Ozonolysis MCQs With Answer

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Ozonolysis MCQs With Answer — tailored for B. Pharm students — offer a focused review of ozonolysis, an essential oxidative cleavage reaction of alkenes and alkynes. This set covers mechanism details (molozonide and ozonide formation), reagents and workups (reductive: Zn, Me2S, PPh3; oxidative: H2O2), stereochemical outcomes, and analytical identification of products. Questions also address applications in organic synthesis, structural elucidation of double-bond positions, safety and experimental conditions (low temperature, ozone generators), and differences from related reactions like dihydroxylation. These MCQs emphasize conceptual understanding and problem-solving skills required in pharmaceutical chemistry and drug synthesis. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the first detectable intermediate formed when ozone reacts with an alkene?

  • Molozonide (primary ozonide)
  • Criegee intermediate
  • Ozonide (trioxolane)
  • Carbocation

Correct Answer: Molozonide (primary ozonide)

Q2. Which product is typically formed from reductive ozonolysis of an internal alkene?

  • Two ketones
  • Two carboxylic acids
  • Two aldehydes
  • Alkane and alcohol

Correct Answer: Two ketones

Q3. Which reagent is commonly used for reductive workup in ozonolysis to give aldehydes instead of carboxylic acids?

  • Zinc in acetic acid
  • Hydrogen peroxide (H2O2)
  • Nitric acid
  • Ozone followed by water

Correct Answer: Zinc in acetic acid

Q4. Oxidative workup of ozonolysis (e.g., with H2O2) converts aldehyde fragments into which functional group?

  • Alcohols
  • Carboxylic acids
  • Ketals
  • Esters

Correct Answer: Carboxylic acids

Q5. Ozonolysis of a terminal alkene followed by reductive workup typically yields which pair of products?

  • Formaldehyde and a ketone
  • Formaldehyde and an aldehyde or ketone depending on substitution
  • Two carboxylic acids
  • One alcohol and one ether

Correct Answer: Formaldehyde and an aldehyde or ketone depending on substitution

Q6. Which step follows molozonide formation in the accepted ozonolysis mechanism?

  • Rearrangement to ozonide via Criegee intermediate
  • Hydrogen abstraction
  • Beta-elimination to form alkyne
  • Direct reduction to alcohol

Correct Answer: Rearrangement to ozonide via Criegee intermediate

Q7. Which condition is most commonly used to trap the ozonide and prevent side reactions?

  • High temperature (above 50°C)
  • Room temperature stirring for hours
  • Low temperature (e.g., –78°C)
  • Strong base at elevated temperature

Correct Answer: Low temperature (e.g., –78°C)

Q8. Which reagent is a common reductive quencher for ozonolysis that is odorless and often preferred in the lab?

  • Dimethyl sulfide (Me2S)
  • Sodium borohydride
  • Chromium trioxide
  • Hydrogen peroxide

Correct Answer: Dimethyl sulfide (Me2S)

Q9. In ozonolysis, what is the Criegee intermediate?

  • A diradical formed after ozone addition
  • A zwitterionic carbonyl oxide formed by molozonide fragmentation
  • A carbocation stabilized by ozone
  • A peracid formed during workup

Correct Answer: A zwitterionic carbonyl oxide formed by molozonide fragmentation

Q10. Ozonolysis of an alkyne followed by oxidative workup generally yields:

  • Keto-enols only
  • Two carboxylic acids (or one acid and CO2 if terminal)
  • Alkenes
  • Alcohols

Correct Answer: Two carboxylic acids (or one acid and CO2 if terminal)

Q11. Which observation suggests incomplete ozonolysis during a reaction?

  • Persistence of starting alkene signal in NMR
  • Formation of only carbon dioxide
  • Immediate appearance of carboxylic acids
  • Colorless reaction mixture

Correct Answer: Persistence of starting alkene signal in NMR

Q12. Why is ozonolysis useful for determining double bond location in unknown molecules?

  • It transforms double bonds into diagnostic carbonyl fragments that can be analyzed
  • It hydrogenates the double bond making NMR simpler
  • It introduces heavy atoms detectable by mass spectrometry only
  • It preserves the double bond but changes stereochemistry

Correct Answer: It transforms double bonds into diagnostic carbonyl fragments that can be analyzed

Q13. Which safety concern is most important when performing ozonolysis?

