Examples of Aromatic Hydrocarbons
CH3
H
H
H
H
H
H
H
H
H
H
Benzene
H
H
Toluene
H
H
H
H
H
H
H
Naphthalene
Some history
11.1
Benzene
1825 Michael Faraday isolates a new
hydrocarbon from illuminating gas.
1834 Eilhardt Mitscherlich isolates same
substance and determines its empirical
formula to be CnHn. Compound comes
to be called benzene.
1845 August W. von Hofmann isolates benzene
from coal tar.
1866 August Kekulé proposes structure of
benzene.
Kekulé Formulation of Benzene
Kekulé proposed a cyclic structure for C6H6
with alternating single and double bonds.
H
11.2
The Structure of Benzene
H
H
H
H
H
Kekulé Formulation of Benzene
Kekulé Formulation of Benzene
Later, Kekulé revised his proposal by suggesting
a rapid equilibrium between two equivalent
structures.
H
H
H
H
H
H
H
H
H
H
H
H
However, this proposal suggested isomers of the
kind shown were possible. Yet, none were ever
found.
X
X
H
X
H
X
H
H
H
H
H
H
Structure of Benzene
All C—C bond distances = 140 pm
Structural studies of benzene do not support the
Kekulé formulation. Instead of alternating single
and double bonds, all of the C—C bonds are the
same length.
140 pm
140 pm
140 pm
140 pm
140 pm
140 pm is the average between the C—C
single bond distance and the double bond
distance in 1,3-butadiene.
Benzene has the shape of a regular hexagon.
Resonance Formulation of Benzene
Kekulé Formulation of Benzene
Instead of Kekulé's suggestion of a rapid
equilibrium between two structures:
H
140 pm
express the structure of benzene as a resonance
hybrid of the two Lewis structures. Electrons are
not localized in alternating single and double bonds,
but are delocalized over all six ring carbons.
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Resonance Formulation of Benzene
The Stability of Benzene
Circle-in-a-ring notation stands for resonance
description of benzene (hybrid of two Kekulé
structures)
Thermochemical Measures of Stability
3 x cyclohexene
heat of hydrogenation: compare experimental
value with "expected" value for hypothetical
"cyclohexatriene"
+ 3H2
”Expected" heat
of hydrogenation
of benzene is
3 x heat of
hydrogenation of
cyclohexene.
Pt
120 kJ/mol
ΔH°= – 208 kJ
360 kJ/mol
3 x cyclohexene
Observed heat of
hydrogenation is
152 kJ/mol less
than "expected."
Benzene is 152
kJ/mol more
stable than
expected;
152 kJ/mol is the
resonance energy
of benzene.
360 kJ/mol
208 kJ/mol
Hydrogenation
of 1,3cyclohexadiene
(+2H2) gives off
more heat than
hydrogenation
of benzene
(+3H2)!
231 kJ/mol
208 kJ/mol
3 x cyclohexene
Cyclic conjugation versus noncyclic conjugation
3H2
Pt
360 kJ/mol
231 kJ/mol
120 kJ/mol
208 kJ/mol
heat of hydrogenation = 208 kJ/mol
3H2
Pt
heat of hydrogenation = 337 kJ/mol
Resonance Energy of Benzene
compared to localized 1,3,5-cyclohexatriene
152 kJ/mol
compared to 1,3,5-hexatriene
129 kJ/mol
11.4
An Orbital Hybridization View
of Bonding in Benzene
Exact value of resonance energy of benzene
depends on what it is compared to, but
regardless of model, benzene is more stable
than expected by a substantial amount.
Orbital Hybridization Model of Bonding in Benzene
Planar ring of 6 sp2 hybridized carbons
Orbital Hybridization Model of Bonding in Benzene
Each carbon contributes a p orbital
Six p orbitals overlap to give cyclic π system;
six π electrons delocalized throughout π system
Orbital Hybridization Model of Bonding in Benzene
High electron density above and below plane
of ring
The π Molecular Orbitals
of Benzene
Benzene MOs
Benzene MOs
Antibonding
orbitals
Energy
Antibonding
orbitals
Energy
Bonding
orbitals
6 p AOs combine to give 6 π MOs
3 MOs are bonding; 3 are antibonding
Bonding
orbitals
All bonding MOs are filled
No electrons in antibonding orbitals
Benzene MOs
Figure 11.4
p 435
General Points
General Points
1) Benzene is considered as the parent and
comes last in the name.
