Alkynes undergo oxidative cleavage, breaking their triple bonds to form carbonyl compounds. This process requires powerful oxidizing agents like ozone or potassium permanganate, and harsh conditions compared to alkene oxidation.

The products of alkyne oxidation depend on the substituents. Internal alkynes yield two ketones, while terminal alkynes produce a carboxylic acid and an aldehyde. This reaction showcases the unique reactivity of carbon-carbon triple bonds.

Oxidative Cleavage of Alkynes

Oxidative cleavage of alkynes

• Breaks carbon-carbon triple bond resulting in formation of two carbonyl compounds (aldehydes or ketones)
• Requires powerful oxidizing agents or harsh reaction conditions compared to alkenes
  • Ozone ($O_3$) followed by reductive workup with reducing agents like zinc ($Zn$) or dimethyl sulfide ($Me_2S$)
  • Potassium permanganate ($KMnO_4$) under acidic conditions
  • Potassium periodate ($KIO_4$) in presence of ruthenium (II) chloride ($RuCl_2$) catalyst
• Mechanism with ozone:
  1. Ozone adds to alkyne forming unstable molozonide intermediate
  2. Molozonide rearranges to form ozonide
  3. Reductive workup cleaves ozonide yielding two carbonyl compounds

Alkyne vs alkene reactivity

• Alkynes less reactive than alkenes towards oxidative cleavage due to higher bond energy of carbon-carbon triple bond compared to double bond
• Alkenes undergo oxidative cleavage with milder oxidizing agents like osmium tetroxide ($OsO_4$) followed by periodate ($IO_4^-$) cleavage
• Products differ:
  • Alkynes yield two carbonyl compounds (aldehydes or ketones)
  • Alkenes yield two carbonyl compounds or one carboxylic acid and one ketone/aldehyde depending on substitution pattern

Products of alkyne oxidation

• Internal alkynes (R-C≡C-R') yield two ketones
  • R and R' groups become alkyl or aryl substituents on resulting ketones
• Terminal alkynes (R-C≡C-H) yield carboxylic acid and aldehyde
  • R group becomes alkyl or aryl substituent on resulting carboxylic acid
  • Hydrogen atom on terminal end oxidized to aldehyde functional group
• Specific products depend on substituents attached:
  • Alkyl groups result in aldehydes or ketones with alkyl substituents (methyl, ethyl)
  • Aryl groups lead to aldehydes or ketones with aryl substituents (phenyl, naphthyl)

Oxidation and Carbon-Carbon Bond Cleavage

• Oxidation involves the loss of electrons or increase in oxidation state
• Carbon-carbon bond cleavage occurs as a result of the oxidation process
• Oxidizing agents (e.g., ozone, potassium permanganate) facilitate the reaction
• Reaction mechanism involves the formation of unstable intermediates leading to bond cleavage