6.9.5 Fragmentation of Aromatics

The presence of an aromatic ring in a compound results in a prominent molecular ion.  A common peak at [M – 1] results from the loss of a hydrogen atom from the benzene ring.  Alkyl substituted benzene rings result in a prominent peak at m/z 91 (Figure 2.12).  In most cases, the peak at m/z 91 is the result of a tropylium ion caused by the following rearrangement.

The peak observed in most aromatic compounds at m/z 65 results from the elimination of an acetylene molecule from the tropylium ion.

         Benzene rings with highly branched substituted groups produce fragments larger than m/z 91 by intervals of 14 units.  The largest of these peaks will result in a highly substituted cation and a large radical, like a simpler branched alkane.  The fragment at m/z 105 in Figure 2.12 is relatively small since it produces a primary carbocation and an unstable methyl radical.  Substituted benzene rings also first undergo α cleavage followed by hydrogen rearrangement producing a grouping of peaks at m/z 77 from C₆H₅⁺, m/z 78 from C₆H₆⁺, and m/z 79 from C₆H₇⁺.

Figure 6.12. Fragmentation of an Aromatic.Spectra from the NIST/EPA/NIH Mass Spectral Library.  Reprinted with permission from NIST.

         Side chains with more than two carbon atoms create a peak at m/z 92 (Figure 6.12).  Unbranched side chains result in a more prevalent peak at m/z 92 than do branched groups.

 

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