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Chapter 12Chapter 12

Substituted benzenesSubstituted benzenes

The Benzene Ring as a SubstituentThe Benzene Ring as a Substituent

••CCCC

CC ••CC

 

benzylic carbon is analogous to allylic carbonbenzylic carbon is analogous to allylic carbon

Resonance in Benzyl RadicalResonance in Benzyl Radical

HH HH••

HH

HHHH

HH

HH

unpaired electron is delocalized betweenunpaired electron is delocalized between

benzylic carbon and the ring carbons that arebenzylic carbon and the ring carbons that are

ortho and para to itortho and para to it

Resonance in Benzyl RadicalResonance in Benzyl Radical

HH HH

HH

HHHH

HH••

HH

unpaired electron is delocalized betweenunpaired electron is delocalized between

benzylic carbon and the ring carbons that arebenzylic carbon and the ring carbons that are

ortho and para to itortho and para to it

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Resonance in Benzyl RadicalResonance in Benzyl Radical

HH HH

HH

HHHH

HH

••

 

HH

unpaired electron is delocalized betweenunpaired electron is delocalized between

benzylic carbon and the ring carbons that arebenzylic carbon and the ring carbons that are

ortho and para to itortho and para to it

Resonance in Benzyl RadicalResonance in Benzyl Radical

HH HH

HH

HHHH

HH••

HH

unpaired electron is delocalized betweenunpaired electron is delocalized between

benzylic carbon and the ring carbons that arebenzylic carbon and the ring carbons that are

ortho and para to itortho and para to it

industrial processindustrial process

FreeFree--radical chlorination of tolueneradical chlorination of toluene

highly regioselective for benzylic positionhighly regioselective for benzylic position

 

CHCH33

ClCl22CHCH22ClCl

or or 

heatheat

TolueneToluene Benzyl chlorideBenzyl chloride

Similarly, dichlorination and trichlorination areSimilarly, dichlorination and trichlorination are

FreeFree--radical chlorination of tolueneradical chlorination of toluene

  ..chlorination gives:chlorination gives:

CClCCl33

 

CHClCHCl22

(Dichloromethyl)benzene(Dichloromethyl)benzene (Trichloromethyl)benzene(Trichloromethyl)benzene

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is used in the laboratory to introduce ais used in the laboratory to introduce a

halogen at the benzylic positionhalogen at the benzylic position

Benzylic BrominationBenzylic Bromination

  CHCH33

+ Br + Br 22

CClCCl44, 80, 80°°CC

lightlight

+ HBr + HBr 

CHCH22Br Br 

NONO22 NONO22

pp--NitrotolueneNitrotoluene pp--NitrobenzylNitrobenzyl

bromide (71%)bromide (71%)

is a convenient reagent for benzylic brominationis a convenient reagent for benzylic bromination

NN--Bromosuccinimide (NBS)Bromosuccinimide (NBS)

CClCCl44

benzo lbenzo l

CHCH22CHCH33

++NNBr Br 

OO CHCHCHCH33

NHNH

OO

++

peroxide,peroxide,

heatheatOO OO

(87%)(87%)

Oxidation ofOxidation of Alkylbenzenes Alkylbenzenes

Site of Oxidation is Benzylic CarbonSite of Oxidation is Benzylic Carbon

CHCH

CHCH22RR

or or 

COHCOH

OONaNa22Cr Cr 22OO77

HH22SOSO44

HH22OO

CHRCHR22

or or 

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ExampleExample

OO

NaNa22Cr Cr 22OO77

HH22SOSO44

HH22OO

heatheat

  33

NONO22

pp--NitrotolueneNitrotoluene

NONO22

pp--NitrobenzoicNitrobenzoic

acid (82acid (82--86%)86%)

ExampleExample

  CH(CHCH(CH33))22 COHCOH

OO

HH22SOSO44

HH22OO

heatheat

CHCH33 COHCOH

(45%)(45%)

OO

Nucleophilic SubstitutionNucleophilic Substitution

inin BenzylicBenzylic HalidesHalides

Primary Benzylic HalidesPrimary Benzylic Halides

CHCH22ClClOO22NN

acetic acidacetic acid

NaOCCHNaOCCH33

OOMechanism is SMechanism is S

NN

22

CHCH22OCCHOCCH33OO22NN

OO

(78(78--82%)82%)

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What about SWhat about SNN1?1?

