Aromatic Compounds II

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1 Org Chem II Part I Lecture 2 Aromatic Compounds II Instructor: Dr. Tanatorn Khotavivattana tanatorn.k@chula.ac.th Recommended Textbook: Chapter 17 in Organic Chemistry, 8 th Edition, L. G. Wade, Jr., 2010, Prentice Hall (Pearson Education)

2 Basic Knowledge in Organic Reactions 1 5 main types of reactions Addition Substitution Elimination Radical Oxidation Reduction Reactions

3 Basic Knowledge in Organic Reactions 2 Nucleophile / Electrophile / Base / Acid / Leaving group

4 Basic Knowledge in Organic Reactions 3 Arrow pushing

5 Basic Knowledge in Organic Reactions 4 Examples

6 Basic Knowledge in Organic Reactions 5 Examples

7 Reactions of Aromatic Compounds 6 Electrophilic Aromatic Substitution Halogenation (usually Br 2 ) Nitration Sulfonation Friedel-Craft Alkylation/Acylation Nucleophilic Aromatic Substitution Reduction of Aromatic Rings Birch reduction Will not cover Catalytic hydrogenation Will not cover Miscellaneous Reaction at the side chain Will not cover + Lecture 5-6 Oxidation of phenol Will not cover

8 Electrophilic Aromatic Substitution 7 Like an alkene, benzene has clouds of pi electrons above and below its sigma bond framework. Although benzene s pi electrons are in a stable aromatic system, they are available to attack a strong electrophile to give a carbocation. (nonaromatic) Loss of aromaticity = highly endothermic nature of this first step. Regains aromaticity by loss of the proton on the tetrahedral carbon atom overall reaction: substitution of an electrophile (E + ) for a proton (H + )

9 Electrophilic Aromatic Substitution 8 Mechanism

10 EAS #1: Halogenation of Benzene 9 Benzene is not as reactive as alkenes, which react rapidly with bromine at room temperature to give addition products

11 EAS #1: Halogenation of Benzene 10 Bromination Bromine itself is not sufficiently electrophilic to react with benzene. A strong Lewis acid such as FeBr 3 catalyzes the reaction by forming a stronger electrophile with a weakened Br Br bond and a partial positive charge on one of the bromine atoms Mechanism

12 EAS #1: Halogenation of Benzene 11 Bromination

13 EAS #1: Halogenation of Benzene 12 Formation of the sigma complex is rate-limiting, and the transition state leading to it occupies the highest-energy point on the energy diagram.

14 EAS #1: Halogenation of Benzene 13 Chlorination Iodination

15 EAS #2: Nitration of Benzene 14 Benzene reacts with a mixture of nitric acid and sulfuric acid to give nitrobenzene (sulfuric acid is a catalyst) Mechanism

16 EAS #2: Nitration of Benzene 15

17 (EAS #2: Nitration of Benzene) 16 Aromatic nitro groups are easily reduced to amino groups by treatment with reducing agents such as tin, zinc, or iron in dilute acid. Nitration followed by reduction is often the best method for adding an amino group to an aromatic ring.

18 EAS #3: Sulfonation of Benzene 17 Arylsulfonic acids are synthesized by sulfonation of benzene derivatives, using fuming sulfuric acid : sulfur trioxide SO 3 in H 2 SO 4

19 EAS #3: Sulfonation of Benzene Mechanism 18

20 EAS #3: Sulfonation of Benzene 19

21 (EAS #3: Sulfonation of Benzene) 20 Desulfonation Sulfonation is reversible, and a sulfonic acid group may be removed from an aromatic ring by heating in dilute sulfuric acid. Mechanism

22 Effects of substituents on EAS 21 Rate: Toluene reacts about 25 times faster than benzene (toluene is activated toward EAS and the methyl group is an activating group). Position: Nitration of toluene gives a mixture of products, primarily those resulting from substitution at the ortho and para positions (the methyl group is an ortho, para-director).

23 Effects of substituents on EAS 22 The rate-limiting step for EAS is the first step In ortho or para substitution of toluene, the positive charge is spread over two secondary carbons and one tertiary (3 ) carbon

24 Effects of substituents on EAS 23

25 Effects of substituents on EAS 24

26 Effects of substituents on EAS 25 1) Activating, Ortho, Para-Directing Substituents 1.1) Alkyl groups:

27 Effects of substituents on EAS 26 1) Activating, Ortho, Para-Directing Substituents 1.2) Alkoxy groups: Anisole undergoes nitration about 10,000 times faster than benzene and about 400 times faster than toluene.

