Two structures of benzene as suggested by Kekulé
If we think of a hybrid of these two structures, then the CÀ ÀC bonds in
benzene are neither single bonds nor double bonds. They should have a
bond order between a single (1.47 A ˚ ) and a double bond (1.33 A ˚ ). It has
actually been proven that benzene is a planar molecule, and all of its CÀ ÀC
bonds are of equal length (1.39 A ˚ ). The bond order (1.39 A ˚ ) is indeed in
between a single and a double bond! Thus, instead of drawing the benzene
structure using alternative single and double bonds, a hybrid structure can be
drawn as follows.
Hybrid structure of benzene
The hybrid structure of benzene is represented by inscribing a circle in the
hexagon as depicted above. With benzene, the circle represents the six
electrons that are delocalized about the six carbon atoms of the benzene ring.
The resonance theory accounts for the much greater stability of benzene
(resonance energy) when compared with the hypothetical 1,3,5-cyclohexatriene. It also explains why there is only one 1,2-dibromobenzene rather
than two. Therefore, the structure of benzene is not really a 1,3,5-cyclohexatriene, but a hybrid structure as shown above.
The molecular orbital explanation of the structure of benzene
The bond angles of the carbon atoms in benzene are 120
. All carbon atoms
are sp
2
-hybridized, and each carbon atom has a single unhybridized p orbital
perpendicular to the plane of the ring. The carbon sp
2 -hybridized orbitals
overlap to form the ring of the benzene molecule. Because the CÀ ÀC bond
lengths are 1.39 A ˚ , the p orbitals are close enough to overlap efficiently and
equally all round the ring.
H
H
H
H
H
H
Benzene structure in the light of molecular orbital theory
+
+
+
+
+
+
_
_
_
_
_
_
π bonds
The six overlapping p orbitals overlap to form a set of six p molecular
orbitals. Six p electrons are completely delocalized around the ring, and
118
CH4 ORGANIC FUNCTIONAL GROUPS
If we think of a hybrid of these two structures, then the CÀ ÀC bonds in
benzene are neither single bonds nor double bonds. They should have a
bond order between a single (1.47 A ˚ ) and a double bond (1.33 A ˚ ). It has
actually been proven that benzene is a planar molecule, and all of its CÀ ÀC
bonds are of equal length (1.39 A ˚ ). The bond order (1.39 A ˚ ) is indeed in
between a single and a double bond! Thus, instead of drawing the benzene
structure using alternative single and double bonds, a hybrid structure can be
drawn as follows.
Hybrid structure of benzene
The hybrid structure of benzene is represented by inscribing a circle in the
hexagon as depicted above. With benzene, the circle represents the six
electrons that are delocalized about the six carbon atoms of the benzene ring.
The resonance theory accounts for the much greater stability of benzene
(resonance energy) when compared with the hypothetical 1,3,5-cyclohexatriene. It also explains why there is only one 1,2-dibromobenzene rather
than two. Therefore, the structure of benzene is not really a 1,3,5-cyclohexatriene, but a hybrid structure as shown above.
The molecular orbital explanation of the structure of benzene
The bond angles of the carbon atoms in benzene are 120
. All carbon atoms
are sp
2
-hybridized, and each carbon atom has a single unhybridized p orbital
perpendicular to the plane of the ring. The carbon sp
2 -hybridized orbitals
overlap to form the ring of the benzene molecule. Because the CÀ ÀC bond
lengths are 1.39 A ˚ , the p orbitals are close enough to overlap efficiently and
equally all round the ring.
H
H
H
H
H
H
Benzene structure in the light of molecular orbital theory
+
+
+
+
+
+
_
_
_
_
_
_
π bonds
The six overlapping p orbitals overlap to form a set of six p molecular
orbitals. Six p electrons are completely delocalized around the ring, and
118
CH4 ORGANIC FUNCTIONAL GROUPS