LiAlH4 reduction MCQs With Answer

LiAlH4 reduction MCQs With Answer is an essential topic for B. Pharm students studying organic and medicinal chemistry. This introduction covers lithium aluminum hydride (LiAlH4) as a powerful reducing agent, its chemoselectivity, common solvent and workup requirements, and typical reductions of carbonyls, esters, amides, nitriles and epoxides. Focused keywords include LiAlH4 reduction MCQs, lithium aluminum hydride, reducing agent, B.Pharm organic chemistry, functional group reduction, mechanism and laboratory safety. Clear understanding of mechanism, reagent handling, and functional-group compatibility helps with synthesis and drug impurity control. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which functional group is readily reduced to a primary alcohol by LiAlH4?

• Aldehydes
• Alkenes
• Nitriles (unchanged)
• Ethers

Correct Answer: Aldehydes

Q2. How many hydride (H-) equivalents are formally available per mole of LiAlH4?

• 1
• 2
• 3
• 4

Correct Answer: 4

Q3. Which solvent is most commonly used with LiAlH4 to avoid decomposition?

• Protic solvents like ethanol
• Water
• Dry ether or anhydrous THF
• Concentrated acid

Correct Answer: Dry ether or anhydrous THF

Q4. Compared to NaBH4, LiAlH4 is best described as:

• Less reactive and more selective
• More reactive and capable of reducing esters and carboxylic acids
• Only useful for reduction of nitro groups
• Inert in aprotic solvents

Correct Answer: More reactive and capable of reducing esters and carboxylic acids

Q5. What is the immediate nucleophile that attacks the carbonyl in LiAlH4 reductions?

• Li+ ion
• AlH4- hydride
• AlCl3
• Proton (H+)

Correct Answer: AlH4- hydride

Q6. Which functional group is typically reduced by LiAlH4 to form a primary amine?

• Amide
• Ester
• Aldehyde
• Ether

Correct Answer: Amide

Q7. How should LiAlH4 reactions be quenched in the laboratory to avoid violent hydrogen evolution?

• Rapidly add concentrated acid at room temperature
• Add water quickly in large excess
• Slow, controlled addition of a protic quench (e.g., water or alcohol) at low temperature
• Mix directly with strong oxidizing agents

Correct Answer: Slow, controlled addition of a protic quench (e.g., water or alcohol) at low temperature

Q8. Which product results when LiAlH4 reduces an ester?

• Aldehyde
• Carboxylic acid
• Primary alcohol(s)
• Ketone

Correct Answer: Primary alcohol(s)

Q9. Upon aqueous workup of a LiAlH4 reaction, what aluminium-containing byproduct is commonly formed?

• AlCl3
• Al2O3
• Al(OH)3 (aluminum hydroxide)
• AlH3

Correct Answer: Al(OH)3 (aluminum hydroxide)

Q10. Which statement best describes LiAlH4’s action on nitriles?

• LiAlH4 oxidizes nitriles to amides
• LiAlH4 reduces nitriles to primary amines
• LiAlH4 leaves nitriles unchanged
• LiAlH4 converts nitriles to carboxylic acids

Correct Answer: LiAlH4 reduces nitriles to primary amines

Q11. What is a common laboratory hazard when handling LiAlH4?

• It is non-reactive and poses no hazard
• It violently reacts with water producing hydrogen gas
• It combusts only in the presence of ozone
• It sublimates at room temperature

Correct Answer: It violently reacts with water producing hydrogen gas

Q12. Which reagent is preferred for selective reduction of an ester to an aldehyde instead of complete reduction to an alcohol?

• LiAlH4
• DIBAL-H at low temperature
• NaBH4 in methanol
• H2 with Pd/C at ambient pressure

Correct Answer: DIBAL-H at low temperature

Q13. In the mechanism of LiAlH4 reduction of a ketone, what intermediate forms immediately after hydride attack?

