Aromatic nitro compounds resonance effects

The resonance spectra of aromatic nitro compounds [55] show inductive effects of substituents on the nitro group. 

Resonance effects are also important in aromatic amines. m-Nitroaniline is a weaker base than aniline, a fact that can be accounted for by the —7 effect of the nitro group. But p-nitroaniline is weaker still, though the —I effect should be less because of the greater distance. We can explain this result by taking into account the canonical form A. Because A contributes to the resonance hybrid, " the electron density of the unshared pair is lower in p-nitroaniline than in m-nitroaniline, where a canonical form such as Ais impossible. The basicity is lower in the para compound for two reasons, both... 

Nitrile oxides have the structure R—C =N+—O- or Ar—C=N+—O-. Aliphatic nitrile oxides usually can be prepared only in situ, while the analogous aromatic compounds, which are resonance-stabilized, generally can be isolated. The most common preparation of nitrile oxides is the dehydration of aliphatic nitro compounds. Figure 12.40 shows in detail how this dehydration can be effected by the reaction of ni-troethane with a mixture of NEt3 and Ph—N=C=0. 

The destabilization of sp -bound fluorine by p-jt repulsion activates fluorinated aromatic compounds totvard nucleophilic attack and subsequent substitution. The susceptibility of the carbon center toward nucleophiles is also enhanced by the negative inductive (—T) effect of fluorine. In particular, if the aromatic compound is also activated by —M electron-withdrawing substituents, for example a nitro or cyano group, in the ortho or para positions the fluorine is easily replaced by a variety of nucleophiles even under very mild conditions via a resonance stabilized Meisenheimer complex (Scheme 2.39). The ease of nucleophilic halogen replacement - F > Cl > Br > I - is in the opposite order to that for aliphatic nucleophilic substitution. 

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