Chapter 05
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Aromatic Compounds
CHEM 240: Spring 2014
Prof. Greg Cook
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©2014 Gregory R Cook
Monday, October 6, 14
Benzene
First isolated in 1825 from whale oil by Michael
Faraday
•
Unsaturated hydrocarbon but did not have the
typical reactivity of alkenes or alkynes.
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•
C6H 6
prismane
Br 2
or
KMnO 4
no reaction
Dewar benzene
2
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Monday, October 6, 14
Benzene
•
In 1865 Kekulé proposed a structure having a sixmembered ring with alternating double bonds.
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1.54 Å
1.34 Å
Kekulé structures
H
H
H
H
H
bonds identical
1.39 Å
H
modern view of benzene's structure
fully conjugated
RESONANCE - not equilibrium
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Aromatic Compounds
CHO
benzaldehyde
almond
CO2H
CH 3
CH 3
benzoic acid
cinnamaldehyde
toluene
xylene
phenol
aniline
polyaromatic hydrocarbons (PAH's)
carcinogens formed from combustion
This PAH is the main carcinogen
found in Tobacco smoke (so quit for
goodness sake!)
naphthalene
cinnamon
moth balls
O
O
O
S
H
H
H
estrone
CH 3
N
CH 3 O
HO
NH 2
CH 3
CHO
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OH
H 3C
N
N
N
H
OCH2CH 3
sildenafil (viagra)
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Aromatic Reactivity
H
+
Br 2
NOT formed
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H
FeBr 3 catalyst
+
Br
Br 2
Br
Br
+
HBr
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Monday, October 6, 14
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Aromatic Stability
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Monday, October 6, 14
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•
Aromatic Stability
To have aromatic stabilization, a molecule must
have three things:
•
•
A planar ring
•
Have 4n+2 number of electrons in the pisystem (Hückel Rule)
A fully conjugated pi-system (a p orbital on
every atom in the ring)
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Monday, October 6, 14
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Aromatic Stability
AROMATIC 4 pi electrons 4 pi electrons AROMATIC
2 pi electrons
6 pi electrons
AROMATIC
6 pi electrons
AROMATIC
6 pi electrons
8 pi electrons
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Naming Aromatic Compounds
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Naming Substituted Benzenes
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Naming Substituted Benzenes
CH 3
Cl
Cl
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orthodichlorobenzene
or
1,2-dichlorobenzene
CO2H
O2N
NO 2
O2N
NO 2
paranitrobenzoic acid
2,4,6-trinitrotoluene
Ph
4-phenyloctane
4-phenyloctane
we often abbreviate phenyl
with Ph when we draw structures
biphenyl
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Electrophilic Aromatic Substitution
FeBr 3
+
Br 2
H
Br
+
Br
HBr
H
Br
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NOT formed
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Monday, October 6, 14
Electrophilic Aromatic Substitution
FeBr 4
H
Br
H
Br
Br
H
Br
Br
-HBr
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FeBr 4
NOT formed
H
e
n
e
r
g
y
Br
H
H
Br
Br
Rate
Determing
Step (RDS)
Br
reaction progress
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Monday, October 6, 14
13
Chlorination and Iodination
FeCl 3
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+
Cl2
FeBr 3
+
I2
+
HCl
+
HBr
+
HI
Br
Br 2
CuI2
+
Cl
I
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Monday, October 6, 14
Nitration
H 2SO4
HNO 3
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+
NO 2
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Monday, October 6, 14
Sulfonation
H 2SO4
SO 3
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+
SO 3H
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Friedel-Crafts Alkylation
AlCl3
R
Cl
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+
R
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Monday, October 6, 14
Limitations of the Friedel-Crafts Alkylation
AlCl3
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+
R
R
Cl
•
•
Only alkyl halides work, not aryl or vinyl halides.
•
Aromatic rings with amine groups will not work
(-NH2) because the amine will react with the alkyl
halide.
Aromatic rings with electron withdrawing groups
already on them will not work (-NO2, -CN, SO3H, -COR, -CO2H).
