Probing kinetic and mechanistic features of bulk azide-alkyne cycloaddition

Bulk azide-alkyne cycloaddition between 1-azidodecane and phenyl propargyl ether is studied in detail under the conditions of linear heating. The reaction mechanism involves two parallel channels that respectively yield isomeric 1,4- and 1,5-adducts. Remarkably, the ratio of the isomer amounts remains practically the same at different heating rates. It is argued that this unusual effect is a sign of the unique kinetics when the parallel channels have equal activation energies. Isoconversional kinetics analysis of differential scanning calorimetry data supports this argument and estimates those activation energies to be 82 ± 1 kJ mol^-1. The analysis has also helped to determine that the process follows the second-order kinetics as well as to estimate the activation entropy of the formation of 1,4- and 1,5-adducts. Large negative values of the activation entropy (-114 and -118 J K^-1 mol^-1) indicate that the reaction proceeds via the Huisgen concerted mechanism. The parallel channels generate the same amount of heat (279 ± 34 and 283 ± 36 kJ mol^-1) as measured by combustion calorimetry.