Consider the resonance forms of the acetate ion 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 can have two resonance forms:

The atoms “X”, “Y”, and “Z” in the general structure could be C, N, O, P, S, or others, and the asterisk (*) serves as a reminder that the p orbital on atom “Z” could be vacant, contain a single electron, or contain 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 negative formal charge on the central carbon atom, which is 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 2.2 Drawing Resonance Forms for an Anion.

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

Strategy. Look for a three-atom grouping where each atom possesses 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 2.3 – 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 porbital.

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.

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=CH–CH₂⁺ (2)

(d) the benzoate anion (2)