15.9 : Mass Spectrometry: Amine Fragmentation

Amines can be identified using mass spectroscopy based on their characteristic fragmentation patterns. The molecular ions of amines undergo fragmentation via ⍺-cleavage. The ⍺-cleavage of the carbon-carbon bonds in amines generates an alkyl radical and resonance-stabilized nitrogen-containing cation.

In amines, the number of nitrogen atoms affects the mass of the molecular ion, which is described by the nitrogen rule of mass spectrometry. This rule states that a compound containing a single or an odd number of nitrogen atoms produces a molecular ion with an odd molecular weight, while a compound with no or an even number of nitrogen atoms generates a molecular ion with an even molecular weight.

For example, the mass spectrum of 3-methyl-1-butanamine, which contains one nitrogen atom, displays a molecular ion having an odd mass-to-charge ratio of 87. Fragmentation of this molecular ion via ⍺-cleavage produces a base peak, which features a mass signal at a mass-to-charge ratio of 30, corresponding with the loss of the 2-methyl propyl radical.

Similarly, the fragmentation of triethylamine via ⍺-cleavage generates a cation at a mass-to-charge ratio of 86. The molecular ion peak shows an odd mass-to-charge ratio of 101, as shown below.