Crossed Cannizzaro reaction MCQs With Answer

Crossed Cannizzaro reaction MCQs With Answer

The Crossed Cannizzaro reaction is a base-induced disproportionation of two different non-enolizable aldehydes, where one aldehyde is reduced to an alcohol and the other oxidized to a carboxylate. This topic is essential for B.Pharm students studying reaction mechanisms, chemoselectivity, hydride transfer, and synthetic applications. Key concepts include choice of base, substrate scope, stoichiometry, reaction kinetics, and factors affecting product distribution. Understanding crossed versus classical Cannizzaro, mixed aldehyde selectivity, and real-world pharmaceutical syntheses improves problem-solving and exam readiness. These MCQs emphasize mechanism, reagent selection, and application in drug-related synthesis. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the defining feature of a crossed Cannizzaro reaction?

• Disproportionation of two identical aldehydes
• Disproportionation of two different non-enolizable aldehydes
• Oxidation of an aldehyde to a ketone
• Reduction of an aldehyde to an alkane

Correct Answer: Disproportionation of two different non-enolizable aldehydes

Q2. Which condition is essential for the classical Cannizzaro and crossed Cannizzaro reactions?

• Strong acid catalyst
• Absence of base
• No alpha-hydrogen in at least one aldehyde
• High temperature and pressure

Correct Answer: No alpha-hydrogen in at least one aldehyde

Q3. In a crossed Cannizzaro reaction, what role does the strong base play?

• Protonates the aldehyde carbonyl
• Generates alkoxide and initiates hydride transfer
• Acts as a nucleophile to form an acetal
• Oxidizes the alcohol product

Correct Answer: Generates alkoxide and initiates hydride transfer

Q4. Which aldehyde pair is most suitable for a crossed Cannizzaro reaction?

• Both aldehydes with alpha-hydrogens
• Both aromatic aldehydes without alpha-hydrogens
• One enolizable and one non-enolizable aldehyde
• Both aldehydes are ketones

Correct Answer: Both aromatic aldehydes without alpha-hydrogens

Q5. What is the typical pair of products from a crossed Cannizzaro reaction?

• Two ketones
• An alcohol and a carboxylate (or carboxylic acid after workup)
• A mixture of esters
• Two alkenes

Correct Answer: An alcohol and a carboxylate (or carboxylic acid after workup)

Q6. Which aldehyde is preferentially reduced in a crossed Cannizzaro when one is more electrophilic?

• The less electrophilic aldehyde
• The more electrophilic aldehyde
• Neither, reduction is random
• Both are reduced equally

Correct Answer: The more electrophilic aldehyde

Q7. What determines the hydride donor in a crossed Cannizzaro reaction?

• The aldehyde with stronger electron-withdrawing groups donates hydride
• The aldehyde that forms a more stable alkoxide tends to donate hydride
• Hydride is donated by solvent
• Hydride is donated by base

Correct Answer: The aldehyde that forms a more stable alkoxide tends to donate hydride

Q8. Which base is commonly used for Cannizzaro reactions in labs?

• Pyridinium chloride
• Sodium hydroxide (NaOH)
• Hydrochloric acid (HCl)
• Silver nitrate (AgNO3)

Correct Answer: Sodium hydroxide (NaOH)

Q9. Why are enolizable aldehydes usually avoided in Cannizzaro reactions?

• They readily form enamines
• They undergo aldol condensation under basic conditions
• They cannot be deprotonated
• They polymerize in acid

Correct Answer: They undergo aldol condensation under basic conditions

Q10. In a crossed Cannizzaro, what happens if stoichiometry is 1:1 of two aldehydes?

• Only one product type forms
• Mixture of disproportionation products with low selectivity
• No reaction occurs
• Complete conversion to esters

Correct Answer: Mixture of disproportionation products with low selectivity

Q11. How can chemoselectivity be improved in a crossed Cannizzaro?

• Using catalytic amounts of weak acid
• Using large excess of one aldehyde
• Removing the base
• Heating to very high temperatures

Correct Answer: Using large excess of one aldehyde

Q12. Which of the following aldehydes would be most likely to undergo Cannizzaro reaction?

