Cannizzaro reaction MCQs With Answer is an essential topic for B.Pharm students studying pharmaceutical chemistry. This introduction covers the Cannizzaro reaction mechanism, scope, and applications with clear keywords like Cannizzaro reaction, MCQs, B.Pharm, aldehydes, hydride transfer, disproportionation, and pharmaceutical synthesis. You’ll learn why non-enolizable aldehydes undergo base-induced hydride transfer to yield a primary alcohol and a carboxylate, how crossed Cannizzaro varies selectivity, and the impact of substituents and reaction conditions on outcomes. The focus is practical: reaction steps, reagents (NaOH/KOH), and synthetic relevance to drug intermediate preparation. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What best defines the Cannizzaro reaction?
- Acid-catalyzed polymerization of aldehydes
- Base-induced disproportionation of non-enolizable aldehydes to alcohols and carboxylates
- Oxidation of aldehydes to ketones using PCC
- Radical-mediated dimerization of aldehydes
Correct Answer: Base-induced disproportionation of non-enolizable aldehydes to alcohols and carboxylates
Q2. Which class of aldehydes typically undergoes Cannizzaro reaction?
- Enolizable aliphatic aldehydes with alpha-hydrogens
- Non-enolizable aldehydes lacking alpha-hydrogens
- All ketones and aldehydes
- Only formaldehyde
Correct Answer: Non-enolizable aldehydes lacking alpha-hydrogens
Q3. What is the primary step in the mechanism of the Cannizzaro reaction?
- Radical formation followed by coupling
- Hydride transfer from one aldehyde to another
- Protonation of the carbonyl to form an oxonium ion
- Enolization and aldol condensation
Correct Answer: Hydride transfer from one aldehyde to another
Q4. Which reagent is most commonly used to carry out the Cannizzaro reaction?
- PCC (pyridinium chlorochromate)
- Strong base such as NaOH or KOH
- Acidic Lewis acids like AlCl3
- Hydrogenation catalysts like Pd/C
Correct Answer: Strong base such as NaOH or KOH
Q5. What are the stoichiometric products of a simple Cannizzaro reaction between two identical aldehyde molecules?
- Two alcohols
- One alcohol and one carboxylate (salt)
- Two carboxylic acids
- An ester and water
Correct Answer: One alcohol and one carboxylate (salt)
Q6. Why do enolizable aldehydes rarely undergo Cannizzaro reaction?
- They readily undergo aldol condensation under basic conditions
- They are too reactive for any reaction under base
- They lack a carbonyl group
- They form stable carbocations instead
Correct Answer: They readily undergo aldol condensation under basic conditions
Q7. Which aromatic aldehyde is a classic substrate for the Cannizzaro reaction?
- Benzaldehyde
- Acetaldehyde
- Propionaldehyde
- Acetone
Correct Answer: Benzaldehyde
Q8. In a crossed Cannizzaro reaction, what controls which aldehyde is reduced?
- Relative enolizability of the aldehydes
- Relative ability to accept hydride (electronic effects) and steric factors
- Only the solvent polarity
- Only the temperature
Correct Answer: Relative ability to accept hydride (electronic effects) and steric factors
Q9. What is typically used to favor selective reduction in crossed Cannizzaro reactions?
- Use of a catalytic amount of acid
- Using formaldehyde as the hydride donor or acceptor partner
- Performing the reaction at very low temperature with no base
- Adding radical initiators
Correct Answer: Using formaldehyde as the hydride donor or acceptor partner
Q10. The hydride donor in the Cannizzaro mechanism is best described as:
- A radical hydrogen atom
- A hydride equivalent delivered by an alkoxide-stabilized intermediate
- A proton from solvent
- Oxygen transfer from water
Correct Answer: A hydride equivalent delivered by an alkoxide-stabilized intermediate
Q11. Which experimental observation supports a hydride-transfer mechanism?
- Isolation of radical coupling products
- Isotopic labelling showing hydrogen transfer between aldehyde molecules
- Formation of enolates prior to product formation
- Observation of carbenium ions by mass spectrometry
Correct Answer: Isotopic labelling showing hydrogen transfer between aldehyde molecules
Q12. Which side product forms in the Cannizzaro reaction when performed in aqueous base without workup?
- Carboxylate salt
- Ester
- Ketone
- Imine
Correct Answer: Carboxylate salt
Q13. How does an electron-withdrawing substituent on benzaldehyde affect the Cannizzaro reaction rate?
- It generally accelerates the reaction by increasing electrophilicity
- It completely inhibits the reaction
- It causes radical pathways to dominate
- It has no effect on the reaction rate
Correct Answer: It generally accelerates the reaction by increasing electrophilicity
Q14. Which statement distinguishes Cannizzaro from Tishchenko reaction?
