Branched alkanes tend to fragment very easily, due to the presence of 2o, 3o, and 4o carbon atoms in the structure. When branched alkanes fragment, stable secondary and tertiary carbocations are formed. For this reason the molecular ion peak is much less intense than in straight chain alkanes. Figure 10 shows the mechanism of fragmentation for isobutane. The mass spectrum for isobutane is contained in Figure 11. Notice the reduced intensity of the molecular ion peak.
Figure 10. Mechanism of fragmentation for isobutane.
Figure 11. Mass spectrum for isobutane.
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