Ortho effects in the dissociation of ionized N-chlorophenyl- and N-bromophenyl-2-aminobenzamidines: intramolecular aromatic substitution with cyclization to protonated 2-(2-aminophenyl)-1H-benzimidazoles

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Eur. J. Mass Spectrom. 8, 27 - 33 (2002)

Maria Anita Mendes, Roberto Rittner and Marcos N. Eberlin^*
Institute of Chemistry, State University of CampinasUNICAMP, CP 6154, 13083-970 Campinas, SP Brazil
Jerzy Suwinski and W. Szczepankiewicz
Institute of Organic Chemistry and Technology, The Silesian Technical University, 44-100 Gliwice, Poland

ABSTRACT:

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.

Keywords: ortho effect, benzamidines, intramolecular aromatic substitution, pentaquadrupole mass spectrometry, MS/MS, MS/MS/MS, gas-phase ion chemistry

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