Gas-phase, pinacol-type rearrangements in the unimolecular decomposition of some isomers of C4H9O+

Abstract

The slow unimolecular reactions undergone by three isomers of C₄H₉O⁺ are explained in terms of a simple ‘mechanistic’ approach. For two such isomer, protonated butanone and protonated isobutyraldehyde equilibration via a pinacol-type rearrangement precedes dissociation. The third isomer, protonated butyraldehyde, reacts differently because the energy needed to cause isomerisation of this to the other ions is greater than that needed to dissociate any of the three isomers considered. Extension of the analysis, using the potential surface approach, explains the non-occurrence of some mechanistically plausible reactions (e.g. C₃H₆ loss) and results of deuterium labelling studies.