Organic Chemistry

Section 7.11 A Molecular Orbital Description of Stability 291

1,3,5-Hexatriene and Benzene

1,3,5-Hexatriene, with six carbon atoms, has six patomic orbitals.

The six patomic orbitals combine to produce six molecular orbitals:

and (Figure 7.11). Half of the MOs ( and ) are bonding MOs, and the

other half ( and ) are antibonding. 1,3,5-Hexatriene’s six electrons occupy

the three bonding MOs ( and ), and two of the electrons (those in ) are de-

localized over all six carbons. Thus, molecular orbital theory and resonance contribu-

tors are two different ways of showing that the electrons in 1,3,5-hexatriene are

delocalized. Notice in the figure that as the MOs increase in energy, the number of

nodes increases, the number of bonding interactions decreases, and the MOs alternate

from being symmetric to being asymmetric.

p

c 1 ,c 2 , c 3 c 1

c 4 ,c 5 , c 6 p

c 5 , c 6 c 1 ,c 2 , c 3

p c 1 ,c 2 ,c 3 ,c 4 ,

1,3,5-hexatriene

CH 2 CH CH CH

resonance hybrid of

1,3,5-hexatriene

CH CH 2 CH 2 CH CH CH CH CH 2

CH 2 CH CH 2 CH 2 CH CH 2

resonance contributors of the allyl anion resonance hybrid

CH CH 2

− − δ− δ−

CH 2

molecular orbitals energy levels

Energy

6 5 4 3 2 1

asymmetric

symmetric

asymmetric

symmetric

asymmetric

symmetric

energy of the

p atomic orbitals

>Figure 7.11

Six patomic orbitals overlap to

produce the six molecular orbitals

of 1,3,5-hexatriene. The six

electrons occupy the three bonding

molecular orbitals c 1 , c 2 ,and c 3.

p

BRUI07-263_297r4 21-03-2003 11:32 AM Page 291