Ortho Effects in the Dissociation of Ionized N -Chlorophenyl- and N -Bromophenyl-2-Aminobenzamidines: Intramolecular Aromatic Substitution with Cyclization to Protonated 2-(2-Aminophenyl)-1 H -Benzimidazoles

Electron ionization (70 eV) mass spectra, double-stage (MS²) 10 eV collision-induced dissociation (CID) product-ion mass spectra of molecular ions and triple-stage (MS³) sequential product-ion mass spectra of major fragment ions are reported for the parent N-phenyl and the isomeric ortho-, meta- and para-N-chlorophenyl- and N-bromophenyl-2-aminobenzamidines. Dissociation is greatly influenced by an ortho effect that favors the loss of ortho H atoms and particularly the loss of the ortho halogen substituents. Dissociation occurs via a two-step intramolecular aromatic substitution reaction and a distonic-ion intermediate inhibits scrambling of ring hydrogens thus favoring the loss of the ortho substituents. The ortho isomers form an abundant and diagnostic [M – X]⁺ fragment ion (X = Cl, Br) and are easily distinguished. Protonated 2-(1H-benzimidazol-2-yl)-phenylammonium ions are likely formed; they dissociate mainly by NH₃ loss upon CID and react readily with pyridine by proton transfer.