Cleavage of Aliphatic Amines

The intensity of the molecular ion of aliphatic amines decreases with increasing molecular weight [29], the reason being their predisposition to a-cleavage. Analogous tendencies are observed for aliphatic ethers [27], hydrocarbons [30], aldehydes [20], and others. 

A comparison of the relative strength of functional groups to cause a-cleavage is summarized in Table 6.5 [31]. This also corresponds to a rough measure of relative charge-stabilizing capability of the respective substituent, e.g., the ratio H2C=0H /H2C=NH2 from 2-amino ethanol molecular ion is 2.3/100 and the ratio of H2C=0H /H2C=SH from 2-thio ethanol molecular ion is 42/70 [16]. 

As with acylium ions and carbenium ions before, the series of homologous im-monium ions is part of the mass spectrometrist s tool box. They can easily be recognized in the mass spectra and have even-numbered m/z values (Table 6.6). In the El spectrum of A-ethyl-A-methyl-propanamine the series is completely present from m/z 30 up to m/z 100. 

Note In the El mass spectra of primary amines the methylene immonium ion, CH2=NH, m/z 30, resulting from a-cleavage either represents the base peak or at least is the by far most abundant of the immonium ion series. 

Even though a-cleavage seems to be a purely electronic process at first sight, the influence of ion internal energy and of the alkyl chain should not be neglected. 

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