2.9: Drawing Resonance Forms

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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:

Two resonance forms in a three-atom grouping of X, Y, and Z with one double bond in the structure shown separated by a double-headed arrow.

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 chemical reaction of 2,4 – pentanedione with a base.

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.

An arrow mechanism of resonance in a 2,4-pentanedione anion.

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.

Three resonance forms of 2,4-pentanedione, separated by two double-headed arrows.

Worked Example 2.2

Drawing Resonance Forms for an Anion

Draw three resonance structures for the carbonate ion, CO₃²^–.

The chemical structure of a carbonate ion.

Strategy

Look 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 $C=O$ double bond, giving the grouping .

Solution

Exchanging the position of the double bond and an electron lone pair in each grouping generates three resonance structures.

Three resonance structures of the carbonate ion, featuring the lone pairs and charges on the oxygen atoms in each case and separated by two double-headed arrows.

Worked Example 2.3

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 (·).

The chemical structure of pentadienyl radical, featuring an unpaired electron on the right-most carbon atom.

Strategy

Find the three-atom groupings that contain a multiple bond next to an atom with a p orbital.

Solution

The unpaired electron is on a carbon atom next to a $C=C$ bond, giving a typical three-atom grouping that has two resonance forms.

Two resonance forms of pentadienyl radical, featuring an unpaired electron on the right-most carbon atom in one case and the central carbon atom in the next, separated by a double-headed arrow.

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.