• Carbocation
• Alkoxide coordinated to aluminum
• Free radical
• Carboxylate

Correct Answer: Alkoxide coordinated to aluminum

Q14. Which functional group is resistant to reduction by LiAlH4 under normal conditions?

• Ester
• Amide
• Alkene (unconjugated)
• Aldehyde

Correct Answer: Alkene (unconjugated)

Q15. What role does Li+ play in LiAlH4 reductions?

• Provides the hydride nucleophile directly
• Coordinates to oxygen-containing intermediates, stabilizing transition states
• Acts as an oxidizing agent
• Serves as a catalyst for hydrogenation

Correct Answer: Coordinates to oxygen-containing intermediates, stabilizing transition states

Q16. LiAlH4 opens epoxides. Which carbon does hydride attack in an unsymmetrical epoxide under typical conditions?

• The more substituted carbon (via carbocation)
• The less substituted carbon (SN2-like attack)
• Neither carbon; epoxide is inert
• Both carbons equally (radical mechanism)

Correct Answer: The less substituted carbon (SN2-like attack)

Q17. Which of the following is true about LiAlH4 reduction of carboxylic acids?

• Carboxylic acids are oxidized to esters
• LiAlH4 reduces carboxylic acids to primary alcohols
• LiAlH4 converts carboxylic acids to ketones
• Carboxylic acids are inert to LiAlH4

Correct Answer: LiAlH4 reduces carboxylic acids to primary alcohols

Q18. When performing a LiAlH4 reduction, why must the reaction mixture be kept anhydrous?

• Water coordinates and increases reaction rate
• Water reacts with LiAlH4 producing hydrogen and destroying reagent
• Water is required for product formation
• Water prevents formation of aluminum hydroxide

Correct Answer: Water reacts with LiAlH4 producing hydrogen and destroying reagent

Q19. Which reduction transformation is LiAlH4 most commonly used for in pharmaceutical synthesis?

• Conversion of primary alcohols to aldehydes
• Reduction of amides to amines
• Selective hydrogenation of aromatic rings
• Ozonolysis of alkenes

Correct Answer: Reduction of amides to amines

Q20. What is the expected product when LiAlH4 reduces an acyl chloride (acid chloride)?

• Carboxylic acid
• Primary alcohol via rapid reduction
• Retention of acyl chloride
• Alkene

Correct Answer: Primary alcohol via rapid reduction

Q21. Which reagent is safer and milder than LiAlH4 for reducing ketones in aqueous methanol?

• NaBH4
• DIBAL-H
• LiAlH(O-t-Bu)3
• H2 with PtO2

Correct Answer: NaBH4

Q22. During LiAlH4 reductions, what type of intermediate coordination occurs between substrate and aluminum?

• Aluminum forms π-complex with aromatic rings
• Aluminum coordinates to oxygen or nitrogen lone pairs forming Lewis acid-base complexes
• Aluminum coordinates to halide ions only
• No coordination occurs; the reaction is purely ionic

Correct Answer: Aluminum coordinates to oxygen or nitrogen lone pairs forming Lewis acid-base complexes

Q23. Which of the following is a typical precaution when scaling up LiAlH4 reductions in industry?

• Quench large batches quickly with excess protic solvent
• Ensure strict anhydrous conditions and controlled quench to manage hydrogen evolution
• Replace all ethers with water to improve safety
• Use open-air stirring to dissipate heat

Correct Answer: Ensure strict anhydrous conditions and controlled quench to manage hydrogen evolution

Q24. LiAlH4 reduces imines to:

• Alcohols
• Amines
• Nitriles
• Carboxylic acids

Correct Answer: Amines

Q25. Which reagent will typically reduce a nitro group to an amine under non catalytic conditions?

• LiAlH4
• NaBH4 in water
• O2
• HNO3

Correct Answer: LiAlH4

Q26. How does LiAlH4 typically affect conjugated α,β-unsaturated carbonyl compounds?

