Organic Chemistry

Section 15.6 A Molecular Orbital Description of Aromaticity and Antiaromaticity 603

Energy

Energy

nonbonding

MOs

a.

b. d.

antibonding c.

MOs

antibonding

MOs

bonding MOs

bonding MOs

bonding MO

bonding MOs

antibonding

MOs

antibonding

MO

Figure 15.2
The distribution of electrons in the molecular orbitals of (a) benzene, (b) the
cyclopentadienyl anion, (c) the cyclopentadienyl cation, and (d) cyclobutadiene. The
relative energies of the molecular orbitals in a cyclic compound correspond to the
relative levels of the vertices. Molecular orbitals below the midpoint of the cyclic structure
are bonding, those above the midpoint are antibonding, and those at the midpoint are
nonbonding.

p

p

Tutorial:

Molecular orbital description

of aromaticity

American scientist who devised this simple method. Notice that the number of
molecular orbitals is the same as the number of atoms in the ring because each ring
atom contributes a porbital. (Recall that orbitals are conserved; Section 7.11.)
The six electrons of benzene occupy its three bonding molecular orbitals, and
the six electrons of the cyclopentadienyl anion occupy itsthree bonding molecu-
lar orbitals. Notice that there is always an odd number of bonding orbitals because one
corresponds to the lowest vertex and the others come in degenerate pairs. This means
that aromatic compounds—such as benzene and the cyclopentadienyl anion—with an
odd number of pairs of electrons have completely filled bonding orbitals and no
electrons in either nonbonding or antibonding orbitals. This is what gives aromatic
molecules their stability. (A more in-depth description of the molecular orbitals in
benzene is given in Section 7.11.)
Antiaromatic compounds have an even number of pairs of electrons. Therefore,
either they are unable to fill their bonding orbitals (cylopentadienyl cation) or they have
a pair of electrons left over after the bonding orbitals are filled (cyclobutadiene).
Hund’s rule requires that these two electrons go into two degenerate orbitals (Section
1.2). The unpaired electrons are responsible for the instability of antiaromatic molecules.

PROBLEM 12

How many bonding, nonbonding, and antibonding molecular orbitals does cyclobuta-

diene have? In which molecular orbitals are the electrons?

PROBLEM 13

Can a radical be aromatic?

PROBLEM 14

Following the instructions for drawing the molecular orbital energy levels of the com-

pounds shown in Figure 15.2, draw the molecular orbital energy levels for the cyclohep-

tatrienyl cation, the cycloheptatrienyl anion, and the cyclopropenyl cation. For each

compound, show the distribution of the electrons. Which of the compounds are aro-

matic? Which are antiaromatic?

p

p

p

p

p

p

p

p

p p

p p

p

Aromatic compounds are stable

because they have filled bonding

Pmolecular orbitals.