Synthesis Using Aromatic Materials
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1 Chapter 10 Synthesis Using Aromatic Materials ELECTROPHILIC AROMATIC SUBSTITUTION AND DIRECTED ORTHO METALATION Copyright 2018 by Nelson Education Limited 1
2 10.2 p Bonds Acting as Nucleophiles Copyright 2018 by Nelson Education Limited 2
3 As described in Chapter 8, the p bond of an alkene can act as a nucleophile. Example: Copyright 2018 by Nelson Education Limited 3
4 When benzene is mixed with bromine, no reaction happens. Addition of a catalyst can induce reactivity. Copyright 2018 by Nelson Education Limited 4
5 10.3 Electrophilic Aromatic Substitution Copyright 2018 by Nelson Education Limited 5
6 Electrophilic aromatic substitution (S E Ar) reactions proceed by a general two-step mechanism: 1. addition of an electrophile (Chapter 8) 2. elimination (explained in detail in Chapter 12) Copyright 2018 by Nelson Education Limited 6
7 Second step restores aromaticity. This drives reaction toward substitution instead of nucleophilic attack. Copyright 2018 by Nelson Education Limited 7
8 Copyright 2018 by Nelson Education Limited 8
9 10.4 Types of Electrophiles Used in Electrophilic Aromatic Substitution Copyright 2018 by Nelson Education Limited 9
10 Halogenation Aromatic rings can be halogenated with bromine, chlorine, or iodine using a Lewis acid catalyst. Copyright 2018 by Nelson Education Limited 10
11 The activated electrophilic halogen reacts with the aromatic ring to form the areniumintermediate. One of the bromines of FeBr 4 acts as a base to remove a proton. Copyright 2018 by Nelson Education Limited 11
12 Nitration A hydrogen atom is replaced with a nitro group ( NO 2 ) via S E Ar. The electrophile in this reaction is the nitronium ion (NO 2+ ). It is generated by dehydration of HNO 3 with H 2 SO 4. Copyright 2018 by Nelson Education Limited 12
13 Nitration mechanism: Provides a route to aromatic amines (details in Ch. 19) Copyright 2018 by Nelson Education Limited 13
14 Sulfonation A hydrogen on the aromatic ring is replaced by a sulfonic acid group ( SO 3 H). The active electrophile is likely SO 3 H +. Copyright 2018 by Nelson Education Limited 14
15 Formation of SO 3 H + : Copyright 2018 by Nelson Education Limited 15
16 Sulfonation mechanism: Sulfonation can be reversed using a strong acid (e.g., H 2 SO 4 ) in water. Copyright 2018 by Nelson Education Limited 16
17 Friedel Crafts alkylation This is a type of S E Ar for adding alkyl groups to aromatic rings. AlCl 3 = Lewis acid Copyright 2018 by Nelson Education Limited 17
18 Friedel Crafts alkylation mechanism: Copyright 2018 by Nelson Education Limited 18
19 Limitations of the Friedel Crafts alkylation 1. Carbocations are not reactive enough to couple with weakly nucleophilic aromatic rings. Copyright 2018 by Nelson Education Limited 19
20 2. Because alkyl groups are electron donating, the product of a Friedel Crafts alkylation is usually more nucleophilic (more reactive) than the starting reactant. Over-alkylation is common for Friedel Crafts alkylation. Copyright 2018 by Nelson Education Limited 20
21 3. Carbocations can rearrange and lead to product mixtures. Carbocation rearrangements (recall Chapter 8): Copyright 2018 by Nelson Education Limited 21
22 Two competing mechanisms result in the formation of the two products: Copyright 2018 by Nelson Education Limited 22
23 Friedel Crafts acylation Very similar to Friedel Crafts alkylation, but adds an acyl group instead. This involves the formation of an acylium ion. Copyright 2018 by Nelson Education Limited 23
24 The product of Friedel Crafts acylation is less reactive than reactant because acyl group is electron withdrawing. Therefore, single acylations are highly feasible. Can provide route to singly-alkylated product via subsequent reduction: Copyright 2018 by Nelson Education Limited 24
25 Friedel Crafts acylation can use anhydride instead of acid chloride: It cannot produce aromatic aldehydes. Copyright 2018 by Nelson Education Limited 25
26 Gatterman Koch reaction can yield aromatic aldehydes: Copyright 2018 by Nelson Education Limited 26
27 10.5 Aromatic Nomenclature and Multiple Substituents Copyright 2018 by Nelson Education Limited 27
28 Many aromatic compounds have trivial names: On aromatic compounds with multiple substituents, relative positions designated by numbers or Greek words: Copyright 2018 by Nelson Education Limited 28
29 10.6 Directing Groups in Electrophilic Aromatic Substitution Copyright 2018 by Nelson Education Limited 29
30 Existing substituents affect the outcome of S E Ar reactions with respect to the following: rate of reaction regioselectivity Functional groups are classified according to particular properties: 1. Activating vs. deactivating Copyright 2018 by Nelson Education Limited 30
31 2. Ortho/para vs. meta directors Ortho/para directors: Favour the formation of ortho and para regioisomers Meta directors: Favour the production of the meta regioisomer Copyright 2018 by Nelson Education Limited 31
32 Copyright 2018 by Nelson Education Limited 32
33 Ortho/para directing groups Electron-donating groups tend to direct regioselectivity toward ortho and/or para products Strong ortho/para directors Copyright 2018 by Nelson Education Limited 33
34 Copyright 2018 by Nelson Education Limited 34
35 Substituent lone pairs stabilize areniumion for ortho/para substitution: Copyright 2018 by Nelson Education Limited 35
36 If initial addition occurs at meta position, lone pair cannot stabilize arenium. Copyright 2018 by Nelson Education Limited 36
