Low-Energy,Low-Temperature Mass Spectra. Part 17: Selected Aliphatic Amides

The 12 eV, 350 K electron impact ionisation mass spectra of 28 aliphatic amides of diverse structure are reported and discussed. All the spectra contain strong molecular ion signals, often corresponding to the base peak, thus providing unequivocal molecular mass information. Structural information is also accessible from specific primary fragmentations associated with the substituent(s) attached to the carbon and nitrogen atom of the amide group. Alkene elimination from the ionised amides takes place most readily from the substituent attached to the carbon atom via a McLafferty rearrangement, but it also occurs to a lesser degree from some ionised amides with no γ-hydrogen atom. In contrast, alkyl radical loss is usually associated with fission of an N-alkyl group, typically by apparent α-cleavage. Other primary fragmentations of significance include expulsion of an alkenyl radical derived from an N-alkyl substituent. In one such process, the eliminated alkenyl radical has one carbon atom fewer than the initial N-alkyl substituent; this fragmentation is preferentially associated with the N-neopentyl group and, to a lesser degree, the N-isobutyl group. A related process involves loss of an alkenyl radical with the same number of carbon atoms as the N-alkyl group; this reaction occurs for all four ionised N-butylformamides, but is most favoured for the isobutyl and t-butyl isomers. Both these processes entail double hydrogen transfer with the formation of an exceptionally stable protonated amide as the ionic product. Further dissociation of the primary fragment ions to give secondary fragment ions occurs only relatively rarely. The differences in the spectra of isomeric amides in which only the structure of an N-alkyl group is varied are often quite pronounced, especially in the case of n-butyl and isobutyl substituents, thus enhancing the analytical value of these low-energy, low-temperature mass spectra.