form two doughnut-shaped clouds of p electrons, one above and one below
the ring.
H
H
H
H
H
H
Doughnut-shaped cloud of π electrons
+
+
+
+
+
+
_
_
_
_
_
_
Six p atomic orbitals, one from each carbon of the benzene ring, combine to
form six p molecular orbitals. Three of the molecular orbitals have energies
lower than that of an isolated p orbital, and are known as bonding molecular
orbitals. Another three of the molecular orbitals have energies higher than
that of an isolated p orbital and are called antibonding molecular orbitals.
Two of the bonding orbitals have the same energy, as do the antibonding
orbitals. Such orbitals are said to be degenerate.
Stability of benzene
Benzene has a closed bonding shell of delocalized p electrons. This closed
bonding shell partly accounts for the stability of benzene. Benzene is more
stable than the Kekule ´ structure suggests. The stability of benzene can be
shown as follows.
+ H 2
+ 2 H 2
+ 3 H 2
P
o
t
e
n
t
i
a
l
e
n
e
r
g
y
Cyclohexane
∆H o = -28.6 kcal/mol
∆H o = -55.4 kcal/mol
∆H o = -85.8 kcal/mol
∆H o = -49.8 kcal/mol
+ 3 H 2
Resonance (stabilization)
energy = 36.0 kcal/mol
The energy required for the hydrogenation of cyclohexene to cyclohexane is
À28.6 kcal/mol. Therefore, in the case of cyclohexadiene, where there are
two double bonds, the energy required for the hydrogenation can be
calculated as 2 Â À28:6 ¼ À57:2 kcal/mol. In practice, the experimental
value is quite close to this calculated value, and is À55.4 kcal/mol. In this
4.6 AROMATIC COMPOUNDS AND THEIR DERIVATIVES
119