Resonance and Delocalization - Organic Chemistry Concepts: An EFL Approach - page 56

Resonance forms differ from each other only in the arrangement of the π
and/or non-bonding lone pairs of electrons. The octet rule of valency is
followed. The relative position of all bonded atoms remains unchanged,
as in Figure 4.1 . For comparison, Figure 4.2 shows a non-resonance
situation.

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FIGURE 4.2
The non - resonance relationship between dienes.

Resonance forms do not have to be equal. As Figure 4.3 shows, each Lewis
structure contributes to the overall resonance hybrid, but these contribu-
tions are not always equal. If resonance forms are not equal, the reso-
nance hybrid will be more like the Lewis structure of the most stable
resonance form. The hybrid is a weighted average of the resonance forms.

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FIGURE 4.3
Resonance description of the carbonyl group.

A resonance situation is more stable than a non-resonance situation. The
resonance hybrid is more stable than any single resonance form.
The more reasonable resonance forms that can be drawn, the greater the
stability of the molecule.

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In the below benzene system, as shown in Figure 4.4 ,

all six C-C bonds are the same length, between average single and double
bond lengths;

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the potential energy of the delocalized hybrid is 152 kJ/mol lower than
the predicted potential energy of either resonance form with a fixed bond-
ing arrangement.

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