15.3 : 质谱法：环烷烃裂解

Cycloalkane molecular ions are more stable than linear or branched alkane molecular ions.

For example, the cyclohexane molecular ion signal at the mass-to-charge ratio of 84 displays stronger intensity than the hexane's molecular ion peak.

The molecular ion of cyclohexane cleaves to produce an ethylene molecule and a new radical cation with a mass-to-charge ratio of 56. This radical cation is relatively stable and is the base peak of the spectrum. 

In the branched cycloalkanes, apart from the fragmentation into ethylene and a butyl radical cation, another fragmentation enforces the loss of the side chain from the molecular ion. For example, the methyl cyclopentane molecular ion loses its methyl side chain to form the cyclopentyl cation.

The cyclopentyl cation further undergoes fragmentation to yield an ethylene molecule and a propyl radical cation.