  • Toxicity of ozone gas and formation of explosive ozonides
  • Flammability of water
  • Formation of inert nitrogen gas
  • High pressure formation of hydrogen

Correct Answer: Toxicity of ozone gas and formation of explosive ozonides

Q14. Which functional group is generally unaffected by mild ozonolysis conditions?

  • Unactivated aromatic ring (under controlled conditions)
  • Alkene directly conjugated to aromatic ring
  • Dialkyl sulfide
  • Thiols

Correct Answer: Unactivated aromatic ring (under controlled conditions)

Q15. Ozonolysis followed by reductive workup is often chosen over oxidative when the goal is:

  • To convert alkenes into carboxylic acids
  • To preserve aldehydes and avoid overoxidation to acids
  • To generate peroxides
  • To form epoxides selectively

Correct Answer: To preserve aldehydes and avoid overoxidation to acids

Q16. Which analytical technique is most directly used to confirm formation of carbonyl products after ozonolysis?

  • IR spectroscopy showing C=O stretch
  • UV-Vis absorption only
  • Polarimetry change
  • Thin-layer chromatography without standards

Correct Answer: IR spectroscopy showing C=O stretch

Q17. In ozonolysis, what is an ozonide (trioxolane)?

  • Stable final carboxylic acid derivative
  • A cyclic peroxide formed from recombination of Criegee intermediate fragments
  • Protonated ozone molecule
  • A sulfide byproduct

Correct Answer: A cyclic peroxide formed from recombination of Criegee intermediate fragments

Q18. Which of the following substrates is most readily cleaved by ozonolysis?

  • Tetra-substituted internal alkene
  • Electron-rich terminal alkene (e.g., vinyl ether)
  • Aromatic hydrocarbon with no double bonds
  • Alkane

Correct Answer: Electron-rich terminal alkene (e.g., vinyl ether)

Q19. Which statement about stereochemistry in ozonolysis of alkenes is correct?

  • Ozonolysis retains alkene stereochemistry in the products
  • Ozonolysis converts stereochemical information into separate carbonyl fragments, so stereochemistry at the C=C is lost
  • It inverts the stereochemistry producing enantiomers
  • It converts cis alkenes exclusively into trans products

Correct Answer: Ozonolysis converts stereochemical information into separate carbonyl fragments, so stereochemistry at the C=C is lost

Q20. Which gas is used to generate ozone for laboratory ozonolysis reactions?

  • Nitrogen
  • Oxygen
  • Argon
  • Chlorine

Correct Answer: Oxygen

Q21. What is the effect of conjugation on the reactivity of an alkene toward ozonolysis?

  • Conjugated alkenes generally react more slowly and give stabilized intermediates
  • Conjugation has no effect on ozonolysis rate
  • Conjugated alkenes explode on contact with ozone
  • Conjugated alkenes always give alcohols

Correct Answer: Conjugated alkenes generally react more slowly and give stabilized intermediates

Q22. Which workup reagent would you choose to ensure oxidation of any aldehyde fragments to acids after ozonolysis?

  • Pyridine
  • Hydrogen peroxide (H2O2)
  • Dimethyl sulfide (Me2S)
  • Triethylamine

Correct Answer: Hydrogen peroxide (H2O2)

Q23. What is a common laboratory sign that ozonolysis reaction has reached completion during bubbling of ozone?

  • Loss of all solvent
  • Persistent blue or pale coloration of the reaction indicating excess ozone
  • Production of white fumes only
  • Immediate precipitation of ozone

Correct Answer: Persistent blue or pale coloration of the reaction indicating excess ozone

Q24. Ozonolysis is particularly useful in carbohydrate chemistry for:

  • Removing protecting groups without affecting sugars
  • Cleaving double bonds to determine glycosidic linkage positions
  • Hydrogenating sugar rings
  • Converting hydroxyls to ethers selectively

Correct Answer: Cleaving double bonds to determine glycosidic linkage positions

Q25. Which of the following is NOT a typical product outcome of ozonolysis of alkenes?

  • Ketones
  • Aldehydes
  • Carboxylic acids
  • Alkanes (unchanged carbon skeleton)

Correct Answer: Alkanes (unchanged carbon skeleton)

Q26. During ozonolysis, why is low temperature used when forming ozonides?

  • To accelerate rearrangement to carbonyls
  • To stabilize reactive intermediates and minimize decomposition or explosion risk
  • To increase solubility of ozone in water
  • To favor formation of alcohols

Correct Answer: To stabilize reactive intermediates and minimize decomposition or explosion risk

Q27. What happens when ozone reacts with an enol ether?

