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Hydrogen randomization precedes the formation of M+--H. and M+. -CH; species from the stilbene molecular ion at 15 eV. The carbon atom involved in the M+. -CH; elimination originates randomly from the whole molecule. The [M -151 ion (m/e 165) in the spectra of stilbene and 9,lO-dihydrophenanthrene is produced from a common ion.

The mass spectra of a variety of alkyl and aryl oxazoles have been determined and the spectra analyzed with the aid of deuterium labelling and high resolution mass spectrometry. In contrast to the corresponding benzenoid compounds, the mass spectra of isomeric alkyl oxazoles are distinctive and in this respect are akin to those of the corresponding pyridines. Further analogy to the pyridines is suggested by the unfavorable nature of a carbonium ion adjacent to the 2-position and this effect may be used to locate alkyl substituents attached to the oxazole nucleus. The loss of carbon monoxide from the molecular ions of 2,5-disubstituted oxazoles probably occurs with ring opening and migration of the C-5 substituent (e.g. Br) to the C-4 position.ALTHOUGH the behaviour of many aromatic five-membered heterocyclic ring systems upon electron impact is now fairly well documented: a detailed study of the mass spectra of oxazoles has not been reported.s This paper deals with the mass spectra of representative alkyl-, phenyl-and alkylphenyl-oxazoles. Details of the spectra are summarized in Table 1 and Figs. 1-12.The mass spectrum (Table 1) of oxazole (I) itself is typical of that of an unsubstituted aromatic compound insomuch as the molecular ion (m/e 69) constitutes the base peak. The most abundant fragment ions in the high mass region occur at M-1 (8 %), M-27 (M-HCN, 13 %), M-28 (M-H,CN and/or M-COY 21 %) and M-29The spectra of the isomeric compounds 2,4-dimethyloxazole (11 , Table 1) and 4,5-dimethyloxazole (111, Fig. 1) show characteristic differences. This behaviour is in contrast to that of the isomeric xylenes4 and ethylmethylbenzenes,6a which give virtually identical spectra, but akin to that of three isomeric ethylmethylpyridines,60 which give distinctive spectra. Apparently, the presence of heteroatoms in the ring system prevents, or greatly retards, those processes by which the molecular ions from the isomeric xylenes, or ethylmethylbenzenes, become equivalent. The M-1 peak from 111 is considerably more abundant (12 %) than that (2 %) from 11. These data parallel those for 3-ethylpyridine versus 2-ethylpyridine, the former exhibiting a much more abundant M-15 the greater stability of carbonium ion a relative to carbonium ion b rationalizes the data, just as in the pyridines. Hence the M-l'peak from 2,4dimethyloxazole (11) is probably formed largely by loss of a hydrogen atom from the 4-methyl group (see c) (the bond fixation within the oxazole nucleus accounts for the greater stability of c relative to b), while the M-1 peak from I11 will probably correspond to the formation of both a and d.(M-CHO, 37%). 13

The mass spectra of substituted pyridazines, phthalazines, and related compounds are reported and discussed. Molecular ions are a prominent feature of all the spectra, and fragmentation modes may be usefully correlated with both the type of heterocycle and its substitution pattern. Fragmentation patterns have been substantiated by extensive high resolution studies and appropriate metastable ions.

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