AROMATICITY. Chapter 8 REACTIONS OF BENZENE ORGANIC CHEMISTRY, 2 ND EDITION PAULA YURKANIS BRUICE

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1 ORGANIC CHEMISTRY, 2 ND EDITION PAULA YURKANIS BRUICE Chapter 8 AROMATICITY REACTIONS OF BENZENE RAED M. AL-ZOUBI, ASSISTANT PROFESSOR OF CHEMISTRY DEPARTMENT OF CHEMISTRY - N 4 L 0 JORDAN UNIVERSITY OF SCIENCE AND TECHNOLOGY

2 Nomenclature 1. Monosubstituted Benzenes Some are named by attaching benzene after the name of the substituent: 2

3 A benzene substituent is called phenyl. A benzene substitutuent with a methylene group is called benzyl. 3

4 Benzene is either the base name or the substituent (phenyl): Aryl group (Ar) is the general term for either an unsubstituted or a substituted phenyl group 4

5 2. Disubstituted Benzenes In disubstituted benzenes, the relative positions of the two substituents are indicated by numbers or by prefixes: 5

6 The two substituents are listed in alphabetical order: 6

7 Common names are preferred in naming certain substituted benzenes, e.g., toluene, aniline, phenol. Do not deconstruct the common name; e.g., do not change toluene to methylbenzene. The substituent that is part of the common name is position 1, but do not label as such in the chemical name. 7

8 Reactions of Benzene 8

9 Aromatic compounds such as benzene undergo electrophilic aromatic substitution reactions (EAS): 9

10 Benzene is a nucleophile that reacts with an electrophile An electrophilic substitution yields an aromatic product, which is significantly more stable than the addition reaction 10

11 Reaction Coordinate Diagrams for the Two Benzene Reactions 11

12 There are five common electrophilic aromatic substitution reactions: 1. Halogenation 2. Nitration 3. Sulfonation 4. Friedel Crafts acylation 5. Friedel Crafts alkylation 12

13 General Mechanism for Electrophilic Aromatic Substitution of Benzene 13

14 1. Halogenation of Benzene 14

15 Lewis acid weakens the Br Br (or Cl Cl) bond, which makes the halogen a better electrophile: 15

16 Mechanism for bromination: B: Bromide or Benzene The catalyst is regenerated: 16

17 17

18 2. Nitration of Benzene 18

19 Nitronium ion formation: Electrophilic aromatic substitution: 19

20 3. Sulfonation of Benzene 20

21 ulfonic acid is a strong acid: 21

22 Reaction coordinate diagram for electrophilic aromatic substitution: 22

23 4. Friedel Crafts Acylation Reactions The electrophile is an acylium ion: 23

24 Mechanism for Friedel Crafts acylation: Must be carried out with more than one equivalent of AlCl 3 : 24

25 5. Friedel Crafts Alkylation of Benzene 25

26 Mechanism for Friedel Crafts alkylation: 26

27 Substitutions with double and triple bonds can undergo catalytic hydrogenation (Sections 4.11 and 6.9) 27

28 Oxidation of an alkyl group bonded to a benzene ring Provided that a hydrogen is bonded to the benzylic carbon, 28

29 Summary of Electrophilic Aromatic Substitution Reactions 29

30 Summary of Friedel Crafts Reactions 30

31 Substituted benzenes undergo the five electrophilic aromatic substitution reactions 31

32 The slow step of an electrophilic aromatic substitution reaction is the formation of the carbocation intermediate: Electron-donating substituents increase the rate of substitution reactions by stabilizing the carbocation intermediate. Electron-withdrawing substituents decrease the rate of substitution reactions by destabilizing the carbocation intermediate. 32

33 Inductive Electron Withdrawal Electron Donation by Hyperconjugation 33

34 Resonance Electron Donation and Withdrawal Substituents such as NH 2, OH, OR, and Cl donate electrons by resonance, but they also withdraw electrons inductively: 34

35 Substituents such as C=O, CΞN, SO 3 H, and NO 2 withdraw electrons by resonance: 35

36 Electron-donating substituents increase the reactivity of the benzene ring toward electrophilic aromatic substitution Electron-withdrawing substituents decrease the reactivity of the benzene ring toward electrophilic aromatic substitution 36

