The mass spectra of most aromatic compounds show distinct molecular ion peaks. This is probably due to the loss of an electron in the p system, leaving the carbon skeleton relatively undisturbed.
When an alkyl side-chain is attached to the ring, fragmentation usually occurs at the benzylic position, producing the tropylium ion (m/e = 91) as shown in figure 20.
Figure 20. Formation of tropylium ion.
However, fragmentation can also occur at the attachment point to the ring producing the phenyl cation (m/e = 77) as shown in figure 21.
Figure 21. Formation of phenyl cation.
If the side-chain is a propyl group or larger, then the McLafferty rearrangement is a possibility, producing a fragment of m/e = 92.
Figure 22. Mechanism of McLafferty rearrangement.
Formation of a substituted tropylium ion is typical for alkyl-substituted benzenes producing a peak at m/e = 105.
Figure 23. Formation of substituted tropylium ion.
The complete mass spectrum for propyl benzene is given in figure 24, which illustrates all of these points.
Figure 24. Mass spectrum for propyl benzene.
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