- Cannizzaro is base-induced disproportionation; Tishchenko is metal-alkoxide-catalyzed ester formation
- Both are identical reactions with different names
- Cannizzaro yields esters while Tishchenko yields alcohols and acids
- Only Tishchenko involves hydride transfer
Correct Answer: Cannizzaro is base-induced disproportionation; Tishchenko is metal-alkoxide-catalyzed ester formation
Q15. Which of the following aldehydes is unlikely to undergo Cannizzaro reaction?
- Benzaldehyde
- p-Nitrobenzaldehyde
- Acetaldehyde
- p-Chlorobenzaldehyde
Correct Answer: Acetaldehyde
Q16. What solvent is commonly used for Cannizzaro reactions in the lab?
- Aqueous alcoholic (e.g., ethanol-water) or water with base
- Pure dichloromethane without base
- Neat hydrogen peroxide
- Anhydrous ether with acid catalyst
Correct Answer: Aqueous alcoholic (e.g., ethanol-water) or water with base
Q17. Which observation indicates a crossed Cannizzaro has occurred selectively?
- Formation of a mixture of many alcohols in equal amounts
- Predominant formation of the reduced product from the more easily reduced aldehyde
- Only polymerization products are formed
- Exclusive formation of esters
Correct Answer: Predominant formation of the reduced product from the more easily reduced aldehyde
Q18. What is the role of the alkoxide intermediate in the Cannizzaro reaction?
- It acts as the hydride acceptor to form a radical
- It stabilizes the transition state and facilitates hydride transfer
- It forms an ester directly with another aldehyde
- It removes protons making the medium acidic
Correct Answer: It stabilizes the transition state and facilitates hydride transfer
Q19. Which experimental modification can suppress competing aldol reactions and favor Cannizzaro?
- Using strong acid rather than base
- Choosing non-enolizable aldehydes or using excess non-enolizable partner
- Working at high temperatures to favor enolization
- Adding catalytic amounts of cyanide
Correct Answer: Choosing non-enolizable aldehydes or using excess non-enolizable partner
Q20. Which pair of products results from Cannizzaro reaction of benzaldehyde under NaOH?
- Benzyl alcohol and sodium benzoate
- Benzyl alcohol and benzoic acid without salt formation
- Toluene and benzene
- Benzoic anhydride and water
Correct Answer: Benzyl alcohol and sodium benzoate
Q21. Does formaldehyde undergo Cannizzaro reaction, and what are its products?
- No, formaldehyde cannot undergo any redox reactions
- Yes; it can produce methanol and formate (as salt) under strong base
- Yes; it yields only carbon dioxide and water
- No; it yields only polymers under base
Correct Answer: Yes; it can produce methanol and formate (as salt) under strong base
Q22. Which mechanistic characteristic explains why Cannizzaro requires at least two aldehyde molecules?
- One aldehyde forms an enolate that reacts with oxygen
- One aldehyde acts as hydride donor and the other as hydride acceptor
- Two aldehydes form a cyclic dimer intermediate before reaction
- Two aldehydes are needed to chelate the base
Correct Answer: One aldehyde acts as hydride donor and the other as hydride acceptor
Q23. Which analytical signal would confirm formation of the carboxylate product in Cannizzaro?
- Strong IR absorption around 1700 cm−1 for free acid carbonyl only
- IR band for carboxylate (as COO−) with asymmetric/symmetric stretches ~1600–1400 cm−1 and absence of strong OH
- Absence of any new IR bands
- Only UV absorption changes
Correct Answer: IR band for carboxylate (as COO−) with asymmetric/symmetric stretches ~1600–1400 cm−1 and absence of strong OH
Q24. Which of the following increases Cannizzaro reaction rate most significantly?
- Weak base like carbonate in dilute solution
- Strong base in concentrated solution
- Strong acid catalyst
- Complete absence of solvent
Correct Answer: Strong base in concentrated solution
Q25. During the Cannizzaro reaction, which intermediate is directly responsible for delivering the hydride?
- Carbanion stabilized by resonance
- Alkoxide derived from nucleophilic attack of base on aldehyde
- Carbocation stabilized by resonance
- Free radical anion
Correct Answer: Alkoxide derived from nucleophilic attack of base on aldehyde
Q26. Which statement about stereochemistry in Cannizzaro reaction is correct?
- Stereochemistry is a major concern because many chiral centers are created
- The reaction typically does not create new stereogenic centers and thus stereochemistry is rarely an issue
- The reaction always yields racemic mixtures
- The reaction inverts configuration at alpha carbon
Correct Answer: The reaction typically does not create new stereogenic centers and thus stereochemistry is rarely an issue
Q27. What historical figure is credited with discovering the Cannizzaro reaction?
