15.1 : 質量分析: 長鎖アルカンのフラグメンテーション

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.