bao cao benzne

8/9/2019 bao cao benzne

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REACTIONS OF BENZENEREACTIONS OF BENZENE

AND ITS DERIVATIVEAND ITS DERIVATIVE

REACTIONS OF BENZENEREACTIONS OF BENZENE

AND ITS DERIVATIVEAND ITS DERIVATIVEGroup 5:Group 5:L Th Dim Quyn

Vin Hng Nhung

o Ngc ThiNguyn Anh Quc


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OutlinesIntrodution

Electrophilic Aromatic Substitution

Disubstitution and Polysubstitution

Nucleophilic Aromatic Substitution


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Introdution to benzene

function groups

The Halogen

The Nitro group (-NO2)The Sulfonic acid group (-SO3H)

The Alkyl group (-R)

The Acyl group (RCO-)


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Electrophilic Aromatic Substitution

The characteristic reaction of benzene is electrophilic aromaticsubstitutiona hydrogen atom is replaced by an electrophile (E+)


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All occur by a common three-step mechanism

Step 1: Generation of the electrophile

Step 2: Reaction of the electrophile with the ring

Step 3: Proton transfer to regenerate the aromatic ring

+

H

EE+

Hslow

Resonance-stabilized cation intermed iate

E

H

E

H

E

H E

Basefast Base-H

Mechanism


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Benzenedoesnotundergoadditionreactionslikeotherunsaturatedhydrocarbons,becauseaddition would

yieldaproductthatisnotaromatic. Substitutionofahydrogenkeepsthearomaticring

intact.


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Chlorination and Bromination

+-

A molecula complex with

a positive cha ge on chlorineand a negative charge on iron

Ferric chlorid e

(a Lewis acid)

Chlorine

(a Lewis base)

+Cl

FeCl

Cl Fe

Cl Cl+

An ion pair containing

a chloroniu m ion

-Cl Fe

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Step 1: Formationofachloroniumion(E+)


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Step 2: Reactionofthechloroniumion withthearomaticring

+

+

+

Resonance-stabil i ed cation intermediate

+

slow, ratedeterm ining

Cl

HH

Cl

H

Cl

Cl +

Step 3: Protontransferto FeCl4- forms HCl,regeneratesthe Lewisacidcatalyst,andgiveschlorobenzene

Cl

H

Cl Cl

Cl

Cl

Fe

Cl H-Cl Fe Cl

Cl

Cl

Chlorobenzene

f a st

Catio nintermed iate

++

+

+-


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Nitration and Sulfonation

Nitrationandsulfonationintroducetwodifferentfunctionalgroupsintothearomaticring.

Nitrationisespeciallyusefulbecausethenitrogroupcanbe

reducedtoan NH2 group.

Nitration


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The electrophile is NO2+, generated in this way

H O NO2 O SO3 HH O NO2H

H

HSO4++ +

Nit ic cidConjugat acidof

nitic

acid

O

H

H NO2 H O

H

NO2++

+

The n it onium

ion


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COOH

NO2

3H2

Ni

COOH

NH2

2H2O

4-Aminobenzoic acid4-Nitrobenzoic acid

+(3 atm)

+

A particular value of nitration is that the nitro group can bereduced to a 1 amino group.

Reduction occurs with other reagents such as an active metal(Fe, Sn or Zn) in HCl.


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Carried out using concentrated sulfuric acidcontaining dissolved sulfur trioxide.

Concentrated sulfuric acid containing dissolved sulfurtrioxide is fuming sulfuric acid.

+ SO3

H2SO4

SO3H

Benzene Benzenesulfonic acid

Sulfonation


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Friedel-CraftsAlkylation

Friedel-Crafts alkylation forms a new C-C bond between anaromatic ring and an alkyl group.

Mixing benzene and alkyl halide, AlCl3 catalyst, form alkylbenzene and HX

ClAlCl3

HCl+

Benzene 2-Chloropropane

(Isopropyl chloride)

Cumene

(Isopropylbenzene)

+


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Step 1: Formation of an alkyl cation as an ion pair.

Step 2: Attack of the alkyl cation on the ring.

Step 3: Proton transfer regenerates aromaticity.

+ R+

R

H

R

H

R

H

A resonance-stabilized cation

+

+

+

H

RCl lCl3 R AlCl3 HCl

R Cl ClAl

Cl

ClR Cl

Cl

Cl

Al Cl R

+

AlCl4-

An ion pair containinga carbocation

+-

+

A molecularcomplex


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Friedel-CraftsAcylation ofBenzene

Friedel-Crafts acylation forms a new C-C bondbetween a benzene ring and an acyl group.

