Gas-phase thermolysis of azines. Part 3. Kinetics and mechanism of pyrolysis of substituted arylidenepyrazin-2-yl- and pyrimidin-2-ylhydrazines

Five N-arylidene-N’-pyrazin-2-ylhydrazines, series (1a-e) and five N-arylidene-N’-pyrimidin-2-ylhydrazines, series (2a-e) were synthesized and characterized: four of the former series and two of the latter are novel compounds. The ten substrates were then subjected to: (i) flow flash vacuum pyrolysis (FVP) at 700 °C and short (ms) contact time; (ii) static sealed-tube pyrolysis (STP) at temperatures in the range of 300–380 °C, and relatively longer substrate residence time of 15–60 min. The products of thermolysis were analyzed, identified, characterized and compared. Further, the kinetics of pyrolysis was investigated over the temperature range ca 550–620 K and 500–615 K, respectively, for substrates (1a,c,e) and (2a,c,e). Rates, Arrhenius log A and Ea, and entropy of activation (∆S≠) were calculated. The nature and composition of the products of pyrolysis were ascertained, and substrate molecular reactivities were adequately explained based on plausible reaction mechanisms, and further rationalized using known effects of common substituent groups and specific nitrogen replacement (N = ) substituents. Concerted six-membered transition state suffices to explain the kinetics and products of FVP/STP reactions of fifteen N-arylidene-N'-azin-2-yl-hydrazines. [Display omitted] •Ten N-aryliden-N`-azinylhydrazines (six novel) were prepared and characterized.•Flow & static pyrolyses of synthesized pyrazinyl- & pyrimidinylhydrazones were studied.•Reaction products were assayed, including key 1,2,4-triazole compounds (four novel).•Pyrolysis kinetics & molecular reactivities were evaluated & mechanistically correlated.