Treatment of cyclooctatetrene with potassium gives you a dianion. Classify the starting material and product as aromatic, antiaromatic or nonaromatic? 1
2
Classify cyclononatetrene and it s various ions as either aromatic, antiaromatic or nonaromatic. 3
4
Background Electrophilic Aromatic Substitution The characteristic reaction of benzene is electrophilic aromatic substitution a hydrogen atom is replaced by an electrophile. 5
Benzene does not undergo addition reactions like other unsaturated hydrocarbons, because addition would yield a product that is not aromatic. Substitution of a hydrogen keeps the aromatic ring intact. There are five main examples of electrophilic aromatic substitution. 6
7
Regardless of the electrophile used, all electrophilic aromatic substitution reactions occur by the same two-step mechanism addition of the electrophile E + to form a resonance-stabilized carbocation, followed by deprotonation with base, as shown below: 8
The first step in electrophilic aromatic substitution forms a carbocation, for which three resonance structures can be drawn. To help keep track of the location of the positive charge: 9
The energy changes in electrophilic aromatic substitution are shown below: 10
Halogenation In halogenation, benzene reacts with Cl 2 or Br 2 in the presence of a Lewis acid catalyst, such as FeCl 3 or FeBr 3, to give the aryl halides chlorobenzene or bromobenzene respectively. Analogous reactions with I 2 and F 2 are not synthetically useful because I 2 is too unreactive and F 2 reacts too violently. 11
Chlorination proceeds by a similar mechanism. 12
Nitration and Sulfonation Nitration and sulfonation introduce two different functional groups into the aromatic ring. Nitration is especially useful because the nitro group can be reduced to an NH 2 group. 13
Generation of the electrophile in nitration requires strong acid. 14
Generation of the electrophile in sulfonation requires strong acid. 15
Friedel-Crafts Alkylation and Friedel-Crafts Acylation In Friedel-Crafts alkylation, treatment of benzene with an alkyl halide and a Lewis acid (AlCl 3 ) forms an alkyl benzene. 16
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. 17
Friedel-Crafts Alkylation and Friedel-Crafts Acylation 18
19
In Friedel-Crafts acylation, the Lewis acid AlCl 3 ionizes the carbon-halogen bond of the acid chloride, thus forming a positively charged carbon electrophile called an acylium ion, which is resonance stabilized. The positively charged carbon atom of the acylium ion then goes on to react with benzene in the two step mechanism of electrophilic aromatic substitution. 20
Three additional facts about Friedel-Crafts alkylation should be kept in mind. [1] Vinyl halides and aryl halides do not react in Friedel- Crafts alkylation. 21
[2] Rearrangements can occur. These results can be explained by carbocation rearrangements. 22
23
Rearrangements can occur even when no free carbocation is formed initially. 24
[3] Other functional groups that form carbocations can also be used as starting materials. 25
Each carbocation can then go on to react with benzene to form a product of electrophilic aromatic substitution. For example: 26
Starting materials that contain both a benzene ring and an electrophile are capable of intramolecular Friedel-Crafts reactions. 27
For Monday, do problems 18.1-18.11. 28
1) Why is benzene less reactive than an alkene? The pi electrons of benzene are delocalized over 6 atoms, thus making benzene more stable and less available for electron donation. While an alkene s electrons are localized between two atoms, thus making it more nucleophillc and more reactive toward electrophiles. 29
2) Show how the other two resonance structures can be deprotonated in step two of electrophillic aromatic substitution. 30
31
3) Draw a detailed mechanism of the chlorination of benzene. Formation of Electrophile Electrophillic Additon 32
Deprotonation 33
4) Draw stepwise mechanism for the sulfonation of A. Formation of Electrophile 34
Electrophillic Addition Deprotonation 35
5) What product is formed when benzene is reacted with each of the following alkyl halides? a) b) 36
c) 37
6) What acid chloride is necessary to produce each product from benzene using a Friedal-Crafts acylation? a) b) 38
c) 39
7) Draw a stepwise mechanism for the following friedal-crafts alkylation? Formation of Electrophile 40
Electrophillic Additoon Protonation 41
8) Which of these halides are reactive in a Friedal-Crafts alkylation? Look at the carbon to which the halogen is attached and determine its hybridization. If sp 2 its unreactive, while sp 3 is reactive. 42
