When an alkane is ionized by EI, it will lose an electron to form a radical cation. This radical cation has the same mass as the parent compound (minus one electron) and is the molecular ion (M+). The type of radical formed follows the stability of radicals:
3o > 2o > 1o > methyl
The alkane molecular ion can further fragment to form a homologous series of cations of mass CnH2n+1. These cations arise from the loss of methyl radical (M - 15), ethyl radical (M - 29), etc. Scheme 3 shows a possible mechanism of fragmentation for pentane; the corresponding mass spectrum of pentane is given in Figure 3.
Scheme 3. Mechanism of fragmentation for pentane.
Notice the appearance of M - 15, M - 29, M - 43, and M - 57 ions in Figure 3. Table 5 shows the typical fragments lost by acyclic alkanes and their respective masses.
| Mass Lost | Fragment Lost |
| 1 | H· |
| 2 | 2 H· |
| 15 | CH3· |
| 29 | C2H5· |
| 43 | C3H7· -or- C2H4 & CH3· |
| 57 | C4H9· -or- C2H4 & C2H5· |
| 71 | C5H11· -or- C3H6 & C2H5· |
The ions of m/z 57 and 43 result from the loss of methyl and ethyl radical, respectively. The ions of m/z 29 and 15 result from the subsequent loss of ethene from these two higher mass fragments. In general, once a radical is lost, the subsequent losses are of neutral molecules. This is called the even electron (EE) ion rule. That is, once an even electron ion is formed, it fragments by rearrangement to give other EE ions. For instance, in decane (see Figure 4): M ® [M – 15] ® [M - 15 – 28] or [M - 15- 42] or [M - 15 – 56]. The same can be said for M - 29 ® [M - 29 – 28], etc. This is how that characteristic EE ion series: 29, 43, 57, 71, 85 arises in hydrocarbon MS.
Figure 3. Mass spectrum of pentane. The hydrocarbon ions are drawn as primary ions although by the time they reach the detector, they have rearranged to the more stable secondary and tertiary carbocations. (Adapted from NIST Mass Spec Data Center, S.E. Stein, director, "Mass Spectra" in NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Eds. P.J. Linstrom and W.G. Mallard, June 2005, National Institute of Standards and Technology, Gaithersburg MD, 20899 (http://webbook.nist.gov))
Ions observed in the mass spectra of straight chain alkanes will usually appear in groups of 14 mass unit intervals (corresponding to one CH2 group difference). The most abundant fragment ion is usually the 3 carbon fragment, with the abundances of higher mass ions decreasing with increasing mass. Often, the M - 15 ion (formed by loss of methyl radical) will be absent. The series of fragments to look for in these spectra are of CnH2n+1+, CnH2n+, and CnH2n-1+. The mass spectrum of n-hexadecane (Figure 4) illustrates these points.
Figure 4. Mass spectrum of n-hexadecane. (Adapted from NIST Mass Spec Data Center, S.E. Stein, director, "Mass Spectra" in NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Eds. P.J. Linstrom and W.G. Mallard, June 2005, National Institute of Standards and Technology, Gaithersburg MD, 20899 (http://webbook.nist.gov))
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