Relative solvolysis rates in aqueous acetoneRelative solvolysis rates in aqueous acetone

 

CC

33

CHCH33

ClCl CC

33

CHCH33

ClClCHCH33

tertiary benzylic carbocation is formedtertiary benzylic carbocation is formed

more rapidly than tertiary carbocation;more rapidly than tertiary carbocation;

therefore, more stabletherefore, more stable

What about SWhat about SNN1?1?

Relative rates of formation:Relative rates of formation:

CC CCCHCH33

33

CHCH33

++

33

CHCH33

++

more s a emore s a e less stableless stable

Compare.Compare.

++CCCC

CC ++CC

 

benzylic carbon is analogous to allylic carbonbenzylic carbon is analogous to allylic carbon

Resonance in Benzyl CationResonance in Benzyl Cation

HH HH++

HH

HHHH

HH

HH

unpaired electron is delocalized betweenunpaired electron is delocalized between

benzylic carbon and the ring carbons that arebenzylic carbon and the ring carbons that are

ortho and para to itortho and para to it

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Resonance in Benzyl CationResonance in Benzyl Cation

HH HH 

HH

HHHH

HH++

HH

unpaired electron is delocalized betweenunpaired electron is delocalized between

benzylic carbon and the ring carbons that arebenzylic carbon and the ring carbons that are

ortho and para to itortho and para to it

Resonance in Benzyl CationResonance in Benzyl Cation

HH HH

HH

HHHH

HH

++

HH

unpaired electron is delocalized betweenunpaired electron is delocalized between

benzylic carbon and the ring carbons that arebenzylic carbon and the ring carbons that are

ortho and para to itortho and para to it

Resonance in Benzyl CationResonance in Benzyl Cation

HH HH

HH

HHHH

HH++

HH

unpaired electron is delocalized betweenunpaired electron is delocalized between

benzylic carbon and the ring carbons that arebenzylic carbon and the ring carbons that are

ortho and para to itortho and para to it

SolvolysisSolvolysis

CC

CHCH33

ClCl

CHCH33

CHCH33CHCH22OHOH

CC

CHCH33

CHCH33

OCHOCH22CHCH33

(87%)(87%)

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PhenolsPhenolsNomenclatureNomenclature

NomenclatureNomenclature

OHOH

named on basis of phenol as parentnamed on basis of phenol as parent

55--ChloroChloro--22--methylmethylphenolphenol

33

ClCl

substituents listed in alphabetical order substituents listed in alphabetical order 

lowest numerical sequence: first point of lowest numerical sequence: first point of 

difference ruledifference rule

NomenclatureNomenclature

OHOH OHOHOHOH

OHOH

OHOH

1,21,2--BenzenediolBenzenediol 1,31,3--BenzenediolBenzenediol 1,41,4--BenzenediolBenzenediol

(common name:(common name:

pyrocatechol)pyrocatechol)

(common name:(common name:

resorcinol)resorcinol)

(common name:(common name:

hydroquinone)hydroquinone)

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NomenclatureNomenclature

  OHOH

pp--Hydroxybenzoic acidHydroxybenzoic acid

COCO22

HH

name on basis of benzoic acid as parentname on basis of benzoic acid as parent

higher oxidation states of carbon outrankhigher oxidation states of carbon outrank

hydroxylhydroxyl groupgroup

The OH group of phenols allows hydrogen bondingThe OH group of phenols allows hydrogen bonding

Physical PropertiesPhysical Properties

to other phenol molecules and to water.to other phenol molecules and to water.