9) Draw a stepwise mechanism for the following reaction. Formation of Electrophile 1,2 H shift 43
Electrophillic Additon Deprotonation 44
10) Draw the product of each reaction a) b) 45
c) d) 46
11) Draw a stepwise mechanism for the intermolecular Friedal-Crafts acylation below 47
Formation of Electrophile 48
Electrophillic addition 49
Deprotonation 50
Substituted Benzenes Many substituted benzene rings undergo electrophilic aromatic substitution. Each substituent either increases or decreases the electron density in the benzene ring, and this affects the course of electrophilic aromatic substitution. 51
Considering inductive effects only, the NH 2 group withdraws electron density and CH 3 donates electron density. 52
Resonance effects are only observed with substituents containing lone pairs or π bonds. An electron-donating resonance effect is observed whenever an atom Z having a lone pair of electrons is directly bonded to a benzene ring. 53
An electron-withdrawing resonance effect is observed in substituted benzenes having the general structure C 6 H 5 -Y=Z, where Z is more electronegative than Y. Seven resonance structures can be drawn for benzaldehyde (C 6 H 5 CHO). Because three of them place a positive charge on a carbon atom of the benzene ring, the CHO group withdraws electrons from the benzene ring by a resonance effect. 54
To predict whether a substituted benzene is more or less electron rich than benzene itself, we must consider the net balance of both the inductive and resonance effects. For example, alkyl groups donate electrons by an inductive effect, but they have no resonance effect because they lack nonbonded electron pairs or π bonds. Thus, any alkyl-substituted benzene is more electron rich than benzene itself. 55
The inductive and resonance effects in compounds having the general structure C 6 H 5 -Y=Z (with Z more electronegative than Y) are both electron withdrawing. 56
These compounds represent examples of the general structural features in electron-donating and electron withdrawing substituents. 57
Electrophilic Aromatic Substitution and Substituted Benzenes. Electrophilic aromatic substitution is a general reaction of all aromatic compounds, including polycyclic aromatic hydrocarbons, heterocycles, and substituted benzene derivatives. A substituent affects two aspects of the electrophilic aromatic substitution reaction: 1. The rate of the reaction A substituted benzene reacts faster or slower than benzene itself. 2. The orientation The new group is located either ortho, meta, or para to the existing substituent. The identity of the first substituent determines the position of the second incoming substituent. 58
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. 59
Consider nitrobenzene It reacts more slowly than benzene in all substitution reactions. The electron-withdrawing NO 2 group deactivates the benzene ring to electrophilic attack. The meta product predominates. The NO 2 group is called a meta director. 60
All substituents can be divided into three general types: 61
62
Keep in mind that halogens are in a class by themselves. Also note that: 63
To understand how substituents activate or deactivate the ring, we must consider the first step in electrophilic aromatic substitution. The first step involves addition of the electrophile (E + ) to form a resonance stabilized carbocation. The Hammond postulate makes it possible to predict the relative rate of the reaction by looking at the stability of the carbocation intermediate. 64
The principles of inductive effects and resonance effects can now be used to predict carbocation stability. 65
The energy diagrams below illustrate the effect of electronwithdrawing and electron-donating groups on the transition state energy of the rate-determining step. Figure 18.6 Energy diagrams comparing the rate of electrophilic substitution of substituted benzenes 66
67
Orientation Effects in Substituted Benzenes There are two general types of ortho, para directors and one general type of meta director. All ortho, para directors are R groups or have a nonbonded electron pair on the atom bonded to the benzene ring. All meta directors have a full or partial positive charge on the atom bonded to the benzene ring. 68
To evaluate the effects of a given substituent, we can use the following stepwise procedure: 69
A CH 3 group directs electrophilic attack ortho and para to itself because an electron-donating inductive effect stabilizes the carbocation intermediate. 70
An NH 2 group directs electrophilic attack ortho and para to itself because the carbocation intermediate has additional resonance stabilization. 71