- Savante Arrhenius
- Stanislao Cannizzaro
- Alfred Nobel
- Friedrich Wöhler
Correct Answer: Stanislao Cannizzaro
Q28. Why is Cannizzaro reaction less favored industrially compared to other reductions?
- It uses expensive catalysts
- Poor atom economy because two aldehyde molecules give one reduced and one oxidized product; stoichiometry and separation problems
- It requires extremely high pressure
- It produces toxic gaseous byproducts
Correct Answer: Poor atom economy because two aldehyde molecules give one reduced and one oxidized product; stoichiometry and separation problems
Q29. How can phase-transfer catalysis affect the Cannizzaro reaction?
- It can improve reaction rates by transferring hydroxide into organic phase, enabling reaction of poorly soluble substrates
- It always leads to elimination reactions instead
- It makes the reaction acid-catalyzed
- It prevents hydride transfer entirely
Correct Answer: It can improve reaction rates by transferring hydroxide into organic phase, enabling reaction of poorly soluble substrates
Q30. Which laboratory precaution is important when performing a Cannizzaro reaction?
- Avoid strong bases as they are inert
- Control addition of base and temperature to limit side reactions like aldol
- Never use aqueous solvents
- Always perform under acidic conditions
Correct Answer: Control addition of base and temperature to limit side reactions like aldol
Q31. What is the expected result when a mixture of benzaldehyde and acetaldehyde is treated with NaOH?
- Selective Cannizzaro of both aldehydes equally
- Aldol condensation of acetaldehyde predominates while benzaldehyde may undergo Cannizzaro
- Only benzoin condensation occurs
- Complete polymerization to complex oligomers only
Correct Answer: Aldol condensation of acetaldehyde predominates while benzaldehyde may undergo Cannizzaro
Q32. In mechanistic studies, what effect does deuterium labeling at the aldehydic hydrogen show?
- No incorporation into products is observed
- Deuterium appears in the reduced alcohol product, evidencing hydride (deuteride) transfer
- Deuterium causes the reaction to become acid-catalyzed
- Deuterium leads to radical coupling products
Correct Answer: Deuterium appears in the reduced alcohol product, evidencing hydride (deuteride) transfer
Q33. Which competitive reaction can occur with benzaldehyde under basic conditions if cyanide is present?
- Benzoin condensation
- Wurtz coupling
- Haloform reaction
- Beckmann rearrangement
Correct Answer: Benzoin condensation
Q34. How does temperature generally influence the Cannizzaro reaction?
- Lower temperatures always stop the reaction completely
- Higher temperatures can increase rates but also promote side reactions like aldol
- Temperature has no effect
- Only cryogenic conditions favor Cannizzaro
Correct Answer: Higher temperatures can increase rates but also promote side reactions like aldol
Q35. Which reagent combination can be used to obtain benzyl alcohol selectively from benzaldehyde in practice?
- NaBH4 in protic solvent
- Strong base alone with no hydride source
- PCC oxidation
- Chromic acid reduction
Correct Answer: NaBH4 in protic solvent
Q36. What product distribution is expected from the Cannizzaro reaction of para-nitrobenzaldehyde under NaOH?
- Mostly para-nitrobenzyl alcohol and para-nitrobenzoate
- Only polymeric nitro compounds
- Only para-nitrotoluene
- Only ester formation via Tishchenko
Correct Answer: Mostly para-nitrobenzyl alcohol and para-nitrobenzoate
Q37. Why is hydride transfer in Cannizzaro considered intramolecular between two aldehyde molecules rather than via free hydride ion?
- Free hydride ions are abundant in solution
- Hydride transfer occurs through a bound alkoxide-aldehyde complex, not free H− in solution
- Hydride is delivered by metal catalyst
- The reaction proceeds via radical hydrogen abstraction
Correct Answer: Hydride transfer occurs through a bound alkoxide-aldehyde complex, not free H− in solution
Q38. Which technique helps separate alcohol and carboxylate products after a Cannizzaro?
- Simple distillation because both are volatile
- Acidification to convert carboxylate to carboxylic acid followed by extraction
- Chromatography is impossible for these compounds
- Freezing the reaction mixture
Correct Answer: Acidification to convert carboxylate to carboxylic acid followed by extraction
Q39. Which factor would most likely decrease the yield of Cannizzaro products?
- Using an excess of non-enolizable aldehyde
- Contamination with acidic impurities
- Using strong base and anhydrous conditions
- Using electron-withdrawing substituents on the aldehyde
Correct Answer: Contamination with acidic impurities
Q40. Can ketones undergo Cannizzaro reaction under normal conditions?
