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 Kekul� structures at the right, are wrong. Benzene has six electrons delocalized around the ring. From a bond-energy viewpoint, how bad is the localized Kekul� model?

If we provisionally accept the Kekul� 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