The nitration reaction of nitrobenzene with nitronium ion yielding ortho‐, meta‐ and para‐dinitrobenzenes has been studied within the Molecular Electron Density Theory, using DFT computational methods at the B3LYP/6‐311G(d,p) level. This electrophilic aromatic substitution (EAS) reaction takes place through a two‐step mechanism involving the formation of a tetrahedric cation intermediate. The electrophilic attack of nitronium ion on nitrobenzene is the rate‐determining step of this EAS reaction, and consequently, responsible for the composition of the reaction mixture. The subsequent proton abstraction from the cation intermediate is barrierless. From the computed activation Gibbs free energies, a relationship 11.0 (ortho) : 87.3 (meta) : 1.7 (para) of the dinitrobenzenes is estimated, in clear agreement with the experimental outcome. The similar nucleophilic activation of the ortho and meta carbons of nitrobenzene makes it possible to question the hypothesis for the orientation in EAS reactions involving nucleophilically deactivated benzenes based on the relative stability of the tetrahedric cation intermediates.
We investigated the flow of electron density along the cyclocondensation reaction between ethyl acetate 2-oxo-2-(4-oxo-4H-pyrido[1.2-a]pyrimidin-3-yl) polyazaheterocycle (1) and ethylenediamine (2) at the ωB97XD/6-311++G(d,p) computational method within of bond evolution theory (BET). The exploration of potential energy surface shows that this reaction has three channels (1-3) with the formation of product 3 via channel-2 (the most favorable one) as the main product and this is in good agreement with experimental observations. The BET analysis allows identifying unambiguously the main chemical events happening along channel-2. The mechanism along first step (TS2-a) is described by a series of four structural stability domains (SSDs), while five SSDs for the last two steps (TS2-b and TS2-c). The first and third steps can be summarized as follows, the formation of N1-C6 bond (SSD-II), then, the restoration of the nitrogen N1 lone pair (SSD-III), and finally, the formation of the last O1-H1 bond (SSD-IV). For the second step, the formation of hydroxide ion is noted, as a result of the disappearance of V(C6,O7) basin and the transformation of C6-N1 single bond into double one
The bonding evolution theory has been used to investigate the flow of
electron density along the reaction pathways of ethyl acetate
2-oxo-2-(4-oxo-4H-pyrido [1.2-a] pyrimidin-3-yl) polyazaheterocycle
(1) and ethylenediamine (2). This reaction has three channels (1-3) and
each one takes place via three or four steps. DFT results reveal that
channel 2, which goes through imine intermediate is by far the most
favorable one, and the main product 3 is more stable than 4 and 5,
showing that this reaction is under kinetic and thermodynamic control,
in clear agreement with the experimental outcomes. The BET analysis
allows identifying unambiguously the main chemical events happening
along channel 2. For this reaction channel, the mechanism along the
first step (TS2-a) is described by a series of four structural stability
domains (SSDs), while five SSDs are required for the second (TS2-b) and
the third (TS2-c) one. The first step can be summarized as follow, the
appearance of V(N1,C6) basin illustrating the formation of N1-C6 bond
(SSD-II), the splitting of N1-H1 bond, followed by the restoration of
the nitrogen N1 lone pair (SSD-III), and finally, the formation of the
last O1-H1 bond (SSD-IV). For the second step, the formation of
hydroxide ion is noted, consequent of the disappearance of V(C6,O7)
basin, the transformation of C6-N1 single bond into double one (SSD-IV).
Finally, the appearance of V(O7,H2) basin leading to the elimination of
water molecule within the last domain. Overall, for the three reaction
steps, the formation of the N-C bond appears always before the O-H one.
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