A compound with delocalized electrons is more stable than it would be if all of its electrons were localized. The extra stability a compound gains from having delocalized electrons is called delocalization energy or resonance energy. Electron delocalization gives a compound resonance, so saying that a compound is stabilized by electron delocalization is the same as saying that it is stabilized by resonance. Since the resonance energy tells us how much more stable a compound is as a result of having delocalized electrons, it is frequently called resonance stabilization.
To understand the concept of resonance energy better, let’s take a look at the resonance energy of benzene. In other words, let’s see how much more stable benzene (with three pairs of delocalized π electrons) is than the unknown, unreal, hypothetical compound “cyclohexatriene” (with three pairs of localized π electrons).
The ΔHᵒ for the hydrogenation of cyclohexene, a compound with one localized double bond, has been determined experimentally to be –28.6 kcal/mol. We would then expect the ΔHᵒ for the hydrogenation of “cyclohexatriene,” a hypothetical compound with three localized double bonds, to be three times that of cyclohexene; that is, 3 X (–28.6) = –85.8 kcal/mol .
When the ΔHᵒ for the hydrogenation of benzene was determined experimenally, it was found to be –49.8 kcal/mol, much less than that calculated for hypothetical “cyclohexatriene.”
Because the hydrogenation of “cyclohexatriene” and the hydrogenation of benzene both form cyclohexane, the difference in the ΔHᵒ values can be accounted for only by a difference in the energies of “cyclohexatriene” and benzene. Figure 7.6 shows that benzene must be 36 kcal/mol (or 151 kJ/mol) more stable than “cyclohexatriene” because the experimental ΔHᵒ for the hydrogenation of benzene is 36 kcal/mol less than that calculated for “cyclohexatriene.”
Because benzene and “cyclohexatriene” have different energies, they must be different compounds. Benzene has six delocalized π electrons, whereas hypothetical “cyclohexatriene” has six localized π electrons. The difference in their energies is the resonance energy of benzene. The resonance energy tells us how much more stable a compound with delocalized electrons is than it would be if its electrons were localized. Benzene, with six delocalized π electrons, is 36 kcal/mol more stable than hypothetical “cyclohexatriene,” with six localized π electrons. Now we can understand why nineteenth-century chemists, who didn’t know about delocalized electrons, were puzzled by benzene’s unusual stability
Since the ability to delocalize electrons increases the stability of a molecule, we can conclude that a resonance hybrid is more stable than the predicted stability of any of its resonance contributors. The resonance energy associated with a compound that has delocalized electrons depends on the number and predicted stability of the resonance contributors: The greater the number of relatively stable resonance contributors, the greater is the resonance energy. For example, the resonance energy of a carboxylate ion with two relatively stable resonance contributors is significantly greater than the resonance energy of a carboxylic acid with only one relatively stable resonance contributor.
Notice that it is the number of relatively stable resonance contributors—not the total number of resonance contributors—that is important in determining the resonance energy. For example, the resonance energy of a carboxylate ion with two relatively stable resonance contributors is greater than the resonance energy of the compound in the following example because even though this compound has three resonance contributors, only one of them is relatively stable:
The more nearly equivalent the resonance contributors are in structure, the greater is the resonance energy. The carbonate dianion is particularly stable because it has three equivalent resonance contributors.
We can now summarize what we know about contributing resonance structures:
- The greater the predicted stability of a resonance contributor, the more it contributes to the resonance hybrid.
- The greater the number of relatively stable resonance contributors, the greater is the resonance energy.
- The more nearly equivalent the resonance contributors, the greater is the resonance energy.
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