Organic Chemistry

orbital can hold two electrons of opposite spin), and Hund’s rule (an electron will

occupy an empty degenerate orbital before it will pair up with an electron that is al-

ready present in an orbital).

1,3-Butadiene and 1,4-Pentadiene

The electrons in 1,3-butadiene are delocalized over four carbons. In other

words, there are four carbons in the system. A molecular orbital description of

1,3-butadiene is shown in Figure 7.9.

Each of the four carbons contributes one patomic orbital, and the four patomic

orbitals combine to produce four molecular orbitals: and Thus, a

molecular orbital results from the linear combination of atomic orbitals (LCAO).

Half of the MOs are bonding MOs ( and ), and the other half are antibonding

MOs ( and ), and they are given the designations and in

order of increasing energy. The energies of the bonding and antibonding MOs are

symmetrically distributed above and below the energy of the patomic orbitals.

Notice that as the MOs increase in energy, the number of nodes increases and the

number of bonding interactions decreases. The lowest-energy MO ( ) has only

the node that bisects the porbitals—it has no nodes between the nuclei because all the

blue lobes overlap on one face of the molecule and all the green lobes overlap on

the other face; has three bonding interactions; has one node between the nuclei

and two bonding interactions (for a net of one bonding interaction); has two nodes

between the nuclei and one bonding interaction (for a net of one antibonding interac-

tion); and has three nodes between the nuclei—three antibonding interactions—and

no bonding interactions. The four electrons of 1,3-butadiene reside in p c 1 and c 2.

c 4

c 3

c 1 c 2

c 1

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

1 p 2 c 1 c 2

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

p

p sp^2

Section 7.11 A Molecular Orbital Description of Stability 287

p atomic orbitals

out-of-phase

in-phase molecular

orbitals

energy levels

possible

alignment

of orbitals

destructive node

overlap

antibonding

* molecular

orbital

energy of the p

atomic orbitals

bonding

molecular

orbital

Energy

2

1

constructive

overlap

Figure 7.8

The distribution of electrons in ethene. Overlapping of in-phase porbitals produces a

bonding molecular orbital that is lower in energy than the patomic orbitals. Overlapping

of out-of-phase porbitals produces an antibonding molecular orbital that is higher in

energy than the patomic orbitals.

resonance contributors

resonance hybrid

CH 2 CH CH CH 2 CH CH CH 2

CH 2 CH CH CH 2

CH 2 CH CH

+

CH 2 CH 2

− + −

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