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. 72
Figure 18.7 The reactivity and directing effects of common substituted benezenes 73
For Wednesday: Draw out stepwise mechanisms for 10b and c. 18.12-18.20 as well. 74
10b) Formation of Electrophile
Electrophillic Addition Deprotonation
10c) Formation of Electrophile
Electrophillic Addition
Deprotonation
12) Identify each group as having an electron donating or electron withdrawing inductive effect. a) CH 3 CH 2 CH 2 CH 2 - Electron donating b) Br- Electron withdrawing c) CH 3 CH 2 O- Electron withdrawing
13) Draw the resonance structures and use them to determine whether there is an electron donating or withdrawing resonance effect. a) Negative charge on ring, electron donating effect
b) Positive charge, electron withdrawing
14) Identify as electron donating or electron withdrawing. a) Lone pair on oxygen, electron donating b) Halogen, electron withdrawing c) Alkyl group, electron donating
15) Predict the products. a) b)
c)
16) Predict the products when reacted with HNO3 and H2SO4. Also state whether the reactant is more or less reactive than benzene. a) Less b) Less
c) More d) Less
d) More
17) Label each compound as less or more reactive than benzene. a) more b) more
c) less d) less
18) Rank each group in order of increasing reactivity. a) b) 2 3 1 2 3 1
19) Draw the resonance structures of ortho attack by NO 2. Label any resonance structure that is especially stable or unstable. a) Most stable
b) Most stable
c) Vey unstable
20) Show why chlorine is an ortho para director. Especially stable, every atom has an octet
Especially stable, every atom has an octet
Limitations in Electrophilic Aromatic Substitutions Benzene rings activated by strong electron-donating groups OH, NH 2, and their derivatives (OR, NHR, and NR 2 ) undergo polyhalogenation when treated with X 2 and FeX 3. 98
A benzene ring deactivated by strong electron-withdrawing groups (i.e., any of the meta directors) is not electron rich enough to undergo Friedel-Crafts reactions. Friedel-Crafts reactions also do not occur with NH 2 groups because the complex that forms between the NH 2 group and the AlCl 3 catalyst deactivates the ring towards Friedel-Crafts reactions. 99
Treatment of benzene with an alkyl halide and AlCl 3 places an electron-donor R group on the ring. Since R groups activate the ring, the alkylated product (C 6 H 5 R) is now more reactive than benzene itself towards further substitution, and it reacts again with RCl to give products of polyalkylation. Polysubstitution does not occur with Friedel-Crafts acylation. 100
Disubstituted Benzenes 1. When the directing effects of two groups reinforce, the new substituent is located on the position directed by both groups. 101
2. If the directing effects of two groups oppose each other, the more powerful activator wins out. 102
3. No substitution occurs between two meta substituents because of crowding. 103
Synthesis of Benzene Derivatives In a disubstituted benzene, the directing effects indicate which substituent must be added to the ring first. Let us consider the consequences of bromination first followed by nitration, and nitration first, followed by bromination. 104
Pathway I, in which bromination precedes nitration, yields the desired product. Pathway II yields the undesired meta isomer. 105
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. As a result, alkyl benzenes undergo selective bromination at the weak benzylic C H bond under radical conditions to form the benzylic halide. 106
107
Note that alkyl benzenes undergo two different reactions depending on the reaction conditions: 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. 108
Oxidation and Reduction of Substituted Benzenes Arenes containing at least one benzylic C H bond are oxidized with KMnO 4 to benzoic acid. Substrates with more than one alkyl group are oxidized to dicarboxylic acids. Compounds without a benzylic hydrogen are inert to oxidation. 109
Ketones formed as products of Friedel-Crafts acylation can be reduced to alkyl benzenes by two different methods: 1. The Clemmensen reduction uses zinc and mercury in the presence of strong acid. 2. The Wolff-Kishner reduction uses hydrazine (NH 2 NH 2 ) and strong base (KOH). 110
We now know two different ways to introduce an alkyl group on a benzene ring: 1. A one-step method using Friedel-Crafts alkylation. 2. A two-step method using Friedel-Crafts acylation to form a ketone, followed by reduction. Figure 18.8 Two methods to prepare an alkyl benzene 111
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. 112
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. 113
For next time, 18.21-18.30