19.11:
Mass Spectrometry of Amines
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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.
19.11:
Mass Spectrometry of Amines
In mass spectroscopy, amines undergo fragmentation to give parent ions with odd molecule weights. This observed mass spectrum follows the nitrogen rule: a molecule with an odd number of nitrogen atoms produces a parent ion with an odd molecular weight. The remaining fragments have an even mass.
Amines undergo fragmentation through α cleavage, producing nitrogen-containing cations—iminium ions—and alkyl radicals. Mass spectra of aromatic and cyclic aliphatic amines exhibit strong molecular ion peaks, but acyclic aliphatic amines show weaker molecular ion peaks.