Mass Spectra of Benzene and Toluene

Mass spectra of aromatic compounds show a strong molecular ion peak. In ionization, the molecule becomes a molecular ion, which appears as a prominent peak in the spectrum. Benzene gives a molecular ion at m/z 78, and toluene gives one at m/z 92.

Benzene’s molecular ion is especially stable. It d...

Aromatic compounds, upon ionization, form molecular ions.

Benzene's molecular ion does not fragment extensively, requiring enormous energy.

On the contrary, molecular ions of alkyl-substituted benzenes, like toluene, fragment at the benzylic carbon, losing a hydrogen atom to generate a resonance-stabilized benzyl carbocation.