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Organic Chemistry OnLine 2000
2000, Paul R. YoungUniversity of Illinois at Chicago, All Rights Reserved
Aromaticity
Organic Chemistry OnLine 2000
Aromaticity: Derivatives of Benzene
H
H
H
H
H
H
Organic Chemistry OnLine 2000
very slow reaction
no reaction
no reaction HCl
Br2/CCl4
H2/Pt
Benzene has four degrees of unsaturation, but undergoesnone of the common reactions of alkenes...
H2/Pt
C6H6
Organic Chemistry OnLine 2000
Heat of Hydrogenation(energy released during reduction with H2/Pt)
Reactant Product H
Cyclohexene Cyclohexane 28.6 kcal/mole1,3-Cyclohexadiene Cyclohexane 55.4 kcal/moleBenzene Cyclohexane 49.8 kcal/mole
H2/Pt
86 kcal/mole expected; 50 kcal/mole observed36 kcal/mole more stable than expected
Isomers of C6H6
Organic Chemistry OnLine 2000
Ladenburg benzene
H
H
H
H
H
H
Kekul benzene
C6H6 isomers as possiblestructures for benzene...
Organic Chemistry OnLine 2000
4 isomers expected for Kekul benzene
Br
Br
Br
Br
Br
Br
Br
Br
3 dibromo isomersBr2/FeBr3
Benzene
Organic Chemistry OnLine 2000
Br
Br
Br
Br
long bond short bond
Organic Chemistry OnLine 2000
Br
Br
Br
Br
BrBr
...three dibromo isomers are predicted.
Ladenburg benzene
Organic Chemistry OnLine 2000
3 isomers expected
Br
Br
Br
Br
Br
Br
Br
Br
3 isomersBr2/FeBr3
identical
Benzene
Organic Chemistry OnLine 2000
Br
Br
Br
Br
Kekul suggested that the two possible 1,2-isomers were in rapid equilibrium...
Organic Chemistry OnLine 2000
Br
Br
Br
Br
Br
Br
Br
Bror
Br
Br
...today we recognize the two structures Kekuldrew as simple resonance forms.
Organic Chemistry OnLine 2000
The conjugated system in the benzene ring results in theformation of a continuous -cloud, above and below theplane of the ring.
An electrostatic potential mapof benzene showing enhancedelectron density above and
below the ring plane.
y 1
y 2 y 3
y 4* y 5*
y 6*The six p-orbitals of benzenecombine to form six -molecularorbitals of increasing energy.
The six bonding electronsoccupy the three lowestenergy molecular orbitals.
Organic Chemistry OnLine 2000
y 6 *
y 4 * y 5 *
y 2 y 3
y 1
Calculated shapes forthe -molecularorbitals of benzene...
Organic Chemistry OnLine 2000
H
H
H
H
H
H
CH3H
H
H
H
H
CH3H
CH3H
H
H
NH2H
H
H
H
H
OH
H
H
H
H
H
NO2H
H
H
H
H
benzene toluene meta-xylene
aniline phenol nitrobenzene
Representative Aromatic Compounds...
Organic Chemistry OnLine 2000
napthalene anthracene
phenanthrene
Some Polycyclic Aromatic Compounds...
Organic Chemistry OnLine 2000
napthalene anthracene
phenanthrene O
DNA damage
Some Polycyclic Aromatic Compounds...
Organic Chemistry OnLine 2000
HO O CH2 C
H
NH2
COOH
I
I
I
I
Thyroxin
DDT
CHC
Cl
Cl
Cl
Cl
Cl
Some Other NotableAromatic Compounds...
Organic Chemistry OnLine 2000
Br CH2CH3 CH3
bromobenzene ethylbenzene methylbenzeneor toluene
Nomenclature
1. Monosubstituted benzene derivatives are names as otherhydrocarbons, using benzene as the parent name.
Organic Chemistry OnLine 2000
2. Disubstituted benzenes are named using ortho-, para- andmeta- to describe the substitution pattern (1,2 1,4 and 1,3respectively) or simply by numbering the substituents.
