Hidrocarburos ariomaticos

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Chapter 17

Reactions of Aromatic Compounds

Jo BlackburnRichland College, Dallas, TX

Dallas County Community College District2003,Prentice Hall

Organic Chemistry , 5th Edition

L. G. Wade, Jr.



Chapter 17 2

Electrophilic

Aromatic SubstitutionElectrophile substitutes for a hydrogen on

the benzene ring.

=>



Chapter 17 3

Mechanism

=>



Chapter 17 4

Bromination of Benzene• Requires a stronger electrophile than Br 2.

• Use a strong Lewis acid catalyst, FeBr 3.

Br Br FeBr 3 Br Br FeBr 3 Animation

Br Br FeBr 3

H

H

H

H

H

H

H

H

H

H

HH

Br + + FeBr 4

_

Animation

Animation Br

HBr +=>

http://d/WADE/CHAP17/ANIMATE/AN17_004.MOV










Chapter 17 5

Energy Diagram

for Bromination

=>



Chapter 17 6

Chlorination

and Iodination• Chlorination is similar to bromination.

Use AlCl3 as the Lewis acid catalyst.

• Iodination requires an acidic oxidizingagent, like nitric acid, which oxidizes the

iodine to an iodonium ion.

H+

HNO3 I21/2 I

+ NO2 H2O+ ++ +

=>



Chapter 17 7

Nitration of BenzeneUse sulfuric acid with nitric acid to form

the nitronium ion electrophile.

H O N

O

O

H O S O H

O

O

+ HSO4

_ H O N

OH

O+

H O N

OH

O+

H2O + N

O

O

+NO2+ then forms asigma complex with

benzene, loses H+ to

form nitrobenzene. =>



Chapter 17 8

Sulfonation

Sulfur trioxide, SO3, in fuming sulfuric acid

is the electrophile.

S

O

O OS

O

O OSO

O OSO

O O

+ + +

_

_ _

S

O

OO

H

SO

O

OH

+

_

S

HOO

O

benzenesulfonic a

=>



Chapter 17 9

Desulfonation

• All steps are reversible, so sulfonic acid

group can be removed by heating in

dilute sulfuric acid.

• This process is used to place deuterium

in place of hydrogen on benzene ring.

Benzene-d 6

=>

D

D

D

D

D

D

D2SO4/D2O

large excess

H

H

H

H

H

H



Chapter 17 10

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.

=>



Chapter 17 11

Sigma Complex

Intermediate

is more

stable if

nitrationoccurs at

the ortho

or para

position.

=>



Chapter 17 12

Energy Diagram

=>



Chapter 17 13

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.

OCH3

H

NO2

+

OCH3

H

NO2

+

=>



Chapter 17 14

Nitration of Anisole

Ortho attack

Meta attack

Para attack

=>






Chapter 17 15

The Amino Group

Aniline reacts with bromine water (without a

catalyst) to yield the tribromide. Sodium

bicarbonate is added to neutralize theHBr that’s also formed.

NH2

Br 23

H2O, NaHCO3

NH2

Br

Br

Br

=>



Chapter 17 16

Summary of

Activators

=>



Chapter 17 17

Deactivating Meta-

Directing Substituents• Electrophilic substitution reactions for

nitrobenzene are 100,000 times slower than for benzene.

• The product mix contains mostly themeta isomer, only small amounts of theortho and para isomers.

• Meta-directors deactivate all positionson the ring, but the meta position is lessdeactivated.

=>



Chapter 17 18

Ortho Substitution

on Nitrobenzene

=>



Chapter 17 19

Para Substitution

on Nitrobenzene

=>



Chapter 17 20

Meta Substitution

on Nitrobenzene

=>



Chapter 17 21

Energy Diagram

=>



Chapter 17 22

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.

=>



Chapter 17 23

Summary of Deactivators

=>



Chapter 17 24

More Deactivators

=>



Chapter 17 25

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. =>



Chapter 17 26

Sigma Complex

for Bromobenzene

Br

E+

Br

H

E

(+)

(+)(+)

Ortho attack

+ Br

E

+

Br

H E

+

(+)

(+)(+)

Para attack

Ortho and para attacks produce a bromonium ion

and other resonance structures.

=>

Meta attack

Br

E+

Br

H

H

E

+

(+)

(+) No bromonium ionpossible with meta attack.



Chapter 17 27

Energy Diagram

=>



Chapter 17 28

Summary of

Directing Effects

=>



Chapter 17 29

Multiple Substituents

The most strongly activating substituent

will determine the position of the next

substitution. May have mixtures.

OCH3

O2 N

SO3

H2SO

4

OCH3

O2 N

SO3H

OCH3

O2 N

SO3H

+

=>



Chapter 17 30

Friedel-Crafts Alkylation

• Synthesis of alkyl benzenes from alkyl

halides and a Lewis acid, usually AlCl3.

• Reactions of alkyl halide with Lewis acidproduces a carbocation which is the

electrophile.

• Other sources of carbocations:alkenes + HF or alcohols + BF3.

