Nitro-benzene electron steps

For imsymmetrically substituted /H-dinitrobenzenes (l-X-2,4-dinitrobenzenes) the first reduction occurs either at the 2-nitro group (X = Cl, Br, I) or at the 4-nitro group (X = COOH, CH3, C2H5) [122]. These differences may be caused by inductive effects and by steric inhibition of coplanarity with the benzene ring of the nitro group or of other substituents. In 2,4-dinitrochlorobenzene the inductive effect from the chlorine on the 2-nitro group probably is responsible for its easier reduction in the first four-electron reduction 2-hydroxylamino-4-nitrochlorobenzene is formed in the second four-electron step, 2,4-dihydroxylaminochlorobenzene and in the last four-electron reduction, 2,4-di-aminochlorobenzene. 

This step is done at about 60°C and with ordinary concentrated nitric and sulfuric acids. The nitro groups on the aromatic molecule, however, exhibit great electron-withdrawing power, and therefore, the mononitrated benzene is very resistant to further nitration because of local suppression of the nitronium ion. 

In toluene, as well as with many other monosubstituted benzenes, the substitution group (methyl, in the case of toluene) acts as an electron donor. This counters the electron-withdrawing effect of the previously substituted nitro groups and allows higher local nitronium ion activity, thus allowing a much easier trinitration step. This is the key, then, to inexpensive synthesis of trinitro-aromatic explosives. Figure 3.5 is the TNT molecule, the first, and most important (as far as quantity of production goes) of the monosubstituted TNBs. 

The reaction mechanism of Lehmstedt-Tanasescu reaction is generally based on an aromatic electrophilic addition of an or/Ao-nitro-substituted benzaldehyde to electron-rich arenes, yielding substituted 3-phenyl 1,2-benzisoxazoles, which are then rearranged to the desired acridones. In the first step of the reaction mechanism the benzaldehyde is protonated by sulfuric acid to form intermediate 5, followed by electrophilic addition to an electron rich arene (benzene in the following scheme), to give compound 6. Intramolecular attack of the formed hydroxyl group to the or/to-positioned... 

---