TOPIC 2. REACTIONS OF AROMATIC COMPOUNDS

45 Pages·2009·0.28 MB·English

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TOPIC 2. REACTIONS OF
AROMATIC COMPOUNDS
(Chapters 15, parts of 20, and 21)
OBJECTIVES
1. Describe the reactions between strong electrophiles and aromatic
compounds (the nucleophilc component) which lead to substitutionof a
hydrogen atom on the aromatic ring.
2. Describe the reactions of anilines (ArNH ) and phenols (ArOH) which take
2
place on the nucleophilic ring or nucleophilic substituent.
3. Outline the mechanisms whereby aryl halides undergo nucleophilic
substitution (NOTS 1 or S 2)
N N
4. Describe various reactions of the side chain of aromatic compounds.
5. Use this knowledge to predict the products of these reactions and to be
able to do single-step syntheses and multi-step syntheses using these
reactions as well as the reactions previously studied.
CHEM2312: Notes: D.M. Collard, 2009
ELECTROPHILIC AROMATIC
S:15.1-2
SUBSTITUTION REACTIONS Review Prob: 15.28,30
Electrophilic Substitution versus Addition
Arenes undergo substitutionwith strong electrophiles…
H
catalyst
+ E-A
E=electrophile; A=anion or leaving group
E-A + catalyst active electrophile
H FeBr
3
+ Br-Br
…whereas alkenes undergo additionreactions
+ Br-Br Br Br
Electrophilic aromatic substitutions
Halogenation
Hal
2
FeHal
3
or Fe
Nitration
HNO
3
H SO
2 4
Sulfonation
SO
3
H SO
2 4
CHEM2312: Notes: D.M. Collard, 2009
Friedel-Crafts Alkylation
R-Hal
AlCl
3
Friedel-Crafts Acylation
O
R Cl
AlCl
3
Mechanism
First step
Compare to:
E
E
H
E E E
H H H
CHEM2312: Notes: D.M. Collard, 2009
Second step
Consider two possibilities
Compare to:
A
H
H A
H
E
A
H
H
E
HALOGENATION OF BENZENE S:15.3
Role of the catalyst (bromine as example): Generation of
Active Electrophile
Br Br FeBr
3
Br Br FeBr
3
Nu Br
Nu Br Br FeBr3
CHEM2312: Notes: D.M. Collard, 2009
Mechanism of Substitution Step
Br Br FeBr
3
Br
H
Br FeBr
3
NITRATION OF BENZENE S:15.4
Role of the Catalyst: Generation of Active Electrophile
O
HO SO H H O N
3
O
H O
O N
H O
CHEM2312: Notes: D.M. Collard, 2009
Mechanism of Substitution Step
O
N
O
NO
2
H
H
O
H
SULFONATION OF BENZENE S:15.5
Generation of Active Electrophile
H SO
2 4
Mechanism of Substitution Step
O
S O
O
SO SO
3 3
H
O
SO H
3
SO2-O H H SO3H
O
H
CHEM2312: Notes: D.M. Collard, 2009
??Prob: 15.18,30??
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FRIEDEL-CRAFTS ALKYLATION S:15.6
Prob: 15.29 a,b
Examples
HF
+
OH
H SO
+ 2 4
AlCl
+ Cl 3
CHEM2312: Notes: D.M. Collard, 2009
Role of the Catalyst: Generation of Active Electrophile
Alkene + strong acid
R
H
Alcohol + strong acid
R O H H
Alkyl chloride + Lewis acid
Cl
R Cl Al Cl
Cl
Mechanism of Substitution Step
R
R R
H + HCl + AlCl
3
Cl Cl
Al
Cl Cl
CHEM2312: Notes: D.M. Collard, 2009
Rearrangement during Friedel-Crafts Alkylation
AlCl
+ Cl 3
FRIEDEL-CRAFTS ACYLATION S:15.7
Examples
O 1. AlCl
3
+
H C Cl 2. H O
3 2
1. AlCl
O O O 3
+
2. H O
R R 2
CHEM2312: Notes: D.M. Collard, 2009
Role of the Catalyst: Generation of Active Electrophile
O Cl
Al Cl
R Cl
Cl
Cl
Al Cl
Cl
O O O
R R
Mechanism of Substitution Step
R C O
O O
R R
H
Cl Cl
Al
+ HCl + AlCl
Cl Cl 3
CHEM2312: Notes: D.M. Collard, 2009