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,
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