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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

2

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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C. Examples

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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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B. General mechanism

C. Comparing Electrophilic Aromatic Substitution and Addition Reaction

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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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(ii) Mechanism

H. Friedel-Crafts acylation and alkylation of benzene

RXAlCl3

R

RCOXAlCl3

CR

O

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(i) What are the electrophiles?

(ii) Example of Friedel-Crafts alkylation

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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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8.4 Chemical Transformation of Some Substituents of Benzene

A. Reduction

H2, Pd/C

H2, Pd/C

NO2

O

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B. Oxidation

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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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B. Trisubstituted Benzene

C. Polysubstituted Benzene

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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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C. Moderate Electron-donating Group

D. Weak Electron-donating Group

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E. Moderate Electron-Withdrawing Group

F. Strong Electron-Withdrawing 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?