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Electrophilic Electrophilic

Electrophilic - PowerPoint Presentation

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Electrophilic - PPT Presentation

Attack Electrophilic Aromatic Substitution Electrophile substitutes for a hydrogen on the benzene ring Mechanism gt Bromination of Benzene Requires a stronger electrophile than Br 2 Use a strong Lewis acid catalyst FeBr ID: 592205

benzene addition sigma electrophilic addition benzene electrophilic sigma acid meta substitution alkyl complex lewis halides hydrogen para ortho electrophile

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Slide1

Electrophilic

AttackSlide2

Electrophilic Aromatic Substitution

Electrophile substitutes for a hydrogen on the benzene ring.Slide3

Mechanism

=>Slide4

Bromination of Benzene

Requires a stronger electrophile than Br

2.Use a strong Lewis acid catalyst, FeBr3.Slide5

Energy Diagram for Bromination

=>Slide6

Chlorination and Iodination

Chlorination is similar to bromination. Use AlCl

3 as the Lewis acid catalyst.Iodination requires an acidic oxidizing agent, like nitric acid, which oxidizes the iodine to an iodonium ion.Slide7

Nitration of Benzene

Use sulfuric acid with nitric acid to form the nitronium ion electrophile.

NO

2

+

then forms a

sigma complex with

benzene, loses H

+

to

form nitrobenzene. =>Slide8

Sulfonation

Sulfur trioxide, SO

3, in fuming sulfuric acid is the electrophile. Slide9

Nitration of Toluene

Toluene reacts 25 times faster than benzene. The methyl group is an activator.

The product mix contains mostly ortho and para substituted molecules.Slide10

Sigma Complex

Intermediate is more stable if nitration occurs at the

ortho or para position.Slide11

Energy Diagram

=>Slide12

Friedel-Crafts Alkylation

Synthesis of alkyl benzenes from alkyl halides and a Lewis acid, usually AlCl

3.Reactions of alkyl halide with Lewis acid produces a carbocation which is the electrophile.Other sources of carbocations: alkenes + HF or alcohols + BF3. Slide13

Examples of Carbocation Formation

=>Slide14

Formation of Alkyl Benzene

+

-Slide15

Limitations of Friedel-Crafts

Reaction fails if benzene has a substituent that is more deactivating than halogen.

Carbocations rearrange. Reaction of benzene with n-propyl chloride and AlCl3 produces isopropylbenzene.The alkylbenzene product is more reactive than benzene, so polyalkylation occurs. Slide16

Friedel-Crafts Acylation

Acyl chloride is used in place of alkyl chloride.

The acylium ion intermediate is resonance stabilized and does not rearrange like a carbocation.The product is a phenyl ketone that is less reactive than benzene. Slide17

Mechanism of AcylationSlide18

Clemmensen Reduction

Acylbenzenes can be converted to alkylbenzenes by treatment with aqueous HCl and amalgamated zinc.Slide19

Gatterman-Koch Formylation

Formyl chloride is unstable. Use a high pressure mixture of CO, HCl, and catalyst.

Product is benzaldehyde.Slide20

Activating,

O-, P-Directing Substituents

Alkyl groups stabilize the sigma complex by induction, donating electron density through the sigma bond.Substituents with a lone pair of electrons stabilize the sigma complex by resonance.Slide21

The Amino Group

Aniline reacts with bromine water (without a catalyst) to yield the tribromide. Sodium bicarbonate is added to neutralize the HBr that’s also formed.

=>Slide22

Summary of ActivatorsSlide23

Deactivating Meta-Directing Substituents

Electrophilic substitution reactions for nitrobenzene are 100,000 times

slower than for benzene.The product mix contains mostly the meta isomer, only small amounts of the ortho and para isomers.Meta-directors deactivate all positions on the ring, but the meta position is less deactivated. Slide24

Ortho Substitution on NitrobenzeneSlide25

Para Substitution on Nitrobenzene

=>Slide26

Meta Substitution on NitrobenzeneSlide27

Energy DiagramSlide28

Structure of Meta-Directing Deactivators

The atom attached to the aromatic ring will have a partial positive charge.

Electron density is withdrawn inductively along the sigma bond, so the ring is less electron-rich than benzene. Slide29

Summary of DeactivatorsSlide30

More DeactivatorsSlide31

Halobenzenes

Halogens are deactivating toward electrophilic substitution, but are ortho, para-directing!

Since halogens are very electronegative, they withdraw electron density from the ring inductively along the sigma bond.But halogens have lone pairs of electrons that can stabilize the sigma complex by resonance. Slide32

Sigma Complex for Bromobenzene

Ortho and para attacks produce a bromonium ion

and other resonance structures.

No bromonium ion

possible with meta attack.Slide33

Energy DiagramSlide34

Summary of Directing EffectsSlide35

Multiple Substituents

The most strongly activating substituent will determine the position of the next substitution. May have mixtures.Slide36
Slide37

37

II. Electrophilic Addition

“Loose” p electrons are nucleophilic (Lewis bases), react with electrophiles (Lewis acids).Slide38

38

II. Electrophilic Addition

A. Addition of hydrogen halides

(X = Cl, Br, I)

Reactivity: HI > HBr > HCl >> HF (stronger acid = better electrophile)Slide39

39

II. Electrophilic Addition

A. Addition of hydrogen halides1. Markovnikov’s rule

In the addition of HX to an alkene, the H goes to the carbon with more H’s.

Question 6-2.

Draw the products. Click on the arrow to check answers.

Check

AnswerSlide40

40

II. Electrophilic Addition

A. Addition of hydrogen halides1. Markovnikov’s rule

In the addition of HX to an alkene, the H goes to the carbon with more H’s.

Answer 6-2.

Slide41

41

II. Electrophilic Addition

A. Addition of hydrogen halides2. mechanism

Mechanistic interpretation of Markovnikov’s rule: The reaction proceeds through the

more stable carbocation intermediate.Slide42

42

II. Electrophilic Addition

A. Addition of hydrogen halides2. mechanism

lower

E

a

faster rate of

formation