To address the differential chemical reactivity of 4-BrPyCN, we conducted detailed computational and experimental studies and proposed a plausible mechanism for the unexpected reaction outcome with 4-BrPyCN (Scheme 1). We performed experiments to screen changes to reaction components namely, (i) structural analogs of 4-BrPyCN, (ii) structural analogs of di-substituted ethane (co-reactant), and (iii) reaction solvent for the formation of Py-N-ene in order to rationalize the differential chemical reactivity of 4-BrPyCN. In addition to the experimental investigations, we also performed Density Functional Theory (DFT) calculations to elucidate differences between the chemical reactivities of 4-BrPyCN and 4-IPyCN under identical reductive reaction environments, leading to Py-N-ene and C4-Iodopyrazolyl cyclopropane carbonitrile, respectively. Specifically, we used Transition State Theory (TST) to predict the stepwise mechanism for 4-bromopyrazolyl cyclopropane carbonitrile and 4-iodopyrazolyl cyclopropane carbonitrile products from 4-BrPyCN and 4-IPyCN, respectively. However, it should be noted that the traditional use of TST to predict reaction mechanisms has limitations because the product formation prediction is directional and is not selective between different forward and reverse routes of reactions, especially for barrierless pathways that includes spontaneous rearrangements of electrons. While such inaccuracies do not affect the use of TST to verify reaction mechanisms, we found that TST failed to provide a complete mechanism for the experimentally observed Py-N-ene and the differential chemical reactivity of 4-BrPyCN. Therefore, we used Molecular Orbital (MO) theory to propose electrocyclic rearrangement around the α-aminonitrilic site of 4-BrPyCN, involving reversible ring-closing and ring-opening events, which was essential to identify new transition states to ultimately predict the formation of Py-N-ene.