A Useful Technique for Drawing Resonance Forms

Look back at the resonance forms of nitromethane and the acetate ion shown in the previous section. The pattern seen in both is a common one that leads to a useful technique for drawing resonance forms. In general, any three-atom grouping with a multiple bond has two resonance forms  

The atoms X, Y, and Z might be C, N, O, P, or S, and the asterisk ( ) might mean that the p orbital on atom Z is vacant, that it contains a single electron, or that it contains a lone pair of electrons. The two resonance forms diifer simply by an exchange of the positions of the multiple bond and the asterisk from one end to the other. 

By recognizing such three-atom pieces within larger structures, resonance forms can be systematically generated. Look, for instance, at the anion produced when is removed from 2,4-pentanedione by reaction with a strong base. How many resonance structures does the resultant anion have  

4-pentanedione anion has a lone pair of electrons and a formal negative charge on the central carbon atom, next to a C=0 bond on the left. The 0=C-C grouping is a typical one for which two resonance structures can be drawn  

Just as there is a C=0 bond to the left of the lone pair, there is a second C=0 bond to the right. Thus, we can draw a total of three resonance structures for the 2,4-pentanedione anion  

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C. A useful technique for drawing resonance forms (Section 2.6). 

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