Synthetic Reagents: Aluminium isopropoxide and applications MCQs With Answer

Introduction: Aluminium isopropoxide is a versatile organoaluminum reagent extensively used in organic synthesis for transfer hydrogenation and oxidation reactions. This quiz collection explores its structure, mechanism, preparation, reaction scope, and practical applications relevant to pharmaceutical synthesis, including the Meerwein–Ponndorf–Verley (MPV) reduction and Oppenauer oxidation. Questions emphasize mechanistic details, substrate selectivity, stereochemical outcomes, reagent handling, solvent and temperature effects, and comparison with alternative reducing agents. Designed for M.Pharm students, the set deepens conceptual understanding and exam preparedness by combining theoretical fundamentals with real-world examples such as stereoselective ketone reductions and protection-group compatibility. Answers are provided to facilitate self-assessment and targeted revision.

Q1. What is the correct molecular formula/representation of aluminium isopropoxide?

• Al(O-iPr)3
• AlCl3·3iPrOH
• Al(OCH3)3
• Al(OH)3

Correct Answer: Al(O-iPr)3

Q2. In the Meerwein–Ponndorf–Verley (MPV) reduction catalyzed by aluminium isopropoxide, what is the role of the reagent?

• Acts as a hydride donor itself without solvent participation
• Serves as a Lewis acid catalyst enabling transfer hydrogenation from isopropanol
• Oxidizes alcohols to ketones by direct electron transfer
• Performs radical hydrogen abstraction

Correct Answer: Serves as a Lewis acid catalyst enabling transfer hydrogenation from isopropanol

Q3. Which transition state is characteristic of the MPV reduction mechanism with aluminium isopropoxide?

• A four-membered cyclic borate intermediate
• A six-membered cyclic alkoxy–aluminium transition state
• A free hydride ion transfer to carbonyl
• A radical-chain transition state

Correct Answer: A six-membered cyclic alkoxy–aluminium transition state

Q4. How is aluminium isopropoxide typically employed in MPV reactions with respect to stoichiometry?

• Used catalytically in small amounts with excess isopropanol
• Always used in large stoichiometric excess relative to substrate
• Used in equimolar amounts to the substrate for single turnover
• Used as a sub-stoichiometric radical initiator

Correct Answer: Used catalytically in small amounts with excess isopropanol

Q5. What is the Oppenauer oxidation as catalyzed by aluminium isopropoxide?

• Oxidation of secondary alcohols to ketones using acetone as hydride acceptor
• Reduction of ketones to alcohols using sodium borohydride
• Oxidative cleavage of vicinal diols to carbonyls
• Hydrogenation of alkenes to alkanes

Correct Answer: Oxidation of secondary alcohols to ketones using acetone as hydride acceptor

Q6. Which solvent choice is commonly used for an Oppenauer oxidation catalyzed by aluminium isopropoxide?

• Toluene (aprotic, non-polar) with excess acetone as hydride acceptor
• Neat water to promote hydrolysis
• Strongly protic solvents like methanol as both solvent and oxidant
• Liquid ammonia to stabilize intermediates

Correct Answer: Toluene (aprotic, non-polar) with excess acetone as hydride acceptor

Q7. Which carbonyl substrate is reduced faster in an MPV reduction using aluminium isopropoxide?

• Aromatic ketone (e.g., acetophenone)
• Aliphatic ketone (e.g., 2-butanone)
• Aldehyde (e.g., benzaldehyde)
• Carboxylic acid

Correct Answer: Aldehyde (e.g., benzaldehyde)

Q8. How does temperature typically influence the MPV reduction catalyzed by aluminium isopropoxide?

• Mild heating (reflux of isopropanol) accelerates the reaction without harsh conditions
• The reaction requires cryogenic temperatures to proceed
• Higher temperature always leads to complete decomposition of aluminium isopropoxide
• Temperature has no effect on rate or selectivity

Correct Answer: Mild heating (reflux of isopropanol) accelerates the reaction without harsh conditions

Q9. Is the hydride transfer step in the MPV reduction stereospecific, and what does this imply?

• No, it proceeds via free hydride leading to racemization
• Yes, it is stereospecific via a concerted six-membered transition state, giving predictable stereochemical outcome
• No, it involves radical intermediates causing random stereochemistry
• Yes, but only for aromatic substrates due to resonance stabilization

Correct Answer: Yes, it is stereospecific via a concerted six-membered transition state, giving predictable stereochemical outcome

Q10. Which of the following is a common laboratory method to prepare aluminium isopropoxide?

