15.2 : 质谱法：支链烷烃碎裂

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.