Brief Review of Resonance

This subject was treated in detail in Chapter 1. Here we only pick up the highlights of that treatment. If anything seems unfamiliar or strange go back to Chapter 1 A good way to check that this important material is under control is to see if you can do Problems 1.18—1.21. 

Remember that a molecule that is best described as a resonance hybrid, does not, repeat not, spend part of its time as one form and part as another. That is chemical equilibrium not resonance. The two are very different phenomena. Resonance is always indicated by the special double headed arrow — , whereas equilibrium is shown by two arrows pointing in opposite directions 

As we pointed out as early as Chapter 1, the real structure of a resonance-stabilized molecule is a hybrid, or combination, of the resonance forms contributing to the structure. These forms represent different electronic descriptions of the molecule. Resonance forms differ only in the distributions of electrons and never in the positions of atoms. If you have moved an atom, you have written a chemical equilibrium. 

It is important to be able to estimate the relative importance of resonance forms in order to get an idea of the best way to represent a molecule. To do so, we can assign a weighting factor, c, to each resonance form. The weighting factor indicates the percent contribution of each resonance form to the overall structure. Some guidelines for assigning weighting factors are summarized below. 

The more bonds in a resonance form, the more stable the form is. 

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