Br
C(CH3)3
NO2
1) Benzene is considered as the parent and
comes last in the name.
2) List substituents in alphabetical order.
3) Number ring in direction that gives lowest
locant at first point of difference.
Bromobenzene
tert-Butylbenzene
Nitrobenzene
Example
Ortho, Meta, and Para
Alternative locants for disubstituted
derivatives of benzene
Cl
Br
F
1,2 = ortho
(abbreviated o-)
2-bromo-1-chloro-4-fluorobenzene
1,3 = meta
(abbreviated m-)
Examples
NO2
Cl
CH2CH3
O
CH
Cl
o-ethylnitrobenzene
m-dichlorobenzene
(1-ethyl-2-nitrobenzene)
(1,3-dichlorobenzene)
Benzaldehyde
1,4 = para
(abbreviated p-)
O
CH
COH
Styrene
Benzoic acid
O
OH
CCH3
Acetophenone
Phenol
CH2
NH2
OCH3
Aniline
Anisole
Names in Can be Used as Parent
OCH3
Easily Confused Names
OCH3
phenyl
phenol
benzyl
OH
CH2—
NO2
Anisole
p-Nitroanisole
or
4-Nitroanisole
a group
a compound
a group
Naphthalene
Resonance energy = 255 kJ/mol
Polycyclic Aromatic Hydrocarbons
Most stable Lewis structure;
both rings correspond to
Kekulé benzene.
Anthracene and Phenanthrene
Physical Properties of Arenes
Anthracene
Phenanthrene
resonance energy:
347 kJ/mol
381 kJ/mol
Physical Properties
Arenes (aromatic hydrocarbons) resemble
other hydrocarbons. They are:
nonpolar
insoluble in water
less dense than water
Reduction of Benzene Rings
1. Reactions involving the ring
H
a) Reduction
Catalytic hydrogenation (Section 11.3)
Birch reduction (Section 11.10)
b) Electrophilic aromatic substitution
(Chapter 12)
c) Nucleophilic aromatic substitution
(Chapter 12)
2. The ring as a substituent (Sections 11.11-11.16)
catalytic
hydrogenation
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Birch reduction
H
H
H
H
H
H
H
H
Birch Reduction of Benzene
H
H
H
H
H
Na, NH3
H
H
H
H
CH3OH
H
H
The Birch Reduction
H
H
H
(80%)
Product is non-conjugated diene.
Reaction stops here. There is no further reduction.
Reaction is not hydrogenation. H2 is not involved in any way.
Mechanism of the Birch Reduction
Mechanism of the Birch Reduction
Step 2: Proton transfer from methanol
Step 1: Electron transfer from sodium
H
H
H
+ • Na
H
H
H
H
H
H
H
•
H
••
–
H
H
H
+
H
•
H
H
H
H
Na+
H
H
H
– ••
•• OCH3
••
•
H
••
–
H
H
H
•• OCH3
••
Mechanism of the Birch Reduction
Mechanism of the Birch Reduction
Step 3: Electron transfer from sodium
H
H
H
•
H
H
Step 4: Proton transfer from methanol
H
+ • Na
H
H
H
H
–
H
H
+
••
H
Birch Reduction of an Alkylbenzene
H
H
H
H
H
Na, NH3
H
H
H
CH3OH
C(CH3)3
H
C(CH3)3
H
H
H
H
H
H
H
H
H
H
(86%)
If an alkyl group is present on the ring, it ends up as
a substituent on the double bond.
H
••
•• OCH3
H
–
Na+
H
H
H
– ••
•• OCH
•• 3
H
H
••
H
H
H
The Benzene Ring as a Substituent
Bond-dissociation energy for C—H bond
is equal to ΔH° for:
C
R—H
C
C•
C•
allylic radical
Benzylic carbon is analogous to allylic carbon.
Bond-dissociation Energies of Propene and Toluene
H 2C
CH
C
H
368 kJ/mol
-H•
H
CH
C
H
H
356 kJ/mol
-H•
Resonance in Benzyl Radical
H
C•
H
•H
The more stable the free radical R•, the weaker
the bond, and the smaller the bond-dissociation
energy.
H
H 2C
+
and is about 400 kJ/mol for alkanes.
benzylic radical
H
R•
•
C
H
H
H
H
H
H
H
C•
H
H
Low BDEs indicate allyl and benzyl radical are
more stable than simple alkyl radicals.