HH OOOO

Hydrogen Bonding in PhenolsHydrogen Bonding in Phenols Physical PropertiesPhysical Properties

Compared to compounds of similar size andCompared to compounds of similar size and

molecular weight, hydrogen bonding in phenolmolecular weight, hydrogen bonding in phenol

raises its melting point, boiling point, andraises its melting point, boiling point, andsolubility in water.solubility in water.

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Physical PropertiesPhysical Properties

CC66HH55CHCH33 CC66HH55OHOH CC66HH55FF

Molecular weightMolecular weight 9292 9494 9696

 –  –9595 4343  –  –4141Melting point (Melting point (°°C)C)

BoilingBoiling

point (point (°°C,1 atm)C,1 atm)

111111 132132 8585

Solubility inSolubility in

HH22O (g/100 mL,25O (g/100 mL,25°°C)C)

0.050.05 8.28.2 0.20.2

 Acidity of Phenols Acidity of Phenols

most characteristic property ofmost characteristic property ofphenols is their acidityphenols is their acidity

CompareCompare

••••••

••••••

•• –  – 

••

KKaa = 10= 10--1010

++HH ++

•• ••

CHCH33CHCH22OO HH••••

••••

aa == --

++HH ++ CHCH33CHCH22OO

••••

••••

 –  – ••••

 

Delocalized negative charge in phenoxide ionDelocalized negative charge in phenoxide ion

•••• OO

••••

HH HH

••••

 –  – •••• OO

••••

HH HH –  – ••••

HH HH

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Delocalized negative charge in phenoxide ionDelocalized negative charge in phenoxide ion

•••• OO

••••

HH HH –  – ••••

  •••• OO

••••

HH HH

••••

HHHH

Delocalized negative charge in phenoxide ionDelocalized negative charge in phenoxide ion

•••• OO

••••

HH HH –  – ••••

  •••• OO

••••

HH HH

••••

 

HHHH

Phenols are converted to phenoxide ionsPhenols are converted to phenoxide ions

in aqueous basein aqueous base

OO HH••••

••••OO••

••••••

••••

 –  – 

++ HOHO –  – 

++ HH22OO

stronger acidstronger acid weaker acidweaker acid

Substituent EffectsSubstituent Effects

on theon the

 Acidity of Phenols Acidity of Phenols

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ElectronElectron--releasing groups have little or no effectreleasing groups have little or no effect

  OHOHOHOH OHOH

CHCH33 OCHOCH33

KKaa:: 1 x 101 x 10--1010 5 x 105 x 10--1111 6 x 106 x 10--1111

ElectronElectron--withdrawing groups increase aciditywithdrawing groups increase acidity

OHOHOHOH OHOH

ClCl NONO22

KKaa:: 1 x 101 x 10--1010 4 x 104 x 10--99 7 x 107 x 10--88

Effect of electronEffect of electron--withdrawing groups is mostwithdrawing groups is most

pronounced at ortho and para positionspronounced at ortho and para positions

OHOHOHOH

NONO22

OHOH

NONO

NONO22

KKaa:: 6 x 106 x 10--88 4 x 104 x 10--99 7 x 107 x 10--88

Effect of strong electronEffect of strong electron--withdrawing groupswithdrawing groups

is cumulativeis cumulative

OHOH OHOH

NONO22

OHOH

NONO22OO22NN

NONO22 NONO22NONO22

KKaa:: 7 x 107 x 10--88 1 x 101 x 10--44 4 x 104 x 10--11

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Resonance DepictionResonance Depiction

••••••

•••••••• –  – 

••••

HH

HH

HH

HH

••••

HH

HH

HH

HH

••

NN••••OO OO

••••

••••••••••••  –  – 

++••••

 –  – 

NN••••OO OO

••••

••••••••••••  –  – 

++

 

Preparation of Aryl EthersPreparation of Aryl Ethers

Typical Preparation is by Williamson SynthesisTypical Preparation is by Williamson Synthesis

SS 22OONaNa ++ RRXX OORR NaNaXX++

Typical Preparation is by Williamson SynthesisTypical Preparation is by Williamson Synthesis

