Nucleophile-substrate interaction nitrogen reaction

Taking into account the fact that the solvation of ambident anions in the activated complex may differ considerably from that of the free anion, another explanation for the solvent effect on orientation, based on the concept of hard and soft acids and bases (HSAB) [275] (see also Section 3.3.2), seems preferable [366]. In ambident anions, the less electronegative and more polarizable donor atom is usually the softer base, whereas the more electronegative atom is a hard Lewis base. Thus, in enolate ions, the oxygen atom is hard and the carbon atom is soft, in the thiocyanate ion the nitrogen atom is hard and the sulfur atom is soft, etc. The mode of reaction can be predicted from the hardness or softness of the electrophile. In protic solvents, the two nucleophilic sites in the ambident anion must interact with two electrophiles, the protic solvent and the substrate RX, of which the protic solvent is a hard and RX a soft acid. Therefore, in protic solvents it is to be expected that the softer of the two nucleophilic atoms (C versus O, N versus O, S versus N) should react with the softer acid RX. 

There Is an Inverse correlation between the gas-phase relative rates of nitration and the generally observed trend of the relative rates of electrophilic aromatic siibstltutlons In solution. Toluene reacts 3 times slower and nitrobenzene 10 times faster than benzene. This suggests that nitration In the gas-phase Is nucleophilic In character, and thus does not correspond at all to solution-phase electrophilic nitration by the NO-" " Ion. We tentatively suggest as a possible rationalization of this unexpected result that the gas-phase reaction Involves primary electrostatic Interaction of the aromatic substrate with the nucleophilic terminal oj gen of the CH20N02" cation, followed by displacement by the aromatic ring on nitrogen, with simultaneous elimination of formaldehyde. 

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