Alkyne oxidation III. Competitive features and degenerate branching

A comparison is made between the slow com bustions of propyne and butyne-2 which are degenerately branched and acetylene which is not. The relative rates of ‘oxidation’ radical attack on these alkynes and a number of the reactive intermediates formed in their slow combustions have been obtained from their separate and competitive oxidations. The results confirm that the same electrophilic radical species (hydroxyl or hydroperoxyl) is involved in the oxidations of acetylene, propyne and butyne-2. It is concluded that the branching intermediate is a peracid formed from an aldehyde in the case of propyne and an acyl radical in butyne-2 oxidation, which produces only traces of aldehydes, by the sequence: RCHO + X˙→ RĊO + HX ; RĊO + O 2 → RCO˙ 3 ; RCO˙ 3 + RʹH→ RCO 3 H + Rʹ˙ ; RCO 3 H → R˙ + CO 2 +˙OH, where X˙ is a chain carrier and RʹH a molecule with a labile hydrogen atom. In acetylene oxidation the hydrogen abstraction reaction is too slow to compete with alternative reactions of the acyl radical and hence it is not branched.