Mass Spectrometry: Carboxylic Acid, Ester, and Amide Fragmentation

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.