1. Aromatic Substitution
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Aromatic Substitution
Chapter 22
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2. Reading Assignment Sections 22.0 through 22.2
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Reading Assignment
Sections 22.0 through 22.2
Sections 22.4 through 22.9
Sections through 22.12
Section 22.17
Section (chapter summary)
Problems
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3. Assigned Problems In-text problems End-of-chapter 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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4. Substitution versus Addition
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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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5. Bromine Addition to Benzene
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Bromine Addition to Benzene
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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6. Bromine Substitution on Benzene
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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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7. Bromine Substitution on Benzene
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Bromine Substitution on Benzene
This is electrophilic aromatic substitution.
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8. 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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9. Electrophilic Aromatic Substitution
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Electrophilic Aromatic Substitution
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10. But first, a brief digression into nomenclature
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But first, a brief digression into nomenclature
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11. IUPAC Nomenclature of Substituted Benzenes
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IUPAC Nomenclature of Substituted Benzenes
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12. IUPAC Nomenclature of Benzene Derivatives
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IUPAC Nomenclature of Benzene Derivatives
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13. Common Nomenclature in Substituted Benzenes
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Common Nomenclature in Substituted Benzenes
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14. Common Nomenclature in Substituted Benzenes
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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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15. Electrophilic Aromatic Substitution
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Electrophilic Aromatic Substitution
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16. Mechanism of 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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17. Mechanism of Electrophilic Aromatic Substitution (Step 1)
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Mechanism of Electrophilic Aromatic Substitution (Step 1)
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18. Mechanism of Electrophilic Aromatic Substitution (Step 2)
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Mechanism of Electrophilic Aromatic Substitution (Step 2)
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19. 4/21/2017
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20. 4/21/2017
All that is required is for us to figure out how to generate the appropriate A+ for any substitution reaction.
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21. Reaction with Acids Use H2SO4 What does this prove?
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Reaction with Acids
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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22. Reaction with Acids Use D2SO4
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Reaction with Acids
Use D2SO4
Can use this method to prepare benzene-d6
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23. Nitration of Benzene What does the H2SO4 do?
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Nitration of Benzene
What does the H2SO4 do?
It reacts with nitric acid to generate the electrophile (see next image)
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24. Formation of Nitronium Ion
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Formation of Nitronium Ion
Nitronium ion (NO2+) is the electrophile that attacks the benzene ring.
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25. Halogenation of Benzene
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Halogenation of Benzene
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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26. Formation of the Halonium Ion
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Formation of the Halonium Ion
X+ is a halonium ion:
Cl+ = chloronium ion
Br+ = bromonium ion
The halonium ion is the electrophile in this reaction.
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27. The Friedel-Crafts Alkylation Reaction
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The Friedel-Crafts Alkylation Reaction
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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28. Formation of a Stable Carbocation Complex
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Formation of a Stable Carbocation Complex
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29. 4/21/2017
Explain this:
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30. The Friedel-Crafts Acylation Reaction
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The Friedel-Crafts Acylation Reaction
The product is an acylbenzene (aromatic ketone)
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31. The Friedel-Crafts Acylation Reaction
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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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32. Formation of a Stable Acylium Ion Complex
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Formation of a Stable Acylium Ion Complex
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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33. The Acylium Ion is Stabilized by Resonance
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The Acylium Ion is Stabilized by Resonance
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34. Sulfonation of Benzene
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Sulfonation of Benzene
The actual electrophile in this reaction is sulfur trioxide.
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35. Why is Sulfur Trioxide Electrophilic?
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Why is Sulfur Trioxide Electrophilic?
sulfur
trioxide
The resulting ion reacts with benzene
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36. Sulfonation of 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).
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