The Diels-Alder (DA) reaction between hexachlorocyclopentadiene and 1,2-dichloroethylene has been studied using the Molecular Electron Density Theory (MEDT) through Density Functional Theory (DFT) calculations at the B3LYP/6-31G(d) level. The electronics structure of the reagents has been characterized through the Electron Localization Function (ELF) and the Conceptual DFT (CDFT). The DA reaction of hexachlorocyclopentadiene with 1,2-dichloroetylene proceeds via a synchronous or low asynchronous one-step mechanism. Based on the conducted research, two-step mechanism with biradical intermediate was completely ruled out. BET study of the DA reaction shows that this reaction is topologically characterized by nine different phases. The reaction begin by the rupture of the double bonds in substrate molecules. Formation of first C-C sigma bond takes place in phase VII, while the second C-C sigma bond take place in the phase IX. Formation of these two single bonds takes place by sharing the non-bonding electron densities of the two pairs of pseudoradical centers.