2.6 Drawing Resonance Forms

David Tully

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 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.

Worked Example 2.2Drawing Resonance Forms for an AnionDraw 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 negativecharge is adjacent to the C=O double bond, giving the grouping.SolutionExchanging the position of the double bond and an electron lone pair in each grouping generates three resonance structures.

Worked Example 2.3Drawing Resonance Forms for a RadicalDraw 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 porbital.

SolutionThe unpaired electron is on a carbon atom next to a C=C 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.