MECHANISTIC STUDY OF THE COLLISIONALLY ACTIVATED DISSOCIATION OF ALKOXIDE ANIONS

The collisionally activated dissociation (CAD) spectra of several alkoxide anions originating from alcohols by O-·/OH- negative chemical ionization were studied. As in unimolecular decomposition, the bimolecular reaction of alkoxide anions leads to the loss of a hydrogen and/or an alkane neutral molecule and formation of an enolate anion. This is a stepwise reaction leading in the first step to an anion-ketone complex, followed by the abstraction of an α-hydrogen from the ketone to yield the enolate anion and a neutral molecule (H2 and/or RH) in the next step. This reaction is thermodynamically controlled, and hence when two different α-hydrogen groups are present in a non-symmetric intermediate ketone, two enolate isomeric ions could result. The acidity of the different α-hydrogen groups in the governing factor of the contribution of each isomer in the resulting enolate ion. The relative intensities of the peaks are discussed in terms of the stability of the ion in the intermediate complex. Concerning the order in which groups leave the intermediate complex, small groups tend to leave more easily than larger ones. For a detailed study, the 2-butoxide anion labelled with deuterium in different positions was chosen. © 1990.