The molecular ions of linear alkanes prefer to fragment at the carbon-carbon bond away from the end of the chain since the cleavage of an inner bond creates a stable carbocation and a stable radical. Consequently, the mass signals of linear alkanes feature intense peaks in the middle of the mass-to-charge ratio plot with weaker peaks on either end. The fragmentation of each carbon-carbon bond with the release of a methyl group in each splitting leads to prominent peaks in the mass spectra separated by 14 u. Smaller mass signals near each prominent peak result arise from secondary fragmentation at the carbon-hydrogen bond.
For example, consider the fragmentation pattern of n-hexane shown in Figure 1 and the corresponding mass spectra in Figure 2.
Figure 1: Fragmentation of n-hexane molecular ion.
Figure 2: Mass spectrum of n-hexane.
The signals at a mass-to-charge ratio of 71, 57, 43, 29, and 15 indicate that the cleavage can occur at all carbon-carbon bonds in the molecule. However, the relative abundance of these peaks varies. The difference between molecular ion signal and signal at 71 indicates the separation of methyl groups (M.W. = 15 u) group from molecular ions. Further release of a CH2 group from that fragment (M.W. = 14 u) results in a signal at 57. The signals at 43, 29, and 15 are due to the subsequent releases of CH2 groups. The abundance of signals at 15 and 71 is minimal due to the least probability for the formation of unstable methyl carbocation or a methyl radical, respectively. The base peak at 43 indicates the most preferred fragmentation, which yields a pair of stable carbocation and radical.
In linear alkanes, the fragmentation of molecular ions, which are radical cations formed by the removal of a single electron from a parent molecule, predominantly occurs away from the end of the chain to form stable carbocations and radicals.
For example, consider the mass spectrum of hexane. The prominent peaks are separated by 14 u, indicating the possibility of initial fragmentation at different adjacent carbon-carbon bonds in hexane.
The fragmentation between a terminal carbon and its adjacent carbon is less feasible due to the unstable methyl carbocation or the methyl radical generated. If this fragmentation occurs, a methyl radical formation is prioritized over methyl carbocation.
Logic dictates that the most feasible fragmentation results in the most stable carbocation, expulsing the most stable radical. In the hexane mass spectra, the butyl carbocation is the most abundant and becomes the base peak.
The subsequent fragmentation of carbocations with hydrogen loss is responsible for the small peaks near each prominent peak.
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