OCl

CH3CCl

O

AlCl3

AlCl3

O

O

HCl

HCl+

Benzene AcetophenoneAcetylchloride

4-Phenylbutanoylchloride

E-Tetralone

+

+


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The electrophile is an acylium ion.


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A special value of Friedel-Crafts acylations is

preparation of unrearranged alkylbenzenes.

+A lCl3

N 2 H 4 , KOH

diethyle eglyc l Is utyl e ze e2-Methyl-1-

phe yl-1-p p e

2-Methylp p ylchl ide

Cl

O

O

Wolff-Kishner reduction


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OtherElectrophilicAromaticAlkylations

Carbocations are also generated from alkenes andalcohols by treatment of an alkene with a protic

acid, most commonly H2SO4, H3PO4, or HF/BF3

CH3CH=CH2H3PO4

Benzene P ene ene

+


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By treating an alkene with a Lewis acid,

By treating an alcohol with H2SO4 or H3PO4.

+

AlCl3

Benzene Cyclohexene Phenylcyclohexane

Benzene

HOH2

H3PO4

++

2-Methyl-2-propanol(tert-Butyl al ohol)

2-Methyl-2-phenylpropane(tert-Butylbenzene)


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Di- and Polysubstitution Two c aracteristics ofa s stit e t

Orientation: Certain substituents direct preferentially to ortho & para

positions; others to meta positions. Substituents are classified as eitherorthoortho--parapara directingdirecting

ormeta directingmeta directing toward furthersubstitution.

Rate: Certain substituents causetherate ofa second

substitution to be greaterthan thatforbenzene itself;others causetherateto belower. Substituents are classified as activatingactivating ordeactivatingdeactivating

toward furthersubstitution.


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-OCH3 is ortho-para directing.

-COOH is meta directing.

O H3

HNO3H3 OOH

O H3

NO2

O H3

NO2

H2 O

p-Nitroa iso e (55%)

o-Nitroa iso e (44%)

iso e

+++

C H

HN H2S

4

N 2

C H C H

N 2N 2

C H

100C

m-N it o-b oi

a i80%)

B n oia i

+ ++

o-N it o-b oi

a i18%)

p-N it o-b o i

a i2%)

Di- and Polysubstitution


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Weakly

activating

Ortho-

araDirecting

Weaklydeactivating

Moderatelyactivating

Stronglyactivating NH NHR NR OH

NH R NH r

OR

OCArOCR

R

F Cl Br I

: : : : :

::

: ::

:

:

:

:

:

:

:

:

:

:

:

::::

Stronglydeactivating

Moderatelydeactivating

CH

O O

CR COH

SO H

CORO

CNH

NO NH+

CF CClMetaDirectin

g

C N

O O O O

OO

Table 1.1. Directing effects of substitution on futher substitution


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DiDi-- and Polysubstitutionand Polysubstitution

Generalizations:Directivity: Alkyl, phenyl,and all substituents in which

theatom bonded to thering has an unshared pairof

electrons are ortho-para directing. All othersubstituentsare meta directing.

Activation: All ortho-para directing groups exceptthe

halogens areactivating toward furthersubstitution. Thehalogens are weakly deactivating.


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DiDi-- and Polysubstitution.and Polysubstitution.

The orderofsteps is important.

CH

3

K2 Cr2 O7

H2 O4

HNO3

H2 O4

CH3

NO2

COOH

H2 O4

HNO3

K2 Cr2O7

H2 O4

COOH

NO2

COOH

NO2

m-N itrobenzoicacid

p-N itrobenzoicacid

Note the key point: transformation of o,p director into m

director.

o,p

m

o,p

m


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Theory of Directing Effects

The rate of EAS is limited by the slowest stepin the reaction

For almost every EAS, the rate-determining

step is attack of E+

on the aromatic ring to givea resonance-stabilized cation intermediate

The more stable this cation intermediate, thefaster the rate-determining step and the faster

the overall reaction


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Theory of Directing EffectsTheory of Directing Effects

-OCH3 is directing: assume ortho-para attack.Here only para attack is shown.

O H3

NO +

fa t

+

( ) ( ) (f)

O H3

H N O

O H3

H NO

O H3

H NO

O H3

H NO

O H3

N O

H+

+

lo

+

+

+

(g)

::::

: : :


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Theory of Directing Effects

-OCH3: assume ortho-para attack

O H3

NO +

fa t

+

( ) ( ) (f)

O H3

H N O

O H3

H NO

O H3

H NO

O H3

H NO

O H3

N O

H+

+

lo

+

+

+

(g)

::::

: : :


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-CO2H is directing; assume meta attack.