• Only reduces the double bond preferentially
• Hydride attacks the carbonyl (1,2-addition) more readily than 1,4-addition under many conditions
• Leaves conjugated systems untouched
• Always gives fully hydrogenated products

Correct Answer: Hydride attacks the carbonyl (1,2-addition) more readily than 1,4-addition under many conditions

Q27. Which workup sequence is commonly recommended after completion of a LiAlH4 reduction?

• Neutralize with strong base immediately
• Careful stepwise addition of water, then dilute acid to destroy aluminum complexes
• Evaporate solvents without quenching
• Add peroxide to decompose aluminum residues

Correct Answer: Careful stepwise addition of water, then dilute acid to destroy aluminum complexes

Q28. Which of these substrates is most likely to be reduced by LiAlH4 to an alcohol?

• Tertiary alcohol
• Carboxylic acid
• Alkane
• Phenyl ether

Correct Answer: Carboxylic acid

Q29. What is the primary driving force for hydride transfer from Al-H to a carbonyl?

• Formation of a stronger C-H bond and stabilization by aluminum coordination to oxygen
• Release of oxygen gas
• Formation of radical species
• Oxidation of aluminum to elemental state

Correct Answer: Formation of a stronger C-H bond and stabilization by aluminum coordination to oxygen

Q30. Which type of alcohol is formed when LiAlH4 reduces a ketone?

• Primary alcohol
• Secondary alcohol
• Tertiary alcohol
• No alcohol is formed

Correct Answer: Secondary alcohol

Q31. For chemoselective reductions where an ester and an aldehyde are present, which reagent generally reduces only the aldehyde?

• LiAlH4
• NaBH4
• DIBAL-H at high temperature
• LiAlH(O-t-Bu)3

Correct Answer: NaBH4

Q32. In mechanistic terms, LiAlH4 reductions are best described as which type of reaction?

• Nucleophilic hydride transfer to electrophilic carbon
• Electrophilic addition of H+
• Radical chain oxidation
• Pericyclic rearrangement

Correct Answer: Nucleophilic hydride transfer to electrophilic carbon

Q33. Which byproduct can complicate isolation of products from LiAlH4 reductions and often requires filtration?

• Alkyl chlorides
• Insoluble aluminum hydroxide/oxide precipitates
• Excess LiAlH4 crystals
• Ammonium salts

Correct Answer: Insoluble aluminum hydroxide/oxide precipitates

Q34. Why is LiAlH4 ineffective for reducing esters to aldehydes under typical conditions?

• Because LiAlH4 is too selective
• LiAlH4 is too powerful and reduces esters all the way to alcohols
• LiAlH4 lacks hydride for ester reduction
• Esters are inert to all hydride reagents

Correct Answer: LiAlH4 is too powerful and reduces esters all the way to alcohols

Q35. Which structural change might occur during LiAlH4 reduction of a β-hydroxy ketone?

• Retro-aldol cleavage in presence of Lewis acidity
• Oxidation to diketone
• Formation of epoxide
• Complete aromatization

Correct Answer: Retro-aldol cleavage in presence of Lewis acidity

Q36. In a multi-functional molecule containing both a nitro group and an ester, what is a likely outcome with LiAlH4?

• Only the ester is reduced
• Both nitro and ester may be reduced to amine and alcohol respectively
• Nitro group is oxidized further
• No reaction occurs

Correct Answer: Both nitro and ester may be reduced to amine and alcohol respectively

Q37. What is a common analytical sign that a LiAlH4 reduction has consumed the reagent completely?

• Persistent evolution of hydrogen gas when small amounts of moisture are added
• No evolution of gas when small amounts of moisture are added
• Solution turns bright green
• Immediate precipitation of metallic aluminum

Correct Answer: No evolution of gas when small amounts of moisture are added

Q38. Which derivative of LiAlH4 is used for more selective reductions (e.g., to control reactivity)?

