2.6 • 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-atom 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 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 bond on the left. The grouping is a typical one for which two resonance structures can be drawn.
Just as there is a bond to the left of the lone pair, there is a second bond to the right. Thus, we can draw a total of three resonance structures for the 2,4-pentanedione anion.
Drawing Resonance Forms for an Anion
Draw three resonance structures for the carbonate ion, CO32–.
StrategyLook for three-atom groupings that contain a multiple bond next to an atom with a p orbital. Then exchange the positions of the multiple bond and the electrons in the p orbital. In the carbonate ion, each singly bonded oxygen atom with three lone pairs and a negative charge is adjacent to the double bond, giving the grouping .
SolutionExchanging the position of the double bond and an electron lone pair in each grouping generates three resonance structures.
Drawing Resonance Forms for a Radical
Draw three resonance forms for the pentadienyl radical, where a radical is a substance that contains a single, unpaired electron in one of its orbitals, denoted by a dot (·).
StrategyFind the three-atom groupings that contain a multiple bond next to an atom with a p orbital.
SolutionThe unpaired electron is on a carbon atom next to a bond, giving a typical three-atom grouping that has two resonance forms.
In the second resonance form, the unpaired electron is next to another double bond, giving another three-atom grouping and leading to another resonance form.
Thus, the three resonance forms for the pentadienyl radical are: