Kekule' The Structure for Benzene
H
C
H
C
H
C
C
C
H
or
H
C
H
Kekule' structure for benzene, C6H6
Resonance Theory Description of Benzene
According to Resonance Theory, the two equivalent Kekule' structures
of benzene are resonance structures that contribute to the hybrid.The
hybrid is 36.0 kcal/mol more stable than the Kekule' structures.
Kekule' structures
Resonance Energy
hybrid
The hexagon with the circle inside is widely
used to represent the hybrid of benzene.
Bond Equivalency in Benzene
Spectroscopic studies indicate
that benzene is planar, with
the geometry of a regular
hexagon, and carbon-carbon
o
bond lengths of 1.39 A.
H
H
120o
H
H
H
H
o
1.39 A
a comparison of C-C bond lengths
The equivalent lengths of
the C-C bonds in benzene
are consistent with the
hybrid of resonance theory,
which describes each C-C
bond in the hybrid as a
mixture of 50% single bond
and 50% double bond
character.
H3C CH 3
H2C=CH2
benzene
o
1.54 A
o
1.34 A
o
1.39 A
hybrid
o
1.09 A
Molecular Orbital Description of the
!-Electronic Structure of Benzene
Consider the !-bond of ethene formed by overlap of p-orbitals on
adjacent carbons. The wave functions of the combining p-orbitals
may be in-phase (bonding molecular orbital) or out-of-phase
(antibonding molecular orbital).
!*
antibonding
(out-of-phase)
Energy
C
C
combine
"
isolated p-orbitals
!bonding
(in-phase)
The quantity ! is the stabilization energy of the electrons when a
"-bond is formed. It measures the stabilization of the two electrons
in a bonding " molecular orbital relative to two electrons in
localized p-orbitals.
Benzene: a 6 !-Electron System
In a similar way, the energies of the six electrons in the bonding
!-molecular orbitals of benzene (a regular hexagon) are compared
with the energies of six electrons in localized p-orbitals.
"6
"4
"5
Antibonding
MOs
Energy
combine
six isolated p-orbitals
!
"2
"3
2!
Bonding
MOs
"1
In the ground electronic state of benzene, the six !-electrons are in
the three low lying bonding molecular orbitals: "1,"2,"3. The
antibonding molecular orbitals are unfilled. Benzene has a closed
bonding shell of !-electrons, which is a stable configuration. The
total stabilization energy is (2 x 2#) + (4 x #) = 8 #.
-anti-bonding!
-bonding!
:
Huckel's Rule: The (4n + 2) !-Electron Rule
for Predicting Aromaticity
:
In 1931, the German physicist Erich Huckel carried out molecular
orbital calculations on planar carbocycles in which each carbon has
a p-orbital. His calculations showed that monocyclic systems
containing (4n + 2) !-electrons, where n = 0, 1, 2, 3..., have closed
shells of !-electrons like benzene. These systems have a single
lowest bonding !-molecular orbital followed by pairs of bonding
!-MOs.
:
The Huckel Formula Prediction
2
Total Number of !-Electrons
6
10
14
:
These closed shell
!-electron systems are
predicted to be "aromatic"
by the Huckel rule.
bonding
MOs
n=
0
1
2
3
A Second Prediction of Aromaticity
A second way to evaluate aromatic systems is to inscribe a regular
polygon inside a circle with one corner of the polygon at the bottom
of the circle. Wherever a corner touches the circle, there is an energy
level.
Examples
Benzene
hexagon
predicted energy
levels
a closed shell system
"aromatic"
Cyclooctatetraene
8!
octagon
predicted energy
levels
an open shell system
nonaromatic
Properties of Cyclooctatetraene
In contrast to benzene, this nonaromatic compound is
very reactive. It is a yellow liquid, bp 152oC, that is stable
at low temperatures, but polymerizes upon heating. It is
slowly oxidized by air, and easily undergoes catalytic
hydrogenation.
Cyclooctatetraene is not planar, as required for an aromatic
compound. It has a tub shape.
o
o
1.34 A
1.48 A
The different bond lengths
indicate localized bonds.
The Annulenes
Annulenes are monocyclic compounds with alternating double and
single bonds. The ring size is given by a number inside square
brackets before the name "annulene."
benzene
[6]annulene
cyclooctatetraene
[8]annulene
Prediction of Aromaticity of the Annulenes
The Huckel (4n+2) rule predicts whether an annulene is expected
to show aromatic properties. Over the past 40 years, many
annulenes have been synthesized and studied.
:
Predictions of Aromaticity by the Huckel Rule: 4n + 2
[10]annulene
Prediction
Observation
Yes
No
[16]annulene
Prediction
Observation
No
No
[14]annulene
Yes
Yes
[18]annulene
Yes
Yes
The Trouble with [10]Annulene
:
[10]Annulene does not show the properties and stability of an
aromatic compound despite the prediction by the Huckel rule. The
required planar structures (with extensive overlap of the p-orbitals)
in both the all-cis (A) and naphthalene-like (B) forms are unstable
because of strain energy.
Planar Structures of [10]Annulene
H
H
A
(all cis)
severe angle strain
B
(naphthalene-like)
severe steric strain
from H-H repulsion
[10]Annulene has a non-planar structure that
prevents the continuous overlap of the p-orbitals
that is required for an aromatic system.
[4]Annulene (1,3-Cyclobutadiene)
Spectroscopic studies on the matrix trapped
1,3-cyclobutadiene confirm that it is not aromatic. It
has a rectangular geometry indicating nonequivalent
single and double bonds.
Aromatic, Antiaromatic, and Nonaromatic Compounds!
Not the same as aromatic vs. not aromatic!
