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.
Amines undergo ⍺-cleavage in the mass spectrometer to yield a resonance-stabilized nitrogen-containing cation and an alkyl radical.
Interestingly for amines, the number of nitrogen atoms in the compound affects the molecular weight of the molecular ion, also called the nitrogen rule of mass spectrometry.
As per the rule, a neutral organic compound containing an odd number of nitrogen atoms will produce a molecular ion with an odd-numbered molecular weight.
For instance, consider the mass spectrum of triethylamine, which has one nitrogen atom.
Here, the molecular ion has an odd mass-to-charge ratio of 101.
Fragmentation of the molecular ion upon ⍺-cleavage frees a methyl radical and generates a base peak at a mass-to-charge ratio of 86.
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