19.11 : Mass Spectrometry of Amines

In mass spectrometry, molecules are ionized and broken down into relatively stable, smaller species.

In a mass spectrometer, amines undergo fragmentation by breaking the bond between α  and β carbons—also called α cleavage— producing an iminium ion and an alkyl radical.

The mass spectrum of an amine follows the nitrogen rule—a compound with an odd number of nitrogen atom generates a parent ion with an odd molecular weight and a fragment ion with an even molecular weight.

For example, butyl propyl amine has a parent ion peak at m/z 115, confirming the presence of odd number of nitrogen atoms in the molecule.

Since the molecule has two alkyl chains connected to N, α cleavage can occur via two modes.

α cleavage on the butyl side chain yields iminium ion with loss of propyl radical. The iminium fragment peak at m/z 72  is the base peak.

α cleavage on the propyl side chain yields an iminium ion with a peak observed at m/z 86, along with the loss of ethyl radical.