• Propionaldehyde (has alpha hydrogens)
• Benzaldehyde (no alpha hydrogens)
• Acetaldehyde (enolizable)
• 2-Butanone (a ketone)

Correct Answer: Benzaldehyde (no alpha hydrogens)

Q13. What is the mechanistic step where hydride is transferred in Cannizzaro?

• Enolate formation
• Intramolecular proton transfer
• Hydride transfer from an alkoxide to the carbonyl carbon of another aldehyde
• Radical electron transfer

Correct Answer: Hydride transfer from an alkoxide to the carbonyl carbon of another aldehyde

Q14. In crossed Cannizzaro, which product originates from hydride acceptor aldehyde?

• Alcohol
• Carboxylate
• Alkene
• Ketone

Correct Answer: Alcohol

Q15. What is a common practical application of crossed Cannizzaro in pharmaceutical chemistry?

• Synthesis of tertiary amines
• Preparation of benzylic alcohols and carboxylic acids from mixed aldehydes
• Formation of peptide bonds
• Hydrogenation of aromatics

Correct Answer: Preparation of benzylic alcohols and carboxylic acids from mixed aldehydes

Q16. Which factor favors oxidation of one aldehyde over the other in crossed Cannizzaro?

• Lower oxidation potential of the aldehyde
• Higher steric hindrance at the carbonyl
• The aldehyde that forms a less stable alkoxide is oxidized
• Relative kinetics of hydride transfer and alkoxide formation

Correct Answer: Relative kinetics of hydride transfer and alkoxide formation

Q17. Which experimental change can suppress the classical Cannizzaro and promote crossed outcome?

• Using equimolar identical aldehydes
• Using a large excess of one distinct aldehyde
• Performing reaction under inert atmosphere
• Adding catalytic acid

Correct Answer: Using a large excess of one distinct aldehyde

Q18. What happens to the carboxylate formed in Cannizzaro upon acidic workup?

• It converts to an ester
• It protonates to form the carboxylic acid
• It reduces to an aldehyde
• It decomposes to CO2

Correct Answer: It protonates to form the carboxylic acid

Q19. Is isotopic labeling (e.g., D or 18O) useful in studying Cannizzaro mechanism?

• No, it gives no useful information
• Yes, it can trace hydride transfer and oxygen sources
• Only 14C is useful
• It prevents the reaction

Correct Answer: Yes, it can trace hydride transfer and oxygen sources

Q20. Which kinetic order is typically associated with the elementary hydride transfer in Cannizzaro?

• Zero order in aldehyde
• First order in nucleophilic alkoxide and first order in acceptor aldehyde (second order overall)
• Third order overall
• Negative order

Correct Answer: First order in nucleophilic alkoxide and first order in acceptor aldehyde (second order overall)

Q21. Why is benzaldehyde often used as a standard substrate in Cannizzaro studies?

• It is enolizable
• It has no alpha-hydrogens and predictable reactivity
• It slowly oxidizes in air
• It forms stable ketones

Correct Answer: It has no alpha-hydrogens and predictable reactivity

Q22. In a competitive crossed Cannizzaro, which aldehyde is more likely to act as hydride donor?

• The one forming a more stabilized alkoxide anion
• The one with highest steric bulk
• The one present in trace amounts
• The one insoluble in solvent

Correct Answer: The one forming a more stabilized alkoxide anion

Q23. Which solvent property is most important for Cannizzaro reactions?

• Ability to donate protons
• Polarity to stabilize charged alkoxide intermediates
• Strong nucleophilicity
• High boiling nonpolar nature

Correct Answer: Polarity to stabilize charged alkoxide intermediates

Q24. How does electron-withdrawing substituent on an aldehyde affect its role in crossed Cannizzaro?

• Makes it a better hydride donor
• Makes it a better hydride acceptor (more electrophilic)
• No effect
• Prevents reaction entirely

Correct Answer: Makes it a better hydride acceptor (more electrophilic)

Q25. What stoichiometry often improves yield of desired alcohol in crossed Cannizzaro?

