This paper describes the SEAr regioselectivity of the 9 chloropyrido[1,2-a]benzimidazole reaction by experimental and quantum chemistry methods. We conducted the halogenation process in sulfuric acid using N-bromosuccin- or N-chlorosuccinimide. Two isomeric products 8-Hal 9 chloropyrido[1,2-a]benzimidazole and 6-Hal-9-chloropyrido[1,2-a]benzimidazole occurred. Predominantly, the introduction of the electrophilic particle occurred at the 8th position of the heterocycle. Using quantum chemistry methods, we found the orbital control of the electrophilic halogenation reaction and determined the orientation of the electrophile particle introduction by the distribution of the boundary electron density in the substrate. This corresponded well with the experimental data.