
  • No stereochemical preference

Correct Answer: Felkin-Anh (non-chelation) approach product

Q20. In comparing NaBH4 and NaBH(OAc)3, which is preferred for reductive amination in mildly acidic media?

  • NaBH4
  • NaBH(OAc)3
  • LiAlH4
  • BH3·THF

Correct Answer: NaBH(OAc)3

Q21. Which functional group can be selectively reduced by NaBH4 in the presence of an ester?

  • Ester to alcohol
  • Aldehyde to alcohol
  • Carboxylic acid to alcohol
  • Nitrile to amine

Correct Answer: Aldehyde to alcohol

Q22. Which experimental parameter most directly controls the rate of NaBH4 reductions?

  • Concentration of substrate and NaBH4
  • Atmospheric pressure only
  • Magnetic stirring speed only
  • Color of reaction flask

Correct Answer: Concentration of substrate and NaBH4

Q23. NaBH4 is commonly supplied as a white powder. How should it be stored?

  • Open to air on bench
  • Under dry, inert atmosphere in a cool place
  • In hot humid conditions
  • Mixed with water

Correct Answer: Under dry, inert atmosphere in a cool place

Q24. When performing a NaBH4 reduction on an α‑halo ketone, what risk should be considered?

  • Nucleophilic substitution at halogen (dehalogenation)
  • Formation of azo compounds
  • Oxidation to diketone
  • No special risk

Correct Answer: Nucleophilic substitution at halogen (dehalogenation)

Q25. Which analytical technique is most useful to monitor completion of an NaBH4 reduction in the lab?

  • Infrared spectroscopy showing disappearance of C=O stretch
  • Thin-layer chromatography (TLC)
  • NMR spectroscopy showing alcohol signals
  • All of the above

Correct Answer: All of the above

Q26. In NaBH4 reductions, why is careful quenching important?

  • To accelerate reaction
  • To safely decompose excess hydride and avoid H2 evolution
  • To increase basicity of mixture
  • Quenching is unnecessary

Correct Answer: To safely decompose excess hydride and avoid H2 evolution

Q27. Which metal counterion can modify reactivity of borohydride reagents making them stronger?

  • Sodium (Na+)
  • Lithium (Li+)
  • Potassium (K+)
  • Calcium (Ca2+)

Correct Answer: Lithium (Li+)

Q28. NaBH4 reduction of a prochiral ketone can lead to which important consideration in drug synthesis?

  • Production of enantiomerically pure material without catalyst
  • Need for stereochemical control to avoid undesired stereoisomers
  • No stereochemical concerns at all
  • Always gives racemic mixture

Correct Answer: Need for stereochemical control to avoid undesired stereoisomers

Q29. Which of the following is a direct product when NaBH4 reduces benzaldehyde?

  • Benzyl alcohol
  • Toluene
  • Benzoic acid
  • Benzyl chloride

Correct Answer: Benzyl alcohol

Q30. NaBH4 will reduce α,β‑unsaturated aldehydes predominantly by which pathway under standard conditions?

  • 1,4‑conjugate addition (Michael)
  • 1,2‑direct carbonyl reduction
  • Oxidative cleavage
  • Diels-Alder reaction

Correct Answer: 1,2‑direct carbonyl reduction

Q31. In a competitive reduction of aldehyde and ketone in same molecule, which is reduced faster by NaBH4?

  • Ketone
  • Aldehyde
  • Both equally
  • Neither

Correct Answer: Aldehyde

Q32. Which of the following statements about NaBH4 and nitro groups is correct?

  • NaBH4 readily reduces nitro groups to amines
  • NaBH4 generally does not reduce nitro groups under mild conditions
  • NaBH4 converts nitro to hydroxyl groups
  • NaBH4 selectively nitrates aromatic rings

Correct Answer: NaBH4 generally does not reduce nitro groups under mild conditions

Q33. What is a common stoichiometric amount of NaBH4 used for simple carbonyl reductions?

  • 0.01 equivalents
  • 1.0–1.5 equivalents
  • 10 equivalents
  • 100 equivalents

Correct Answer: 1.0–1.5 equivalents

Q34. Which observation indicates over-reduction or decomposition when using NaBH4?

  • Rapid H2 evolution and darkening of mixture
  • Clear colorless solution with no change
  • Increase in melting point
  • Precipitation of pure product without gas

Correct Answer: Rapid H2 evolution and darkening of mixture

Q35. Which advanced transformation can be achieved by combining NaBH4 with transition-metal catalysts?

  • Hydroboration
  • Transfer hydrogenation and selective hydrogenolysis
  • Nitration
  • Direct halogenation

Correct Answer: Transfer hydrogenation and selective hydrogenolysis

Q36. Which waste product is commonly generated after aqueous workup of NaBH4 reductions?