28 Effects of substituents on EAS 27

29 Effects of substituents on EAS 28 1) Activating, Ortho, Para-Directing Substituents 1.2) Alkoxy groups: A methoxy group is so strongly activating that anisole quickly brominates in water without a catalyst. In the presence of excess bromine, this reaction proceeds to the tribromide.

30 Effects of substituents on EAS 29 1) Activating, Ortho, Para-Directing Substituents 1.3) Amine groups: Like an alkoxyl group, a nitrogen atom with a nonbonding pair of electrons serves as a powerful activating group. Aniline undergoes a fast bromination (without a catalyst) in bromine water to give the tribromide.

31 Effects of substituents on EAS 1.3) Amine groups: 30

32 Effects of substituents on EAS 31 2) Deactivating, Meta-Directing Substituents Nitrobenzene is about 100,000 times less reactive than benzene, giving the meta isomer as the major product Nitro group is a strong deactivating group. - The positively charged nitrogen inductively withdraws electron density

33 Effects of substituents on EAS 32

34 Effects of substituents on EAS 33

35 Effects of substituents on EAS 34 2) Deactivating, Meta-Directing Substituents Carbonyl groups Examples:

36 3) Halogen Substituents: Deactivating, but Ortho, Para-Directing the halogens are strongly electronegative, withdrawing electron density from a carbon atom through the sigma bond (inductive withdrawal) 2. the halogens have nonbonding electrons that can donate electron density through pi bonding (resonance donation).

37 Effects of substituents on EAS 36 3) Halogen Substituents: Deactivating, but Ortho, Para-Directing

38 Effects of substituents on EAS 37

39 Effects of substituents on EAS 38 Effects of Multiple Substituents Rate Position

40 Effects of substituents on EAS 39 Effects of Multiple Substituents

41 Effects of substituents on EAS 40 Effects of Multiple Substituents

42 Problem #1 41

43 EAS #4: Friedel Crafts alkylation 42 Carbocations are perhaps the most important electrophiles, because this substitution forms a new carbon carbon bond. In the presence of Lewis acid catalysts such as aluminum chloride or ferric chloride alkyl halides were found to alkylate benzene to give alkylbenzenes. Example

44 EAS #4: Friedel Crafts alkylation 43 Carbocations are perhaps the most important electrophiles, because this substitution forms a new carbon carbon bond. In the presence of Lewis acid catalysts such as aluminum chloride or ferric chloride alkyl halides were found to alkylate benzene to give alkylbenzenes. Example

45 Mechanism 44

46 EAS #4: Friedel Crafts alkylation 45 Alkenes can be used as a carbocation sources Alcohols can be used as a carbocation sources

47 EAS #4: Friedel Crafts alkylation 46 Limitation 1: - Susceptible to carbocation rearrangements

48 EAS #4: Friedel Crafts alkylation 47 Limitation 2: - alkyl groups are activating substituents, the product of the Friedel Crafts alkylation is more reactive than the starting material. Multiple alkylations are hard to avoid. - The problem of overalkylation can be minimized by using a large excess of benzene.

49 Problem #2 48

50 Problem #3 49

51 EAS #5: Friedel Crafts acylation 50 An acyl group is a carbonyl group with an alkyl group attached. Acyl groups are named systematically by dropping the final -e from the alkane name and adding the -oyl suffix. An acyl chloride is an acyl group bonded to a chlorine atom.

52 EAS #5: Friedel Crafts acylation 51 In the presence of aluminum chloride, an acyl chloride reacts with benzene (or an activated benzene derivative) to give a phenyl ketone

53 EAS #5: Friedel Crafts acylation 52 Mechanism

54 EAS #5: Friedel Crafts acylation 53 Para substitution usually prevails (when the aromatic substrate has an ortho, para-directing group), possibly because the electrophile is too bulky for effective attack at the ortho position. One of the most attractive features of the Friedel Crafts acylation is the deactivation of the product toward further substitution the acylation stops after one substitution

55 The Clemmensen Reduction: Synthesis of Alkylbenzenes 54 Acylbenzene can be reduced to alkylbenzene by treatment with aqueous HCl and amalgamated zinc (zinc treated with mercury salts). This two-step sequence can synthesize many alkylbenzenes that are impossible to make by direct alkylation.

56 Electrophilic Aromatic Substitution 55 Example

57 Problem #4 56

58 Nucleophilic Aromatic Substitution 57 Nucleophiles can displace halide ions from aryl halides, particularly if there are strong electron-withdrawing groups ortho or para to the halide.

59 Nucleophilic Aromatic Substitution 58 Mechanism

60 Nucleophilic Aromatic Substitution 59

61 Problem #5 60 Propose mechanisms and show the expected products of the following reactions.

62 Homework #1 61

63 Homework #2 62

64 Homework #3 62

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