Nitromethane resonance hybrids

The mechanisms of the condensation and many other reactions of nitroalkanes are formulated nowadays in terms of carbanions, as in the case of reactions involving CH activated by other groups6. The ion generated from a nitroalkane by the action of base is regarded as a resonance hybrid e.g. for nitromethane ... 

The best way to think about resonance forms is to realize that a molecule like nitromethane is no different from any other. Nitromethane doesn t jump hack and forth between two resonance forms, spending part of its time looking like one and the rest of its time looking like the other. Rather, nitromethane has a single unchanging structure that is a resonance hybrid of the two individual forms and has characteristics of both. The only problem with nitromethane is that we can t draw it accurately using a familiar Kekule line-bond structure. Line-bond structures just don t work well for resonance hybrids. The difficulty, however, lies with the representation of nitromethane on paper, not with nitromethane itself. 

RULE 1 Individual resonance forms are imaginary, not real. The real structure is a composite, or resonance hybrid, of the different forms. Species such as nitromethane, benzene, and the acetate ion are no different from any other. They have single, unchanging structures, and they do not switch back and forth between resonance forms. The only difference between these and other substances is in the way they must be represented on paper. 

Some substances, such as nitromethane, benzene, and acetate ion, can t be represented by a single Lewis or line-bond structure and must be considered as a resonance hybrid of two or more structures, neither of which is correct by itself. The only difference between two resonance forms is in... 

Resonance forms that are equivalent contribute equally to a resonance hybrid. The two forms of nitromethane in Figure 1.27 and the two forms of the allyl cation (A and A ) in Figure 1.32 are good examples. In both cases the resonance forms are completely equal to each other (indistinguishable, same number of bonds, same number of charges, charges on same atoms). 

Resonance forms differ only in the placement of their tt or nonbonding electrons. Neither the position nor the hybridization of any atom changes from one resonance form to another. In nitromethane, for example, the nitrogen atom is s/j -hybridized and the oxygen atoms remain in exactly the same place in both resonance forms. Only the positions of the tt electrons in the N=0 double bond and the lone-pair electrons on oxygen differ from one form to anothor. This movement of electrons on going from one resonance structure to another is sometimes indicated by using curved arrows. A curved arrow always indicates the movement of electrons, not the movement of atoms. An arrow shows that a pair of electrons moves from the atom or bond at the tail of the arrow to the atom or bond at the head of the arrow. 

---