Drawing Resonance Forms

Look back at the resonance forms of the acetate ion and the acetone anion shown in the previous section. The pattern seen there is a common one that leads to a useful technique for drawing resonance forms. In general, any three atoms grouping with a p orbital on each atom has two resonance forms:

The atoms X, Y, and Z in the general structure might be C, N, O, P, S, or  others, 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 differ simply by an exchange in position of both the multiple bond and the asterisk from one end of the three-atom grouping to the other.

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

The 2,4-pentanedione anion has a lone pair of electrons and a formal negative charge on the central carbon atom, next to a C=O bond on the left. The  O=C-C:^- grouping is a typical one for which two resonance structures can be drawn.

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