1. 1 Benzene and Aromatic Compounds Buckminsterfullerene—Is it Aromatic? The two most common elemental forms of carbon are diamond and graphite. Their physical characteristics are very different because their molecular structures are very different.

2. 2 Figure 18.1 Five examples of electrophilic aromatic substitution

3. 3 Electrophilic Aromatic Substitution Chlorination proceeds by a similar mechanism. Halogenation

4. 4 Electrophilic Aromatic Substitution Generation of the electrophile in nitration requires strong acid. Nitration and Sulfonation

5. 5 Electrophilic Aromatic Substitution Generation of the electrophile in sulfonation requires strong acid. Nitration and Sulfonation

6. 6 Electrophilic Aromatic Substitution Friedel-Crafts Alkylation and Friedel-Crafts Acylation

7. 7 Electrophilic Aromatic Substitution In Friedel-Crafts acylation, a benzene ring is treated with an acid chloride (RCOCl) and AlCl 3 to form a ketone. Because the new group bonded to the benzene ring is called an acyl group, the transfer of an acyl group from one atom to another is an acylation. Friedel-Crafts Alkylation and Friedel-Crafts Acylation

8. 8 Electrophilic Aromatic Substitution Friedel-Crafts Alkylation and Friedel-Crafts Acylation [2]Rearrangements can occur. These results can be explained by carbocation rearrangements.

9. 9 Electrophilic Aromatic Substitution Friedel-Crafts Alkylation and Friedel-Crafts Acylation

10. 10 Electrophilic Aromatic Substitution Electrophilic Aromatic Substitution and Substituted Benzenes. Consider toluene—Toluene reacts faster than benzene in all substitution reactions. The electron-donating CH 3 group activates the benzene ring to electrophilic attack. Ortho and para products predominate. The CH 3 group is called an ortho, para director.

11. 11 Electrophilic Aromatic Substitution Orientation Effects in Substituted Benzenes A CH 3 group directs electrophilic attack ortho and para to itself because an electron-donating inductive effect stabilizes the carbocation intermediate.

12. 12 Electrophilic Aromatic Substitution Orientation Effects in Substituted Benzenes An NH 2 group directs electrophilic attack ortho and para to itself because the carbocation intermediate has additional resonance stabilization.

13. 13 Electrophilic Aromatic Substitution Orientation Effects in Substituted Benzenes With the NO 2 group (and all meta directors) meta attack occurs because attack at the ortho and para position gives a destabilized carbocation intermediate.

14. 14 Disubstituted Benzenes 3. No substitution occurs between two meta substituents because of crowding. Electrophilic Aromatic Substitution

15. 15 Synthesis of Benzene Derivatives Pathway I, in which bromination precedes nitration, yields the desired product. Pathway II yields the undesired meta isomer. Electrophilic Aromatic Substitution

16. 16 Halogenation of Alkyl Benzenes Benzylic C—H bonds are weaker than most other sp 3 hybridized C—H bonds, because homolysis forms a resonance-stabilized benzylic radical. Electrophilic Aromatic Substitution As a result, alkyl benzenes undergo selective bromination at the weak benzylic C—H bond under radical conditions to form the benzylic halide.

17. 17 Halogenation of Alkyl Benzenes Electrophilic Aromatic Substitution

18. 18 Halogenation of Alkyl Benzenes Note that alkyl benzenes undergo two different reactions depending on the reaction conditions: Electrophilic Aromatic Substitution With Br 2 and FeBr 3 (ionic conditions), electrophilic aromatic substitution occurs, resulting in replacement of H by Br on the aromatic ring to form ortho and para isomers. With Br 2 and light or heat (radical conditions), substitution of H by Br occurs at the benzylic carbon of the alkyl group.

19. 19 Oxidation and Reduction of Substituted Benzenes Arenes containing at least one benzylic C—H bond are oxidized with KMnO 4 to benzoic acid. Electrophilic Aromatic Substitution Substrates with more than one alkyl group are oxidized to dicarboxylic acids. Compounds without a benzylic hydrogen are inert to oxidation.

20. 20 Oxidation and Reduction of Substituted Benzenes Although the two-step method seems more roundabout, it must be used to synthesize certain alkyl benzenes that cannot be prepared by the one-step Friedel-Crafts alkylation because of rearrangements. Electrophilic Aromatic Substitution

21. 21 Oxidation and Reduction of Substituted Benzenes A nitro group (NO 2 ) that has been introduced on a benzene ring by nitration with strong acid can readily be reduced to an amino group (NH 2 ) under a variety of conditions. Electrophilic Aromatic Substitution