COO

O+

COO

O

COO

O

COO

O

-+

COO

O

+ slow

fast

(a) ( ) (c)


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-CO2H is directing: assume ortho-para attack.COOH

NO2+

COOH

H NO2

COOH

H NO2

COOH

H NO2

-H+

COOH

NO2

+ slow

fast

(d) (e) (f)

Themostdisfa oredcontribu tin structure


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Activating-Deactivating

Any resonance effectAny resonance effect, such as that of -NH2, -OH,and -OR, that delocalizes the positive charge onthe cation intermediate lowers the activation

energy for its formation, and has an activatingeffect toward further EAS

Any resonance or inductive effectAny resonance or inductive effect, such as that of-NO2, -CN, -CO, or -SO3H, that decreases

electron density on the ring deactivates the ringtoward further EAS


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Aryl halides, dont undergo substitution by either of the

SN1 or SN2 pathways that are characteristic of nucleophilic

aliphatic substitution

Undergo nucleophilic aromatic substitution under certain

condition but by mechanisms quite different from those for

nucleophilic aliphatic substitution

Nucleophilic aromatic substitutions are far less common

than electrophilic aromatic substitutions

Nucleophilic Aromatic Substitution


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Benzyne IntermediatesBenzyne Intermediates

When heated at 3000C under hight pressure with aqueous

NaOH, chlorobenzene is converted to sodium phenoxide.

Neutralization of this salt with aqueous Acid gives phenol

Cl

2 NaOHH2O

O-

Na

NaCl H2O

So i me o i e

C loro -be ze e

++re ss re, 300oC

+


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3-M e thy lphe n ol(m-Cre so l)

2-M eth ylp he no l(o-Cre so l)

+

CHCl OH

CH CH

OH

1 . NaOH,he at ,p ressure

. HCl, H O

BenzyneBenzyne IntermediatesIntermediates

At pressure high and temperature atAt pressure high and temperature at 50050000CC reactionreaction

appears to involveappears to involve nucleophilicnucleophilic substitution of group OH forsubstitution of group OH for

groupgroup ClCl on the benzene ringon the benzene ring

For example oFor example o--chlorotoluenechlorotoluene under these conditionsunder these conditions

gives a mixture of ogives a mixture of o--cresol and mcresol and m--cresolcresol


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Benzyne IntermediatesBenzyne Intermediates

The same type of reaction can be brought about usingof sodiumamide in liquid ammonia

Under these condition p-chlorotoluene gives amixture

of 4-methylaniline and 3-methylaniline

CH3

Cl

NaNH2NH3 (l)

CH3

NH2

CH3

NH2

NaCl

3-Met yla ili e(m-Tol i i e)

4-Met yla ili e(p-Tol i i e)

++(-33

oC)

+


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Mechainsm

Step1: Dehydrohalogenation of the benzene ring gives a

benzyne intermediate

When elimination of HX gives a benzyne intermediate, thatthen adds the nucleophile to give new products


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Step 2: Addition to either carbon of the triple is possible

Step 3: Proton transfer from ammonia to the carbanion



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AdditionAddition-- eliminationelimination

When an aryl halide contains electron-

withdrawing NO2 groups ortho and/or para to X,

nucleophilic aromatic substitution takes placereadily

Aromatic halides are normally inert to the

types of nucleophiles that readily displace

halide ions from alkyl halides


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When 1-chloro-2,4-dinitrobenzene is heated at reflux in aqueous Na2CO3

Hence product react with acid

form 2,4-dinitrophenol

ClNO 2

NO 2

N a2 CO 3 , H 2 O

O-

Na+

NO 2

NO 2

1 0 0oC

S o i m 2 ,4 -d i i tro -e o id e

1 -C loro -2 ,4 -d i i tro b e z e e


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Mechainsm


Step 1: the nucleophile adds to the aromatic ring at the carbon

bearing the halogen

Step 2:Elimination of halide ion regenerates the aromatic ring and

gives the observed product


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2,6-di-tert-butyl-4-methylphenol

Or Butylated

hydroxytoluene (BHT)In food industry, it is anantioxidation in food toretard spoilage

ProblemsProblems

Problem 22.23: Mechanism

OH

2,6-di-tert-butyl-4-methylphenol


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Starting materials :

4-methylphenol (p-cresol)2-methylpropene

H3PO4 catalystOH

p-cresol2-methylpropene


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Summary

Reaction sof

Benzene


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Summary


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References


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Thank for your

attention

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