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The first resonance structure has a carbon atom that lacks an octet (C+), while all of the atoms in the second resonance structure have filled octets. Therefore, the second resonance structure is the major contributor. You may also notice that the second resonance structure has more covalent bonds than the first resonance structure. So another way to state this rule is as follows: The most important resonance contributors have the greatest number of covalent bonds. This is because a resonance structure with more covalent bonds will have a greater number of filled octets. As seen in the first resonance structure above, it is common to encounter a carbon atom with a positive charge, even though it lacks an octet. In contrast, oxygen is much more electronegative than carbon, so you should never draw a resonance structure in which an oxygen atom lacks an octet. This can be illustrated with the following example: OOO OOO Major Minor Insignificant contributor contributor

In the first resonance structure, all of the atoms have filled octets, so this resonance structure is the major contributor. The second resonance structure is a minor contributor because the carbon atom lacks an octet. The third resonance structure shown is insignificant because the positively charged oxygen atom lacks an octet. Avoid drawing insignificant resonance structures. Rule2 Thestructurewithfewerformalchargesismoreimportant.Considerthefollowingexample: OOO NNN Most significant Major Minor contributor contributor contributor

In this example, the first resonance structure is the most significant resonance structure (it is the greatest contributor to the resonance hybrid) because it has filled octets and no formal charges. The second resonance structure is still a major contributor since it has filled octets, but it is less important than the first because it has formal charges. The third resonance structure is a minor contributor because it has a carbon atom that lacks an octet.

2.8 ASSESSING THE RELATIVE IMPORTANCE OF RESONANCE STRUCTURES

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In cases where there is an overall net charge, as seen in the example below, the creation of new charges is not favorable. For such charged compounds, the goal in drawing resonance structures is to delocalize the charge – relocate it to as many different positions as possible. OOO CH3 C CH2 CH3 C CH2 CH3 C CH2 Insignificant delocalized negative charge

In this case, the third resonance structure above is insignificant (and should not be drawn). Rule 3 Other things being equal, a structure with a negative charge on the more electronegative element will be more important. To illustrate this, let’s revisit the previous example, in which there are two significant resonance structures. The first resonance structure has a negative charge on oxygen, while the second resonance structure has a negative charge on carbon. Since oxygen is more electronegative than carbon, the first resonance structure is the major contributor: OO CH3 C CH2 CH3 C CH2 Major contributor Minor contributor

Similarly, a positive charge will be more stable on the less electronegative element. In the following example, both resonance structures have filled octets, so we consider the location of the positive charge. Nitrogen is less electronegative than oxygen, so the resonance structure with N + is the major contributor: HH OO HH NN HH

Minor contributor Major contributor

Rule 4 Resonance structures that have equally good Lewis structures are described as equivalent and contribute equally to the resonance hybrid. As an example, consider the carbonate ion (CO 2−), 3

shown here: OOOO OCO OCO OCO OCO

Equivalent and major contributors Insignificant

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CHAPTER 2 RESONANCE

This ion has a net charge, so recall from Rule 2 that the goal is to delocalize the charges as much as possible and to avoid creating new charges. In the actual structure of the carbonate ion (the resonance hybrid), the two negative charges are shared equally among all three oxygen atoms. EXERCISE 2.61 Draw all significant resonance structures for the following compound. N

Answer The only resonance pattern evident in the enamine is a lone pair next to a pi bond, so we draw the following two curved arrows, giving a resonance structure with charge separation: NN

As we inspect this resonance structure, we find the same pattern again–a lone pair next to a pi bond. So, once again, we draw the following two curved arrows, giving another resonance structure: NN

In total, there are three significant resonance structures: NNN

In each of the three resonance structures, all of the atoms have filled octets. The first is the most significant because it is has no formal charges, but the other two resonance structures do certainly contribute some character to the resonance hybrid. Be careful – DON’T draw the third structure shown below, which has both C+ and C−, and is therefore insignificant: NNN Insignificant

2.64

2.66 2.67 2.68 N 2.8 ASSESSING THE RELATIVE IMPORTANCE OF RESONANCE STRUCTURES

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This resonance structure suffers from TWO major deficiencies: 1) it does not have filled octets, while the other resonance structures shown above all have filled octets, and 2) it has a negative charge on a carbon atom (which is not an electronegative atom). Either of these deficiencies alone would render the resonance structure a minor contributor. But with both deficiencies together (C+ and C−), this resonance structure is insignificant. The same is true for any resonance structure that has both C+ and C−. Such a resonance structure will generally be insignificant. PROBLEMS For each of the following compounds, draw all of the significant resonance structures.

N

2.62 2.63 O

____________________________________________________________________ 2.65 O

____________________________________________________________________ ____________________________________________________________________ N

____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ O O

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