SS 22OONaNa ++ RRXX OORR NaNaXX++

but the other combinationbut the other combination

++ aa

fails because aryl halides are normally unreactivefails because aryl halides are normally unreactive

toward nucleophilic substitutiontoward nucleophilic substitution

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ExampleExample

 

heatheatOONaNa ++ CHCH33II OOCHCH33

(95%)(95%)

Carboxylation of PhenolsCarboxylation of Phenols

 Aspirin and Aspirin and

thethe

KolbeKolbe--SchmittSchmitt

OCCHOCCH33

OO

eac oneac on COHCOH

OO

 Aspirin is prepared from salicylic acid Aspirin is prepared from salicylic acid

By esterificationBy esterification

OO

COHCOH

OOCCHCCH33

HH22SOSO44COHCOH

OOHHCHCH33COCCHCOCCH33

how is salicylic acid prepared?how is salicylic acid prepared?

Preparation of Salicylic AcidPreparation of Salicylic Acid

OHOHONaONa

125125°°C, 100 atmC, 100 atm CONaCONa

OO

  --

acidification converts the sodium salt shownacidification converts the sodium salt shown

above to salicylic acidabove to salicylic acid

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acidacid--base considerations provide an explanation:base considerations provide an explanation:

What Drives the Reaction?What Drives the Reaction?

 

++ COCO22

OO   –  – 

••••

•••••••• OO

 –  – ••••••••CC OO

••••

••••HH

stronger base:stronger base:ppKKaa of conjugateof conjugateacid = 10acid = 10

weaker base:weaker base:ppKKaa of conjugateof conjugateacid = 3acid = 3

••••

OO•••• ••

••

Preparation of Salicylic AcidPreparation of Salicylic Acid

OHOHONaONa

how does carbonhow does carbon--carbon bond form?carbon bond form?

125125°°C, 100 atmC, 100 atm CONaCONa

OO

recall electron delocalization in phenoxide ionrecall electron delocalization in phenoxide ion

negative charge shared by oxygen and by thenegative charge shared by oxygen and by the

ring carbons that are ortho and para to oxygenring carbons that are ortho and para to oxygen

•••• OO

••••

HH HH

••••

 –  – •••• OO

••••

HH HH –  – ••••

HH HH

•••• OO

••••  ••

•• OO••••

HH

HH

HH

HH

HH –  – ••••

HH

HH

HH

HH

HH

 –  – ••••

Mechanism ofMechanism of orthoortho CarboxylationCarboxylation

••••••••OO

•••• –  – 

OO••••

••••

OO

HH

••••

••••OO••

••

HH

 –  – 

••••

••••••••

CC

OO

OO•••• ••••

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Mechanism ofMechanism of orthoortho CarboxylationCarboxylation

••••OO••••

  •••• –  – 

 

OO••••

••••

OO

HH

••••

••••OO••

••

HH

 –  – 

••••

••••••••

CC

OO

OO•••• ••

•• 

••••

 –  – 

••••

••••••••CC

OO

OO

••••

•••• ••

••

Reimer Reimer--Tiemann reaction.Tiemann reaction.

Synthesis of phenolic aldehydesSynthesis of phenolic aldehydes

DichlorocarbenesDichlorocarbenes

Reimer Reimer--Tiemann rxnTiemann rxn

OH O- O- OH

CHCl3, aq. NaOH

70 C

CHCl2 CHO

HCl

CHO

Mechanism of Reimer Mechanism of Reimer--TiemannTiemann

OH- + CHCl3 H2O + Cl3C:- Cl- + Cl2C:

O- O-

CCl2

O-

CHCl2

+ :CCl2 H

Strong ElectrophileStrong Electrophile

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Oxidation of Phenols:Oxidation of Phenols:

QuinonesQuinones

The most common examples of phenol oxidationsThe most common examples of phenol oxidationsare the oxidations of 1,2are the oxidations of 1,2-- and 1,4and 1,4--benzenediolsbenzenediols

QuinonesQuinones

to give quinones.to give quinones.