Nomenclature
1. Monosubstituted benzene derivatives are names as otherhydrocarbons, using benzene as the parent name.
Br
Br
CH2CH3
CH2CH3
CH3
H3C
ortho-dibromobenzeneor 1,2-dibromobenzene
meta-diethylbenzeneor 1,3-diethylbenzene
para-dimethylbenzene,1,4-dimethylbenzene
or para-xylene
Organic Chemistry OnLine 2000
3. Substituents are numbered to give the lowest possiblenumber sequence at the first point of difference, assigningpriorities alphabetically if there is a tie.
CH2CH3
CH2CH3
Cl
1-chloro-3,5-diethylbenzene
1 3
5
2. Disubstituted benzenes are named using ortho-, para- andmeta- to describe the substitution pattern (1,2 1,4 and 1,3respectively) or simply by numbering the substituents.
Nomenclature
1. Monosubstituted benzene derivatives are names as otherhydrocarbons, using benzene as the parent name.
Organic Chemistry OnLine 2000
4. When a benzene ring is a substituent on another chain, itis referred to as a phenyl group.
1-phenylheptane
1
3. Substituents are numbered to give the lowest possiblenumber sequence at the first point of difference, assigningpriorities alphabetically if there is a tie.
2. Disubstituted benzenes are named using ortho-, para- andmeta- to describe the substitution pattern (1,2 1,4 and 1,3respectively) or simply by numbering the substituents.
Nomenclature
1. Monosubstituted benzene derivatives are names as otherhydrocarbons, using benzene as the parent name.
Organic Chemistry OnLine 2000
CH3H
H
NO2
H
H
CH3CH3
H
H
Br
H
CH3H
Cl
H
H
H
...provide IUPAC names for each of the molecules shown above.
Organic Chemistry OnLine 2000
CH3H
H
NO2
H
H
CH3CH3
H
H
Br
H
CH3H
Cl
H
H
H
4-bromo-1,2-dimethylbenzene
Organic Chemistry OnLine 2000
CH3H
H
NO2
H
H
CH3CH3
H
H
Br
H
CH3H
Cl
H
H
H
4-bromo-1,2-dimethylbenzene1-chloro-3-methylbenzene or 3-chlorotoluene
Organic Chemistry OnLine 2000
CH3H
H
NO2
H
H
CH3CH3
H
H
Br
H
CH3H
Cl
H
H
H
4-bromo-1,2-dimethylbenzene
1-methyl-4-nitrobenzene or 4-nitrotoluene
1-chloro-3-methylbenzene or 3-chlorotoluene
Organic Chemistry OnLine 2000
CH2CH3
Cl
H
NO2
H
H
Cl
CH2CH3
H
NO2
H
H
Cl
H
CH2CH3NO2
H
H
...provide IUPAC names for each of the molecules shown above.
Organic Chemistry OnLine 2000
CH2CH3
Cl
H
NO2
H
H
Cl
CH2CH3
H
NO2
H
H
Cl
H
CH2CH3NO2
H
H
1
2
4
2-chloro-1-ethyl-4-nitrobenzene
Organic Chemistry OnLine 2000
CH2CH3
Cl
H
NO2
H
H
Cl
CH2CH3
H
NO2
H
H
Cl
H
CH2CH3NO2
H
H
1-chloro-2-ethyl-4-nitrobenzene
1
2
4
1
2
4
2-chloro-1-ethyl-4-nitrobenzene
Organic Chemistry OnLine 2000
CH2CH3
Cl
H
NO2
H
H
Cl
CH2CH3
H
NO2
H
H
Cl
H
CH2CH3NO2
H
H
2-chloro-1-ethyl-4-nitrobenzene1-chloro-2-ethyl-4-nitrobenzene
4-chloro-2-ethyl-1-nitrobenzene
1
2
4
1
2
4
1
2
4
Organic Chemistry OnLine 2000
CH3
Cl
H
NO2
H
H
Cl
CH3
H
NO2
H
H
Cl
H
CH3NO2
H
H
...name as derivatives of toluene.