1. Aromatic: cyclic, planar, fully conjugated, +2)
electrons (4n (n = 0, 1, 2, .). Electronic delocalization
leads to great stability.!
2. Antiaromatic: cyclic, planar, fully conjugated, 4n
electrons (n = 1, 2, 3, ). Electronic delocalization
leads to great instability.!
3. Nonaromatic: does not meet requirements for
aromatic or antiaromatic structure. Energy is not
increased or decreased by electronic conguration.!
Benzene is Aromatic!
Cycloactatetraene is Antiaromatic if it is planar!
A nonplanar cyclooctatetraene is
nonaromaric!
Comparison to open chain analogues !
Aromatic Ions
:
Monocyclic species that carry either a positive or negative charge
often show unusual stability when they have closed !-electron shells
of 4n+2 !-electrons (Huckel's rule). Such systems are "aromatic."
Cyclopentadienyl Anion
Cyclopentadiene is unusually acidic (pKa = 16) for a hydrocarbon.
The acidity is due to the unusual stability of its conjugate base, the
cyclopentadienyl anion.
+ B:-
+ BH
:
-
HH
cyclopentadiene
pKa = 16
H
cyclopentadienyl anion
(unusually stable)
comparable to water
Resonance Theory: An Inadequate Explanation of Acidity
Resonance theory does not adequately explain the unusual stability of
the cyclopentadienyl anion. Although a series of resonance structures
can be drawn that contribute to and stabilize the hybrid, even more
resonance structures can be drawn for the cycloheptatrienyl anion that
shows no unusual stability.
:
-BH
-
-
etc
:
+ B:-
-
:
HH
cyclopentadiene
pKa = 16
unusually stable
:
-
-
-BH
:
+
B:-
H
H
H
etc
:
-
HH
cycloheptatriene
pKa = 36
H
H
H
not unusually stable
Molecular Orbital Theory
According to molecular orbital theory, the cyclopentadienyl anion
is an aromatic system with unusual stability. It is planar with
continuous overlap of 5 p-orbitals with 6 electrons.
also represented as
-
6 ! electrons
:
The cyclopentadienyl anion is a 6 !-electron system with a closed
shell configuration that is aromatic according to the Huckel rule.
6!
a closed
shell system
"Aromatic"
The Cycloheptatrienyl Anion: A Nonaromatic 4n ! System
8!
an open shell system
with two electrons
in antibonding MOs
Cycloheptatrienyl anion is not an aromatic system,
and therefore displays no special stability.
Cycloheptatrienyl Cation (Tropylium Ion): A (4n + 2) ! System
Removal of a hydride ion (H:-) from cycloheptatriene yields the
unusually stable cycloheptatrienyl cation, also called the tropylium ion.
-H:-
+
+
HH
H
tropylium ion: 6 ! electrons
Cycloheptatrienyl Cation (Tropylium Ion): A (4n + 2) ! System
6!
a closed shell system
"aromatic"
Successful Predictions of MO Theory
6!
8!
cyclopentadienyl anion cycloheptatrienyl anion
"aromatic"
"nonaromatic"
6!
cycloheptatrienyl cation
"aromatic"
Two-Electron Reduction and Oxidation of 4n Systems
:
Annulenes with 4n ! electrons are nonaromatic (Huckel's rule).
Such compounds typically undergo two-electron redox reactions
producing (4n + 2) ! electron systems that are aromatic. In gaining
(reduction) or losing (oxidation) two electrons, closed shell !
electron systems are formed.
An Example: The two-Electron Reduction of Cyclooctatetraene
K
THF
cyclooctatetraene
[8]annulene
a 4n ! system
open shell
nonaromatic
-
2 K+
cyclooctatetraene dianion
a (4n + 2) ! system
closed shell
aromatic
Polynuclear Benzenoid Aromatic Compounds
Many compounds are known with two or more benzene rings
fused together. These compounds are called polynuclear
aromatic compounds.
naphthalene
C10H8
anthracene
C14H10
phenanthrene
C14H10
These structures are planar with continuous overlap of the
p-orbitals. Calculations and heat of hydrogenation data indicate
considerable stabilization energy in these compounds.
Heterocyclic Aromatic Compounds
Cyclic organic compounds where a ring carbon is replaced by an
atom such as nitrogen, oxygen or sulfur are called heterocycles.
When these compounds show unusual stability and reactivity
because of !"electronic features as found in aromatic structures,
they are called heterocyclic aromatic compounds. Four important
examples are shown below as Kekule' structures.
..
N
..
pyridine
N
H
pyrrole
..
O
..
..
S.
.
furan
thiophene
The above heterocycles have !"electronic structures analogous to
previously discussed "aromatics" when the heteroatom is sp2
hybridized.
Pyridine has a sextet of !"electrons (one from each of the 5 carbon
and single nitrogen atoms) with the nonbonding electron pair on
nitrogen residing in an sp2 orbital in the plane of the ring. Note that
the 90o angle between the !"electrons and the nonbonding electrons
means there is no direct interaction. The nonbonding electrons are
localized on the nitrogen.
N
..
pyridine
N
:
The pyridine !"electrons are stabilized,
as in benzene, because they reside in a
closed shell structure.
In, pyrrole, furan and thiophene, the four ring carbons contribute
four electrons to the !"electronic structure while the heteroatom
(N,O,S) contributes a nonbonding pair to give a sextet of !"electrons.
The "aromaticity" of these five-membered ring heterocycles results
from a closed shell electronic structure as is found in the
cyclopentadiene anion. Note again the heteroatom is sp2 hybridized.
..
N-H
..
O:
O:
NH
pyrrole
furan
..
S:
S:
thiophene
six !"electron systems
These heterocyclic aromatic compounds are
widely found in biological structures.