AbstractThe highly trans‐stereoselective reaction of ammonium salt (AS 2) toward (E)‐N‐benzylidene‐4‐methylbenzenesulfonamide (IM 4) in the presence of Na2CO3 leading to an aziridine derivative, trans‐Az 6, was theoretically studied using Molecular Electron Density Theory (MEDT) at the B3LYP/6‐31G(d) computational level to probe energy transformation, selectivities, and molecular mechanism. The reaction starts by a nucleophilic substitution reaction between 1,4‐diazabicyclo [2.2.2]octane (DABCO) and phenacyl bromide (PB) to form AS 2 which undergoes a proton abstraction by Na2CO3 to produce Ammonium Ylide (AY 3). Subsequently, nucleophilic addition of AY 3 to the double bond in IM 4 leads to form a betaine‐like intermediate, namely, IN‐Ta which named IN‐Ta. Finally, trans‐Az 6 is produced as a result of the nucleophilic attack of the negatively charged nitrogen atom on the carbon atom bearing DABCO in IN‐Ta. Analysis of the relative Gibbs free energies shows that the ring closure step is the rate‐determining step (RDS). By an investigation of the conceptual density functional theory, CDFT reactivity indices AY 3 and IM 4 are classified as a strong nucleophilic molecular system and as a strong electrophilic molecular system, respectively, which implies that the addition step of AY 3 to IM 4 has a high polar character. An analysis of the calculated electrophilic and nucleophilic Parr functions at the reactive sites of reagents clarifies the regioselectivity observed experimentally within the C1–C2 bond creation process.