1. Aromatic Substitution Chapter 22 WWU - Chemistry

2. 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 WWU -- Chemistry

3. 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 WWU -- Chemistry

4. Substitution versus Addition • Benzene does not undergo addition reactions. • To illustrate, consider the addition reaction of benzene and Br2: WWU -- Chemistry

5. 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. WWU -- Chemistry

6. 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. WWU -- Chemistry

7. Bromine Substitution on Benzene This is electrophilic aromatic substitution. WWU -- Chemistry

8. 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). WWU -- Chemistry

9. Electrophilic Aromatic Substitution WWU -- Chemistry

10. But first, a brief digression into nomenclature WWU -- Chemistry

11. IUPAC Nomenclature of Substituted Benzenes WWU -- Chemistry

12. IUPAC Nomenclature of Benzene Derivatives WWU -- Chemistry

13. Common Nomenclature in Substituted Benzenes WWU -- Chemistry

14. 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). WWU -- Chemistry

15. Electrophilic Aromatic Substitution WWU -- Chemistry

16. 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). WWU -- Chemistry

17. Mechanism of Electrophilic Aromatic Substitution (Step 1) WWU -- Chemistry

18. Mechanism of Electrophilic Aromatic Substitution (Step 2) WWU -- Chemistry

19. WWU -- Chemistry

20. All that is required is for us to figure out how to generate the appropriate A+ for any substitution reaction. WWU -- Chemistry

21. 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: WWU -- Chemistry

22. Reaction with Acids • Use D2SO4 • Can use this method to prepare benzene-d6 WWU -- Chemistry

23. Nitration of Benzene • What does the H2SO4 do? • It reacts with nitric acid to generate the electrophile (see next image) WWU -- Chemistry

24. Formation of Nitronium Ion Nitronium ion (NO2+) is the electrophile that attacks the benzene ring. WWU -- Chemistry

25. 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. WWU -- Chemistry

26. 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. WWU -- Chemistry

27. 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. WWU -- Chemistry

28. Formation of a Stable Carbocation Complex WWU -- Chemistry

29. Explain this: WWU -- Chemistry

30. The Friedel-Crafts Acylation Reaction The product is an acylbenzene (aromatic ketone) WWU -- Chemistry

31. 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. WWU -- Chemistry

32. 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. WWU -- Chemistry

33. The Acylium Ion is Stabilized by Resonance WWU -- Chemistry

34. Sulfonation of Benzene • The actual electrophile in this reaction is sulfur trioxide. WWU -- Chemistry

35. Why is Sulfur Trioxide Electrophilic? sulfur trioxide The resulting ion reacts with benzene WWU -- Chemistry

36. 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). WWU -- Chemistry