wwu - chemistry aromatic substitution chapter 22
TRANSCRIPT
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Aromatic Substitution
Chapter 22
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Reading Assignment
Sections 22.0 through 22.2 Sections 22.4 through 22.9 Sections 22.10 through 22.12 Section 22.17 Section 22.18 (chapter summary) Problems
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Assigned Problems
In-text problems 22-1 through 22-2 22-4 through 22-13 22-23 through 22-29
End-of-chapter problems 1 through 19
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Substitution versus Addition
Benzene does not undergo addition reactions. To illustrate, consider the addition reaction of
benzene and Br2:
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Bromine Addition to Benzene
Br Br+
Br
HH
Br
As a result of the bromine addition, the benzene ring has lost its aromatic character -- it has lost its 150 kJ/mole of resonance energy. The reaction, as shown, is energetically unfavorable.
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Bromine Substitution on Benzene
If, on the other hand, we allow a bromine atom to replace a hydrogen atom of benzene (i.e., a substitution reaction), the aromatic character of the benzene ring is retained.
This type of process remains energetically favorable.
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Bromine Substitution on Benzene
H
Br Br
Br
H Br+catalyst
+
This is electrophilic aromatic substitution.
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Electrophilic Aromatic Substitution
In general, the reactions of benzene follow the same pattern, as seen in the following image.
A+, in this reaction, is used to represent any electrophile (Lewis acid).
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Electrophilic Aromatic Substitution
H
+ A
A
+ H
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But first, a brief digression into nomenclature
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IUPAC Nomenclature of Substituted Benzenes
X
1
2
3
4
5
6
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IUPAC Nomenclature of Benzene Derivatives
Learn Nomenclature Either on Your OwnLearn Nomenclature Either on Your Ownor Use the Organic Nomenclature Softwareor Use the Organic Nomenclature Software
CH3
methylbenzene
CH3
Cl
1-chloro-3-methylbenzene
(toluene)
Br
NO2
1-bromo-3-nitrobenzene
CH3
CH3
1,4-dimethylbenzene
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Common Nomenclature in Substituted Benzenes
Ripso
ortho
meta
para
orthoortho,, metameta and and parapara Positions Positions
CH3
NO2
m-nitrotoluene
1-methyl-3-nitro-benzeneo-
m-
p-
Cl
Cl
p-dichlorobenzene
1,4-dichlorobenzene
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Common Nomenclature in Substituted Benzenes The ortho, meta, and para names can only be
used when there are two substituents. If there are three or more substituents, you must
use the IUPAC name (with numbers).
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Electrophilic Aromatic Substitution
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Mechanism of Electrophilic Aromatic Substitution Nearly all the substitution reactions of benzene
follow the same mechanism. In the scheme that follows, A+ represents the
electrophile (Lewis acid).
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Mechanism of Electrophilic Aromatic Substitution (Step 1)
H
+ Aslow
H
A
H
A
H
A
a resonance-stabilized arenium ion
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Mechanism of Electrophilic Aromatic Substitution (Step 2)
H
A + Bfast
A
+ B H
this represents theresonance hybridof the arenium ion
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Some Substitution Reactions of BenzeneSome Substitution Reactions of Benzene
FeCl3
CH3
Cl2
Cl
CH3ClAlCl3
RC
Cl
O AlCl3 C R
O
OH NO
O
H2SO4
N O
O
S
O
OH
OOH S
O
O
OHSO3
+
+
+
+
+
Halogenation
Friedel-CraftsAlkylation
Friedel-CraftsAcylation
Nitration
Sulfonation
+ +
-
-
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All that is required is for us to figure out how to generate the appropriate A+ for any substitution reaction.
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Reaction with Acids
H
+ H
H
+ H
K = 1
•Use H2SO4
•What does this prove?
•The hydrogens of benzene are exchangeable.
•We can make “practical” use of this reaction in the following scenario:
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Reaction with Acids
H
+ D
D
+ H
K = 1
•Use D2SO4
•Can use this method to prepare benzene-d6
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Nitration of Benzene
H
+ HNO3
H2SO4
NO2
+O
H H
•What does the H2SO4 do?
•It reacts with nitric acid to generate the electrophile (see next image)
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Formation of Nitronium Ion
HO N
O
O
+ 2 H2SO4
H
OH H
+
O
NO
+ 2 HSO4-Nitronium ion
2+
Nitronium ion (NO2+) is the electrophile that attacks
the benzene ring.
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Halogenation of Benzene
H
+ X2
FeX3
X
+ XH
•X can be either bromine or chlorine
•Other catalysts might include elemental iron (Fe0) or aluminum chloride (AlCl3)
•The ferric halide serves to generate the electrophile in this reaction.
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Formation of the Halonium Ion
X2 + FeX3 X FeX4
a "stable" complex
•X+ is a halonium ion:
•Cl+ = chloronium ion
•Br+ = bromonium ion
•The halonium ion is the electrophile in this reaction.
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The Friedel-Crafts Alkylation Reaction
H
+AlCl3
R
+ XHR X
•The product is an alkylbenzene
•The role of the anhydrous aluminum chloride is to generate a stable carbocation complex.
•The carbocation is the electrophile in this process.
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Formation of a Stable Carbocation Complex
R X + AlCl3 R AlCl4
a stable carbocation complex
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Explain this:
H
+
CH3 CH2 CH2 CH2 Cl
AlCl3
C CH3
CH3
CH3
+
H Cl
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The Friedel-Crafts Acylation Reaction
H
+AlCl3
C
+
XHR C Cl
O
R
O
The product is an acylbenzene (aromatic ketone)
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The Friedel-Crafts Acylation Reaction
The role of the anhydrous aluminum chloride is to generate a stable carbocation complex.
The carbocation is the electrophile in this process.
In this case, the carbocation is an acylium ion.
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Formation of a Stable Acylium Ion Complex
R C + AlCl3 AlCl4
a stable acylium ion complex
Cl
O
R C
O
•The acylium ion is the electrophile in this reaction
•Acylium ions do not rearrange
•The same result is obtained if one uses an acid anhydride instead of an acid chloride.
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The Acylium Ion is Stabilized by Resonance
R C
O
R C
O
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Sulfonation of Benzene
H
+
SO3
S
+
OH
OH
O
O
HHO S OH
O
O
(fuming)
•The actual electrophile in this reaction is sulfur trioxide.
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Why is Sulfur Trioxide Electrophilic?
S O
O
O
+ - +
: :..
:..: :
: :
: :..
..H2SO4
S O
O
O
H
sulfurtrioxide
The resulting ion reacts with benzene
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Sulfonation of Benzene
The protonated SO3 is the electrophile in this reaction.
The reaction requires fuming sulfuric acid (i.e., sulfuric acid saturated with sulfur trioxide)
This reaction is reversible (unlike the other examples).