The fragmentation patterns of cycloalkanes may show mass clusters arranged in a homologous series, as in the alkanes. However, the most significant mode of cleavage of the cycloalkanes involves the loss of ethene from the parent molecule or from intermediate radical-ions. Additionally, if the cycloalkane has a side chain, loss of that side chain is also a favorable mode of fragmentation. Figure 12 provides the mechanism for the fragmentation for cyclopentane. The mass spectrum of cyclopentane (Figure 13) has an intense peak at m/e = 42 due to the loss of ethene.
Figure 12. Mechanism of fragmentation for cyclopentane.
Figure 13. Mass spectrum for cyclopentane.
In order for cycloalkanes to fragment, two carbon-carbon bonds must be broken. This process may require a significant amount of time (relative to the amount of time it takes an ion to reach the detector in a mass spectrometer), therefore, a significant amount of the molecular ion will reach the detector resulting in a large molecular ion peak.
Cycloalkanes tend to cleave in CnH2n+, CnH2n-1+, and CnH2n-2+ fragments. The larger number of even numbered mass fragments of cycloalkanes helps to distinguish this functional group from the acyclic alkanes.
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