The fragmentation patterns observed for compounds such as carboxylic acids, esters, and amides in the mass spectra include ⍺-cleavage and McLafferty rearrangement. Fragmentation by ⍺-cleavage preferentially occurs at the carbon-carbon bond at the ⍺-position next to the carboxylic group to generate a neutral radical and a cation. Long chain compounds with hydrogen at their γ-carbon undergo McLafferty rearrangement to give a radical cation and a neutral alkene.
For example, the fragmentation of butyramide is demonstrated in Figure 1. The molecular ion peak of butyramide, which contains one nitrogen atom, shows an odd mass-to-charge ratio of 87. Fragmentation of butyramide molecular ion by ⍺-cleavage produces a cation at a mass-to-charge ratio 44. In addition to ⍺-cleavage, butyramide molecular ion also undergoes fragmentation via McLafferty rearrangement. The base peak at the mass-to-charge ratio 59 arises due to McLafferty rearrangement.
Carboxylic acids, esters, and amides undergo fragmentation in the mass spectrometer by ⍺-cleavage and McLafferty rearrangement.
Fragmentation by ⍺-cleavage yields a neutral radical and a cation.
On the other hand, fragmentation by McLafferty rearrangement yields an alkene and a radical-cation.
Consider the mass spectrum of butanoic acid where the fragmentation of the molecular ion upon ⍺-cleavage forms a [COOH]+ cation at a mass-to-charge ratio of 45.
Notably, the McLafferty rearrangement produces a base peak at the mass-to-charge ratio of 60.
Similarly, the mass spectrum of methyl butanoate shows peaks at mass-to-charge ratios of 71 and 59 arising from ⍺-cleavage. In contrast, the peak at a mass-to-charge ratio of 74 is produced due to McLafferty rearrangement.
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