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TRANSCRIPT
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Chapter 8. Aromaticity
Learning objectives:
1. Name benzene derivatives.
2. Recognize aromatic compounds.
3. Explain the trend for the acidity of substituted phenol using the concept of electron-
donating and electron-withdrawing groups, and inductive and resonance effects.
4. Write the oxidation of the benzylic groups with C-H bond(s) to benzoic acid.
5. Write the electron-pushing (arrow-pushing) mechanisms for the electrophilic aromatic
substitution of benzene including halogenation, nitration, sulfonation, Friedel-Crafts
alkylation, and Friedel-Crafts acylation.
6. Explain the trend for the reactivity of substituted benzene toward electrophilic
aromatic substitution using the concept of electron-donating and electron-withdrawing
groups.
7. Recognize the reagents for reduction of nitro group to amino group.
Sections to be covered
8.1 Introduction of Aromatic Compounds
8.2 Nomenclature of Monosubstituted Benzenes
8.3 Electrophilic Aromatic Substitution
8.4 Chemical Transformation of Some Substituents of Benzene
8.5 Nomenclature of Disubstituted and Polysubstituted Benzene
8.6 Reactivity of Substituted Benzene
8.7 The Effect Substituent on pKa
8.1 Introduction of Aromatic Compounds
A. Aromatic compounds are unusually stable
The resonance of benzene provides additional stabilization.
H2Ni
H = -28.6 kcal/mol
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Energy
-28.6 kcal/mol
cyclohexatriene(hypothetical)
-85.8 kcal/mol(-28.6x3)
+ H2
+ 3H2
+ 3H2
-49.8 kcal/mol
36 kcal/mol
B. The Criteria of Being Aromatic
(i) Cyclic molecule
(ii) Every atom has p orbital
(iii) Planar molecule
(iv) Number of delocalizable electrons complies with 4n+2 rule (as compared with 4n
rule)
Integral numbers that comply with the 4n+2 equation:
2, 6, 10, 14, 18, 22, 26 • • • • • • 4n+2
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D. Heterocyclic Compounds
Know the type of lone-pair electrons that are delocalizable.
HN
N
HN O S
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D. Self-assessment Questions
Can you describe the criteria for aromaticity?
Can you use the criteria for aromaticity to identify aromatic and non-aromatic
compounds?
Can you use orbital hybridization to identify whether the electron pair can
contribute and form the aromatic compound?
8.2 Nomenclature of Monosubstituted Benzenes
Know the name for all of the monosubstituted benzenes.
ClBr
NO2
CH3 OHNH2
SO3HOCH3
CHOCO2H
CHCH3
CH3
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Know the name for all of Benzene-based Substituents.
As substituent:
A. Self-assessment Questions
Can you name all the monosubstituted benzenes (12) and benzene-based
substituents (3) listed in this section?
8.3 Electrophilic Aromatic Substitution
Know why it’s called electrophilic aromatic substitution.
A. General reaction equation
+ E+ + H+
E
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D. Reaction coordinate diagram
E. Halogenation of Benzene
Cl2 (Br2)FeCl3 (FeBr3)
Cl (Br)
(i) How to derive the mechanism for halogenation of benzene?
Retrosynthetic analysis
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(ii) The role(s) of FeCl3 or FeBr3?
(iii) Mechanism of halogenation
F. Nitration of Benzene
HNO3
H2SO4
NO2
(i) What is the electrophile needed and how to get it?
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(ii) What is the role of H2SO4?
(iii) Mechanism of nitration
G. Sulfonation of Benzene
SO3
H2SO4
SO3H
fumingH2SO4
(i) What is the electrophile needed and how to get it?
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(iii) Example of Friedel-Crafts acylation
I. Self-assessment Questions
Can you explain what is electrophilic aromatic substitution?
Can you derive the general mechanism of electrophilic aromatic substitution
and identify the rate-determining step
Can you write the electron-pushing mechanisms for the electrophilic aromatic
substitution of benzene including halogenation, nitration, sulfonation, Friedel-
Crafts alkylation, and Friedel-Crafts acylation.
Can you identify the reagents and acid catalysts needed for various electrophilic
aromatic substitutions, and complete these reactions?
Can you explain the difference between electrophilic aromatic substitution and
addition reaction?
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C. Self-assessment Questions
Can you recognize all the reagents used for reduction and oxidation in this
section?
Can you complete the reduction and oxidation with similar functional groups
attached to benzene as shown in this section and provide the corresponding
products?
Can you combine all the reactions you have learned so far and complete a
multi-step synthesis?
8.5 Nomenclature of Disubstituted and Polysubstituted Benzene
A. Disubstituted Benzenes
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D. Self-assessment Questions
Can you name all the disubstituted benzenes using ortho (O), meta (m) and
para (p), or numbering systems?
8.7 Reactivity of Substituted Benzene
Know what are Electron-donating group (EDG) and electron-withdrawing group
(EWG)
Know how to judge EDG and EWG can predict the possible outcomes from a reaction.
A. The Influence of EDG and EWG on the Electron Density of Substituted Benzene
Know how to use resonance effect and inductive effect
B. Strong Electron-donating Group
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G. Influence of EDG and EDW on the Reaction Rate of Benzene
(i) What is the correct order of decreasing reactivity (fastest to slowest) toward the
electrophilic aromatic substitution for the following compounds?
NO2Cl CH3 OCH3
I II III IV V
Resonance effect > inductive effect (mostly)
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H. Self-assessment Questions
Can you describe what are resonance effect and inductive effect?
Can you recognize EDG and EWG using resonance and inductive effects?
Can you explain the trend for the reactivity of substituted benzene toward
electrophilic aromatic substitution using the concept of electron-donating and
electron-withdrawing groups?
8.7 The Effect of Substituents on pKa
Know the influence of EDG and EWG on the electron density of substituted benzene.
Know the influence of EDG and EWG on the stability of conjugate bases.
A. What Influence that EDG and EWG Will Have on Acidity?
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B. Substituted Phenols
OHOH OH OH
I II III IV V
pKa 10.20 10.19 9.95 9.38
H3CO H3C
OH
VI
OH
ClO2N
H
O
7.66 7.14
C. Substituted Benzoic Acids
CO2HCO2H CO2H CO2H
I II III IV V
pKa 4.47 4.34 4.20 4.00
H3CO H3C
CO2H
VI
CO2H
BrO2N
CH3
O
3.70 3.44
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D. Substituted Prononated Anilines
NH3NH3 NH3 NH3
I II III IV V
pKa 5.29 5.07 4.58 3.91
H3CO H3C
NH3
VI
NH3
BrO2N
H
O
1.76 0.98
Which of the following amine is most basic?
E. Self-assessment Questions
Can you describe the influence of EDG and EWG on the electron density of
substituted benzenes?
Can you describe the influence of EDG and EWG on the stability of the
conjugate bases for the substituted benzenes ?
Can you explain the trend of acidity for the substituted benzenes using the
concept of electron-donating and electron-withdrawing groups?