  • Metallic sodium
  • Sodium borate salts
  • Hydrazine
  • Boric acid exclusively as gas

Correct Answer: Sodium borate salts

Q37. For stereoselective reductions near coordinating substituents, adding which metal can induce chelation control?

  • Sodium chloride
  • Titanium or aluminum Lewis acids
  • Noble gases
  • Organic dyes

Correct Answer: Titanium or aluminum Lewis acids

Q38. In a pharmaceutical lab, which precaution is most important when weighing NaBH4?

  • Weigh in presence of open flame
  • Avoid contact with moisture and wear PPE
  • Mix with oxidizers before weighing
  • Weigh directly into water

Correct Answer: Avoid contact with moisture and wear PPE

Q39. Which substrate will require activation or a stronger reducing agent than NaBH4?

  • Benzaldehyde
  • Methyl benzoate (an ester)
  • Cyclohexanone
  • A simple imine

Correct Answer: Methyl benzoate (an ester)

Q40. How does electron-withdrawing substitution on an aromatic aldehyde affect NaBH4 reduction rate?

  • Decreases rate (less reactive)
  • Increases rate (more electrophilic)
  • No effect
  • Causes oxidation instead

Correct Answer: Increases rate (more electrophilic)

Q41. Which technique helps isolate alcohol product after NaBH4 reduction?

  • Direct distillation without quench
  • Aqueous quench followed by extraction and chromatography
  • Heating with strong base
  • Evaporation of water only

Correct Answer: Aqueous quench followed by extraction and chromatography

Q42. NaBH4-mediated reduction of a benzylidene acetal will most likely:

  • Reduce acetal to diol under mild conditions
  • Cleave acetal and reduce to benzyl alcohol under forcing conditions
  • Oxidize acetal
  • Leave acetal untouched

Correct Answer: Cleave acetal and reduce to benzyl alcohol under forcing conditions

Q43. Which factor differentiates NaBH4 and NaBH(OAc)3 in reactivity?

  • NaBH(OAc)3 is more reactive in basic media
  • NaBH(OAc)3 is milder and tolerant of weak acid conditions for reductive amination
  • NaBH4 is non-nucleophilic
  • NaBH4 is only used for oxidations

Correct Answer: NaBH(OAc)3 is milder and tolerant of weak acid conditions for reductive amination

Q44. Which product results from NaBH4 reduction of cyclohexenone under standard conditions (no additives)?

  • Cyclohexanone
  • Cyclohexenol (allylic alcohol)
  • Cyclohexane
  • No reaction

Correct Answer: Cyclohexenol (allylic alcohol)

Q45. During scale-up of a NaBH4 reduction for API synthesis, what main issue must be controlled?

  • Exotherm and safe gas evolution
  • Color of product only
  • Type of glassware exclusively
  • Magnet size

Correct Answer: Exotherm and safe gas evolution

Q46. Which statement about NaBH4’s compatibility with halogenated solvents is true?

  • NaBH4 is highly reactive with chloroform
  • NaBH4 is typically used in alcohols or THF rather than reactive halogenated solvents
  • NaBH4 dissolves and reacts vigorously with hexane
  • Halogenated solvents increase selectivity for esters

Correct Answer: NaBH4 is typically used in alcohols or THF rather than reactive halogenated solvents

Q47. Which of these is an advantage of NaBH4 in medicinal chemistry?

  • Extreme reactivity with all functional groups
  • High selectivity allowing stepwise functional group manipulation
  • Causes racemization of all centers
  • Always eliminates the need for protecting groups

Correct Answer: High selectivity allowing stepwise functional group manipulation

Q48. If NaBH4 reduction gives a mixture of diastereomers, which technique can help separate them?

  • Preparative chromatography
  • Heating to decompose one diastereomer
  • Adding more NaBH4
  • Filtration through celite only

Correct Answer: Preparative chromatography

Q49. Which reagent combination is often used to reduce an acyl chloride to an alcohol selectively?

  • NaBH4 alone
  • NaBH4 in presence of methanol cautiously or NaBH4 with buffered conditions
  • KMnO4
  • Nitric acid

Correct Answer: NaBH4 in presence of methanol cautiously or NaBH4 with buffered conditions

Q50. What is the main mechanistic step in NaBH4 reduction of a carbonyl?

  • Electrophilic aromatic substitution
  • Nucleophilic hydride transfer to the carbonyl carbon
  • Radical chain propagation
  • Pericyclic rearrangement

Correct Answer: Nucleophilic hydride transfer to the carbonyl carbon

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