reaction of aromatic chapter18-ppt

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Electrophilic Aromatic Substitution

OBJECTIVES

• Understand the reaction mechanisms of Benzene

• Predict the product of each reaction• Know the different catalyst used in each

reaction

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

Background

• The loosely held p electrons make the benzene ring electron rich.

so, it reacts with electrophiles.• Reactions that keep the aromatic ring intact are favored.

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The General Mechanism

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• The energy changes in electrophilic aromatic substitution are shown below:

The General Mechanism

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Five examples of electrophilic aromatic substitution

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• 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.

Halogenation

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Mechanism of Halogenation

• Chlorination proceeds by a similar mechanism.

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Nitration and Sulfonation

• Generation of the electrophile in both nitration and sulfonation requires strong acid.

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Nitration and Sulfonation

Formation of the electrophile

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

Friedel-Crafts Alkylation and Friedel-Crafts Acylation

A. General Features

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

• The transfer of an acyl group from one atom to another is an acylation.

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Friedel-Crafts Alkylation and Friedel-Crafts Acylation

B. Mechanism : generation of carbocation!

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Other functional groups that form carbocations can also be used as starting materials.

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D. Intramolecular Friedel-Crafts Reactions

Friedel-Crafts Acylation: No rearrangement

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Electrophilic Aromatic Substitution of Substituted Benzenes.

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.

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Electrophilic Aromatic Substitution of Substituted Benzenes.

• 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.

How does the substitution affects the outcome?

Further stabilizing (destabilizing) the cationic intermediate

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

Substituted Benzenes

Inductive Effects

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

Electron-donating resonance effect : An atom Z having a lone pair of electrons is directly bonded to a benzene ring

Resonance Effects


stabilization

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• Electron-withdrawing resonance effect : the general structure C6H5-Y=Z, where Z is more electronegative than Y.

• 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.

destabilization

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• 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.

• Any alkyl-substituted benzene: more electron rich than benzene itself.

Considering Both Inductive and Resonance Effects


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• C6H5-Y=Z (with Z more electronegative than Y): Both the inductive and resonance effects are electron withdrawing.

• C6H5-Z : Depends on the net balance of two opposing effects

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• Examples of the general structural features in electron-donating and electron withdrawing substituents.

Let’s try this!

1. Draw the three products that might form on reaction of toluene ( methylbenzene) with Br2.

2. Show the mechanism of the reaction benzene with nitric acid and sulfuric acid yield nitrobenzene.

3. Chlorination of o-xylene ( o-dimethylbenzene) yields a mixture of two products, but chlorination of p-xylene yields a single product. Explain.

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Let’s try this!

4. How many products might be formed on

chlorination of m-xylene?

5. What products would you expect t obtain from the reaction of the following to obtain from the reaction of the following compounds with chloroethane and AlCl3?

A. Benzene B. P-Xylene

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Let’s try this!

6. What products would you expect to obtain from the reaction of benzene with the following reagents?

a. (CH3)3CCl, AlCl3

b. CH3CH2COCl, AlCl3

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• 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.

Toluene

Substitution Reactions of Substituted Benzenes

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• 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.

Nitrobenzene

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All substituents can be divided into three general types:

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• Keep in mind that halogens are in a class by themselves.• Also note that:

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• 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.

Why Substituents Activate or Deactivate a Benzene Ring

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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.

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To evaluate the effects of a given substituent, we can use the following stepwise procedure:

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A. The CH3 Group – An Ortho, para Director

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


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

B. The NH2 Group – An Ortho, para Director

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

C. The NO2 Group – A meta Director

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Limitations on Electrophilic Substitution Reactions with Substituted Benzenes

• 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.

A. Halogenation of Activated Benzenes

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• 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.

B. Limitations in Friedel-Crafts Reactions

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• 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.

No Further Reaction

Reactivity control !!

• Polysubstitution does not occur with Friedel-Crafts acylation.

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Disubstituted Benzenes

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

Synergistic effect

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

3. No substitution occurs between two meta substituents because of crowding.

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Synthesis of Benzene Derivatives

For the synthesis of 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.

Synthetic strategy !!

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- Pathway I, in which bromination precedes nitration, yields the desired product.

- Pathway II yields the undesired meta isomer.

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Halogenation of Alkyl Benzenes : radical reaction

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.

Other reactions on aromatic compounds

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• 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.

Halogenation of Alkyl Benzenes

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Halogenation of Alkyl Benzenes

mechanism

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Oxidation and Reduction of Substituted Benzenes

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

A. Oxidation of Alkyl Benzenes

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

Effectively turns into Friedel-Craft alkylation product

1. The Clemmensen reduction—uses zinc and mercury in the presence of strong acid.

B. Reduction of Aryl Ketones to Alkyl Benzenes

2. The Wolff-Kishner reduction—uses hydrazine (NH2NH2) and strong base (KOH).

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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.

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Control of the reactivity

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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.

C. Reduction of Nitro Groups

Working backwards

Synthetic analysis

SEATWORK

1. Draw and name the three products that might form on reaction of toluene

( methylbenzene) with Br2.

2. Show the mechanism of the reactions of benzene with nitric acid and sulfuric acid to yield nitrobenzene.

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SEATWORK

3. Chlorination of o-xylene ( o-dimethylbenzene) yields a mixture of two products, but chlorination of p-xylene yields a single products. Explain.

4. How many products might be formed on chlorination of m-xylene?

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SEATWORK

5. What products would you expect to obtain from the reaction of the following compounds with chloroethane and AlCl3?

a. Benzene b. p-xylene

6. What products would you expect to obtain from the reaction of benzene with the following reagents?

a. (CH3)3CCl,AlCl3

b. CH3CH2COCl,AlCl369

reaction of aromatic chapter18-ppt

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