Organic Chemistry

292 CHAPTER 7 Electron Delocalization and Resonance• More About Molecular Orbital Theory

6

4 5

2 3

1

Energy

energy of the p

atomic orbitals

energy levels

p atomic orbitals of benzene

Figure 7.12N

Benzene has six molecular

orbitals, three bonding

and three antibonding

The six electrons occupy the three

bonding molecular orbitals.

p

(c 4 ,c 5 ,c 6 ).

(c 1 , c 2 ,c 3 )

p

Figure 7.13 shows that there are sixbonding interactions in the lowest-energy MO

of benzene—one more than in the lowest-energy MO of 1,3,5-hexatriene

(Figure 7.12). In other words, putting the three double bonds into a ring is accompa-

nied by an increase in stabilization. Each of the other two bonding MOs of benzene

( and ) has a node in addition to the node that bisects the porbitals. These two or-

bitals are degenerate: has four bonding interactions and two antibonding interac-

tions, for a net of two bonding interactions, and also has two bonding interactions.

Thus, and are bonding MOs, but they are not as strongly bonding as

The energy levels of the MOs of ethene, 1,3-butadiene, 1,3,5-hexatriene, and benzene

are compared in Figure 7.14. You can see that benzene is a particularly stable mole-

cule—more stable than 1,3,5-hexatriene and much more stable than a molecule with one

or more isolated double bonds. Compounds such as benzene that are unusually stable

because of large delocalization energies are called aromatic compounds. The structural

features that cause a compound to be aromatic are discussed in Section 15.1.

PROBLEM 17

How many bonding interactions are there in the and molecular orbitals of the fol-

lowing compounds?

a. 1,3-butadiene b. 1,3,5,7-octatetraene

c 1 c 2

c 2 c 3 c 1.

c 3

c 2

c 2 c 3

1 c 12

PROBLEM 16

Answer the following questions for the molecular orbitals of 1,3,5-hexatriene:

a. Which are the bonding MOs and which are the antibonding MOs?

b. Which MOs are symmetric and which are asymmetric?

c. Which MO is the HOMO and which is the LUMO in the ground state?

d. Which MO is the HOMO and which is the LUMO in the excited state?

e. What is the relationship between the HOMO and the LUMO and symmetric and asym-

metric orbitals.

Like 1,3,5-hexatriene, benzene has a six-carbon system. The six-carbon sys-

tem in benzene, however, is cyclic. The six patomic orbitals combine to produce six

molecular orbitals (Figure 7.12). Three of the MOs are bonding ( and ) and

three are antibonding ( and ). Benzene’s six electrons occupy the three

lowest-energy MOs (the bonding MOs). The two electrons in are delocalized over

the six carbon atoms. The method used to determine the relative energies of the MOs

of compounds with cyclic systems is described in Section 15.6.p

c 1

c 4 ,c 5 , c 6 p

c 1 ,c 2 , c 3

p

p p

p

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