Reduction processes: catalytic hydrogenation and metal hydride reductions MCQs With Answer

Reduction processes are central to pharmaceutical process chemistry, especially catalytic hydrogenation and metal hydride reductions used in synthesis, protection/deprotection and scale-up. This quiz collection covers mechanistic principles, reagent selection, chemoselectivity, stereoselectivity, catalyst poisoning, safety and practical workup considerations relevant to M.Pharm students. Questions are designed to test both conceptual understanding and applied decision-making for choosing catalysts or hydride reagents for specific functional groups and transformations. Review these MCQs to strengthen your ability to plan reductions, predict products, and anticipate operational hazards encountered in laboratory and industrial settings.

Q1. Which homogeneous catalyst is classically used for selective hydrogenation of alkenes under mild, soluble conditions?

• Pd/C (palladium on carbon)
• Wilkinson’s catalyst (RhCl(PPh3)3)
• Raney nickel
• PtO2 (Adams catalyst)

Correct Answer: Wilkinson’s catalyst (RhCl(PPh3)3)

Q2. Which catalyst is specifically “poisoned” to give syn-hydrogenation of alkynes to cis-alkenes without further reduction to alkanes?

• Lindlar’s catalyst (Pd/CaCO3, Pb-poisoned)
• Raney nickel
• Wilkinson’s catalyst
• PtO2

Correct Answer: Lindlar’s catalyst (Pd/CaCO3, Pb-poisoned)

Q3. Which sequence of elementary steps best describes the mechanism of homogeneous metal-catalyzed hydrogenation?

• Oxidative addition → migratory insertion → reductive elimination
• Ligand dissociation → homolytic cleavage → radical recombination
• Surface adsorption → H-atom transfer → desorption
• Nucleophilic attack → protonation → elimination

Correct Answer: Oxidative addition → migratory insertion → reductive elimination

Q4. Which hydride reagent is commonly used to reduce esters to primary alcohols under standard anhydrous conditions?

• LiAlH4 (lithium aluminum hydride)
• NaBH4 (sodium borohydride)
• DIBAL-H at −78 °C
• NaBH3CN (sodium cyanoborohydride)

Correct Answer: LiAlH4 (lithium aluminum hydride)

Q5. Which reducing agent selectively reduces aldehydes and ketones in the presence of esters at ambient temperature?

• NaBH4 (sodium borohydride)
• LiAlH4 (lithium aluminum hydride)
• DIBAL-H at −78 °C
• BH3·THF (borane–THF)

Correct Answer: NaBH4 (sodium borohydride)

Q6. Which reagent reliably reduces nitriles to primary amines in a straightforward two-electron hydride transfer?

• LiAlH4 (lithium aluminum hydride)
• NaBH4 (sodium borohydride)
• DIBAL-H
• NaBH3CN

Correct Answer: LiAlH4 (lithium aluminum hydride)

Q7. Which hydride reagent is typically used to convert esters to aldehydes under controlled low-temperature conditions?

• DIBAL-H (diisobutylaluminum hydride) at −78 °C
• LiAlH4 (lithium aluminum hydride)
• NaBH4 (sodium borohydride)
• BH3·THF (borane–THF)

Correct Answer: DIBAL-H (diisobutylaluminum hydride) at −78 °C

Q8. Which of the following statements about NaBH4 is FALSE?

• NaBH4 reduces aldehydes and ketones readily in protic solvents.
• NaBH4 reacts violently and uncontrollably with trace water at room temperature.
• NaBH4 is significantly less reactive toward esters than LiAlH4.
• NaBH4 can be used in methanol or ethanol as the solvent for many reductions.

Correct Answer: NaBH4 reacts violently and uncontrollably with trace water at room temperature.

Q9. Which catalytic system is emblematic for asymmetric hydrogenation of prochiral ketones giving high enantioselectivity?

• Noyori’s Ru–BINAP (Ru catalysts with chiral diphosphine ligands)
• Wilkinson’s catalyst (RhCl(PPh3)3)
• Pd/C heterogeneous catalyst
• Raney nickel

Correct Answer: Noyori’s Ru–BINAP (Ru catalysts with chiral diphosphine ligands)

Q10. Which method is most commonly employed to remove a benzyl (Bn) protecting group by hydrogenolysis?