Organic Chemistry OnLine 2000
CH3
Cl
H
NO2
H
H
Cl
CH3
H
NO2
H
H
Cl
H
CH3NO2
H
H
2-chloro-4-nitrotoluene
Organic Chemistry OnLine 2000
CH3
Cl
H
NO2
H
H
Cl
CH3
H
NO2
H
H
Cl
H
CH3NO2
H
H
2-chloro-4-nitrotoluene 2-chloro-5-nitrotoluene
Organic Chemistry OnLine 2000
CH3
Cl
H
NO2
H
H
Cl
CH3
H
NO2
H
H
Cl
H
CH3NO2
H
H
2-chloro-4-nitrotoluene 2-chloro-5-nitrotoluene
5-chloro-2-nitrotoluene
Organic Chemistry OnLine 2000
The resonance description of benzene will explain thegeometry of the molecule and the isomer distribution
of benzene derivatives, but does not explain theunusual stability of benzene and its derivatives.
The stability of benzene is suggested to arise from thefact that the conjugated system is planar andcontains 4n + 2 electrons (with n = 1), and it issuggested that all compounds having planar,
conjugated systems containing 4n + 2 electrons willshare this stability. This property, described
originally by Hckel, is referred to as aromaticity.
Organic Chemistry OnLine 2000
4n + 2 Rule
2,3-Dimethylcyclobutadiene
2,3-Dimethylcyclobutadiene
Planar...
2,3-Dimethylcyclobutadiene
Planar...4 p electrons,
2,3-Dimethylcyclobutadiene
Planar...4 p electrons,therefore not aromatic (antiaromatic)
2,3-Dimethylcyclobutadiene
Planar...4 p electrons,therefore not aromatic (antiaromatic)
Organic Chemistry OnLine 2000
4 p electronsantiaromatic
4n + 2 Rule
Benzene
Benzene
Planar...
Benzene
Planar...6 p electrons,
Benzene
Planar...6 p electrons,therefore aromatic
Benzene
Planar...6 p electrons,therefore aromatic
Organic Chemistry OnLine 2000
4 p electronsnot aromatic
6 p electronsaromatic
4n + 2 Rule
Cyclooctatetraene
Cyclooctatetraene
Not planar...
Cyclooctatetraene
Not planar...8 p electrons,
Cyclooctatetraene
Not planar...8 p electrons,therefore not aromatic
Cyclooctatetraene
Cyclooctatetraene
Organic Chemistry OnLine 2000
4 p electronsnot aromatic
6 p electronsaromatic
8 p electronsnot aromatic
4n + 2 Rule
Cyclopentadienyl Anion
Cyclopentadienyl Anion
Planar...
Cyclopentadienyl Anion
Planar...6 p electrons,
Cyclopentadienyl Anion
Planar...6 p electrons,therefore aromatic
Cyclopentadienyl Anion
Planar...6 p electrons,therefore aromatic
Organic Chemistry OnLine 2000
4 p electronsnot aromatic
6 p electronsaromatic
8 p electronsnot aromatic
6 p electronsaromatic
4n + 2 Rule
Cycloheptatrienyl Cation
+
Cycloheptatrienyl Cation
+
Planar...
Cycloheptatrienyl Cation
+
Planar...6 p electrons,
Cycloheptatrienyl Cation
+
Planar...6 p electrons,therefore aromatic
Cycloheptatrienyl Cation
+
Planar...6 p electrons,therefore aromatic
Organic Chemistry OnLine 2000
4 p electronsnot aromatic
6 p electronsaromatic
4n + 2 Rule
Organic Chemistry OnLine 2000
4n + 2 Rule
Organic Chemistry OnLine 2000
N
N N
N
NH2
HNO
O
CH3
H
NH
Adenine Uracil
4n + 2 Rule
Organic Chemistry OnLine 2000
N
N N
N
NH2
H
4n + 2 Rule
Organic Chemistry OnLine 2000
N
N N
N
NH2
H
4n + 2 Rule
Adenine:10 electrons
Aromatic
Organic Chemistry OnLine 2000
N
N N
N
NH2
H
Adenine:10 electrons
Aromatic
4n + 2 Rule
The lone pairs on thenitrogens are orthogonal to
the -system and do notparticipate in themolecular orbital.