• 1:1 stoichiometry always best
• Large excess of hydride acceptor aldehyde
• Large excess of hydride donor aldehyde
• Equimolar with catalytic aldehyde

Correct Answer: Large excess of hydride acceptor aldehyde

Q26. Which analog of Cannizzaro involves intramolecular hydride transfer within a single molecule?

• Benzoin condensation
• Tishchenko reaction
• Meerwein–Ponndorf–Verley reduction
• Intramolecular Cannizzaro (disproportionation within one dialdehyde)

Correct Answer: Intramolecular Cannizzaro (disproportionation within one dialdehyde)

Q27. In pharmaceutical synthesis, why might crossed Cannizzaro be preferred over separate redox steps?

• It avoids use of external oxidants and reductants
• It requires high-energy radiation
• It always gives single product
• It is the only way to form carboxylic acids

Correct Answer: It avoids use of external oxidants and reductants

Q28. Which analytical technique is useful to monitor product distribution in crossed Cannizzaro?

• Infrared spectroscopy alone
• Gas chromatography (GC) or HPLC combined with NMR
• Melting point only
• Polarimetry only

Correct Answer: Gas chromatography (GC) or HPLC combined with NMR

Q29. Which statement about base strength and Cannizzaro is correct?

• Weaker bases give faster Cannizzaro
• Stronger bases generally accelerate alkoxide formation and the reaction
• Base strength has no effect
• Acidic conditions are required

Correct Answer: Stronger bases generally accelerate alkoxide formation and the reaction

Q30. Can crossed Cannizzaro be catalytic in base?

• Yes, commonly catalytic in strong acids
• No, base is consumed stoichiometrically forming alkoxide salts
• Yes, using organometallic catalysts only
• Only in gas phase

Correct Answer: No, base is consumed stoichiometrically forming alkoxide salts

Q31. What is the fate of the alkoxide intermediate in Cannizzaro?

• It undergoes nucleophilic attack on another aldehyde, transferring hydride
• It eliminates to form an alkene
• It is immediately protonated and lost
• It forms a stable radical

Correct Answer: It undergoes nucleophilic attack on another aldehyde, transferring hydride

Q32. Which structural feature in aldehyde increases tendency to be oxidized in crossed Cannizzaro?

• Strong electron-donating groups on the aromatic ring
• Electron-withdrawing groups that stabilize the carboxylate
• Presence of alpha-hydrogens
• Long alkyl chains

Correct Answer: Electron-withdrawing groups that stabilize the carboxylate

Q33. In a crossed Cannizzaro between p-nitrobenzaldehyde and benzaldehyde, which is more likely reduced?

• p-Nitrobenzaldehyde
• Benzaldehyde
• Both equally
• Neither reacts

Correct Answer: Benzaldehyde

Q34. Why might phase transfer catalysts be used in Cannizzaro reactions?

• To introduce acid catalysts
• To transfer hydroxide into organic phase and increase reaction rate
• To prevent reaction by complexation
• They are not useful

Correct Answer: To transfer hydroxide into organic phase and increase reaction rate

Q35. Which product ratio is expected if a very electron-poor aldehyde is mixed with a less electron-poor aldehyde?

• Electron-poor aldehyde mainly reduced
• Less electron-poor aldehyde mainly reduced
• Equal amounts of both alcohols
• No reaction occurs

Correct Answer: Less electron-poor aldehyde mainly reduced

Q36. What safety concern is important when performing Cannizzaro reactions on scale?

• Evolution of flammable hydrogen gas
• Handling strong bases and exothermic neutralization
• Formation of explosive peroxides
• Temperature-independent reactions

Correct Answer: Handling strong bases and exothermic neutralization

Q37. How can one favor formation of alcohol from a specific aldehyde in a crossed Cannizzaro?

• Lower concentration of hydride acceptor
• Use excess of the aldehyde intended to be reduced (hydride donor)
• Use acidic conditions
• Remove solvent entirely

Correct Answer: Use excess of the aldehyde intended to be reduced (hydride donor)

Q38. Which thermodynamic factor influences Cannizzaro product distribution?

• Relative stabilities of alcohol and carboxylate products and their salts
• Bond dissociation energy of C–H in the solvent
• Boiling point of aldehydes only
• Color of the reactants

Correct Answer: Relative stabilities of alcohol and carboxylate products and their salts

Q39. What is a competing reaction when attempting Cannizzaro with aldehydes that have alpha-hydrogens?