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Friedel-Crafts Acylation
O
O
AlCl3
+
Cl
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R
R
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Monday, October 6, 14
Summary of EAS
Br 2
FeBr 3
Br
+
Br 2
FeBr 3
Br
+ FeBr 4
electrophile
Cl2
FeCl 3
Cl
+
Cl2
FeCl 3
Cl
+ FeCl 4
electrophile
I2
CuI2
I
+
I2
CuI2
+ CuI3
I
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electrophile
HNO 3
H 2SO 4
NO 2
O
HO
+
N
H 2SO 4
O
O N O
electrophile
SO3
H 2SO 4
O
SO 3H
O
+
O S
H 2SO 4
O S
O
OH
electrophile
R-Cl
AlCl3
R
R Cl
+
+ AlCl4
R
AlCl3
electrophile
O
R
O
Cl
AlCl3
O
R
R
O
Cl
+
AlCl3
R
Cl
electrophile
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Monday, October 6, 14
+ AlCl4
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Substituent Effects
Y
Y
Y
Y
E
+
E+
+
+
E
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E
•
How does Y affect the reactivity of the benzene
ring?
•
How does Y affect the ratio of products?
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Substituent Effect on Reaction Rate
OH
Cl
>
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relative
reactivity
>
>
1
1000
NO2
6x10-8
0.033
Activating
Electron Donating Groups
Deactivating
Electron Withdrawing Groups
O
alkyl
OR
NR2
SR
X
CN
NO2
SO3H
R
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Electron Density in the Ring
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Substituent Effect on Regioselectivity
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Electron Donating Groups
OH
OH
E+
OH
OH
E
+
+
E
E
ortho addition
OH
OH
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E+
OH
OH
OH
E
E
E
E
H
H
H
H
especially stable
para addition
OH
OH
OH
OH
H
especially stable E H
OH
OH
OH
E+
E
H
E
E
H
meta addition
OH
OH
E+
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Monday, October 6, 14
H
H
H
E
E
E
none especially stable
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Electron Withdrawing Groups
NO 2
NO 2
E+
NO 2
OH
E
+
+
E
E
ortho addition
O
N
O
O
N
O
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E+
O
O
N
O
E
E
E
H
H
H
especially UNstable
para addition
O
N
O
N
O
O
N
O
O
N
O
O
N
O
E+
E
E
H
N
H
especially UNstable
meta addition
O
E
H
O
O
N
O
O
N
O
O
N
O
E+
H
H
H
E
E
E
none especially UNstable - the lesser of the evils
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Monday, October 6, 14
26
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Halogens - deactivating, but o,p-directors?
Cl
Cl
E
E
H
H
halogens, while inductively electron withdrawing
groups, they can stabilze plus charge through
resonance. Thus, they are an exception and are
ortho-para directors
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©2014 Gregory R Cook
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Summary of Substituent Effects
Deactivating
Electron Withdrawing Groups
meta directors
ortho, para directors
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NO 2 strongest (resonance)
SO 3H
O
R
CN
Activating
Electron Donating Groups
OR
X weak
ortho, para directors
but deactivating
strongest (resonance)
NR 2
SR
alkyl
weakest (inductive)
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Oxidation and Reduction
CH3
KMnO4
O
C
O
O
OH
KMnO4
C
OH
O
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KMnO4
C
OH
KMnO4
No Reaction
H2
Pd/C
H2
Pt, ethanol
130 atm
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Fused Aromatic Rings
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Aromatic Carbon - Graphite
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Aromatic Carbon - Graphite
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Aromatic Carbon - other forms
carbon nanotubes
buckminsterfullerene
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Aromatic Heterocycles
lone pair
in resonance
with aromatic
pi system
N
H
N
N
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N
NH2
N
N
H
N
N
Adenine
H
O
lone pair
in resonance
with aromatic
pi system
O
O
N
N
H
N
N
Guanine
NH2
N
H
N
H3C
lone pair
in resonance
with aromatic
pi system
N
H
NH2
O
H
N
N
N
H
Thymine
H
O
N
N
H
O
Cytosine
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Organic Synthesis
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•
•
Create molecules by design
•
•
Compare target and starting material
Consider reactions that allow you to build
molecules - may need to change functional
groups along the way
Retrosynthetic Analysis
•
Work backwards from the product - immediate
precursors
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Organic Synthesis
•
Starting from benzene, how would you synthesize pbromobenzoic acid?
CO2H
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Br
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Organic Synthesis
CH 3
SO 3H
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O2N
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