Organic Chemistry OnLine 2000
N
N N
N
NH2
H
Adenine:10 electrons
Aromatic
4n + 2 Rule
NO
O
CH3
H
NH
Organic Chemistry OnLine 2000
NO
O
CH3
H
NH
N
N N
N
NH2
H
Adenine:10 electrons
Aromatic
4n + 2 Rule
Organic Chemistry OnLine 2000
NO
O
CH3
H
NH
N
N N
N
NH2
H
Adenine:10 electrons
Aromatic
4n + 2 Rule
Uracil:6 electrons
Aromatic
Organic Chemistry OnLine 2000
CH2
CH3CH CH2
CH3
CH CH2H3C
H3C
CH2
N
N N
N
CH2CH2COOHCOOH
Fe
Heme
4n + 2 Rule
Organic Chemistry OnLine 2000
CH2
CH3CH CH2
CH3
CH CH2H3C
H3C
CH2
N
N N
N
CH2CH2COOHCOOH
Fe
4n + 2 Rule
Organic Chemistry OnLine 2000
CH2
CH3CH CH2
CH3
CH CH2H3C
H3C
CH2
N
N N
N
CH2CH2COOHCOOH
Fe
4n + 2 Rule
Heme: 22 electrons; Aromatic (n = 5)
Heme
Heme
Planar...
Heme
Planar...22 p electrons,
Heme
Planar...22 p electrons,therefore aromatic
Organic Chemistry OnLine 2000
Buckminsterfullerene
C60 with 31 double bonds
Buckminsterfullerene
Buckminsterfullerene
62 p electrons, (4n + 2 with n = 15)
Buckminsterfullerene
62 p electrons, (4n + 2 with n = 15)not planar with considerable ring strain...
Buckminsterfullerene
62 p electrons, (4n + 2 with n = 15)not planar with considerable ring strain...the molecule appears aromatic, but is much morereactive than benzene.
Organic Chemistry OnLine 2000
Would you predict cyclodecapentaene to be anaromatic compound?
Cyclodecapentaene
Organic Chemistry OnLine 2000
Would you predict cyclodecapentaene to be anaromatic compound?
Cyclodecapentaene
...consider thegeometry of the
two centralhydrogens.
Organic Chemistry OnLine 2000
Cyclodecapentaene
HH
Would you predict cyclodecapentaene to be anaromatic compound?
Steric repulsion between the opposing hydrogens forces the system out of planarity, therefore the molecule is not aromatic.
Organic Chemistry OnLine 2000
Azulene, C10H8, is a deep blue hydrocarbon with a large dipolemagnetic moment.
a. is azulene aromatic using the Hckel definition?b. utilize resonance forms for azulene to explain the largedipole moment.
Organic Chemistry OnLine 2000
10 p -electronsAromatic
Azulene, C10H8, is a deep blue hydrocarbon with a large dipolemagnetic moment.
a. is azulene aromatic using the Hckel definition?b. utilize resonance forms for azulene to explain the largedipole moment.
Organic Chemistry OnLine 2000
The two major (neutral) resonance forms for azulene...
Organic Chemistry OnLine 2000
The two major (neutral) resonance forms for azulene...
...and the very stable Zwiterionic form.
Organic Chemistry OnLine 2000
The two major (neutral) resonance forms for azulene...
...and the very stable Zwiterionic form.
cycloheptatrienecation-like
Organic Chemistry OnLine 2000
The two major (neutral) resonance forms for azulene...
...and the very stable Zwiterionic form.
cycloheptatrienecation-like
cyclopentadieneanion-like
Organic Chemistry OnLine 2000
dd
d
d
d
d
dd
The contribution from the Zwitterionic resonance formleads to the presence of a large dipole moment, which
is associated with color in conjugated molecules.
Organic Chemistry OnLine 2000
dd
d
d
d
d
dd
Contrast the electron density map of azulene with that ofnapthalene, which has no significant Zwitterionic component.
