Synthesis and mass spectral studies of some 1- [2-benzimidazolyl] or [6-methyl-4 [1H] -pyrimidinon-2-yl] -5-substituted phenyl-3-methylpyrazoles

New 1-[2-benzimidazolyl] 5-substituted phenyl-3- methylpyrazoles [IV] and 1-[6-methyl-4 [1H] -pyrimidinon-2-yl]-5-substituted phenyl-3- methylpyrazoles [V] have been synthesized by the condensation of beta- diketones [III] and 2-hydrazinobenzimidazole [I] or 2-hydrazino-6- methyluracil [II], respectively. Bromination study of IV and V in Br2/ACOH has revealed that electrophilic attack of bromine occurs in pyrazole ring at position 4 and in uracil ring at position 5. The general spectral fragmentation mode of these compounds has been studied