2.6 Drawing Resonance Forms

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 $\text{C=O}$ bond on the left. The $\text{O=C–C}{\mathbf{\text{:}}}^{\text{−}}$ grouping is a typical one for which two resonance structures can be drawn.

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

Worked Example 2.2

Drawing Resonance Forms for an Anion

Draw three resonance structures for the carbonate ion, CO₃^2–.

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 $\text{C=O}$ 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.

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 (·).

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 $\text{C=C}$ 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.

Thus, the three resonance forms for the pentadienyl radical are:

Problem 2-9

Which of the following pairs of structures represent resonance forms, and which do not? Explain.

(a) (b)

Problem 2-10

Draw the indicated number of resonance forms for each of the following substances:

(a)

The methyl phosphate anion, CH₃OPO₃^2– (3)

(b)

The nitrate anion, NO₃^– (3)

(c)

The allyl cation, H₂C$\text{=}$CH–CH₂⁺ (2)

(d)

The benzoate anion (2)