Mass Spectrometry: Aromatic Compound Fragmentation

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.

The benzyl cation further rearranges to a more stable tropylium ion, which exhibits a strong peak in the mass spectra.

Alternatively, the molecular ions of alkylbenzenes with larger alkyl groups fragment via cleavage of the side chain to initially form a benzyl cation, which then rearranges to a tropylium ion.

If the side chain has three or more carbons and at least one hydrogen on the γ carbon, a McLafferty rearrangement occurs, detected at a mass-to-charge ratio of 92.

Polyalkylated compounds fragment and lose a hydrogen atom to form a methyltropylium ion, which gives a medium peak.

The loss of one methyl group forms the tropylium ion.

Notably, isomers of such disubstituted rings have identical mass spectra. So, mass spectrometry cannot determine the substitution patterns of polyalkylated benzenes.