Protonated N-Alkyl-2-nitroanilines Undergo Intramolecular Oxidation of the Alkyl Chain upon Collisional Activation

The collisional activation of protonated N-propyl-2-nitroaniline obtained by electrospray ionization shows two major competitive dissociation pathways: the elimination of the elements of propionic acid, [M + H - C₃H₆O₂]⁺ to give an m/z 107 ion, and of the elements of ethanol, [M + H - C₂H₆O]⁺ to give an m/z 135 ion. The mechanistic study reported here addresses these unusual fragmentations to reveal that both occur via a common intermediate formed by the transfer of an oxygen atom from the nitro group to the first carbon atom of the propyl group, allowing elimination of propionic acid and (H₂O + ethene), respectively. The corresponding loss of CH₄O does not occur when the propyl group is replaced by an ethyl group, but elimination of the elements of propanol does occur when propyl is replaced by a butyl group. Further, the product ions of m/z 107 and 135 are also formed when the propyl chain is replaced with a hexyl group.