关键词: 2,6-Diaminopyridine, Nitration mechanism, Active intermediates, Thermal effects, Negative water effects

Abstract: Mechanistic understanding of the active intermediates of 2,6-diaminopyridine (DAP) dinitration in the concentrated nitric-sulfuric acid system is of crucial importance for the selectivity control of target product, i.e., 2,6-diamino-3,5-dinitropyridine (DADNP). The active intermediates determining the product selectivity are theoretically studied. The HSO₄^--NO₂⁺ complex is proposed as the dominant active nitrating intermediate for the first time, which shows low energy barrier (i.e., 10.19 kcal·mol^-1,1 kcal = 4.186 kJ) for direct dinitration of DAP to DADNP. The formed water during the reaction results in not only the formation of less active SO₄^2--NO₂⁺ complex, but also the occurance of DAP sulfonation (DAP-SO₃H intermediate) to facilitate the formation of mononitration byproduct. Meanwhile, the accompanied thermal effects cause the generation of undesirable pyridine-NHNO₂ intermediate, which is difficult to be rearranged to yield DADNP, inhibiting the reaction and thus giving low DAP conversion. The insights reported here elucidates the importance of thermal effects elimination and water content control, confirmed experimentally in the batch- and micro-reaction systems.

Key words: 2,6-Diaminopyridine, Nitration mechanism, Active intermediates, Thermal effects, Negative water effects