• Tishchenko reaction
• Aldol condensation
• Benzoin condensation only occurs in base
• Wolff–Kishner reduction

Correct Answer: Aldol condensation

Q40. Which mechanistic probe would distinguish a hydride-transfer step from a radical mechanism in Cannizzaro?

• Use of radical traps to see if reaction is inhibited
• Measuring pH only
• Changing temperature and observing color
• UV–Vis spectroscopy alone

Correct Answer: Use of radical traps to see if reaction is inhibited

Q41. What is the role of counterions (e.g., Na+ vs. K+) in Cannizzaro reactions?

• No effect on reaction rate or selectivity
• They can influence solubility and aggregation of alkoxide, affecting reactivity
• They convert aldehydes to ketones
• They always inhibit hydride transfer

Correct Answer: They can influence solubility and aggregation of alkoxide, affecting reactivity

Q42. Which structural modification of an aldehyde would increase its likelihood of being the hydride acceptor?

• Adding electron-donating methoxy groups
• Adding electron-withdrawing nitro groups
• Introducing bulky alkyl groups at alpha position
• Converting it to a hemiacetal

Correct Answer: Adding electron-withdrawing nitro groups

Q43. What effect does dilution typically have on crossed Cannizzaro outcomes?

• No effect
• May slow bimolecular hydride transfer and affect selectivity
• Always increases reaction rate
• Causes spontaneous polymerization

Correct Answer: May slow bimolecular hydride transfer and affect selectivity

Q44. Which reagent combination could transform an aldehyde pair into an alcohol and acid via crossed Cannizzaro?

• Aldehyde pair + weak acid catalyst
• Aldehyde pair + strong base like NaOH in aqueous medium
• Aldehyde pair + hydrogenation catalyst
• Aldehyde pair + dehydrating agent

Correct Answer: Aldehyde pair + strong base like NaOH in aqueous medium

Q45. How does temperature usually affect Cannizzaro reactions?

• Elevated temperature can accelerate reaction but may favor side reactions
• Temperature has no effect
• Lower temperatures always increase rate
• Only cryogenic conditions allow reaction

Correct Answer: Elevated temperature can accelerate reaction but may favor side reactions

Q46. Which experimental observation suggests a successful crossed Cannizzaro?

• Complete disappearance of starting aldehydes with formation of corresponding alcohol and carboxylic acid after workup
• Formation of polymeric residue only
• Color change without product isolation
• No change in IR spectra

Correct Answer: Complete disappearance of starting aldehydes with formation of corresponding alcohol and carboxylic acid after workup

Q47. Can aromatic heterocyclic aldehydes participate in crossed Cannizzaro?

• No, heterocycles never react
• Yes, provided they lack enolizable alpha-hydrogens and are compatible with base
• Only in acidic media
• Only when converted to ketones first

Correct Answer: Yes, provided they lack enolizable alpha-hydrogens and are compatible with base

Q48. In designing a synthetic route, why might a chemist avoid crossed Cannizzaro for complex molecules?

• The reaction is too selective
• Functional group incompatibility with strong base and low chemoselectivity may complicate outcome
• It always gives high yields
• It requires precious metal catalysts

Correct Answer: Functional group incompatibility with strong base and low chemoselectivity may complicate outcome

Q49. Which post-reaction treatment is commonly used after performing a Cannizzaro?

• Oxidative workup with chromic acid immediately
• Acidic workup to protonate carboxylate to carboxylic acid and neutralize base
• Drying with molecular sieves only
• Direct distillation without neutralization

Correct Answer: Acidic workup to protonate carboxylate to carboxylic acid and neutralize base

Q50. How does understanding crossed Cannizzaro benefit a B.Pharm student?

• It only helps in inorganic chemistry
• It develops mechanistic reasoning for redox and synthetic planning relevant to drug synthesis
• It is irrelevant to pharmaceuticals
• It replaces the need to learn spectroscopy

Correct Answer: It develops mechanistic reasoning for redox and synthetic planning relevant to drug synthesis

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