37 Moderate ortho/para directors In some cases, lone pair on substituent may already be engaged in delocalization. Copyright 2018 by Nelson Education Limited 37
38 Weak ortho/para directors 1. Alkyl groups Ø have weak electron-donating character (hyperconjugation) 2. Aromatic rings Copyright 2018 by Nelson Education Limited 38
39 Deactivating ortho/para directors Halogens deactivate due to high electronegativity poor orbital overlap between 2p ring orbitals and lone pair p orbitals for Cl, Br, and I (3p, 4p, and 5p respectively) They act as ortho/para directors due to weak resonance effect. Copyright 2018 by Nelson Education Limited 39
40 Meta directing groups Electron-withdrawing groups typically: deactivate an aromatic ring toward S E Ar favour meta regioisomer Copyright 2018 by Nelson Education Limited 40
41 Copyright 2018 by Nelson Education Limited 41
42 Moderately deactivating meta directors These groups contain polar p bonds connected to electronegative atoms and conjugated to the ring Strongly deactivating meta directors Substituents that are electron withdrawing due to strong inductive effects. Copyright 2018 by Nelson Education Limited 42
43 Modifying reactivity in electrophilic aromatic substitutions Substituents can be temporarily modified to control reactivity. Copyright 2018 by Nelson Education Limited 43
44 In some cases, incidental modification can lead to unexpected products. Copyright 2018 by Nelson Education Limited 44
45 Strength of activation on polysubstituted benzenes When multiple substituents are present, the collective effects of directing groups must be considered. Copyright 2018 by Nelson Education Limited 45
46 10.7 Electrophilic Aromatic Substitution of Polycyclic and Heterocyclic Aromatic Compounds Copyright 2018 by Nelson Education Limited 46
47 Reactivity of polycyclic aromatic compounds Regioselectivity is controlled by stability of arenium ion. These two products result from the most stable arenium ions. Copyright 2018 by Nelson Education Limited 47
48 Mechanism for formation of 1-nitronaphthalene: Copyright 2018 by Nelson Education Limited 48
49 Mechanism for formation of 2-nitronaphthalene: Copyright 2018 by Nelson Education Limited 49
50 Reactivity of substituted polyaromatics Directing group effects still apply. Example: Because the methyl group is electron donating, the ring to which it is attached is more nucleophilic. Copyright 2018 by Nelson Education Limited 50
51 Reactivity of heterocyclic compounds The presence of heteroatoms can have a strong influence on reactivity and regioselectivity. Examples: Copyright 2018 by Nelson Education Limited 51
52 Mechanism for furan nitration: Copyright 2018 by Nelson Education Limited 52
53 Indole preferentially reacts via path that preserves aromaticity: Copyright 2018 by Nelson Education Limited 53
54 Pyridine is an electron-poor aromatic ring. The electrophilic nitrogen atom destabilizes the positive charge in the addition intermediate. Copyright 2018 by Nelson Education Limited 54
55 10.8 Directed Ortho Metalation as an Alternative to Electrophilic Aromatic Substitution Copyright 2018 by Nelson Education Limited 55
56 Directed ortho metalation (DOM): an alternate route to aromatic substitution steps occur in opposite order relative to S E Ar DMG = directed metalation group: a substituent that favours deprotonation at the adjacent ortho position Copyright 2018 by Nelson Education Limited 56
57 A closer look: A very strong base must be used in the first step. Recall: BuLi = n-butyllithuim TMEDA (tetramethylethylenediamine) is used to enhance basicity of BuLi (complexes with Li). DMG stabilizes aryllithium (ArLi). Copyright 2018 by Nelson Education Limited 57
58 Common directed metalation groups Many DMGs are carbonyl-based functional groups: Others are derived from heteroatom substituents: Copyright 2018 by Nelson Education Limited 58
59 Once an aryllithium intermediate has formed, a variety of electrophiles can be added. Copyright 2018 by Nelson Education Limited 59
60 10.9 Retrosynthetic Analysis in Aromatic Synthesis Copyright 2018 by Nelson Education Limited 60
61 Synthesis: assembling new substances by reacting different molecules to combine in a controlled manner Retrosynthesis: technique for planning synthesis in which the target is analyzed in terms of what it can be made from Disconnection: a retrosynthetic step, an imaginary reverse reaction Copyright 2018 by Nelson Education Limited 61
62 Retrosyntheses for p-nitro isopropylbenzene: Forward synthesis: Copyright 2018 by Nelson Education Limited 62
63 Using synthons in synthesis Synthon: an imaginary component that captures the overall reactivity pattern of a series of compounds Copyright 2018 by Nelson Education Limited 63
64 10.10 Patterns in Electrophilic Aromatic Substitution Reactions General reaction: Copyright 2018 by Nelson Education Limited 64
65 Copyright 2018 by Nelson Education Limited 65
66 Chapter summary Aromatic rings can be modified via S E Ar. These reactions all follow the same general mechanism. Friedel Crafts alkylation and acylation can be used to introduce alkyl and acyl groups, respectively. Substituents can be activating or deactivating; ortho/para directing or meta directing. Retrosynthesis is commonly used in planning chemical synthesis. Copyright 2018 by Nelson Education Limited 66
Organic Chemistry. Second Edition. Chapter 19 Aromatic Substitution Reactions. David Klein. Klein, Organic Chemistry 2e
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