Aldehydes and Ketones: Ozonolysis

  • Introduction to ozonolysis
  • Purpose of ozonolysis reaction
  • Importance in organic synthesis
  • Overview of the reaction mechanism
  • Different products formed in ozonolysis

Reaction Conditions

  • Ozonolysis is carried out under specific conditions:
    • Low temperature (-70°C to -78°C)
    • Presence of an inert solvent (such as CH2Cl2 or CH2Cl2/MeOH)

Mechanism of Ozonolysis

  • Step 1: Formation of Ozone Trioxide (O3)
    • O2 reacts with O3 in the presence of UV light or electrical discharge to form O3
  • Step 2: Initial Reaction with the C=C bond
    • Ozone reacts with the C=C bond of the alkene to form a primary ozonide
    • The ozonide can be unstable and further reacts to produce two fragments
  • Step 3: Rearrangement or Decomposition
    • The primary ozonide can undergo different reactions:
      • If it is stable, it can rearrange to form a more stable compound
      • If it is unstable, it can undergo decomposition to form aldehydes, ketones, or carboxylic acids

Formation of Aldehydes

  • Primary ozonides derived from alkenes containing a terminal C=C bond can undergo further decomposition to give aldehydes
  • Example: Propene ozonolysis
    • Primary ozonide forms
    • Decomposition leads to formation of formaldehyde (HCHO)
    • Overall reaction: CH3CH=CH2 + O3 → CH2O + CH3CHO + O2

Formation of Ketones

  • Secondary ozonides derived from alkenes with a C=C bond in the middle of the chain can also undergo decomposition to give ketones
  • Example: 2-Butene ozonolysis
    • Secondary ozonide forms
    • Decomposition leads to formation of 2-butanone (CH3COCH3)
    • Overall reaction: CH3CH=CHCH3 + O3 → CH3COCH3 + CH3COCH3 + O2

Formation of Carboxylic Acids

  • Tertiary ozonides derived from alkenes with two substituents on each carbon of the C=C bond can undergo decomposition to give carboxylic acids
  • Example: 2-Methylpropene ozonolysis
    • Tertiary ozonide forms
    • Decomposition leads to formation of propanoic acid (CH3CH2COOH)
    • Overall reaction: (CH3)2C=CH2 + O3 → CH3CH2COOH + CH3COOH + O2

Limitations of Ozonolysis

  • Ozonolysis is not applicable to alkenes with substituents hindered by steric effects
  • Ozonolysis is not selective and can produce multiple products for complex alkene structures
  • Ozonolysis requires specific conditions and can be challenging to control

Applications in Organic Synthesis

  • Ozonolysis is an important tool in organic synthesis
  • Useful for carbon-carbon bond cleavage and functional group transformations
  • Helps in the identification of unknown compounds
  • Plays a role in the synthesis of various organic compounds, such as aldehydes, ketones, and carboxylic acids

Ozonolysis Examples

  • Example 1: Ozonolysis of Ethene
    • Initial reaction: CH2=CH2 + O3 → CH2O + O2
    • Overall reaction: CH2=CH2 + 1/2 O3 → CH2O + O2
  • Example 2: Ozonolysis of Cyclohexene
    • Initial reaction: C6H10 + O3 → C6H10O + O2
    • Overall reaction: C6H10 + O3 → C6H10O + O2

Summary

  • Ozonolysis is a key reaction in organic synthesis for the cleavage of carbon-carbon bonds.
  • The reaction involves the formation of ozonide intermediates, which can then decompose to yield aldehydes, ketones, or carboxylic acids.
  • Ozonolysis is carried out under specific conditions, and its products depend on the nature of the alkene used.

