Fragmentation Patterns of Amines

The value of the mass of the molecular ion can be of great help in identUying a substance as an amine. As stated in Section 3.6, a compound with an odd number of nitrogen atoms must have an odd molecular weight. On this basis, it is possible to quickly determine whether a substance could be an amine. Unfortunately, in the case of aliphatic amines, the molecular ion peak may be very weak or even absent. 

The most intense peak in the mass spectrum of an aliphatic amine arises from a-cleavage  

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When there is a choice of R groups to be lost through this process, the largest R group is lost preferentially. For primary amines that are not branched at the carbon next to the nitrogen, the most intense peak in the spectrum occurs at miz = 30. It arises from a-cleavage  

The presence of this peak is strong, although not conclusive, evidence that the test substance is a primary amine. The peak may arise from secondary fragmentation of ions formed from the fragmentation of secondary or tertiary amines as well. In the mass spectrum of ethylamine (Fig. 4.55), the mIz = 30 peak can be seen clearly. 

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Attention should be paid to the appearance of spurious peaks in the fragmentation patterns of amines determined by GC-MS, when the analytes came into contact with methanol or ethanol as solvents. Thus, for example, Schiff bases may be formed on condensation of a primary amine with traces of formaldehyde or acetaldehyde present in the solvent. Although the peaks of such product may be unresolved in the chromatogram, they may appear as ions with mass increments of +12 or +26 in the mass spectrogram, complicating the identification of the analyte, as was the case with some amphetamine drugs99. 

The mass spectral fragmentation patterns of amines also aid in structural assignments. For example, the mass spectrum of V,V-diethylethanamine (triethylamine) shows the peak for the molecular ion at m/z = 101 (Figure 21-5). However, the more prominent base peak is at m/z = 86 and is caused by the loss of a methyl group by a-cleavage (Section 11-10). Such fragmentation is favored because it results in a resonance-stabilized iminium ion. 

C. Mass and N.M.R. Spectra of Steroidal Amines.—A study has recently been made18 of the mass spectral fragmentation pattern of the dimer (28), obtained by reduction of the cyanamide of conanine with lithium aluminium hydride.19 As with certain benzylamines, a major fragment has been interpreted to arise from a heterolytic rupture of the C—N= bond. 

Fragmentation patterns of free 1-boraadamantanes are unknown. For the complexes of 1-boraadamantanes with amines, the most characteristic are [M —H]" ions formed as a result of easy hydride elimination <88IZV2282). 

A comparative study of the fragmentation pattern of holacurtine (59) by electronic impact and by chemical ionization has been carried out. " These two mass spectrometric techniques gave complementary information. By electronic impact, the base peak is m/e 87, indicating the close vicinity of the ether and amine... 

Mass spectrum of 3-methyl-l-butanamine (isopentylamine). The most characteristic fragmentation pattern of aliphatic amines is )3-cleavage. 

The compounds involved and detected by analytical GC-MS workflows like volatiles, PCBs, or pesticides are discussed with representative mass spectra in the following sections. These compounds do not necessarily belong to the same functional compound class with a chemical classification for instance of amines, ester, phenols, hydrocarbons and others. Many standard references cover these characteristic fragmentation patterns of specific functional groups and compound classes in detail (Howe, 1981 McLafferty and Turecek, 1993 Budzikiewicz, 1998). 

An example of how information from fragmentation patterns can be used to solve structural problems is given in Worked Example 12.1. This example is a simple one, but the principles used are broadly applicable for organic structure determination by mass spectrometry. We ll see in the next section and in later chapters that specific functional groups, such as alcohols, ketones, aldehydes, and amines, show specific kinds of mass spectral fragmentations that can be interpreted to provide structural information. 

The application of CID(+) to the polyglycol amines in the APCI-FIA-MS-MS(+) mode showed a product ion pattern with ions equally spaced with A m/z 44 starting at m/z 212 as shown in Fig. 2.5.6(c). In addition, the inset also contains the fragmentation scheme of the fatty acid polyglycol amine parent ion at m/z 670. 

The mass spectrum of pyrrolopyridine 44, prepared from the reaction of A -hydroxylquinolinimide with aryl amines at elevated temperatures (250 °C), shows interesting fragmentation patterns corresponding to the loss of CO (Scheme 1) <1998SC2871>. 

Tandem fragmentation-cyclization of bicyclic ketones connected to unsaturated side chains have been used to construct hi-, tri-, and spirocyclic ketones [334], Cossy et al. have shown that depending upon the substitution pattern, ketyl radical anions obtained from photochemically induced electron transfer from amines to cyclopropylketones lead either to the formation of 3-substituted cycloalkanones or to ring expanded products (Scheme 78) [335],... 

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