=>



Chapter 17 31

Examples of

Carbocation Formation

CH3 CH CH3

Cl

+ AlCl3

CH3

C

H3C H

Cl AlCl3+ _

H2C CH CH3

HFH3C CH CH3

F+

_

H3C CH CH3

OHBF3

H3C CH CH3

OH BF3+

H3C CH CH3

++ HOBF3

_

=>



Chapter 17 32

Formation of

Alkyl Benzene

C

CH3

CH3

H+

H

H

CH(CH3)2+

H

H

CH(CH3)2

B

F

F

F

OH

CH

CH3

CH3

+

HF

BF

OHF

=>

+

-



Chapter 17 33

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 reactivethan benzene, so polyalkylation occurs.

=>



Chapter 17 34

Friedel-Crafts

Acylation• Acyl chloride is used in place of alkyl

chloride.

• The acylium ion intermediate isresonance stabilized and does not

rearrange like a carbocation.

• The product is a phenyl ketone that isless reactive than benzene.

=>



Chapter 17 35

Mechanism of Acylation

R C

O

Cl AlCl3 R C

O

AlCl3Cl+ _

R C

O

AlCl3Cl+ _ AlCl4 +

_ +R C O R C O

+

C

O

R +

H

C

H

O

R

+Cl AlCl3

_ C

O

R +

HCl

AlCl3

=>



Chapter 17 36

Clemmensen Reduction

Acylbenzenes can be converted to

alkylbenzenes by treatment with

aqueous HCl and amalgamated zinc.

+ CH3CH2C

O

Cl1) AlCl3

2) H2O

C

O

CH2CH3Zn(Hg)

aq. HCl

CH2CH2CH3

=>



Chapter 17 37

Gatterman-Koch

Formylation• Formyl chloride is unstable. Use a high

pressure mixture of CO, HCl, and catalyst.

• Product is benzaldehyde.

CO + HCl H C

O

ClAlCl3/CuCl

H C O+

AlCl4

_

C

O

H

+ CO

H+ HCl+

=>



Chapter 17 38

Nucleophilic

Aromatic Substitution• A nucleophile replaces a leaving group

on the aromatic ring.

• Electron-withdrawing substituents

activate the ring for nucleophilic

substitution.

=>



Chapter 17 39

Examples of

Nucleophilic Substitution

=>



Chapter 17 40

Addition-Elimination

Mechanism

=>



Chapter 17 41

Benzyne Mechanism

• Reactant is halobenzene with no electron-withdrawing groups on the ring.

• Use a very strong base like NaNH2.

=>



Chapter 17 42

Benzyne Intermediate

CH3

HH

H NH2

CH3

HH

H NH2

_

or

CH3

HH

H

NH2

CH3

HH

H

NH2 _

meta-toluidine para-toluidine =>



Chapter 17 43

Chlorination of Benzene

• Addition to the benzene ring may occur with high heat and pressure (or light).

• The first Cl2 addition is difficult, but the

next 2 moles add rapidly.• The product, benzene hexachloride, is

an insecticide.

=>



Chapter 17 44

Catalytic Hydrogenation

• Elevated heat and pressure is required.

• Possible catalysts: Pt, Pd, Ni, Ru, Rh.

•Reduction cannot be stopped at anintermediate stage.

=>

CH3

CH3

Ru, 100°C1000 psi3H2,

CH3

CH3



Chapter 17 45

Birch Reduction:

Regiospecific

• A carbon with an e--withdrawing group

is reduced.

• A carbon with an e--releasing group

is not reduced.

C

O

OH Na, NH3

CH3CH2OH

C

O

O

H

_

OCH3 Li, NH3

(CH3)3COH, THF

OCH3

=>



Chapter 17 46

Birch Mechanism

=>



Chapter 17 47

Side-Chain Oxidation

Alkylbenzenes are oxidized to benzoic

acid by hot KMnO4 or Na2Cr 2O7/H2SO4.

CH(CH3)2

CH CH2

KMnO4, OH-

H2O, heat

COO

COO

_

_

=>



Chapter 17 48

Side-Chain Halogenation

• Benzylic position is the most reactive.

• Chlorination is not as selective as

bromination, results in mixtures.• Br 2 reacts only at the benzylic position.

=>

CHCH2CH3

Br

h Br 2,

CH2CH2CH3



Chapter 17 49

SN1 Reactions

• Benzylic carbocations are resonance-

stabilized, easily formed.

• Benzyl halides undergo SN1 reactions.

CH2Br CH3CH2OH, heat

CH2OCH2CH3

=>



Chapter 17 50

SN2 Reactions

•Benzylic halides are 100 times morereactive than primary halides via SN2.

• Transition state is stabilized by ring.

=>



Chapter 17 51

Reactions of Phenols

• Some reactions like aliphatic alcohols:

phenol + carboxylic acid ester

phenol + aq. NaOH phenoxide ion

• Oxidation to quinones: 1,4-diketones.

OH

CH3

Na2Cr 2O7, H2SO4

CH3

O

O

=>



Chapter 17 52

Quinones

• Hydroquinone is used as a developer for film. It reacts with light-sensitized

AgBr grains, converting it to black Ag.• Coenzyme Q is an oxidizing agent

found in the mitochondria of cells.

=>

El hili



Chapter 17 53

Electrophilic

Substitution of Phenols• Phenols and phenoxides are highly reactive.

• Only a weak catalyst (HF) required for

Friedel-Crafts reaction.

• Tribromination occurs without catalyst.

• Even reacts with CO2.O

_

CO2, OH-

O

C

O

O _

_

H+

OH

C

O

OH

salicylic acid

=>



Chapter 17 54

End of Chapter 17

Hidrocarburos ariomaticos

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