OHOH OO

NaNa22Cr Cr 22OO7,7, HH22SOSO44

(76(76--81%)81%)

OHOH OO

HH22OO

The most common examples of phenol oxidationsThe most common examples of phenol oxidationsare the oxidations of 1,2are the oxidations of 1,2-- and 1,4and 1,4--benzenediolsbenzenediols

QuinonesQuinones

to give quinones.to give quinones.

OO

OO Ag Ag22OO

OHOH

OHOH

(68%)(68%)

diethyl ether diethyl ether 

CHCH33 CHCH33

Some quinones are dyesSome quinones are dyes

OHOH

 Alizarin Alizarin(red pigment)(red pigment)

OO

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Some quinones are important biomoleculesSome quinones are important biomolecules

  OO

CHCH33OO

CHCH33OO

OO

CHCH33

nn

Ubiquinone (Coenzyme Q)Ubiquinone (Coenzyme Q)nn = 6= 6--1010

involved in biological electron transportinvolved in biological electron transport

Some quinones are important biomoleculesSome quinones are important biomolecules

OO

CHCH33

OOCHCH

33CHCH

33CHCH

33CHCH

33

CHCH33

Vitamin KVitamin K(blood(blood--clotting factor)clotting factor)

 Aryl Diazonium Salts Aryl Diazonium Salts

Nitrosation of Primary ArylaminesNitrosation of Primary Arylamines

gives aryl diazonium ionsgives aryl diazonium ions

aryl diazonium ions are much more stable thanaryl diazonium ions are much more stable than

alkyl diazonium ionsalkyl diazonium ions

most aryl diazonium ions are stable under themost aryl diazonium ions are stable under theconditions of their formation (0conditions of their formation (0--1010°°C)C)

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Nitrosation of Primary ArylaminesNitrosation of Primary Arylamines

gives aryl diazonium ionsgives aryl diazonium ions

aryl diazonium ions are much more stable thanaryl diazonium ions are much more stable than

alkyl diazonium ionsalkyl diazonium ions

most aryl diazonium ions are stable under themost aryl diazonium ions are stable under the

conditions of their formation (0conditions of their formation (0--1010°°C)C)

 ArN ArN NN++

RNRN NN++ fastfast

slowslow

RR++ ++ NN22

 Ar  Ar ++ ++ NN22

Example:Example:

33 22 22

NaNONaNO22, H, H22SOSO44

HH22O, 0O, 0--55°°CC

(CH(CH33))22CHCH NN NN++

HSOHSO44 –  – 

Synthetic Origin of Aryl Diazonium SaltsSynthetic Origin of Aryl Diazonium Salts

 Ar  Ar  HH

 Ar  Ar  NNOO22

 Ar  Ar  NNHH22

33,, 22 44

Fe, HCl or Pd, HFe, HCl or Pd, H22

 Ar  Ar  NN NN++

NaNONaNO22, HCl, H, HCl, H22OO

00--55°°CC

Synthetic Transformations of Aryl Diazonium SaltsSynthetic Transformations of Aryl Diazonium Salts

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Transformations of Aryl Diazonium SaltsTransformations of Aryl Diazonium Salts

 Ar  Ar  Br Br  Ar  Ar  ClCl

 Ar  Ar  NN NN++

 Ar  Ar  FF Ar  Ar  CNCN

 Ar  Ar  HH

 Ar  Ar  OHOH

 Ar  Ar  II

Preparation of PhenolsPreparation of Phenols

hydrolysis of a diazonium salthydrolysis of a diazonium salt

 Ar  Ar  NN NN++

 Ar  Ar  OHOH

22 ,,

ExampleExample

(CH(CH33))22CHCH NNHH22

2. H2. H22O, heatO, heat

1. NaNO1. NaNO22, H, H22SOSO44

HH22O, 0O, 0--55°°CC

 

(CH(CH33))22CHCH OOHH

(73%)(73%)

