Fragmentation Patterns of Carbonyl-Containing Compounds

The molecular ion peak of an aliphatic aldehyde is usually observable, although at times it may be fairly weak. Principal modes of fragmentation include a-cleavage and P-cleavage. If the carbon chain attached to the carbonyl group contains at least three carbons, McLafferty rearrangement is also commonly observed. 

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FIGURE4.40 EI-MS spectrum of (Z)-9-octadecen-4-olide and its dimethyldisulfide derivative showing. Cleavage between thioether groups (from Cosse, A. A., et al, Journal of Chemical Ecology 27 (2001) 1841-1853.) 

The appearance of an M - 1 peak due to the loss of one hydrogen atom is very characteristic of aldehydes. This peak is observed at miz = 85 in the mass spectrum of valeraldehyde (Fig. 4.41). The peak due to the formation of HCO can be observed at mIz = 29 this is also a very characteristic peak in the mass spectra of aldehydes. The second important mode of fragmentation for aldehydes is known as P-cleavage (inductive cleavage). In the case of valeraldehyde, P-cleavage creates a propyl cation (m/z = 43). 

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