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.

From Chapter 15:

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15.8 : Mass Spectrometry: Aromatic Compound Fragmentation

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15.1 : Mass Spectrometry: Long-Chain Alkane Fragmentation

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15.2 : Mass Spectrometry: Branched Alkane Fragmentation

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15.3 : Mass Spectrometry: Cycloalkane Fragmentation

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15.5 : Mass Spectrometry: Cycloalkene Fragmentation

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15.7 : Mass Spectrometry: Alcohol Fragmentation

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15.9 : Mass Spectrometry: Amine Fragmentation

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15.10 : Mass Spectrometry: Alkyl Halide Fragmentation

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15.11 : Mass Spectrometry: Aldehyde and Ketone Fragmentation

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15.12 : Mass Spectrometry: Carboxylic Acid, Ester, and Amide Fragmentation

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15.13 : Chemical Ionization (CI) Mass Spectrometry

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15.14 : Electrospray Ionization (ESI) Mass Spectrometry

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15.15 : Matrix-Assisted Laser Desorption Ionization (MALDI)

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