This lesson delves into the mass spectrometry of branched alkane fragmentation. Branched alkanes possess secondary or tertiary carbon atoms, which generate relatively stable carbocations if the cleavage occurs at the branching point. The high stability of carbocations drives the instant fragmentation of branched alkanes. Accordingly, the branched alkane's molecular ion peak is very weak or invisible in the mass spectra, especially in comparison to a linear alkane.
Figure 1. Fragmentation pathway of 2-methylbutane (top), neopentane (middle), and n-pentane (bottom) molecular ions.
Figure 1 shows the most feasible fragmentation pathway observed in 2-methyl butane, neopentane, and n-pentane molecular ions. 2-methyl butane and neopentane fragment to yield secondary and tertiary carbocations, respectively. The stability of these carbocations drives the fragmentation reaction, even though the co-produced methyl radical is relatively unstable. In contrast, the cleavage of n-pentane leading to the methyl radical is difficult since the stability of a primary carbocation is low.
Figure 2. Fragmentation of 2,2-dimethylpentane.
As shown in Figure 2, the fragmentation of 2,2-dimethyl pentane involves the loss of either methyl or propyl radical, resulting in a tertiary carbocation. Here, the stability of the co-produced radical determines the cleaving bond. So, the cleavage that produces a propyl radical is favored, and the signal from the 2-methyl propyl carbocation becomes the base peak.
The fragmentation of branched alkanes primarily occurs at the branching point, which generates stable secondary or tertiary carbocations.
For instance, consider the mass spectra of 2-methyl butane and 2,2-dimethyl propane. The loss of a methyl radical from 2-methyl butane and 2,2-dimethyl propane molecular ions produces a secondary and tertiary carbocation, respectively.
While the molecular ion peak is visible in the mass spectrum of 2-methyl butane, it is absent in 2,2-dimethyl propane. The increased stability of the tertiary carbocation over the secondary carbocation makes the fragmentation of 2,2-dimethyl propane extensive, leaving no molecular ion to detect.
Here, the increased stability of secondary and tertiary carbocations, in comparison to primary carbocations, drives the fragmentation, leading to a methyl radical, unlike linear alkanes.
In 2,2-dimethyl pentane, fragmentation on either side of the branching center yields a tertiary carbocation. Here, the stability of the co-produced radical determines the fragmentation pattern. Accordingly, the fragmentation leading to a primary radical rather than a methyl radical is more likely.
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