12. Heat, Energy, and Chemical        Bonds   Previous PageNext Page
       Benzene and Resonance

One can calculate heats of formation from bond-energy values for hundreds of molecules, and never be in error more than a kilocalorie or two. However, in those cases where the discrepancy is large, one can learn something new about the nature of chemical bonding. Benzene is a good illustration of this.

Let us try to calculate the standard heat of formation of benzene, C6H6. As we saw in Chapter 9, benzene is an example of a molecule for which simple single bond and double bond ideas are inadequate, and structures using them, such as the Kekulé structures at the right, are wrong. Benzene has six electrons delocalized around the ring. From a bond-energy viewpoint, how bad is the localized Kekulé model?

If we provisionally accept the Kekulé structures, then benzene has three C-C single bonds, three C=C double bonds, and six C-H single bonds. The energy involved in taking one mole of benzene molecules apart into atoms is

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