The stepwise reaction mechanism of the [1+4] cycloaddition reaction between alkyl isocyanides (contains methyl benzene isocyanide, tert-butyl isocyanide, cyclohexyl isocyanide) with 3-Benzylidene-2,4-pentanedione both in the gas phase and in solvent was studied theoretically with B3LYP method using 6-311++G(d,p) basis set. The potential energy of all structures participated in the reaction path has been evaluated. The quantum mechanical calculation was performed to gain a better understanding of the most important geometrical parameters, determine the solvent effect, and the effect of different substituted groups on the potential energy surfaces. Unexpectedly, the second step of all reactions was recognized as the rate-determining step. The best product configuration was recognized based upon the quantum mechanical calculations. The natural bond orbital method (NBO) applied for a better understanding of molecular interaction. The final rate equation shows that the overal reaction rate is second order which depends on the R1 and R2 concentration. The order of reaction with respect to each reactant is one. تفاصيل المقالة