Reactions with Alkoxide Anions

The development of negative chemical ionization (NCI) as a technique in the field of analytical mass spectrometry has stimulated interest in detailed studies of processes involving negative ions such as O-·, OH-, alkoxide and halide anions [1]. In a recent publication, we have demonstrated the potential use of the reactant ion 0-· to probe the structure of aliphatic alcohols [2]; it was found that the product distribution in these systems allows for analysis of structural isomers. On the other hand, reaction of OH- with alcohols resulted in formation of the alkoxide anion, thus allowing for the determination of molecular weights; only minor formation of the corresponding enolate anion was observed. It could be that the latter product ion might be formed via pyrolytic dehydrogenation of the alcohol molecule to the corresponding aldehyde or ketone that would in turn transfer a proton to a strong base such as OH to form the enolate anion. Preliminary results [2] indicated that under our conditions (very diluted alcohol.N₂O or alcohol/H₂O mixtures), pyrolysis plays a minor role and that the formation of enolate occurs in a collisionary induced loss of H₂ from the alkoxide anion, reaction 1, in which M stands for a bath gas molecule.