The electron impact mass spectral behaviour of 2-[acyl(alkyl)amino]oxazoles has been investigated. When the acyl group is aliphatic, the major fragment ions arise either from cleavage of the substituent or fragmentation of the nucleus with simultaneous elimination of a hydroxyl radical. The elimination of a heterocyclic atom in this way is diagnostic, but does not occur from the molecular ion. Analysis with compounds containing a deuterium label has confirmed that the hydrogen lost in the elimination originates from both a site on the N-alkyl chain and a position adjacent to the carbonyl group. A mechanism accounting for the 1 : 1 participation of these hydrogen atoms is suggested. For alpha,o-bis [N-acyl-N-(4-methyloxazol-2-yl) amino] -alkanes the elimination of a hydroxyl radical depends on the length of the methylene bridge and for 2--dialkylamino-oxazoles the elimination occurs also from the molecular ion. Although the presence of 5-alkyl substituents does not affect the fragmentation of 2-[acyl(alkyl)amino]-oxazoles, introduction of oxygen into the 5-substituent effects significant changes in mass spectral behaviour; these are detailed for various groupings. N-Butyl-N-(4-methyloxazol-2-yl)benzamides only lose a hydroxyl radical following extrusion of carbon monoxide. A correlation between the ratio of the relative abundance of the molecular ion to that of the acylium ion, and the substituent o constants of groups in the aromatic ring is observed for these molecules at an ionising voltage of 10 eV. The electron impact mass spectral behaviour of further five-membered heterocycles containing an acyl-(alkyl)amino-substituent has also been investigated. Minimal ring scission is observed for compounds with one heteroatom in the ring. 2-[Acyl(alkyl)amino] thiazoles show elimination of an [SH] radical, whilst isothiazoles only show the elimination when the acylamino-substituent is adjacent to the sulphur atom. Thiazoles and oxadiazoles show the loss of methyl cyanide from the ring in addition to the loss of either [SH] or [OH] radicals. Elimination of nitrogen is only observed for a tetrazole compound. Some one hundred and twenty mass spectra are discussed, and representative examples of one quarter of these are appended (as low resolution spectra) for reference
