Upon ionization, aromatic compounds generate a molecular ion that is observed as a prominent peak in their mass spectra. For example, the molecular ion peak for benzene appears at a mass-to-charge ratio of 78, while toluene is observed at a mass-to-charge ratio of 92. The molecular ion benzene is highly stable and does not readily undergo further fragmentation due to the significant amount of energy required to disrupt the aromatic stability of the benzene ring. In contrast, the molecular ion of alkyl‐substituted benzenes, such as toluene, can undergo fragmentation by losing a hydrogen atom, forming a benzylic carbocation. This carbocation can rearrange to form the resonance-stabilized tropylium cation, which appears in the mass spectra as a strong peak. In the case of toluene, this strong peak is observed at a mass-to-charge ratio of 91.
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.
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