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Treatment of cyclooctatetrene with potassium gives you a dianion. Classify the starting material and product as aromati PowerPoint Presentation
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Treatment of cyclooctatetrene with potassium gives you a dianion. Classify the starting material and product as aromatic, antiaromatic or nonaromatic? . Classify cyclononatetrene and it’s various ions as either aromatic, antiaromatic or nonaromatic. Electrophilic Aromatic Substitution.

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slide1

Treatment of cyclooctatetrene with potassium gives you a dianion. Classify the starting material and product as aromatic, antiaromatic or nonaromatic?

slide5

Electrophilic Aromatic Substitution

Background

  • The characteristic reaction of benzene is electrophilic aromatic substitution—a hydrogen atom is replaced by an electrophile.
slide6

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.
slide8

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:

slide9

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:

slide11

Halogenation

  • In halogenation, benzene reacts with Cl2 or Br2 in the presence of a Lewis acid catalyst, such as FeCl3 or FeBr3, to give the aryl halides chlorobenzene or bromobenzene respectively.
  • Analogous reactions with I2 and F2 are not synthetically useful because I2 is too unreactive and F2 reacts too violently.
slide13

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 NH2 group.
slide16

Friedel-Crafts Alkylation and Friedel-Crafts Acylation

  • In Friedel-Crafts alkylation, treatment of benzene with an alkyl halide and a Lewis acid (AlCl3) forms an alkyl benzene.
slide17

In Friedel-Crafts acylation, a benzene ring is treated with an acid chloride (RCOCl) and AlCl3 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.
slide20

In Friedel-Crafts acylation, the Lewis acid AlCl3 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.
slide21

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.

slide22

[2] Rearrangements can occur.

These results can be explained by carbocation rearrangements.

slide26

Each carbocation can then go on to react with benzene to form a product of electrophilic aromatic substitution. For example:

slide27

Starting materials that contain both a benzene ring and an electrophile are capable of intramolecular Friedel-Crafts reactions.

slide29

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.

slide30

2) Show how the other two resonance structures can be deprotonated in step two of electrophillic aromatic substitution.

slide32

3) Draw a detailed mechanism of the chlorination of benzene.

Formation of Electrophile

Electrophillic Additon

slide38

6) What acid chloride is necessary to produce each product from benzene using a Friedal-Crafts acylation?

a)

b)

slide40

7) Draw a stepwise mechanism for the following friedal-Crafts alkylation?

Formation of Electrophile

slide42

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 sp2 its unreactive, while sp3 is reactive.

slide46

c)

d)

slide51

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.

slide52

Considering inductive effects only, the NH2 group withdraws electron density and CH3 donates electron density.

slide53

Resonance effects are only observed with substituents containing lone pairs or  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.

slide54

An electron-withdrawing resonance effect is observed in substituted benzenes having the general structure C6H5-Y=Z, where Z is more electronegative than Y.

  • Seven resonance structures can be drawn for benzaldehyde (C6H5CHO). 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.
slide55

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  bonds.
  • Thus, any alkyl-substituted benzene is more electron rich than benzene itself.
slide56

The inductive and resonance effects in compounds having the general structure C6H5-Y=Z (with Z more electronegative than Y) are both electron withdrawing.

slide57

These compounds represent examples of the general structural features in electron-donating and electron withdrawing substituents.

slide58

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:
    • The rate of the reaction—A substituted benzene reacts faster or slower than benzene itself.
    • 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.
slide59

Consider toluene—Toluene reacts faster than benzene in all substitution reactions.

  • The electron-donating CH3 group activates the benzene ring to electrophilic attack.
  • Ortho and para products predominate.
  • The CH3 group is called an ortho, para director.
slide60

Consider nitrobenzene—It reacts more slowly than benzene in all substitution reactions.

