It has been shown that flash thermolysis of ohydroxyacetophenone occurs with keto-enol rearrangement and subsequent elimination of water to yield benzofuran' (Scheme 1). This elimination has not been found to occur from the molecular ion of ohydroxyacetophenone in the mass spectrometer.' Since the first step of this process, i.e. enolization of the ketone, has not been observed for molecular ions of aromatic ketones: it is not surprising that upon electron impact o-hydroxyacetophenone eliminates a methyl radical like unsubstituted acetophenone3 and not a molecule of water. 0OH

No abstract

It is shown by "N and specific 13C labelling that -50% of the molecules of hydrogen cyanide, eliminated within -10-6s upon electron impact of benzonitrile, contains the original cyano carbon atom, whereas the remaining percentage contains one of the phenyl ring carbon atoms at random. This is even more dramatic for the molecular ions of benzonitrile which decompose in the first and second field-free regions of the VG Micromass ZAB-2F high-field mass spectrometer used. Then only 5-7% of the eliminated molecules of hydrogen cyanide contains the original cyano carbon atom. A cycloaddition-cycloreversion process in the molecular ions, leading to ionized l-cyano-1,3-hexadien-5-yne as an intermediate in the hydrogen cyanide loss, is proposed to explain this.

The mechanism of propene loss from the metastable [M ϩ D] ϩ ions of isomeric 2-, 3-, and 4-n-propoxypyridines and the related isopropoxypyridines has been examined by chemical ionization (CI) and tandem mass spectrometry in combination with deuterium labeling. The (1) The loss of propene by ion-neutral complex formation and the occurrence of a substantial isotope effect in the subsequent proton/deuteron transfer within the complex, and/or (2) the loss of propene by a 1,2-elimination type reaction.

The loss of CHO,' from the molecular ion of phenoxyacetic acid and the expulsion of an H' atom from ionized anisole lead to phenoxymethylene ions, which fragment predominantly by CO loss on the microsecond time-scale. Carbon-13 labelling reveals that -90% of the CO molecules expelled from the metastable ions derived from phenoxyacetic acid incorporate the carbon atom from the 1-position of the phenyl group of the parent compound, whereas the residual C O molecules contain one of the other carbon atoms of the aromatic ring. The 2-fluoroand 2-methylphenoxymethylene ions derived from the appropriate aryloxyacetic acids behave similarly, i.e. the carbon atom of the methylene group of the parent compound is not incorporated in the expelled CO molecules. In contrast, -45% of the C O molecules eliminated from the metastable phenoxymethylene ions formed from ionized anisole contain the carbon atom of the methyl group, while the remaining part contains the carbon atom from the 1position of the phenyl ring of the parent compound. This result is taken as evidence for the Occurrence of a skeletal rearrangement of the anisole molecular ion leading to an interchange between the carbon atom of the methyl group and the carbon atom at the 1-position of the ring. The elimination of C O from the metastable ions generated from either phenoxyacetic acid or anisole gives rise to a composite metastable peak. Conclusive evidence as to the formation of [ C,H,O) + isomers other than the phenoxymethylene ion is not obtained, indicating that the composite metastable peak is a result of two competing reactions both leading to CO loss. Possible mechanisms of these reactions are discussed together with the mechanism of the skeletal rearrangement of the molecular ion of anisole prior to H' loss.