• LiAlH4·H2O
• LiAlH(O-t-Bu)3
• NaBH4
• AlCl3

Correct Answer: LiAlH(O-t-Bu)3

Q39. During pharmaceutical impurity control, LiAlH4 reduction can be used to:

• Oxidize impurities to higher oxidation state
• Convert reducible impurities to more easily characterized reduced forms
• Quantify water content directly
• Create new impurities intentionally

Correct Answer: Convert reducible impurities to more easily characterized reduced forms

Q40. What happens when LiAlH4 encounters moisture during storage or handling?

• It forms stable hydrates harmlessly
• It reacts, slowly releasing hydrogen and decomposing
• It becomes a liquid at room temperature
• It turns into NaBH4

Correct Answer: It reacts, slowly releasing hydrogen and decomposing

Q41. Which technique is commonly used to remove fine aluminum hydroxide solids after quench?

• Distillation of the crude mixture without filtration
• Filtration through a Celite or Buchner funnel after proper quench
• Direct injection into GC
• Adding concentrated base to dissolve solids

Correct Answer: Filtration through a Celite or Buchner funnel after proper quench

Q42. Which substrate is least likely to be directly reduced by LiAlH4?

• Carboxylic acid
• Ester
• Alkyl chloride (simple)
• Aldehyde

Correct Answer: Alkyl chloride (simple)

Q43. In the context of stereochemistry, LiAlH4 reductions of prochiral ketones typically give:

• A racemic mixture if no chiral induction is present
• Enantiomerically pure alcohol always
• Only one diastereomer regardless of substrate
• No alcohol formation

Correct Answer: A racemic mixture if no chiral induction is present

Q44. What is the expected outcome when LiAlH4 reduces a lactone?

• Conversion to corresponding diol (ring opening to alcohols)
• Retention of the lactone ring unchanged
• Formation of an aromatic compound
• Oxidation to diketone

Correct Answer: Conversion to corresponding diol (ring opening to alcohols)

Q45. Which of the following best explains why LiAlH4 is used in dry ether rather than methanol?

• Methanol promotes faster reduction
• Methanol reacts with LiAlH4, consuming reagent and producing hydrogen
• Ether is a stronger acid to protonate products
• Methanol is too non-polar

Correct Answer: Methanol reacts with LiAlH4, consuming reagent and producing hydrogen

Q46. For converting an amide to an amine, which reagent is typically effective?

• LiAlH4
• Na2CO3
• KMnO4
• Ozone

Correct Answer: LiAlH4

Q47. Which analytical method is commonly used to monitor completion of a LiAlH4 reduction?

• Thin-layer chromatography (TLC) or HPLC to follow disappearance of starting material
• X-ray crystallography during reaction
• Mass spectrometry of the reaction headspace only
• pH meter reading only

Correct Answer: Thin-layer chromatography (TLC) or HPLC to follow disappearance of starting material

Q48. Which is true about using LiAlH4 in presence of aldehyde and ester on the same molecule?

• LiAlH4 will selectively reduce only the aldehyde and leave ester unchanged
• LiAlH4 will likely reduce both the aldehyde and the ester
• LiAlH4 will preferentially oxidize both groups
• LiAlH4 will polymerize the substrate

Correct Answer: LiAlH4 will likely reduce both the aldehyde and the ester

Q49. Which safety practice is essential when adding LiAlH4 to a reaction flask?

• Add LiAlH4 quickly in large portions to speed the reaction
• Weigh and add LiAlH4 under an inert atmosphere and dry conditions
• Always add water to the flask before LiAlH4 to stabilize it
• Work in an open bench with no protective equipment

Correct Answer: Weigh and add LiAlH4 under an inert atmosphere and dry conditions

Q50. When designing a synthesis in medicinal chemistry, why might a chemist choose LiAlH4 reduction?

• For mild, selective hydrogenation of aromatic rings
• To perform strong, broad reductions of carbonyls, carboxylic acids, esters and amides to access alcohols and amines
• To introduce halogen atoms
• To oxidize alcohols to ketones

Correct Answer: To perform strong, broad reductions of carbonyls, carboxylic acids, esters and amides to access alcohols and amines

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.

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