  • The electron-withdrawing NO2 group deactivates the benzene ring to electrophilic attack.
  • The meta product predominates.
  • The NO2 group is called a meta director.
slide64

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.
slide65

The principles of inductive effects and resonance effects can now be used to predict carbocation stability.

slide66

The energy diagrams below illustrate the effect of electron-withdrawing 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

slide68

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.
slide70

A CH3 group directs electrophilic attack ortho and para to itself because an electron-donating inductive effect stabilizes the carbocation intermediate.

slide71

An NH2 group directs electrophilic attack ortho and para to itself because the carbocation intermediate has additional resonance stabilization.

slide72

With the NO2 group (and all meta directors) meta attack occurs because attack at the ortho and para position gives a destabilized carbocation intermediate.

slide73

Figure 18.7

The reactivity and directing

effects of common substituted

benezenes

slide74

For Wednesday:

Draw out stepwise mechanisms for 10b and c.

18.12-18.20 as well.

slide75

10b)

Formation of Electrophile

slide77

10c)

Formation of Electrophile

slide80

Identify each group as having an electron donating or electron withdrawing inductive effect.

  • a) CH3CH2CH2CH2-

Electron donating

b) Br-

Electron withdrawing

c) CH3CH2O-

Electron withdrawing

slide81

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

slide82

b)

Positive charge, electron withdrawing

slide83

Identify as electron donating or electron withdrawing.

  • a)

Lone pair on oxygen, electron donating

Halogen, electron withdrawing

b)

c)

Alkyl group, electron donating

slide86

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

slide87

c)

More

d)

Less

slide88

d)

More

slide90

c)

less

d)

less

slide92

19) Draw the resonance structures of ortho attack by NO2. Label any resonance structure that is especially stable or unstable.

a)

Most stable

slide93

b)

Most stable

slide94

c)

Vey unstable

slide95

20) Show why chlorine is an ortho para director.

Especially stable, every atom has an octet

slide98

Limitations in Electrophilic Aromatic Substitutions

  • Benzene rings activated by strong electron-donating groups—OH, NH2, and their derivatives (OR, NHR, and NR2)—undergo polyhalogenation when treated with X2 and FeX3.
slide99

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 NH2 groups because the complex that forms between the NH2 group and the AlCl3 catalyst deactivates the ring towards Friedel-Crafts reactions.
slide100

Treatment of benzene with an alkyl halide and AlCl3 places an electron-donor R group on the ring. Since R groups activate the ring, the alkylated product (C6H5R) 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.
slide101

Disubstituted Benzenes

  • When the directing effects of two groups reinforce, the new substituent is located on the position directed by both groups.
slide102

2. If the directing effects of two groups oppose each other, the more powerful activator “wins out.”

slide104

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.

slide105

Pathway I, in which bromination precedes nitration, yields the desired product. Pathway II yields the undesired meta isomer.

slide106

Halogenation of Alkyl Benzenes

Benzylic C—H bonds are weaker than most other sp3 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.

slide108

Note that alkyl benzenes undergo two different reactions depending on the reaction conditions:

  • With Br2 and FeBr3 (ionic conditions), electrophilic aromatic substitution occurs, resulting in replacement of H by Br on the aromatic ring to form ortho and para isomers.
  • With Br2 and light or heat (radical conditions), substitution of H by Br occurs at the benzylic carbon of the alkyl group.
slide109

Oxidation and Reduction of Substituted Benzenes

Arenes containing at least one benzylic C—H bond are oxidized with KMnO4 to benzoic acid.

Substrates with more than one alkyl group are oxidized to dicarboxylic acids. Compounds without a benzylic hydrogen are inert to oxidation.

slide110

Ketones formed as products of Friedel-Crafts acylation can be reduced to alkyl benzenes by two different methods:

  • The Clemmensen reduction—uses zinc and mercury in the presence of strong acid.
  • The Wolff-Kishner reduction—uses hydrazine (NH2NH2) and strong base (KOH).
slide111

We now know two different ways to introduce an alkyl group on a benzene ring:

  • A one-step method using Friedel-Crafts alkylation.
  • 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

slide112

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.

slide113

A nitro group (NO2) that has been introduced on a benzene ring by nitration with strong acid can readily be reduced to an amino group (NH2) under a variety of conditions.