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 molecular ion with an odd molecular weight.
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.
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 atoms generates a molecular ion with an odd 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.
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