We studied the thermodynamic and kinetic aspects of guanine alkylation by five derivatives of formononetin nitrogen mustard using density functional theory (DFT). Investigation of the reactivity pattern of the aziridinium ion intermediates as well as drug-guanine adducts were performed using Density functional reactivity theory (DFRT). Aziridinium ion formation was observed to be endothermic, in contrast, the drug-guanine adduct formation was exothermic. A significant interaction of the aziridinium ion with guanine has been observed. The results illustrate the applicability of maximum hardness principle (MHP) and minimum electrophilicity principle (MEP).