Branched chain hydrocarbon spectra

Fragmentation of branched-chain alkanes leads preferentially to the formation of secondary and tertiary carbocations, and because these cations are more easily formed than methyl and primary carbocations, extensive fragmentation is likely. For this reason, the molecular ion of branched-chain hydrocarbons is often very weak or absent entirely from the spectrum. The molecular ion corresponding to miz 114 is not observed, for example, in the mass spectrum of the highly branched... 

As with long-chain ethers, the hydrocarbon pattern may dominate the spectrum of long-chain sulfides the CnH2n peaks seem especially prominent. In branched-chain sulfides, cleavage at the branch may reduce the relative intensity of the characteristic sulfide peaks. 

Figure 18.2 is the mass spectrum of a branched hydrocarbon. Note the intensity of the molecular ion peak and the presence of an M - 15 peak. The M - 15 peak is typical, particularly if the side chain is a methyl group. The position of the side chain is indicated by the m/z 112 and 113 ions (CH3(CH2)5CHCH3). Usually the site of branching is more difficult to establish. 

The advent of 13C NMR allowed an analysis of branching at a far greater level of detail because the 13C shifts of paraffinic hydrocarbons depend strongly on their proximity to tertiary carbons (i.e., the branch points). As a result, the 13C NMR spectrum of a (short chain) branched polyethylene sample is rich in detail, as can be seen in the example shown in Figure 7-24. It was originally thought that... 

Fig. 29. The ESCA valence band spectra of (a) upper surface of green leaf (b) upper surface of yellow-green leaf and (c) lower surface of yellow-green leaf. The heights of key spectral features reflect relative quantification, (d) Most of these spectral features are present in the spectrum of a representative hydrocarbon, low-density polyethylene [35]. Alteration of a hydrocarbon by adding extra links to the (C—C) chain or introducing branching or multiple bonds blunts the peaks and valley features, as seen in (aMc). On the other hand, carbon-oxygen bonds produce O (2s)-dominated peaks at ca. 27.5 eV if C—O—C species are present (first arrow) and/or add a shoulder at ca. 24 eV if >C=0 species are present (second arrow). Reprinted with permission from T. L. Barr, S. Seal, S. E. Hardcastle, M. A. Maclauran, L. M. Chen, and J. Klinowski, Bull. Pol. Acad. Sci. 45, 1 (1997).

Example The El mass spectrum of n-decane is typical for this class of hydrocarbons (Fig. 6.18a). Branching of the aliphatic chain supports cleavage of the bonds adjacent to the branching point, because then secondary or tertiary carbenium ions and/or alkyl radicals are obtained (Fig. 6.18b,c). This allows for the identification of isomers to a certain degree. 

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