• Reaction of aluminium metal with isopropanol under reflux to give Al(O-iPr)3
• Direct oxidation of isopropanol with KMnO4
• Grignard reaction of isopropylmagnesium chloride with Al2O3
• Electrochemical reduction of aluminium chloride in water

Correct Answer: Reaction of aluminium metal with isopropanol under reflux to give Al(O-iPr)3

Q11. What is the primary safety/handling concern for aluminium isopropoxide in the laboratory?

• It is strongly oxidizing and can ignite organic solvents on contact
• It is moisture sensitive and hydrolyzes to aluminium hydroxide and isopropanol
• It is an inert solid with no special handling requirements
• It spontaneously polymerizes at room temperature

Correct Answer: It is moisture sensitive and hydrolyzes to aluminium hydroxide and isopropanol

Q12. Which functional group is generally unaffected (not reduced) by MPV reduction conditions using aluminium isopropoxide?

• Aldehyde
• Ketone
• Alkene
• Ester

Correct Answer: Ester

Q13. Which of the following pharmaceutical synthesis transformations commonly employs aluminium isopropoxide catalysis?

• Stereoselective Oppenauer oxidation of secondary alcohols in steroid modification
• Radical bromination of side chains in peptides
• Hydrogenolysis of benzyl protecting groups with palladium
• Nitration of aromatic rings in API intermediates

Correct Answer: Stereoselective Oppenauer oxidation of secondary alcohols in steroid modification

Q14. Compared to sodium borohydride (NaBH4), aluminium isopropoxide reductions are generally:

• Stronger and less selective, reducing many functional groups
• Milder and more chemoselective, allowing reductions tolerant of some sensitive groups
• Radical-based and require peroxides to initiate
• Only effective in aqueous media

Correct Answer: Milder and more chemoselective, allowing reductions tolerant of some sensitive groups

Q15. In an MPV reduction using aluminium isopropoxide, what is the role of isopropanol?

• Solvent only, it does not participate in the reaction
• Hydride donor (gets oxidized to acetone) and often the solvent
• Oxidant that converts carbonyl to carboxylic acid
• Neutral ligand that deactivates the catalyst

Correct Answer: Hydride donor (gets oxidized to acetone) and often the solvent

Q16. Which substrate type is typically most reactive toward MPV reduction catalyzed by aluminium isopropoxide?

• Conjugated enone (α,β-unsaturated ketone)
• Aliphatic aldehyde
• Carboxylic acid
• Nitroalkane

Correct Answer: Aliphatic aldehyde

Q17. Is the MPV reduction reversible, and what practical implication does this have?

• No, it is irreversible so product isolation is straightforward
• Yes, it is reversible and equilibrium position depends on hydride donor/acceptor strengths
• No, it proceeds through a radical chain and cannot be reversed
• Yes, but only at cryogenic temperatures

Correct Answer: Yes, it is reversible and equilibrium position depends on hydride donor/acceptor strengths

Q18. How do bulky substituents near the carbonyl affect the rate of MPV reduction with aluminium isopropoxide?

• They accelerate the reaction by increasing Lewis acidity
• They slow the reaction due to steric hindrance in the cyclic transition state
• They convert the mechanism to a radical pathway
• They have no effect on rate or selectivity

Correct Answer: They slow the reaction due to steric hindrance in the cyclic transition state

Q19. Can enantioselective MPV reductions be achieved using modified aluminium alkoxides?

• No, aluminium-based catalysts cannot induce enantioselectivity
• Yes, chiral aluminium alkoxides (e.g., BINOL-derived) can provide enantioselective reductions
• Only enzymatic catalysts can induce enantioselectivity in transfer hydrogenations
• Yes, but only when performed under high pressure hydrogen gas

Correct Answer: Yes, chiral aluminium alkoxides (e.g., BINOL-derived) can provide enantioselective reductions

Q20. When isopropanol is used as the hydride donor in an MPV reduction catalyzed by aluminium isopropoxide, what is the major organic by-product formed?

• Propanol (n-propanol)
• Acetone
• Acetic acid
• Isopropyl chloride

Correct Answer: Acetone

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