Unpaired electron is delocalized between
benzylic carbon and the ring carbons that are
ortho and para to it.
Resonance in Benzyl Radical
H
H
C
Resonance in Benzyl Radical
H
H
•
H
H
H
H
H
C
H
H
•
H
H
H
Unpaired electron is delocalized between
benzylic carbon and the ring carbons that are
ortho and para to it.
Unpaired electron is delocalized between
benzylic carbon and the ring carbons that are
ortho and para to it.
Resonance in Benzyl Radical
Free-radical Chlorination of Toluene
H
H
C
H
•
H
CH3
H
H
H
Unpaired electron is delocalized between
benzylic carbon and the ring carbons that are
ortho and para to it.
Toluene
Cl2
CH2Cl
light
or
heat
Benzyl chloride
industrial process
highly regioselective for benzylic position
Benzylic Bromination
Free-radical Chlorination of Toluene
Similarly, dichlorination and trichlorination are
selective for the benzylic carbon. Further
chlorination gives:
is used in the laboratory to introduce a
halogen at the benzylic position
CH2Br
CH3
CHCl2
(Dichloromethyl) benzene
CCl3
(Dichloromethyl) benzene
N-Bromosuccinimide (NBS)
is a convenient reagent for benzylic bromination
Br
CH2CH3
O
NBr +
O
CCl4
benzoyl
peroxide,
heat
CHCH3
O
NH +
O
(87%)
+ Br2
NO2
p-Nitrotoluene
CCl4, 80°C
+ HBr
light
NO2
p-Nitrobenzyl
bromide (71%)
Site of Oxidation is Benzylic Carbon
Example
O
CH3
or
CH2R
or
CH3
Na2Cr2O7
H2SO4
O
COH
H 2O
heat
p-Nitrotoluene
Example
O
CH(CH3)2
COH
Na2Cr2O7
H2SO4
H 2O
heat
CH3
H 2O
heat
NO2
CHR2
COH
O
(45%)
COH
Na2Cr2O7
H2SO4
NO2
p-Nitrobenzoic
acid (82-86%)
Benzylic SN1 Reactions
Benzylic SN1 Reactions
Relative solvolysis rates in aqueous acetone:
CH3
C
CH3
Cl
CH3
CH3
C
CH3
Cl
CH3
C+
CH3
CH3
CH3
620
Relative rates of formation:
1
C+
CH3
more stable
less stable
Tertiary benzylic carbocation is formed
more rapidly than tertiary carbocation;
therefore, more stable.
Compare
Resonance in Benzyl Cation
H
C
C
C+
allylic carbocation
+
C
H
H
H
H
H
C+
benzylic carbocation
Benzylic carbon is analogous to allylic carbon.
H
Positive charge is delocalized between
benzylic carbon and the ring carbons that are
ortho and para to it.
Resonance in Benzyl Cation
H
H
Resonance in Benzyl Cation
H
C
H
H
+
H
H
H
C
H
H
+
H
H
H
H
Positive charge is delocalized between
benzylic carbon and the ring carbons that are
ortho and para to it.
Resonance in Benzyl Cation
Positive charge is delocalized between
benzylic carbon and the ring carbons that are
ortho and para to it.
Solvolysis
CH3
H
H
C
H
+
H
C
H
H
H
Positive charge is delocalized between
benzylic carbon and the ring carbons that are
ortho and para to it.
Cl
CH3
CH3CH2OH
CH3
C
CH3
OCH2CH3
(87%)
Primary Benzylic Halides
O2N
CH2Cl
Mechanism is SN2
O
Like allylic halide,
substitution is faster
than a normal
primary halide.
NaOCCH3
acetic acid
O
O2N
CH2OCCH3
(78-82%)
Dehydrogenation
• Industrial preparation of styrene
• Almost 12 billion lbs. produced annually in U.S.
