Chapter 17' Reactions of Aromatic Compounds

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Chapter 17. Reactions of Aromatic Compounds
A. Electrophilic Aromatic Substitution
1. General reaction
General mechanism - two steps
Step 1
a stabilized carbocation the cyclohexadienyl
cation
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Step 2
In terms of an energy/reaction path diagram
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2. Reaction with strong acids (H2SO4, HBr, etc)
E H
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But why not addition reactions????
nonaromatic
This is a common pattern for aromatic compounds -
substitution regenerates aromatic, stabilized
compounds
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3. Sulfonation a. overall reaction - fuming
sulfuric acid
b. mechanism
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c. The reverse reaction - we will see why this
reaction is important a little later -
desulfonation
4. A horse of a different color - nitration a.
overall reaction - note use of H2SO4!
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b. mechanism - setting up th electrophile
(from H2SO4)
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5. Halogenation
a. overall reaction
b. mechanism
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c. iodination
6. Friedal - Crafts Alkylation
a. overall reaction
X Cl, Br
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b. mechanism
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c. potential problems.
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7. Friedal - Crafts Acylation
a. overall reaction
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(a key feature)
b. mechanism
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c. Gatterman - Koch formylation
formyl chloride
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B. Directing effects in electrophilic aromatic
substitution reactions For di-substituted
arenes recall that there are three isomers
So, for anisole
The methoxy group directs ortho, para.
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There are two ways to look at these patterns. 1.
p-donating substituents - lets use E as the
electrophile
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ortho substitution
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But -Cl, -Br, -I slow down electrophilic
substitution
This brings up the other way to look at o, p
activating groups
E prefers to attack electron rich carbons
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2. p-electron withdrawing
E prefers to attack electron rich carbons - so
meta substitution is preferred!
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The alternative argument goes like this
(same thing happens at ortho attack)
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Lets review!!
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3. Overview
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C. Polysubstitution of aromatic compounds. A
general rule of thumb 1. Activating o,p
directors are stronger than the meta directing
deactivators. 2. The alkyl groups and
halides are in-between. 3. Steric effects can be
important.
Some simple examples - the directors work
together
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Some not - so - simple examples and tricks!
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junk, tar
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D. Clemenson reduction
1. general reaction
2. synthetic uses strategy
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E. Nucleophilic Aromatic Substitution
NOT A GENERAL NOR A GOOD SYNTHETIC METHOD!!!
1. general reaction
BUT
and
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2. mechanism - there are TWO!!

• Normal path - for activated arenes (ones
• with a number of electron withdrawing groups
• and one leaving group).

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b. the benzyne mechanism - elimination/ addition
sequence - needs a very, very strong base and a
leaving group.
Mechanism was proven by J. D. Roberts using
13C labels
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F. Other addition reactions - reduction 1.
catalytic reduction
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2. Birch reduction a. overall reaction
b. mechanism
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c. Substituted variants p-donors stay at CC p
electron withdrawers get left at reduced
positions
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G. Reactions at the benzylic position 1.
oxidation
(R can be any alkyl group)
2. halogenation
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3. nucleophilic substitution
R- relative rate CH3CH2-
1.0 (CH3)2CH- 0.04 allyl-
33 benzyl 78
(SN2)
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H. Reactions of phenols 1. acidity
Why??
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3. oxidants

• Summary
• 1. General features of electrophilic aromatic
  substitution
• a. mechanism - general
• b. directing effects which are which, why

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2. Specific reactions a. sulfonation b.
nitration c. halogenation d. Friedal-Crafts
alkylation e. Friedal-Crafts acylation f.
Gatterman-Koch synthesis 3. Synthesis of
poly-substituted aromatic compounds 4. Clemenson
reduction 5. Nucleophilic aromatic
substitution a. addition-elimination
mechanism b. benzyne mechanism 6. Hydrogenation
of arenes a. catalytic hydrogenation b. Birch
reduction
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7. Benzylic reactions a. oxidation b.
halogenation c. nucleophilic substitution 8.
Reactions of phenols a. use as a nucleophile b.
formation of quinones