Chemical ionization mass spectra of mononitroarenes

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Abstract
The H2 and CH4 chemical ionization mass spectra of a selection of substituted nitrobenzenes have been determined. It is shown that reduction of the nitro group to the amine is favoured by high source temperatures and the presence of water in the ion source. The H2 chemical ionization mass spectra are much more useful for distinguishing between isomeric compounds than the CH4 CI mass spectra because of the more extensive fragmentation. For ortho substituents bearing a labile hydrogen abundant [MH  H2O]+ fragments are observed. When the substituent is electron‐releasing both ortho and para substituted nitrobenzenes show abundant [MHOH]+ fragment ions while meta substituted compounds show abundant loss of NO and NO2 from [MH]+. The latter fragmentation is interpreted in terms of protonation para to the substituent or ortho to the vitro function, while the first two fragmentation routes arise from protonation at the nitro group. When the substituent is electron‐attracting the chemical ionization mass spectra of isomers are very similar except for the H2O loss reaction for ortho compounds.