CH2CH3
630°C
ZnO
CH
+ H2
CH2
Dehydrohalogenation
Acid-Catalyzed Dehydration of
Benzylic Alcohols
Cl
Br
KHSO4
CHCH3
CH2CHCH3
H 3C
Cl
CH
heat
CH2
NaOCH2CH3
(80-82%)
OH
ethanol, 50°C
Cl
CH
H 3C
CHCH3
+
CHCH3
(99%)
Hydrogenation
CH3
Addition Reactions of Alkenylbenzenes
C
CH3
CHCH3
CHCH2CH3
H2
Pt
Br
Br
(92%)
Halogenation
Addition of Hydrogen Halides
Cl
CH
CH2
HCl
Br2
CH
CH2
Br
Br
(82%)
(75-84%)
+
via benzylic carbocation
Free-Radical Addition of HBr
CH
CH2
HBr
peroxides
CH2CH2Br
Polymerization of Styrene
CH
•
CH2Br
via benzylic radical
Polymerization of Styrene
H 2C
CH2
CH
C 6H 5
C 6H 5
••
RO ••
•
CHC6H5
CH2
CH
Mechanism
CH2
H 2C
CH
CHC6H5
C 6H 5
polystyrene
Mechanism
••
RO ••
H 2C
Mechanism
••
RO ••
H 2C
CHC6H5
•
H 2C
CHC6H5
CHC6H5
•
H 2C
CHC6H5
Mechanism
••
RO ••
H 2C
••
RO ••
H 2C
CHC6H5
H 2C
CHC6H5
Mechanism
••
RO ••
CHC6H5
H 2C
CHC6H5
H 2C
CHC6H5
H 2C
CHC6H5
•
H 2C
H 2C
Mechanism
CHC6H5
•
H 2C
CHC6H5
CHC6H5
•
H 2C
CHC6H5
120
Heats of Hydrogenation
Heats of Hydrogenation
To give cyclohexane (kJ/mol):
To give cyclooctane (kJ/mol):
231
208
Heat of hydrogenation of benzene is 152 kJ/mol,
less than 3 times heat of hydrogenation of
cyclohexene.
Structure of Cyclooctatetraene
Cyclooctatetraene is not planar;
has alternating long (146 pm)
and short (133 pm) bonds.
97
205
303
410
Heat of hydrogenation of cyclooctatetraene is
more than 4 times heat of hydrogenation of
cyclooctene.
Structure of Cyclobutadiene
MO calculations give alternating short and long
bonds for cyclobutadiene.
H
H
135 pm
156 pm
H
H
Stability of Cyclobutadiene
Cyclobutadiene is observed to be highly reactive,
and too unstable to be isolated and stored in the
customary way.
Requirements for Aromaticity
Cyclic conjugation is necessary, but not sufficient.
Not only is cyclobutadiene not aromatic, it is
antiaromatic.
An antiaromatic substance is one that is destabilized
by cyclic conjugation.
aromatic
Antiaromatic
when square.
Conclusion
There must be some factor in addition
to cyclic conjugation that determines
whether or not a molecule is aromatic.
Antiaromatic
when planar.
Hückel's Rule
Among planar, monocyclic, completely
conjugated polyenes, only those with 4n + 2 π electrons possess special stability (are
aromatic).
n
4n+2
0
2
1
6
2
10
3
14
4
18
Benzene!
Hückel's Rule
Hückel restricted his analysis to planar,
completely conjugated, monocyclic polyenes.
He found that the π molecular orbitals of
these compounds had a distinctive pattern.
One π orbital was lowest in energy, another
was highest in energy, and the others
were arranged in pairs between the highest
and the lowest.
Frost's Circle
Hückel's Rule
Frost's circle is a mnemonic that allows us to
draw a diagram showing the relative energies of
the π orbitals of a cyclic conjugated system.
1) Draw a circle.
2) Inscribe a regular polygon inside the circle
so that one of its corners is at the bottom.
3) Every point where a corner of the polygon
touches the circle corresponds to a π electron
energy level.
4) The middle of the circle separates bonding
and antibonding orbitals.
Antibonding
Bonding
π MOs of Benzene
π-MOs of Benzene
π-MOs of Benzene
Antibonding
Benzene
Antibonding
Benzene
Bonding
6 p orbitals give 6 π orbitals.
3 orbitals are bonding; 3 are antibonding.
Bonding
6 π electrons fill all of the bonding orbitals.
All π antibonding orbitals are empty.
π-MOs of Cyclobutadiene
(Square Planar)
π-MOs of Cyclobutadiene
(Square Planar)
Antibonding
Cyclobutadiene
4 p orbitals give 4π orbitals.
1 orbital is bonding, one is antibonding, and 2
are nonbonding.
Bonding
Antibonding
Cyclobutadiene
Bonding
4 π electrons; bonding orbital is filled; other 2
π electrons singly occupy two nonbonding orbitals.