Organic Chemistry OnLine 2000
stable very unstable
2
Account for the differences in reactivity between pentalene (anunknown compound) and the well-known pentalene dianion:
Organic Chemistry OnLine 2000
8 -electrons;not aromatic
10 -electrons;aromatic
2
Organic Chemistry OnLine 2000
Reactions of Aromatic Sidechains
1. Oxidation with neutral MnO4-
2. Allylic bromination with NBS
Organic Chemistry OnLine 2000
Oxidation withNeutral MnO4-
CH3
CH2CH2CH3
MnO4-/H2O, heat
MnO4-/H2O, heat
MnO4-/H2O, heat
Organic Chemistry OnLine 2000
CH3
CH2CH2CH3
MnO4-/H2O, heat
MnO4-/H2O, heat
MnO4-/H2O, heat
COOH
...benzylic carbonmust have at least
one hydrogen.
Organic Chemistry OnLine 2000
MnO4-/H2O, heat
CH3
CH3
MnO4-/H2O, heat
Organic Chemistry OnLine 2000
MnO4-/H2O, heat
COOH
COOH
CH3
CH3
MnO4-/H2O, heat
COOH
COOH
Organic Chemistry OnLine 2000
MnO4-/H2O, heat
COOH
COOH
CH3
CH3
MnO4-/H2O, heat
COOH
COOH
Organic Chemistry OnLine 2000
NBS/CCl4"radical initiator"
NBS/CCl4"radical initiator"
NBS/CCl4"radical initiator"
CH2CH2CH3
CH3 Free RadicalBromination of Alkyl
Side-Chains
Organic Chemistry OnLine 2000
O
OO
O
Benzoyl Peroxide (PhCO 2 )2 N Br
O
O N-bromo-succinimide
Br
CH2
H
CH2HBr +
NBSCH2
Br
O
O2
Br
Organic Chemistry OnLine 2000
NBS/CCl4"radical initiator"
NBS/CCl4"radical initiator"
NBS/CCl4"radical initiator"
CH2CH2CH3
CH3 Free RadicalBromination of Alkyl
Side-Chains
Organic Chemistry OnLine 2000
NBS/CCl4"radical initiator"
NBS/CCl4"radical initiator"
NBS/CCl4"radical initiator"
CH2CH2CH3
CH3
Br
CHCH2CH3Br
CH2 Br
Organic Chemistry OnLine 2000
NBS/CCl4"radical initiator"
NBS/CCl4"radical initiator"
NBS/CCl4"radical initiator"
CH2CH2CH3
CH3
Br
CHCH2CH3Br
CH2 Br
Organic Chemistry OnLine 2000
NBS/CCl4"radical initiator"
NBS/CCl4"radical initiator"
NBS/CCl4"radical initiator"
CH2CH2CH3
CH3
Br
CHCH2CH3Br
CH2 Br
Organic Chemistry OnLine 2000
Dissolving Metal Reduction of Benzene DerivativesThe Birch Reduction
Li/NH3
Organic Chemistry OnLine 2000
Li/NH3
CH3Li/NH3
CH3
OCH3Li/NH3
OCH3
With substituted arenes,the Birch Reduction will
proceed to place thedouble bond on a ring
carbon which is attachedto electron donating
substituents...
Dissolving Metal Reduction of Benzene DerivativesThe Birch Reduction
Organic Chemistry OnLine 2000
Li/NH3
CH3Li/NH3
CH3
OCH3Li/NH3
OCH3
Dissolving Metal Reduction of Benzene DerivativesThe Birch Reduction
Organic Chemistry OnLine 2000
Li/NH3
CH3Li/NH3
CH3
OCH3Li/NH3
OCH3
Dissolving Metal Reduction of Benzene DerivativesThe Birch Reduction
Organic Chemistry OnLine 2000
MnO4-/H2O
Predict the products of the following reactions.
Li/NH3
NBS/CCl4"radical initiator"
Organic Chemistry OnLine 2000
Br
Br
MnO4-/H2O
Predict the products of the following reactions.
Li/NH3
NBS/CCl4"radical initiator"
Organic Chemistry OnLine 2000
COOH
HOOC
Br
Br
MnO4-/H2O
Predict the products of the following reactions.
Li/NH3
NBS/CCl4"radical initiator"
Organic Chemistry OnLine 2000
COOH
HOOC
Br
Br
MnO4-/H2O
Predict the products of the following reactions.
Li/NH3
NBS/CCl4"radical initiator"