- Yes, ketones readily undergo Cannizzaro
- No, ketones generally do not undergo Cannizzaro because hydride transfer between ketones is unfavorable
- Only tertiary ketones undergo Cannizzaro
- Ketones give only radical products
Correct Answer: No, ketones generally do not undergo Cannizzaro because hydride transfer between ketones is unfavorable
Q41. Which modification can convert a Cannizzaro-type process into a catalytic, more atom-economical protocol?
- Using stoichiometric strong bases only
- Transition-metal-catalyzed transfer hydrogenation approaches pairing aldehyde oxidation with catalytic hydrogen shuttles
- Adding excess inorganic salts
- Running the reaction at extremely low concentration
Correct Answer: Transition-metal-catalyzed transfer hydrogenation approaches pairing aldehyde oxidation with catalytic hydrogen shuttles
Q42. Which diagnostic NMR change indicates formation of benzyl alcohol from benzaldehyde?
- Disappearance of aldehydic proton signal near 9–10 ppm and appearance of benzylic CH2 or CH signal and OH signal
- Appearance of a strong signal at 200 ppm
- No change in the NMR spectrum
- Only appearance of aliphatic methyl peaks
Correct Answer: Disappearance of aldehydic proton signal near 9–10 ppm and appearance of benzylic CH2 or CH signal and OH signal
Q43. During a Cannizzaro reaction, why might ester byproducts form in some conditions?
- Because of competing Tishchenko or condensation reactions catalyzed by metal alkoxides or dehydration
- Because aldehydes spontaneously decompose to esters
- Because bases directly convert aldehydes to esters
- Ester formation is impossible under all Cannizzaro conditions
Correct Answer: Because of competing Tishchenko or condensation reactions catalyzed by metal alkoxides or dehydration
Q44. Which observation would indicate that hydride transfer is the rate-determining step?
- Rate is independent of aldehyde concentration
- Substantial kinetic isotope effect when aldehydic hydrogen is replaced by deuterium
- Reaction rate increases with radical initiator
- No change when substituents are altered
Correct Answer: Substantial kinetic isotope effect when aldehydic hydrogen is replaced by deuterium
Q45. In designing a synthetic route, why might a chemist choose Cannizzaro over direct hydrogenation?
- Cannizzaro provides both oxidation and reduction products in a single step which might be synthetically useful
- Direct hydrogenation is always cheaper and better
- Cannizzaro requires high-pressure hydrogen gas
- Cannizzaro avoids formation of any carboxylates
Correct Answer: Cannizzaro provides both oxidation and reduction products in a single step which might be synthetically useful
Q46. What happens to the carboxylate formed in a Cannizzaro reaction upon acidification?
- It is converted to the corresponding carboxylic acid
- It becomes an alcohol
- It decomposes into CO2
- It remains as carboxylate regardless of pH
Correct Answer: It is converted to the corresponding carboxylic acid
Q47. Which experimental approach demonstrates the involvement of alkoxide intermediates?
- Observation of radical chain reactions
- Trapping alkoxide intermediates by quenching and analyzing salts or using spectroscopic detection of alkoxide species
- Measuring only final product yields with no intermediate study
- Adding acid to accelerate the reaction
Correct Answer: Trapping alkoxide intermediates by quenching and analyzing salts or using spectroscopic detection of alkoxide species
Q48. Which substituent on benzaldehyde would most likely slow the Cannizzaro reaction?
- Strong electron-withdrawing group like –NO2
- Strong electron-donating group like –OMe
- Halogen like –Cl mildly withdrawing
- No substituent at all
Correct Answer: Strong electron-donating group like –OMe
Q49. Which outcome is expected if a Cannizzaro reaction is performed in the presence of an excess of a primary alcohol solvent?
- Alcohol solvent may convert carboxylate into ester under certain catalytic conditions
- Alcohol solvent prevents any reaction from occurring
- Only ketones are formed
- Reaction yields pure aldehyde recovery
Correct Answer: Alcohol solvent may convert carboxylate into ester under certain catalytic conditions
Q50. What practical tip should B.Pharm students remember when studying Cannizzaro for exams and labs?
- Always assume all aldehydes behave the same
- Focus on key points: non-enolizable substrates, base-induced hydride transfer, product pair (alcohol + carboxylate), and competing aldol/Tishchenko pathways
- Ignore reaction conditions as they are unimportant
- Memorize only product names without understanding mechanism
Correct Answer: Focus on key points: non-enolizable substrates, base-induced hydride transfer, product pair (alcohol + carboxylate), and competing aldol/Tishchenko pathways