37 Electron-Donating Substituents Electron donation into the benzene ring by resonance is more significant than inductive electron withdrawal from the ring: 37

38 Resonance donation into the benzene ring competes with resonance donation into the carbonyl Inductive withdrawal into the benzene ring also occurs Overall, these substituents weakly release electrons 38

39 These substituents are less effective in donating electrons into the ring because 39

40 Alkyl, aryl, and CH=CHR groups are weakly activating substituents because they are slightly more electron donating than they are electron withdrawing: 40

41 These substituents donate into the ring by resonance and withdraw electrons from the ring inductively: They withdraw electrons inductively more strongly than they donate electrons by resonance 41

42 These substituents withdraw electrons both inductively and by resonance: 42

43 These substituents are powerful electron-withdrawing groups: These substituents withdraw electrons both inductively and by resonance Except for the ammonium ions. 43

44 The substituent already attached to the benzene ring determines the location of the new substituent: 44

45 All activating substituents are ortho para directors: 45

46 The weakly deactivating halogens are ortho para directors: 46

47 All substituents that are more deactivating than halogens are meta directors: 47

48 An ortho,para-directing substituent: 48

49 An ortho,para-directing substituent: 49

50 An meta-directing substituent: 50

51 The ortho para product ratio decreases with an increase in the size of the substituents: 51

52 52

53 Methoxy and hydroxy substituents are so strongly activating that halogenation is carried out without a Lewis acid: The presence of Lewis acid and excess bromine generates the tribromo derivative: 53

54 A benzene ring with a meta director cannot undergo a Friedel Crafts reaction: 54

55 Aniline and N-substituted anilines do not undergo Friedel Crafts reaction: Phenol and anisole undergo Friedel Crafts reactions at the ortho and para positions Aniline cannot be nitrated, because it is oxidized by nitric acid 55

56 In designing a disubstituted benzene, consider the order of substitution: 56

57 The Friedel Crafts acylation must be carried out first, because the nitro group is strongly deactivating: 57

58 In the synthesis of para-chlorobenzoic acid from toluene, the methyl group is oxidized after hlorination: In the synthesis of meta-chlorobenzoic acid, the methyl group is oxidized before chlorination: 58

59 The Effect of Substituents on pk a Electron-withdrawing groups stabilize a base and therefore increase the strength of its conjugate acid Electron-donating groups destabilize a base and thus decrease the strength of its conjugate acid 59

60 The more electronic deficient a substituent on phenol, the stronger the acid: To understand the relative pk a values, consider the delocalization of the phenolate anion (stars show anion distribution): Unstable More stable anion = lower pk a Stable: through resonance of anion into nitro 60

61 The more electronic deficient a substituent on benzoic acid, the stronger the acid: Substituent effect on pk a is minimal in benzoic acids because only inductive electronic effects are present: Why? Because the benzene ring is cross conjugated with the carboxylate anion 61

62 The more electronic deficient a substituent on a protonated aniline, the stronger the acid: To understand the relative pk a values, consider the delocalization of the aniline lone pair of the conjugate base (stars show anion distribution): More stable lone pair = lower pk a Unstable Stable: through resonance of lone pair into nitro 62

63 Q: Which of the following statements regarding electrophilic aromatic substitution is incorrect? A. Halogenation: Br, Cl, or I substitutes for a hydrogen. B. Nitration: -NH 2 substitutes for a hydrogen. C. Sulfonation: -SO 3 H substitutes for a hydrogen. D. Friedel Crafts acylation: RC=O (acyl) substitutes for a hydrogen. E. Friedel Crafts alkylation: R (alkyl) substitutes for a hydrogen.

64 Q: Which combination of reagents is best suited to prepare n-propylbenzene? A. Benzene, CH 3 CH 2 CH 2 Cl, AlCl 3 catalyst B. Benzene, CH 3 CH=CH 2, HF catalyst C. Benzene, 1. CH 3 CH 2 COCl, AlCl 3 cat. 2. Zn(Hg) HCl D. Benzene, CH 3 CH 2 CH 2 OH, H 2 SO 4 E. Chlorobenzene, CH 3 CH=CH 2, AlCl 3 catalyst