• Hydrogenation with H2 over Pd/C
• Reduction with LiAlH4
• Transfer hydrogenation with NaBH4
• Oxidation with PCC

Correct Answer: Hydrogenation with H2 over Pd/C

Q11. Which reagent is particularly useful to reduce carboxylic acids selectively to alcohols under milder, chemoselective conditions?

• BH3·THF (borane–tetrahydrofuran)
• NaBH4 (sodium borohydride)
• DIBAL-H at −78 °C
• Wilkinson’s catalyst

Correct Answer: BH3·THF (borane–tetrahydrofuran)

Q12. Which impurity class most strongly poisons palladium catalysts used in hydrogenation, decreasing activity?

• Sulfur-containing compounds (thiols, sulfides)
• Trace water
• Alkali metal salts
• Trace oxygen in small amounts

Correct Answer: Sulfur-containing compounds (thiols, sulfides)

Q13. Which hydride reagent is commonly used to reduce amides to the corresponding amines?

• LiAlH4 (lithium aluminum hydride)
• NaBH4 (sodium borohydride)
• BH3·THF (borane–THF)
• DIBAL-H

Correct Answer: LiAlH4 (lithium aluminum hydride)

Q14. Which reagent is frequently employed for reductive amination under mildly acidic conditions to form secondary amines with good chemoselectivity?

• NaBH3CN (sodium cyanoborohydride)
• LiAlH4 (lithium aluminum hydride)
• NaBH4 (sodium borohydride)
• BH3·THF (borane–THF)

Correct Answer: NaBH3CN (sodium cyanoborohydride)

Q15. Which organoaluminum hydride reagent is notably pyrophoric (requires strict inert-atmosphere handling) and commonly supplied as a hydrocarbon solution?

• DIBAL-H (diisobutylaluminum hydride)
• NaBH4 (sodium borohydride)
• LiAlH4 (lithium aluminum hydride)
• BH3·THF (borane–THF)

Correct Answer: DIBAL-H (diisobutylaluminum hydride)

Q16. Hydrogenation of alkynes with Lindlar catalyst yields which stereochemical outcome for the resulting alkene?

• Cis-alkene (Z configuration)
• Trans-alkene (E configuration)
• Mixture of cis and trans alkenes
• Direct formation of alkane without isolable alkene

Correct Answer: Cis-alkene (Z configuration)

Q17. Which of the following is a common hydrogen donor for transfer hydrogenation in the laboratory (no gaseous H2 required)?

• Isopropanol (secondary alcohol)
• H2 gas cylinder
• LiAlH4
• NaBH4

Correct Answer: Isopropanol (secondary alcohol)

Q18. In heterogeneous catalytic hydrogenation the initial activation of molecular hydrogen most commonly involves which step on the metal surface?

• Dissociative adsorption of H2 to form surface metal hydrides
• Oxidative addition into a homogeneous metal center
• Nucleophilic attack by hydride ion from solution
• Single-electron transfer leading to radicals

Correct Answer: Dissociative adsorption of H2 to form surface metal hydrides

Q19. Which reagent/system is classically used to effect conjugate (1,4-) reduction of α,β-unsaturated carbonyl compounds rather than direct 1,2-carbonyl reduction?

• Copper(I) hydride reagents (e.g., Stryker’s reagent or CuH systems)
• NaBH4 (sodium borohydride) alone
• DIBAL-H at −78 °C
• BH3·THF (borane–THF)

Correct Answer: Copper(I) hydride reagents (e.g., Stryker’s reagent or CuH systems)

Q20. What is the safest commonly recommended initial quench sequence after completing a LiAlH4 reduction on scale?

• Cool reaction, add a nonprotic quench such as ethyl acetate or tert-butanol slowly, then add water/dropwise acid
• Add excess water quickly to neutralize LiAlH4 at once
• Evaporate solvent to dryness and then expose solid to air
• Add concentrated HCl immediately to protonate all species

Correct Answer: Cool reaction, add a nonprotic quench such as ethyl acetate or tert-butanol slowly, then add water/dropwise acid

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