cook.chem.ndsu.nodak.edu/chem341
©2012 Gregory R Cook
Chapter 11Arenes and Aromaticity
CHEM 341: Spring 2012
Prof. Greg Cook
cook.chem.ndsu.nodak.edu/chem341
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©2012 Gregory R Cook
Aromatic Compounds
2
CHO CO2H CH3 CH3
CH3
OH NH2
benzaldehyde benzoic acid toluene xylene
phenol aniline naphthalene
polyaromatic hydrocarbons (PAH's)carcinogens formed from combustionThis PAH is the main carcinogen found in Tobacco smoke (so quit for goodness sake!)
cinnamaldehyde
CHO
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©2012 Gregory R Cook
Benzene - a new hydrocarbon (1800’s)
3
• 1825 - Michael Faraday isolated a new hydrocarbon from whale oil
• Empirical Formula determined to be CnHnMF found to be C6H6
• All the C’s and H’s are equivalent
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©2012 Gregory R Cook
Benzene Reactivity
4
• Highly unsaturated - but UNREACTIVE toward Br2 . . . But . . .
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©2012 Gregory R Cook
Benzene Structure
5
• Structures proposed for C6H6
1.34 Å1.54 Å
prismane Dewar benzene Kekulé structures
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©2012 Gregory R Cook
Benzene Structure
5
• Structures proposed for C6H6
1.34 Å1.54 Å
prismane Dewar benzene Kekulé structures
HHH
HH H
bonds identical1.39 Å
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©2012 Gregory R Cook
Benzene Structure
5
• Structures proposed for C6H6
1.34 Å1.54 Å
prismane Dewar benzene Kekulé structures
HHH
HH H
bonds identical1.39 Å
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©2012 Gregory R Cook
Benzene Stability
6
• Benzene is unusually stable
H2Pd/C NO REACTION
H2Ni
100-200°C100 atm pressure
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©2012 Gregory R Cook
Benzene Stability
7
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©2012 Gregory R Cook
pi Molecular Orbitals of Benzene
8
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©2012 Gregory R Cook
Nomenclature for Benzene Derivatives
9
CHO CO2H CH3 CH3
CH3
OH NH2
benzaldehyde benzoic acid toluene xylene
phenol aniline naphthalene
polyaromatic hydrocarbons (PAH's)carcinogens formed from combustionThis PAH is the main carcinogen found in Tobacco smoke (so quit for goodness sake!)
cinnamaldehyde
CHO
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©2012 Gregory R Cook
Nomenclature for Benzene Derivatives
10
CH3
toluenemethylbenzene
Br
bromobenzene
NO2
nitrobenzene
OH
phenolhydroxybenzene
CH3
o-xylene1,2-dimethylbenzene
CH3CH3
m-xylene1,3-dimethylbenzene
CH3
p-xylene1,4-dimethylbenzene
CH3CH3
p-nitrobenzaldehyde o-difluorobenzene TNT2,4,6-trinitrotoluene
Br
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©2012 Gregory R Cook
Benzene as a substituent
11
Ph
4-phenyloctane biphenyl
H2C
benzyl substitutent
4-phenyloctane
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©2012 Gregory R Cook
Polycyclic Aromatic Hydrocarbons
12
naphthalene
Anthracene
phenanthrene benzo[a]pyrene
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©2012 Gregory R Cook
C60 - Buckminsterfullerene
13
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©2012 Gregory R Cook
Graphene
14
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©2012 Gregory R Cook
Carbon Nanotubes
15
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©2012 Gregory R Cook
Arene Reactivity
16
• Reduction
• Electrophilic Aromatic Substitution
• Reactions of substituents
H2Ni
100-200°C100 atm pressure
Br2+FeBr3 HBr+
Br
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©2012 Gregory R Cook
Birch Reduction
17
Na, NH3
CH3OH
R RH H
H H
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©2012 Gregory R Cook
Oxidation of Alkylbenzenes
18
Na2Cr2O7
H2SO4, H2O, heatNO REACTION
Na2Cr2O7
H2SO4, H2O, heatNO REACTION
Na2Cr2O7
H2SO4, H2O, heat
OH
O
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©2012 Gregory R Cook
Aromatic Stability of Benzene
19
6 π electrons 6 π electrons
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©2012 Gregory R Cook
What about other sized rings?
20
highly unstable and Reactive
1.56 Å
1.35 Å
no "special" stabilityeasily hydrogenated
not flat
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©2012 Gregory R Cook
Hückel’s Rule
21
• Three requirements for aromatic stability
• Fully planar ring
• Fully conjugated
and
• Must have 4n+2 number of pi electrons
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©2012 Gregory R Cook
Hückel’s Rule
22
4 π electrons6 π electrons 8 π electrons
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©2012 Gregory R Cook
Aromatic Ions
23
H H
pKa = 16
H H
H H
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©2012 Gregory R Cook
Larger rings - Annulenes
24
HH
HH
HH
C10H10not planar C18H18
aromatic
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©2012 Gregory R Cook
Aromatic Heterocycles
25
N NH
O
N
NH
N H O N HN N
N
NN
N
NH2
HN
NN
N
O
H
H
NH2 N
N
NH2
OH
N
N
OH
O
H3C
HGuanine Thymine CytosineAdenine
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©2012 Gregory R Cook
Aromatic Heterocycle Basicity
26
N N H
N
NH
N N H
HA
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