Aromatic compounds mass spectra

To distinguish between azobenzene and benzophenone, assuming reference spectra are not available for these compounds, it is a good idea to examine the mass spectra of aromatic ketones, such as acetophenone, butyrophenone, diphenyldiketone, and so forth. Complete identification is assured by obtaining or synthesizing the suspected component and analyzing it on the GC/MS system under the same GC conditions. If the retention time and the mass spectrum agree, then the identification is confirmed. 

The mass spectrum of 2-methylbenzaldehyde suggests an aromatic compound because of the intensity of the molecular ion and peaks at m/z 39, 51, and 65 (see Figure 6.2). The loss of hydrogen atoms and loss of 29 Daltons from the molecular ion indicate that this is an aromatic aldehyde. Looking up m/z 91 in Part III suggests the following structure ... 

D. Sample Mass Spectrum of an Aromatic Halogenated Compound... 

Figure 17.2 is an example of a mass spectrum of an aromatic dichloro compound. The intensity of the molecular ion indicates that an aromatic compound is present. The isotope pattern is that of two chlorines, and subtracting 70 mass units from the molecular ion gives the formula QHj. (See Example 2.3 in Chapter 2 for another example of isotope abundances in the molecular ion region.)... 

Enhanced molecular ion implies reduced matrix interference. An SMB-El mass spectrum usually provides information comparable to field ionisation, but fragmentation can be promoted through increase of the electron energy. For many compounds the sensitivity of HSI can be up to 100 times that of El. Aromatics are ionised with a much greater efficiency than saturated compounds. Supersonic molecular beams are used in mass spectrometry in conjunction with GC-MS [44], LC-MS [45] and laser-induced multiphoton ionisation followed by time-of-flight analysis [46]. 

Leurosine (75) (Scheme 20) is the most abundant of the dimeric antitumor alkaloids isolated from Catharanthus roseus G. Don. Several species of Strep-tomyces produced a common metabolite of the alkaloid, and S. griseus (UI1158) was incubated with 400 mg of leurosine sulfate to obtain 28 mg of pure metabolite (180). The metabolite was identified as 76 primarily on the basis of its H-NMR spectrum. The mass spectrum indicated that the metabolite contained one oxygen atom more than 75. The H-NMR spectrum contained all of the aromatic proton signals of the vindoline portion of the molecule, and aromatic proton signals for the Iboga portion of the compound occurred as a doublet of doublets... 

Check which ionization method was used and examine the general appearance of the mass spectrum. Is the molecular ion peak intensive (as with aromatic, heterocyclic, polycyclic compounds) or weak (as with aliphatic and multifunctional compounds) Are there typical impurities (solvent, grease, plasticizers) or background signals (residual air, column bleed in GC-MS) ... 

Mass spectral data have frequently been used in the structural determination of boron heterocycles. One paper has been devoted to the mass spectra of some six-membered boron-nitrogen systems. It was concluded that the spectra could be interpreted analogously to their hydrocarbon counterparts. In all cases the molecular peak was the base peak of the spectrum (68T6755). Doubly charged molecular ions, a feature typical of aromatic compounds, are often encountered. It should be noted, however, that some certainly non-aromatic aminoboranes give such doubly charged ions as well. 

The following table lists the most common substituents encountered in benzene rings and the neutral particles lost and observed on the mass spectrum.1 Complex rearrangements are often encountered and enhanced by the presence of one or more heteroatomic substituent(s) in the aromatic compound. All neutral particles that are not the product of rearrangement appear in parentheses and are produced alongside the species that are formed via rearrangement. Prediction of the more abundant moiety is not easy, as it is seriously affected by factors that dictate the nature of the compound. These include the nature and the position of any other substituents, as well as the stability of any intermediate(s) formed. Correlations of the data with the corresponding Hammett a constants have been neither consistent nor conclusive. 

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