Formation of 2-methylbenzopyrylium ions from the molecular ions of benzalacetones

Abstract

It can be shown by the method of metastable ion characteristics that different C 10H 9O + ions do not rearrange to a common reactive form. The metastable ion characteristics of C 10H 9O + ions can therefore be used to determine the structure of the ion. Using these methods, metastable C 10H 9O + derived from the molecular ions of substituted benzalacetones 1 are shown to correspond to 2-methyl-benzopyrylium ions 3. This confirms that the loss of a hydrogen atom or a substituent from the phenyl group of the molecular ions of 1 occurs by an intramolecular aromatic substitution reaction. 3 is formed not only from ortho-substituted benzalacetones, but also from the meta- and para-substituted derivatives. In the latter cases a sufficiently long-lived intermediate must be formed, which reacts to produce 3 by hydrogen migrations prior to elimination of the substituent. This reaction model is corroborated by a calculation of the dependence of ion abundances on the electron energy, and of the frequency factors of the gas phase reactions of the three isomeric chlorobenzalacetones.