  • The enol ether is generally cleaved to give an ester and an aldehyde/ketone depending on substitution
  • It forms an epoxide exclusively
  • It reduces to an alkane
  • No reaction occurs under any conditions

Correct Answer: The enol ether is generally cleaved to give an ester and an aldehyde/ketone depending on substitution

Q28. Which of the following correctly contrasts ozonolysis with ozonation by permanganate oxidation?

  • Ozonolysis cannot cleave double bonds while permanganate can
  • Ozonolysis cleaves double bonds to carbonyls; permanganate often oxidizes to diols or further to acids
  • Both reactions always give identical products
  • Permanganate is used only for hydrogenation

Correct Answer: Ozonolysis cleaves double bonds to carbonyls; permanganate often oxidizes to diols or further to acids

Q29. What product results from ozonolysis of cyclohexene with reductive workup?

  • Adipic acid
  • Two molecules of 1,6-hexanedial
  • One dialdehyde: adipaldehyde (hexane-1,6-dial)
  • Cyclohexane

Correct Answer: One dialdehyde: adipaldehyde (hexane-1,6-dial)

Q30. Which statement best describes the role of solvents in ozonolysis?

  • Solvent choice is irrelevant
  • Polar aprotic solvents or dichloromethane at low temperature help control reactivity and solubility of ozone
  • Water must always be used to generate ozonides
  • Strong acids are required as solvents

Correct Answer: Polar aprotic solvents or dichloromethane at low temperature help control reactivity and solubility of ozone

Q31. What is the fate of a terminal alkyne under oxidative ozonolysis?

  • It yields two aldehydes
  • It yields one carboxylic acid and CO2
  • It forms an internal alkene
  • It remains unchanged

Correct Answer: It yields one carboxylic acid and CO2

Q32. Which reagent can be used as an alternative to Me2S for reductive workup to avoid smelly byproducts?

  • Triphenylphosphine (PPh3)
  • Hydrogen peroxide
  • Sodium hydroxide
  • Chromic acid

Correct Answer: Triphenylphosphine (PPh3)

Q33. Which of the following best describes why ozonolysis is classified as an oxidative cleavage?

  • Because ozone acts as a reducing agent
  • Because the C=C bond is cleaved and carbon atoms are oxidized to carbonyl-containing fragments
  • Because it adds hydrogen across the double bond
  • Because it forms radicals only

Correct Answer: Because the C=C bond is cleaved and carbon atoms are oxidized to carbonyl-containing fragments

Q34. In retrosynthetic analysis, ozonolysis is often used to:

  • Introduce new stereocenters
  • Disconnect molecules at double bonds to reveal simpler carbonyl fragments
  • Convert carboxylic acids to alkenes
  • Protect functional groups

Correct Answer: Disconnect molecules at double bonds to reveal simpler carbonyl fragments

Q35. What analytical evidence would confirm formation of an ozonide intermediate?

  • IR band corresponding to a strong O–O stretch and disappearance of alkene C=C signal at low temperature
  • Immediate boiling of the reaction mixture
  • Appearance of a sharp singlet at 10 ppm in 1H NMR
  • Formation of a strong UV absorption at 300 nm only

Correct Answer: IR band corresponding to a strong O–O stretch and disappearance of alkene C=C signal at low temperature

Q36. When performing ozonolysis on a molecule with multiple C=C bonds, selectivity is determined mainly by:

  • The highest boiling point double bond
  • Electronic and steric properties of each double bond
  • Color of the substrate
  • Solubility in water

Correct Answer: Electronic and steric properties of each double bond

Q37. Which product results from ozonolysis of trans-2-butene with reductive workup?

  • Two molecules of acetone
  • Two molecules of ethanal (acetaldehyde)
  • Butane
  • 2-butanone

Correct Answer: Two molecules of ethanal (acetaldehyde)

Q38. Which of the following is a limitation of ozonolysis in complex molecule synthesis?

  • It cannot cleave double bonds
  • Sensitivity of other functional groups and safety concerns due to ozonide formation
  • It only works on aromatic compounds
  • It always gives racemic mixtures

Correct Answer: Sensitivity of other functional groups and safety concerns due to ozonide formation

Q39. What is the typical colour change when ozone is bubbled into a solvent until excess ozone is present?

