Electron impact induced fragmentation of β-allenic and γ-acetylenic alcohols

The main fragmentation pathway of ionized hydroxyallenes (1) consists of a methyl loss. Extensive deuterium-labelling experiments indicate that the terminal allenic carbon is implied in this fragmentation. Collisional activation spectra indicate a propenyl-acylium structure (a) for these [M – CH₃]⁺ ions which can originate from a 1,4-hydroxyl migration followed by hydrogen rearrangements. Isomeric hydroxyacetylenes (2) behave similarly, also giving rise, by methyl loss, to acylium ions a. It is proposed that 2^{+ ˙} is irreversibly isomerized into 1^{+ ˙} by a 1,3-hydrogen transfer ‘catalysed’ by the hydroxy group. The proposed internal proton-bound complex justifies also the easier loss of water from 2⁺˙. Ethyl loss is also a prominent fragmentation for the hydroxyallene and hydroxy-acetylene homologues.