Question

a. Draw resonance contributors for the following species. Indicate which are major contributors and which are minor contributors to the resonance hybrid.

b. Do any of the species have resonance contributors that all contribute equally to the resonance hybrid?

Answer 1

(a) To draw the resonance contributors, the π electrons, or lone pair of electrons, are moved toward a sp² carbon atom. The sp² carbon atom is either a positively charged carbon or a double-bonded carbon.

Each resonance contributor must have the same net charge. The movement of the electrons is shown using a curved arrow. The tail of the curved arrow shows where the electrons start from and the head of the curved arrow shows where the electrons are going.

The greater the predicted stability of the resonance contributor, the more it contributes to the structure of the resonance hybrid.

(1) Consider the given structure.

The following resonance contributors are obtained by moving the lone pair of electrons on the oxygen atom toward a sp² carbon atom:

In structure A, none of the atoms carry any charge. In structure B, the positive charge is on an oxygen atom, whereas the negative charge is on a carbon atom. In structure B, there is a charge separation.

The structures that do not have a charge separation are more stable than the structures with a charge separation. So, structure A is more stable than structure B. Therefore, structure A is the major contributor, whereas structure B is the minor contributor to the resonance hybrid.

(2) Consider the given structure.

The following resonance contributors are obtained by moving the π electrons toward an sp² carbon atom:

In structure A and structure B, none of the atoms carry any charge. The electrons are delocalized over the entire ring. Both of the structures are equally stable and contribute equally to the resonance hybrid.

(3) Consider the given structure.

The following resonance contributors are obtained by moving the lone pair of electrons on the carbon atom toward a sp² carbon atom:

In structure A, the negative charge is on a carbon atom. In structure B, the negative charge is on a nitrogen atom. The structure in which the negative charge is on the more electronegative atom is more stable than the structure in which the negative charge is on the less electronegative atom.

A nitrogen atom is more electronegative than a carbon atom. So, structure B is more stable than structure A. Therefore, structure B is the major contributor, whereas structure A is the minor contributor to the resonance hybrid.

(4) Consider the given structure.

The following resonance contributors are obtained by moving the π electrons toward an sp² carbon atom with a positive charge.

In structure A, the positive charge is on a primary carbon atom. In structure B the positive charge is on a secondary carbon atom. In a structure in which the positive charge is carried by a carbon atom, the stability of the carbocation is to be considered.

In the case of carbocations, a secondary allylic carbocation is more stable than a primary allylic carbocation. So, structure B is more stable than structure A. Therefore, structure B is the major contributor, whereas structure A is the minor contributor to the resonance hybrid.

(5) Consider the given structure.

The following resonance contributors are obtained by moving the π electrons toward a sp² carbon atom with a positive charge.

In all the structures, there is a carbon atom with a single positive charge. The π electrons are delocalized over the entire ring of five carbon atoms. All the structures are equally stable and contribute equally to the resonance hybrid.

(6) Consider the given structure.

The following resonance contributors are obtained by moving the lone pair of electrons on the oxygen atom toward a sp² carbon atom:

In structure A and structure E none of the atoms carry any charge. In structure B, C and D the positive charge is on an oxygen atom, whereas the negative charge is on a carbon atom. The structures that do not have a charge separation are more stable than the structures with a charge separation.

So, structures A and E are more stable than structures B, C and D.

In structure A and structure E the π electrons are delocalized over the entire ring of six carbon atoms. So, the structures A and E are equally stable and contribute equally to the resonance hybrid.

Thus, structures A and E are major contributors, whereas structures B, C and D are minor contributors.

(b) The following two species have resonance contributors that all contribute equally to the resonance hybrid. In both of the species, all the resonance contributors are equally stable and contribute equally to the resonance hybrid.