Cleavage of Amines, Ethers, and Alcohols

The intensity of the molecular ion of aliphatic amines decreases regularly with increasing molecular weight. [33] Analogous general behavior has also early been noted for other compound types such as the aliphatic ethers [31], hydrocarbons [34] and aldehydes [24], and others. The reason for the low stability of amine molecular ions is their predisposition to a-cleavage. 

A comparison of the relative strength of functional groups to cause a-cleavage is summarized in Table 6.5. [6] 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. [20] 

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 (Tab. 6.6). In the El spectrum of iV-ethyl-iV-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=NIT, 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. 

Restricting to the more common elements in organic mass spectrometry (H, B, C, N, O, Si, S, P, F, Cl, Br, I, etc.), a simple rule holds valid With the exception of N, all of the above elements having an odd number of valences also possess an odd mass number and those having an even number of valences have even mass numbers. This adds up to molecular masses fulfilling the nitrogen rule (Tab. 6.7). 

---