Molecular ions of N-alkyl substituted derivatives of o-nitroaniline and some of their fragment ions undergo several fragmentations which are examples of new types of ortho-effects. A fragmentation common to all studied compounds with an unbranched alkyl chain –CH2–(CH2)n–H is the formal loss of H–(CH2)n–COOH (n = 0–3). Other important primary decomposition processes are eliminations of H2O, OH• and HO2• fragments. Reaction mechanisms for these reactions, supported by metastable and collision-induced decomposition spectra of selected model compounds and by deuterium labeling, are proposed. Other fragmentations of the molecular ion of N-alkyl-o-nitroanilines depend on the type of alkyl substituent. For alkyl larger than methyl an important process is the α-cleavage of the C–C bond next to the nitrogen atom yielding the corresponding immonium cation.
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