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Aldehyde Formation: Example 1

  • Ozonolysis of Propene
  • Initial reaction: Propene + Ozone -> Primary Ozonide
  • Decomposition of primary ozonide leads to formaldehyde and acetaldehyde formation
  • Overall reaction: Propene + Ozone -> Formaldehyde + Acetaldehyde + Oxygen

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Aldehyde Formation: Example 2

  • Ozonolysis of Butene
  • Initial reaction: Butene + Ozone -> Primary Ozonide
  • Decomposition of primary ozonide leads to butanal formation
  • Overall reaction: Butene + Ozone -> Butanal + Oxygen

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Ketone Formation: Example 1

  • Ozonolysis of 2-pentene
  • Initial reaction: 2-Pentene + Ozone -> Secondary Ozonide
  • Decomposition of secondary ozonide leads to 2-pentanone formation
  • Overall reaction: 2-Pentene + Ozone -> 2-Pentanone + Oxygen

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Ketone Formation: Example 2

  • Ozonolysis of 3-methyl-2-pentene
  • Initial reaction: 3-Methyl-2-pentene + Ozone -> Secondary Ozonide
  • Decomposition of secondary ozonide leads to 3-methyl-2-pentanone formation
  • Overall reaction: 3-Methyl-2-pentene + Ozone -> 3-Methyl-2-pentanone + Oxygen

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Carboxylic Acid Formation: Example 1

  • Ozonolysis of 2-methyl-1-butene
  • Initial reaction: 2-Methyl-1-butene + Ozone -> Tertiary Ozonide
  • Decomposition of tertiary ozonide leads to propanoic acid formation
  • Overall reaction: 2-Methyl-1-butene + Ozone -> Propanoic acid + Acetic acid + Oxygen

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Carboxylic Acid Formation: Example 2

  • Ozonolysis of 2,3-dimethyl-1-butene
  • Initial reaction: 2,3-Dimethyl-1-butene + Ozone -> Tertiary Ozonide
  • Decomposition of tertiary ozonide leads to 2-methylpropanoic acid formation
  • Overall reaction: 2,3-Dimethyl-1-butene + Ozone -> 2-Methylpropanoic acid + Acetic acid + Oxygen

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Limitations of Ozonolysis

  • Ozonolysis may not work for alkenes with substituted groups that hinder reaction
  • Requires specific conditions, such as low temperature and inert solvent
  • Not a selective reaction, can lead to multiple products
  • Limited control over the reaction outcome

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Applications of Ozonolysis

  • Useful in organic synthesis for the cleavage of carbon-carbon bonds
  • Enables the synthesis of aldehydes, ketones, and carboxylic acids
  • Helps identify unknown compounds through reaction patterns
  • Used to study and understand the structure and reactivity of organic molecules

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Ozonolysis Examples

  • Example 1: Ozonolysis of Ethene
    • Initial reaction: Ethene + Ozone -> Formaldehyde + Oxygen
  • Example 2: Ozonolysis of Cyclohexene
    • Initial reaction: Cyclohexene + Ozone -> Cyclohexanone + Oxygen

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Summary

  • Ozonolysis is a valuable reaction in organic synthesis for the cleavage of carbon-carbon bonds.
  • The reaction mechanism involves the formation of ozonide intermediates that can decompose to yield aldehydes, ketones, and carboxylic acids.
  • Ozonolysis has limitations and requires specific conditions for successful execution.
  • It finds applications in organic synthesis, compound identification, and the study of organic molecule reactivity.

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Problem Solving Session: Aldehydes and Ketones - Ozonolysis

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  • Problem 1: Predict the major products of the ozonolysis reaction for the following compounds:
    • a) 1-hexene
    • b) 2-methyl-2-pentene

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  • Problem 2: Draw the structures of the primary ozonide, secondary ozonide, and tertiary ozonide for propene.

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  • Problem 3: In the ozonolysis of cyclohexene, what are the possible products that can be formed? Explain briefly.

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  • Problem 4: Perform the ozonolysis of 2-methyl-2-butene. Write the overall reaction, and identify the major products obtained.

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  • Problem 5: What conditions are required for a successful ozonolysis reaction? Explain why these conditions are necessary.

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  • Problem 6: Compare and contrast aldehyde formation and ketone formation in ozonolysis reactions. Provide examples for each.

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  • Problem 7: Identify the type of ozonolysis product (aldehyde, ketone, or carboxylic acid) that would be formed from the ozonolysis of 1-octene.

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  • Problem 8: Can ozonolysis be used for the cleavage of carbon-carbon double bonds in aromatic compounds? Explain your answer.

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  • Problem 9: What are the limitations of ozonolysis as a reaction? Discuss briefly.