Ozonolysis of Akynes

Ozonolysis of Alkynes

Ozonolysis of alkynes is a reaction that cleaves the triple bond, resulting in the formation of carboxylic acids (or ketones, in specific cases). Here's a breakdown:

General Reaction

Alkynes react with ozone (O₃) followed by a reductive workup (typically using zinc in acetic acid or dimethyl sulfide, (CH₃)₂S) to produce carboxylic acids. If the alkyne is internal (not at the end of the chain), two carboxylic acids are formed. If the alkyne is terminal (at the end of the chain), a carboxylic acid and carbon dioxide (CO₂) are formed.

Mechanism

The mechanism is complex but involves the initial addition of ozone across the triple bond, followed by rearrangements and ultimately cleavage of the multiple bonds. The reductive workup is crucial to prevent further oxidation of the carboxylic acids.

Examples

• Internal Alkyne:
  R-C≡C-R' + O₃ → R-COOH + R'-COOH
  (followed by reductive workup)
• Terminal Alkyne:
  R-C≡CH + O₃ → R-COOH + CO₂
  (followed by reductive workup)

Specific Examples

• 2-Butyne + O₃ (followed by reductive workup) → 2 CH₃COOH (acetic acid)
• Propyne + O₃ (followed by reductive workup) → CH₃COOH (acetic acid) + CO₂
• 5-Decyne + O₃ (followed by reductive workup) → CH₃(CH₂)₃COOH (pentanoic acid) + CH₃(CH₂)₄COOH (hexanoic acid)

Important Considerations

• Reductive Workup: The use of a reducing agent in the workup is essential. Oxidative workup (e.g., using H₂O₂) would lead to further oxidation of the carboxylic acids to, in the case of terminal alkynes, carboxylic acids and CO₂.
• Terminal Alkynes: The terminal carbon of a terminal alkyne is oxidized all the way to CO₂.
• Internal Alkynes: Internal alkynes are cleaved to give two carboxylic acids.