MASS-SPECTRAL BEHAVIOR OF 5(6)-SUBSTITUTED BENZIMIDAZOLES

Three general classes of 5(6)-substituted benzimidazoles were compared according to common or similar fragmentation pathways in the mass spectrometer. The 5(6)-alkyl derivatives fragment through a common intermediate of m/e 131 as demonstrated by metastable ion ratios for the 2-13C-labeled compounds. It is suggested that this intermediate possesses a ring-expanded structure resembling that of 1, 3-diazaazulene whose fragmentation behavior is very similar. For both species, competitive pathways exist for loss of the 2 carbon and carbocyclic ring carbons with HCN or CN. fragments. Moreover, the expected loss of the 2 carbon of the imidazole ring with these fragments is not the predominant process. The second general group of derivatives fragments by complete loss of the 5(6) substituent (NO2, Cl, CO2H, COCH3) to give a common ion of m/e 117. Again, the metastable losses of HCN and H13CN from the 2-13C-labeled derivatives confirms the common structure of this ion and indicates predominant loss of carbocyclic ring carbons. Finally, the similar behavior of several 5(6)-alkenylbenzimidazoles implies fragmentation through a common 143 ion which may result from a ring-expansion process similar to that of styrene. The three main fragmentation pathways observed here should be general for a variety of benzimidazole derivatives. More importantly, the metastable ratio technique for common ion identification is found to be much more reliable for 13C-labeled compounds than for those with 2H labeling. Increased availability of 13C-enriched reagents makes this technique one of broad applicability in mass spectral investigations. © 1978, American Chemical Society. All rights reserved.