  • Colorless to bright red
  • Colorless to pale blue or bluish
  • Clear to black
  • Yellow to green

Correct Answer: Colorless to pale blue or bluish

Q40. Which of the following statements about ozonolysis of conjugated dienes is true?

  • Both double bonds are always cleaved equally and independently
  • Conjugation can lead to complex mixtures and rearranged products due to delocalized intermediates
  • Conjugated dienes are inert to ozone
  • Ozonolysis converts dienes exclusively to alkenes

Correct Answer: Conjugation can lead to complex mixtures and rearranged products due to delocalized intermediates

Q41. In an ozonolysis reaction, which byproduct originates from dimethyl sulfide (Me2S) reductive workup?

  • Dimethyl sulfoxide (DMSO)
  • Dimethyl sulfoxide dioxide
  • Dimethyl sulfone (DMSO2)
  • Dimethyl sulfide oxide

Correct Answer: Dimethyl sulfoxide (DMSO)

Q42. For a terminal alkene RCH=CH2, reductive ozonolysis typically yields which carbonyl fragments?

  • R–CHO and HCHO (formaldehyde)
  • R–CO2H and methane
  • Two equivalents of R–CHO
  • Only one ketone

Correct Answer: R–CHO and HCHO (formaldehyde)

Q43. Which precaution reduces the risk of forming explosive peroxides when handling ozonides?

  • Heating ozonide mixtures to accelerate decomposition
  • Quenching ozonides promptly with a reductant or performing controlled oxidative workup at low temperature
  • Adding concentrated acids to concentrate ozonides
  • Storing ozonides neat in warm conditions

Correct Answer: Quenching ozonides promptly with a reductant or performing controlled oxidative workup at low temperature

Q44. Which structural change results from ozonolysis of a trisubstituted internal alkene under oxidative conditions?

  • Formation of a mixture of carboxylic acids and ketones depending on substitution
  • Formation of alkanes only
  • Retention of the alkene with added oxygen
  • Conversion to alcohols exclusively

Correct Answer: Formation of a mixture of carboxylic acids and ketones depending on substitution

Q45. Which reagent pair is suitable for ozonolysis where the goal is to avoid thioether smell and obtain aldehydes?

  • Ozone then Me2S
  • Ozone then PPh3 (triphenylphosphine)
  • Ozone then H2O2
  • Ozone then nitric acid

Correct Answer: Ozone then PPh3 (triphenylphosphine)

Q46. Why might ozonolysis be preferred over ozidation methods for locating double bonds in natural product structure elucidation?

  • It leaves double bonds intact for analysis
  • It cleanly cleaves double bonds to carbonyl fragments that are easier to isolate and identify by spectroscopy
  • It is the only method that can cleave carbon–carbon single bonds
  • It is non-destructive to sensitive functional groups

Correct Answer: It cleanly cleaves double bonds to carbonyl fragments that are easier to isolate and identify by spectroscopy

Q47. Ozonolysis of styrene (vinylbenzene) under oxidative workup yields which major product?

  • Benzoic acid and formaldehyde
  • Phenol and acetone
  • Benzaldehyde only
  • Benzene

Correct Answer: Benzoic acid and formaldehyde

Q48. Which change in mass spectrometry would indicate successful ozonolytic cleavage of an alkene?

  • Appearance of peaks corresponding to expected carbonyl fragment masses
  • Complete loss of all peaks
  • Only inert gas peaks
  • No change in spectrum

Correct Answer: Appearance of peaks corresponding to expected carbonyl fragment masses

Q49. When planning an ozonolysis in a multi-step synthesis, which protective strategy might be necessary?

  • Protecting sensitive aldehydes or alcohols because ozone or workup may oxidize them
  • Ensuring no solvents are used
  • Avoiding use of any reductant
  • Heating the reaction to 100°C

Correct Answer: Protecting sensitive aldehydes or alcohols because ozone or workup may oxidize them

Q50. Which advantage makes ozonolysis a valuable tool in pharmaceutical research?

  • It selectively adds halogens to molecules
  • It enables precise cleavage of C=C bonds to generate or identify carbonyl-containing drug fragments, aiding structure elucidation and synthetic planning
  • It replaces all functional groups with nitro groups
  • It always increases molecular weight

Correct Answer: It enables precise cleavage of C=C bonds to generate or identify carbonyl-containing drug fragments, aiding structure elucidation and synthetic planning

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