π-MOs of Cyclooctatetraene (Planar)
π-MOs of Cyclooctatetraene (Planar)
Antibonding
Cyclooctatetraene
Bonding
8 p orbitals give 8 π orbitals.
3 orbitals are bonding, 3 are antibonding, and 2
are nonbonding.
π-Electron Requirement for Aromaticity
4 π electrons
6 π electrons
8 π electrons
not
aromatic
aromatic
not
aromatic
Antibonding
Cyclooctatetraene
Bonding
8 π electrons; 3 bonding orbitals are filled; 2
nonbonding orbitals are each half-filled.
Completely Conjugated Polyenes
6 π electrons;
completely conjugated
6 π electrons;
not completely
conjugated
H
aromatic
H
not
aromatic
Annulenes
Annulenes are planar, monocyclic, completely
conjugated polyenes. That is, they are the
kind of hydrocarbons treated by Hückel's
rule.
[10]Annulene
[10]Annulene
van der Waals
strain between
these two hydrogens
Predicted to be aromatic by Hückel's rule,
but too much angle strain when planar and
all double bonds are cis.
10-sided regular polygon has angles of 144°.
Incorporating two trans double bonds into
the ring relieves angle strain but introduces
van der Waals strain into the structure and
causes the ring to be distorted from planarity.
[14]Annulene
[16]Annulene
HH
HH
14 π electrons satisfies Hückel's rule.
16 π electrons does not satisfy Hückel's rule.
van der Waals strain between hydrogens inside
the ring.
Alternating short (134 pm) and long (146 pm) bonds.
[18]Annulene
H
H
H H
H
H
18 π electrons satisfies Hückel's rule.
Resonance energy = 418 kJ/mol.
Bond distances range between 137-143 pm.
Is an antiaromatic 4n π-electron system.
Cycloheptatrienyl Cation
H
H
H
H
H
+
H
H
Cycloheptatrienyl Cation
6 π electrons delocalized
over 7 carbons
positive charge dispersed
over 7 carbons
very stable carbocation
also called tropylium cation
H
H
H
H
+
H
H
H
+
H
H
H
H
Cyclopentadienide Anion
H
Br–
H
Ionic
H
H
Cycloheptatrienyl Cation
+
H
Br
H
H
••
Covalent
Tropylium cation is so stable that tropylium
bromide is ionic rather than covalent.
mp 203 °C; soluble in water; insoluble in
diethyl ether
H
–
H
6 π electrons delocalized
over 5 carbons
negative charge dispersed
over 5 carbons
stabilized anion
Cyclopentadienide Anion
Acidity of Cyclopentadiene
H
H
H
••
H
H
H
–
H
H
H
H
H
••
–
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
••
H
••
H
–
H
H
H
H
H
–
H
H
H
H
–
••
H
H
H
H
–
Compare Acidities of
Cyclopentadiene and Cycloheptatriene
– ••
H
••
H
Cyclopentadiene is unusually
acidic for a hydrocarbon.
Increased acidity is due to
stability of cyclopentadienide
anion.
Ka = 10-16
H
H
H
pKa = 16
Electron Delocalization in Cyclopentadienide Anion
H+ +
H
H
H
H
H
H
–
H
H
H
H
H
H
H
pKa = 16
pKa = 36
Ka = 10-16
Ka = 10-36
Cyclopropenyl Cation
Compare Acidities of
Cyclopentadiene and Cycloheptatriene
H
H
H
H
H
H
also written as
+
H
H
••
–
H
H
Aromatic anion
6 π electrons
H
H
••
–
H
H
H
+
H
H
n=0
4n +2 = 2 π electrons
H
Antiaromatic anion
8 π electrons
Examples
N
••
••
N
••
••
O
S
Furan
Thiophene
••
••
H
Pyridine
Pyrrole
Examples
N ••
N
••
Quinoline
Isoquinoline
Pyridine
Pyrrole
6 π electrons in ring.
Lone pair on nitrogen is in an
N
••
sp2 hybridized
orbital;
not part of π system of ring.
Lone pair on nitrogen must be part
••
N
H
of ring π system if ring is to have
6 π electrons.
Lone pair must be in a p orbital
in order to overlap with ring π
system.
Furan
Two lone pairs on oxygen.
One pair is in a p orbital and is part
••
O
••
of ring π system; other is in an
sp2 hybridized orbital and is not
part of ring π system.