Mass Spectrometric Fragmentation Reactions of Aliphatic Carboxamides. A Reinvestigation Using Metastable Ions Reveals New Rearrangements

The mass spectrometric fragmentations of aliphatic carboxamides have been reinvestigated by using caproamide 2 and its N-methyl-and N,N-dimethyl derivatives 3 and 4 as model compounds and by studying the unimolecular reactions of metastable molecular ions. Whereas the EI (70 eV) mass spectra of caproic acid 1 and the amides 2–4 suggest a very similar behavior of the unstable molecular ions of these compounds, the mass analyzed ion kinetic energy (MIKE) spectra of metastable molecular ions indicate distinct differences. Of the three fragmentation pathways observed in the MIKE spectra, by far the most important for the molecular ions of the acid 1 is the loss of an ethyl radical which results in the formation of protonated methacrylic acid by a 1,2–shift of the protonated carboxyl group in intermediate distonic ions. The metastable molecular ions of the amides 2–4 also fragment to a significant extent by loss of a propyl radical and by the McLafferty rearrangement, the latter in particular for the tertiary molecular ions of 4. An even more significant difference is the formation of protonated vinylacetamides instead of protonated methacrylamides from the molecular ions of the amides 2–4. An analysis of the MIKE spectra of specifically deuterated derivatives of 2–4 shows that first, in contrast to the reactions of metastable molecular ions of the acid 1, the McLafferty rearrangement of the amide ions 2–4 occurs without significant H/D exchange reactions involving D atoms at the γ-position. This implies that the McLafferty rearrangement of the amide ions is energetically more favored. Second, it is shown that H-shifts from the alkyl chain to the amide group are followed by 1,5– and 1,6–shifts (“long-distance shifts”) of the protonated amide group in the intermediate distonic ions. Thus, the metastable molecular ions of aliphatic carboxamides are mainly distinguished from those of the corresponding carboxylic acids and methyl esters by a preference for a fragmentation by the McLafferty rearrangement and for long-distance shifts of the protonated functional group in intermediate distonic ions.