Methylcyclopentane, mass spectrum

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 a molecule of ethene, either from the parent molecule or from intermediate radical-ions. The peak at mie = 42 in cyclopentane and the peak at mie = 56 in methylcyclopentane result from the loss of ethene from the parent molecule. Each of these fragment peaks is the most intense in the mass spectrum. 

When the cycloalkane bears a side chain, loss of that side chain is a favorable mode of fragmentation. The fragment peak at mJe = 69 in the mass spectrum of methylcyclopentane is due to the loss of the CH3 side chain. A secondary carbocation results from the loss of the methyl group. 

The most common fragmentation patterns of cycloalkanes are loss of side chains and loss of ethylene, CH2=CH2. The peak at miz 69 in the mass spectrum of methylcyclopentane (Figure 14.7) is the result of the loss of the one-carbon side chain to give the cyclopentyl cation, CjHg. The base peak at miz 56 is caused by the loss of ethylene and corresponds to a cation of molecular formula Note... 

The base peak at miz 56 in the mass spectrum of methylcyclopentane corresponds to loss of ethylene to give a radical cation with the molecular formula Pro-... 

---