Alkanes fragmentation mass

Lavanchy, A. Houriet, R. Gaumann, T. The Mass Spectrometric Fragmentation of n-Alkane. Org. Mass Spectrom. 1979, 74, 79-85. 

Branched alkanes fragment more readily than straight chain alkanes, so branched alkanes are less likely to show a molecular ion peak than n-alkanes. Cycloalkanes show a strong molecular ion peak and also show the characteristic peaks separated by 14 Da. Figure 10.14 is the mass spectmm of hexane, CeHi4, MW = 86, and Fig. 10.15 is the mass spectmm of cyclohexane, CeHi2, MW = 84. Cyclohexane has a stronger molecular ion... 

The mass spectrum in Figure 4.7 contains what appears to be a molecular ion at m/z 100 and typical alkane fragments at m/z 43, 57, and 71. With no particular fragment indicating preferred fragmentation, this spectrum corresponds to n-heptane... 

FIGURE 29, The GC/MS/C analysis of the saturated hydrocarbon fraction of an Israeli crude oil (a) total ion current (TIC) (b) trace of fragment mjz 133.133, typical of open-chain alkanes (c) trace of fragment (mass fragmentogram) m/z 133.133, typical for most tri-, tetra- and pentacyclic terpanes, dominated by the hopanes (d) the m/z 217.196 mass fragmentogram, steranes, and (e) the m/z 218.203 fragmentogram, also steranes Traces b-e are SIM of accurate mass (AM) with RP of 5000... 

FIGURE 15.12 As illustrated by the mass spectrum of 2,2-dimethylpropane, alkanes fragment to give the most stable carbocation possible. 

A. Lavanchy, R. Houriet, T. Gaumann, The mass spectrometric fragmentation of n-alkanes, Org. Mass Spectrom. 1979,14, 79. 

Some classes of compounds are so prone to fragmentation that the molecular ion peak IS very weak The base peak m most unbranched alkanes for example is m/z 43 which IS followed by peaks of decreasing intensity at m/z values of 57 71 85 and so on These peaks correspond to cleavage of each possible carbon-carbon bond m the mol ecule This pattern is evident m the mass spectrum of decane depicted m Figure 13 42 The points of cleavage are indicated m the following diagram... 

For linear alkanes, the initial fragment lost is an ethyl group (never a methyl group), followed by propyl, butyl, and so on. An intense peak at mass 43 suggests a chain longer than butane. 

Nepeta (Lamiaceae) is a genus of perennial or annual herbs found in Asia, Europe and North Africa. About 250 species of Nepeta are reported of which, 67 species are present in Iran. Some species of this genus are important medicinal plants and their extracts have been used for medicinal purposes. Aerial parts of Nepeta sintenisii Bornm. was subjected to hydrodistillation and the chemical composition of isolated essential oil has been analyzed by GC/MS method for first time. Identification of components of the volatile oil was based on retention indices relative to n-alkanes and computer matching with the Wiley275.L library, as well as by comparison of the fragmentation patterns of the mass spectra with those reported in the literature. 

The molecular ion intensity decreases with increased branching, therefore the molecular ion peak may be nonexistent. The loss of 15 Daltons from the molecular ion indicates a methyl side chain. The mass spectra of branched alkanes are dominated by the tendency for fragmentation at the branch points, and hence are difficult to interpret. 

Duffield and coworkers65 studied the El- induced mass spectra of five arene- (215-219) and four alkane sulfonylthioureas (220-223) and observed two rearrangement processes, namely loss of S02 from 215-219 and the elimination of ArS02 and RS02 with the thione sulfur atom from 215-223. The other fragmentations involved simple bond cleavages with and without hydrogen transfer (equation 48). The loss of H2S was evident for all the compounds studied except 221 and 222. It was, however, found to be a thermal and not an ionization process. 

The majority of marine isonitriles are sesquiterpenes with the molecular formula, C16H25N. Often cyclic, these are alkanes or alkenes possessing only a single isocyano-related functional group. In the mass spectrum, they exhibit a molecular ion at w/z 231, or an intense fragment ion at m/z 204, indicative of M+-HCN. Some are crystalline (see Table 2). With few exceptions most of the isothiocyano and formamido analogs are minor noncrystalline metabolites (see Table 3). 

The correct analysis of the homologous ion series has certain limitations. Low abundances of peaks in some series require the attention and experience of a researcher. Usually alkane series are dominated in the mass spectra of the most various compounds. Fragmentation initiated by one functional group may completely suppress or notably camouflage other reactions of polyfunctional substances. In the latter case it is useful to consider IR-spectroscopy data in mass spectral interpretation. 

Figure 8.6 Mass spectrum of C34 w-alkane (C34H70). The complete molecule appears at M = 478 and various fragment ions (m/z = 57, 71, etc.) at lower masses. The fragmentation pattern is shown on the molecular structure.

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