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.