How to Draw Resonance Contributors

We have seen that an organic compound with delocalized electrons is generally represented as a stracture with localized electrons to let us know how many ir electrons it has. For example, nitroethane is usually represented with a nitrogen-oxygen double bond and a nitrogen-oxygen single bond. 

Delocalized electrons result from a p orbital overlapping the p orbitals of two adjacent atoms. 

The resonance hybrid shows that the two tt electrons are shared by three atoms. The resonance hybrid also shows that the two nitrogen-oxygen bonds are identical and that the negative charge is shared equally by both oxygens. Thus, we need to visualize and mentally average both resonance contributors to appreciate what the actual molecule— the resonance hybrid— looks like. 

Notice that delocalized electrons result from a p orbital overlapping the p orbitals of two adjacent atoms. For example, in nitroethane, the p orbital of nitrogen overlaps the p orbital of each of two adjacent oxygens in the carboxylate ion, the p orbital of carbon overlaps the p orbital of each of two adjacent oxygens and in benzene, the p orbital of carbon overlaps the p orbital of each of two adjacent carbons. 

To draw a set of resonance contributors for a molecule, first draw a Lewis structure. This is the first resonance contributor. Then, following the rules listed next, move electrons to generate the next resonance contributor. 

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