Alkanes mass spectra

As pointed out, carbenium ions make up the largest portion of alkane mass spectra and dissociate further by alkene loss. This type of fragmentation is rather specific, nevertheless isomerizations of the carbenium ion can happen before. Generally, such processes will yield a more stable isomer, e.g., the tcrt-butyl ion instead of other [C4H9] isomers (Fig. 6.19). 

Straight-Chain Alkanes Mass spectra of straight-chain hydrocarbons are characterized by low-abundant M+ ions. These compounds undergo principal ionization at the C—C o-bond and fragment all along the hydrocarbon chain by... 

Alkane mass spectra tend to show firagmentation that occurs toward the middle of unbranched chains, forming a series of fragments differing by 14 amu. 

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... 

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). 

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.

DDT converts in part to p,p -DDE over time in the environment, especially in sediments [151, 171]. An example of the total aliphatic extract of a sediment from the Los Angeles Bight contaminated with p,p -DDE is shown in Fig. 11. The TIC trace shows a major UCM and the minor resolved peaks are normal alkanes (primarily from higher plant wax), with mature 17a(H),21/ (H)-hopanes (from petroleum residues as is the UCM). The mass spectrum ofp,p -DDE is shown in Fig. 12a, registering the molecular ion cluster at m/z 316-320. DDE is detected in the m/z 246 fragmentogram (Fig. lid), appearing as a small peak in the TIC trace and DDT is not detectable in this sample. 

The structures deduced from the mass spectra must account for the observed equivalent chain length (ECL) or Kovats indices (KI) (Carlson et al., 1998 Katritzky and Chen, 2000 Zarei and Atabati, 2005). The values for monomethylalkanes (Mold et al., 1966 Szafranek et al., 1982) can be used to estimate the expected ECL or KI for a di-, tri- or tetramethyl-alkane structure proposed from a mass spectrum. For example, a dimethylalkane, such as 3,11 -dimethylnonacosane, with 31 carbons, would have its elution time decreased by about 0.3 carbons for the 3-methyl group and about 0.7 carbons for the 11-methyl group. Thus, the predicted ECL is approximately 30 and this is the ECL observed. 

Figure 15.9 shows the mass spectrum of 2-methylpentane. Branched alkanes tend to cleave on either side of the branch because more stable carbocations result. The two primary cleavage pathways for 2-methylpentane are shown in the following equations ... 

The mass spectrum of hexane (Figure 12-18) shows several characteristics typical of straight-chain alkanes. Like other compounds not containing nitrogen, the molecular ion (M+) has an even-numbered mass, and most of the fragments are odd-numbered. The base peak (m/z 57) corresponds to loss of an ethyl group, giving an ethyl radical and a... 

Cation and radical stabilities help to explain the mass spectra of branched alkanes as well. Figure 12-19 shows the mass spectrum of 2-methylpentane. Fragmentation of a branched alkane commonly occurs at a branch carbon atom to give the most highly substituted cation and radical. Fragmentation of 2-methylpentane at the branched carbon atom can give a secondary carbocation in either of two ways ... 

Another true story.) A student who was checking into her lab desk found an unlabeled sample from a previous student. She was asked to identify the sample. She did an IR spectrum and declared, It looks like an alkane. But it seemed too reactive to be an alkane, so she did a GC-MS. The mass spectrum is shown next. Identify the compound as far as you can, and state what part of your identification is uncertain. Propose fragments corresponding to the numbered peaks. 

Figure 9.1.7 shows the mass spectrum of triacontane (17) (TIC retention time 21.62 min, Scan No. 1329). This spectrum exhibits prominent ion peaks corresponding to m/z (43 + 14 x n) where n = 0, 1,..., 13, 14, indicating that the compound is a n-alkane. Although the molecular ion peak is not present, the spectrum coincides with the standard spectrum of docosane with 70% probability as determined by a library computer search. Therefore, this compound is only tentatively identified. 

Another compound which gave identical mass spectra in two of the extracts, WYOl and ALBl, appears to be one of the tricyclic alkanes, m/e = 276. An intense peak at m/e 247 corresponds to M-29. The mass spectrum was an excellent match with that presented by Philip, et al. (10) for which the structure shown below was proposed. An intense peak at m/e 123 was noted and is probably due to the fragmentation shown. 

A fairly similar material has been obtained in an FTT synthesis extended over 6 months (Hayatsu et al., 1977). Upon pyrolysis, it gave a mass spectrum resembling that of the Murchison polymer (Fig. 9). The spectrum shows mainly benzene, naphthalene, and their alkyl derivatives, as well as alkyl-indanes, fluorene, anthra-cene/phenanthrene, alkenes, alkanes, and alkylphenols (Hayatsu et al., 1977). This material has not been studied by the more informative, gentle oxidation methods. 

The compounds identified in the pyrolysate are alkanes, alkenes and alkadienes. However, the distinction based on the mass spectra between the different isomers for hydrocarbons with a larger number of carbon atoms is not simple, and a number of peaks in the chromatogram remained unnamed. The assignment of a given mass spectrum to a specific hydrocarbon can be done in some instances only tentatively. 

In the vacuum pyrolysis of PTFE, the mass spectrum (Fig. 37, bottom trace) appears nearly identical to that of the monomer (Fig. 36, bottom trace) but different from that of the perfluoro-n-alkanes (Fig. 36, top and middle traces). This agrees with the known fact that pyrolysis generates almost the monomer fragments [36]. 

For a straight-chain, or normal, alkane, a peak corresponding to the molecular ion can be observed, as in the mass spectra of butane (Fig. 8.5) and octane (Fig. 8.6). As the carbon skeleton becomes more highly branched, the intensity of the molecular ion peak decreases. You will see this effect easily if you compare the mass spectrum of butane with that of isobutane (Fig. 8.7). The mol-... 

The fragmentation patterns of cycloalkanes may show mass clusters arranged in a homologous series, as in the alkanes. However, the most significant mode of cleavage of the cycloalkanes involves the loss of a molecule of ethene, either from the parent molecule or from intermediate radical-ions. The peak at mie = 42 in cyclopentane and the peak at mie = 56 in methylcyclopentane result from the loss of ethene from the parent molecule. Each of these fragment peaks is the most intense in the mass spectrum. 

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