RESONANCE STRUCTURES AND CURLY ARROWS
45
two benzene rings fused together, and anthracene
has three fused rings. The heterocycle quinoline (see
Section 11.8.1) is a fusion of benzene and pyridine.
These ring systems are undoubtedly aromatic, and
they display the enhanced stability and reactivity
associated with simple aromatic compounds like
benzene.
naphthalene
anthracene
naphthalene
(10 π electrons)
anthracene
(14 π electrons)
these structures are strictly incorrect if
the circle represents six π electrons
N
quinoline
naphthalene
(10 π electron system)
anthracene
(14 π electron system)
the π electron system may involve just
the periphery of the molecule
Molecular orbital calculations suggest that the π
electrons in naphthalene are delocalized over the
two rings and this results in substantial stabilization.
These molecules are planar, and all p orbitals are
suitably aligned for overlap to form π bonding
molecular orbitals. Although we can draw Kekul´ e
structures for these compounds, it is strictly incorrect
to use the circle in hexagon notation since the
circle represents six π electrons. Naphthalene has 10
carbons, and therefore 10 π electrons, and anthracene
has 14 π electrons. The circle notation suggests 12 or
18 π electrons. Note that H¨ uckel’s rule applies only to
monocyclic compounds, and although 10 π electrons
(naphthalene) and 14 π electrons (anthracene) seem
to be meet the criteria for aromaticity, there is good
evidence to suggest we should consider the aromatic
system not as a combination of benzene rings, but as
a single ring involving the periphery of the molecule.
2.10 Resonance structures and curly
arrows
The molecular orbital picture of benzene proposes
that the six π electrons are no longer associated
with particular bonds, but are effectively delocalized
over the whole molecule, spread out via orbitals
that span all six carbons. This picture allows us to
appreciate the enhanced stability of an aromatic ring,
and also, in due course, to understand the reactivity
of aromatic systems. There is an alternative approach
based on Lewis structures that is also of particular
value in helping us to understand chemical behaviour.
Because this method is simple and easy to apply, it is
an approach we shall use frequently. This approach
is based on what we term resonance structures.
Let us go back to the two Kekul´ e representations
for benzene. The Lewis structure for benzene has
alternating single and double bonds, but there are two
ways of writing this. In one form, a particular bond
is single; in the other form, this bond has become
double. Resonance theory suggests that these two
structures are both valid representations, and that each
contributes to the structure of benzene, but the true
structure is something in between, a lower energy
hybrid of the two Kekul´ e forms, a resonance hybrid.
If this is the case, then each bond is neither single
nor double, but, again, something in between. As we
have already seen, all C–C bond lengths in benzene
are 1.40 ˚
A, which is in between the bond lengths for
single (1.47 ˚
A) and double (1.34 ˚
A) bonds.
Kekulé representations
of benzene
curly arrow represents the
movement of two electrons
we could also have written
curly arrows like this
double-headed arrow is used to
indicate resonance structures
45
two benzene rings fused together, and anthracene
has three fused rings. The heterocycle quinoline (see
Section 11.8.1) is a fusion of benzene and pyridine.
These ring systems are undoubtedly aromatic, and
they display the enhanced stability and reactivity
associated with simple aromatic compounds like
benzene.
naphthalene
anthracene
naphthalene
(10 π electrons)
anthracene
(14 π electrons)
these structures are strictly incorrect if
the circle represents six π electrons
N
quinoline
naphthalene
(10 π electron system)
anthracene
(14 π electron system)
the π electron system may involve just
the periphery of the molecule
Molecular orbital calculations suggest that the π
electrons in naphthalene are delocalized over the
two rings and this results in substantial stabilization.
These molecules are planar, and all p orbitals are
suitably aligned for overlap to form π bonding
molecular orbitals. Although we can draw Kekul´ e
structures for these compounds, it is strictly incorrect
to use the circle in hexagon notation since the
circle represents six π electrons. Naphthalene has 10
carbons, and therefore 10 π electrons, and anthracene
has 14 π electrons. The circle notation suggests 12 or
18 π electrons. Note that H¨ uckel’s rule applies only to
monocyclic compounds, and although 10 π electrons
(naphthalene) and 14 π electrons (anthracene) seem
to be meet the criteria for aromaticity, there is good
evidence to suggest we should consider the aromatic
system not as a combination of benzene rings, but as
a single ring involving the periphery of the molecule.
2.10 Resonance structures and curly
arrows
The molecular orbital picture of benzene proposes
that the six π electrons are no longer associated
with particular bonds, but are effectively delocalized
over the whole molecule, spread out via orbitals
that span all six carbons. This picture allows us to
appreciate the enhanced stability of an aromatic ring,
and also, in due course, to understand the reactivity
of aromatic systems. There is an alternative approach
based on Lewis structures that is also of particular
value in helping us to understand chemical behaviour.
Because this method is simple and easy to apply, it is
an approach we shall use frequently. This approach
is based on what we term resonance structures.
Let us go back to the two Kekul´ e representations
for benzene. The Lewis structure for benzene has
alternating single and double bonds, but there are two
ways of writing this. In one form, a particular bond
is single; in the other form, this bond has become
double. Resonance theory suggests that these two
structures are both valid representations, and that each
contributes to the structure of benzene, but the true
structure is something in between, a lower energy
hybrid of the two Kekul´ e forms, a resonance hybrid.
If this is the case, then each bond is neither single
nor double, but, again, something in between. As we
have already seen, all C–C bond lengths in benzene
are 1.40 ˚
A, which is in between the bond lengths for
single (1.47 ˚
A) and double (1.34 ˚
A) bonds.
Kekulé representations
of benzene
curly arrow represents the
movement of two electrons
we could also have written
curly arrows like this
double-headed arrow is used to
indicate resonance structures