Resonance, push-pull

Push-Pull Resonance. The Basicity of para-Nitroaniline.203... 

Compare the geometry of para-nitroaniline to those of both aniline and nitrobenzene. Is there any evidence for push-pull resonance contributors Is there shortening of bonds to the amino and nitro groups Are the bonds in the ring localized Is the dipole moment for para-nitroaniline smaller, larger or about the same as the sum of the dipole moments for aniline and nitrobenzene What does your result say about the importance of push-pull resonance contributors ... 

It has been postulated that the stability of free radicals is enhanced by the presence at the radical center of both an electron-donating and an electron-with-drawing group.This is called the push-pull or captodative effect (see also pp. 159). The effect arises from increased resonance, for example ... 

Case 1. (90°) 8> s (0°) (Fig. 1). This is a normal push-pull system, and a donor group A has different environments in the energy minima near 0° and near 180°, provided that the acceptor groups X and Y are different. The energy required to pass across the tt barrier can be measured by monitoring the band shape of the A resonance when the preexponential lifetimes are intermediate on the NMR time scale. 

For example, we recall discussions of o- and p-directing vs m-directing groups for electrophilic aromatic substitution, the importance of o- andp-quinonoid resonance structures vs the irrelevance of m-quinonoid long bonded resonance structures for stabilization of push-pull disubstituted benzenes. 

Quite fascinating compounds are the super-basic nitriles102. The cyanamides are the most basic nitriles presently known103,104. The high basicity of such compounds is explained by a so-called push-pull mechanism, described schematically by the resonance structures. 

Besides desiring agreement between experiment and theory, we also note that one cannot draw any resonance structure in which both amino and both keto groups are totally conjugated with each other in a push-pull manner. 

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