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 HSO

4

-

-NO

2

+

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 SO

4

2-

-NO

2

+

complex

but also the occurance of DAP sulfonation (DAP-SO

3

H intermediate) to facilitate the formation of mononitration byproduct. Meanwhile

the accompanied thermal effects cause the generation of undesirable pyridine-NHNO

2

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 co

ntrol

confirmed experimentally in the batch- and micro-reaction systems.