Quantitative Evaluations of Resonance Stabilization in Benzene

Although we can appreciate that delocalization of electrons is an energy-lowering phenomenon, it is not easy to see intuitively whether this effect is big or small. In this section, we see two experimental procedures for determining the magnitude of resonance stabilization. 

FIGURE 13.13 The value for the special stability of benzene (red arrow) as derived from measured and calculated heats of hydrogenation. The 1,3.5-cyclohexatriene is drawn with exaggerated bond lengths. 

We calculated the heat of hydrogenation of the unreal, unknown, hypothetical molecule 1,3,5-cyclohexatriene by using values for the heats of hydrogenation of the real molecules cyclohexene and 1,3-cyclohexadiene.The heat of hydrogenation of 1,3-cyclohexadiene was calculated quite accurately, so we can be rather confident that our estimate for 1,3,5-cyclohexatriene won t be far off. That s important, because we can never make the cyclic triene to check the calculation. Any synthetic route will inevitably produce the much more stable real molecule, benzene. The heat of hydrogenation for the real molecule, benzene, is far lower than our estimate. 

5b Heats of Formation The delocalization energy of benzene can be estimated in another way, using heats of formation (A// ). The chemistry of 1,3,5,7-cyclooctatetraene tells us there is no special stabilization in this molecule. Hydrogenation is fast and addition reactions occur with ease. The hHf for cyclooctatetraene is -1-71.23 kcal/mol, or -1-8.9 kcal/mol per CH unit. 

The for benzene is +19.82 kcal/mol, or 3.3 kcal/mol per CH unit (Fig. 13.14). Therefore, benzene is about 5.6 kcal/mol more stable per CH unit than a cyclic polyene with no special stability (8.9 — 3.3 = 5.6 kcal/mol). Because there are six CH units in benzene we can estimate the special stability (resonance or delocalization energy) to be 6 X 5.6 = 33.6 kcal/mol. Note that the two methods, which use the related heats of hydrogenation and heats of formation, are in rather good agreement. 

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