Assessing the Relative Importance of Resonance Structures

Not all resonance structures are always important. Although there are often many valid resonance structures for a single compound (meaning that those structures do not violate the two commandments), usually many of these resonance structures are not significant. For example, the following resonance structures are not significant even though they are valid structures  

Let s see the three rules for finding which resonance structures are significant. 

Rule 1 Minimize charges. The best kind of structure is one without any charges. It is OK to have two charges, but you should try to avoid structures that have more than two charges. Compare the following two cases  

Both compounds have a pi bond between a carbon atom and an electronegative atom (C = 0), and both compounds have a lone pair next to the pi bond. So we would expect their resonance structures to be similar, and we would expect these compounds 

The first and last structures are OK. but the second resonance structure is bad because there are loo many charges. So we don t draw it when drawing resonance structures. It is not significant. 

Not all resonance structures are equally significant. A compound might have many valid resonance structures (that do not violate the two commandments), but it is possible that one or more resonance structures might be insignificant To understand what we mean when we say insignificant, let s revisit the analogy we used in the beginning of the chapter. 

Recall that we used the analogy of a nectarine (being a hybrid between a peach and plum) to explain the concept of resonance. Now, imagine that we create a new type of fruit that is a hybrid between three fruits a peach, a plum, and a kiwi. Suppose that the hybrid fruit that we produce has the following character 65% peach character, 34% plum character, and 1% kiwi character. This hybrid fruit will look almost exactly like a nectarine, because the amount of kiwi character is too small to have an effect on the nature of the resulting hybrid. Even though this fhiit is actually a hybrid of three fruits, nevertheless it will look like a hybrid of only two fruits— because the kiwi character is insignificant.  

A similar concept exists when comparing resonance structures. One compound might have three resonance structures, but all three resonance structures might not contribute equally to the overall resonance hybrid. One resonance structure might be the major contributor (like the peach), while another resonance structure might be insignificant (like the kiwi). In order to understand the true nature of 

There are three simple rules to follow when comparing resonance structures. At this point, you are probably thinking that it is hard enough to keep track of 

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