Organic Chemistry

Section 29.2 Molecular Orbitals and Orbital Symmetry 107

molecular orbitals ground state

Energy

6

excited state

5

4

3

2

1

energy levels

energy of the

p atomic orbitals

Figure 29.3
Six patomic orbitals interact to give the six molecular orbitals of 1,3,5-hexatriene.p

The ground-state HOMO and the

excited-state HOMO have opposite

symmetries.

asymmetric. Therefore,the ground-state HOMO and the excited-state HOMO always
have opposite symmetries—one is symmetric and the other is asymmetric.A molecu-
lar orbital description of 1,3,5-hexatriene, a compound with three conjugated double
bonds, is shown in Figure 29.3. As a review, examine the figure and note

• the distribution of electrons in the ground and excited states


• that the number of bonding interactions decreases and the number of nodes
  increases as the molecular orbitals increase in energy


• that the molecular orbitals alternate from symmetric to asymmetric as the
  molecular orbitals increase in energy

Although the chemistry of a compound is determined by all its molecular orbitals,
we can learn a great deal about the chemistry of a compound by looking at only
the highest occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO). These two molecular orbitals are known as the frontier
orbitals. We will now see that simply by evaluating oneof the frontier molecular
orbitals of the reactant(s) in a pericyclic reaction, we can predict the conditions under
which the reaction will occur (thermal or photochemical, or both) and the products
that will be formed.