Transformations of Aryl Diazonium SaltsTransformations of Aryl Diazonium Salts

 Ar  Ar  Br Br  Ar  Ar  ClCl

 Ar  Ar  NN NN

++

 Ar  Ar  FF Ar  Ar  CNCN

 Ar  Ar  HH

 Ar  Ar  OHOH

 Ar  Ar  II

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Preparation of Aryl IodidesPreparation of Aryl Iodides

reaction of an aryl diazonium salt withreaction of an aryl diazonium salt with

 Ar  Ar  NN NN++

po ass um o epo ass um o e

KIKI

 Ar  Ar  II

ExampleExample

2. KI, room temp.2. KI, room temp.

1. NaNO1. NaNO22, HCl, HCl

HH22O, 0O, 0--55°°CC

22

Br Br  Br Br 

--

Transformations of Aryl Diazonium SaltsTransformations of Aryl Diazonium Salts

 Ar  Ar  Br Br  Ar  Ar  ClCl

 Ar  Ar  NN NN

++

 Ar  Ar  FF Ar  Ar  CNCN

 Ar  Ar  HH

 Ar  Ar  OHOH

 Ar  Ar  II

Preparation of Aryl FluoridesPreparation of Aryl Fluorides

 Ar  Ar  NN NN

++

 Ar  Ar  FF

heat the tetrafluoroborate salt of a diazonium ion;heat the tetrafluoroborate salt of a diazonium ion;

process is called the Schiemann reactionprocess is called the Schiemann reaction

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ExampleExample

22

CCHCCH22CHCH33

2. HBF2. HBF44

. a. a 22, ,, ,

HH22O, 0O, 0--55°°CC

CCHCCH22CHCH33

(68%)(68%)

OO..

OO

Transformations of Aryl Diazonium SaltsTransformations of Aryl Diazonium Salts

 Ar  Ar  Br Br  Ar  Ar  ClCl

 Ar  Ar  NN NN++

 Ar  Ar  FF Ar  Ar  CNCN

 Ar  Ar  HH

 Ar  Ar  OHOH

 Ar  Ar  II

Preparation of Aryl Chlorides and BromidesPreparation of Aryl Chlorides and Bromides

 Ar  Ar  Br Br  Ar  Ar  ClCl

 Ar  Ar  NN NN

++

 

heating a diazonium salt with copper(I) chloride orheating a diazonium salt with copper(I) chloride or

bromidebromide

substitutions of diazonium salts that use copper(I)substitutions of diazonium salts that use copper(I)

halides are calledhalides are called Sandmeyer Sandmeyer reactionsreactions

ExampleExample

22

NONO22

2. CuCl, heat2. CuCl, heat

. a. a 22, ,, ,

HH22O, 0O, 0--55°°CC

NONO22

(68(68--71%)71%)

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ExampleExample

2. CuBr, heat2. CuBr, heat

. a. a 22, r,, r,

HH22O, 0O, 0--1010°°CCNNHH22

ClCl

Br Br 

ClCl

(89(89--95%)95%)

Transformations of Aryl Diazonium SaltsTransformations of Aryl Diazonium Salts

 Ar  Ar  Br Br  Ar  Ar  ClCl

 Ar  Ar  NN NN++

 Ar  Ar  FF Ar  Ar  CNCN

 Ar  Ar  HH

 Ar  Ar  OHOH

 Ar  Ar  II

Preparation of Aryl NitrilesPreparation of Aryl Nitriles

 Ar  Ar  NN NN

++

 Ar  Ar  CNCN

 

salt with copper(I) cyanidesalt with copper(I) cyanide

this is another type of Sandmeyer reactionthis is another type of Sandmeyer reaction

ExampleExample

2. CuCN, heat2. CuCN, heat

. a. a 22, ,, ,

HH22O, 0O, 0°°CCNNHH22

CHCH33

CNCN

CHCH33

(64(64--70%)70%)

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Transformations of Aryl Diazonium SaltsTransformations of Aryl Diazonium Salts

 Ar  Ar  Br Br  Ar  Ar  ClCl

 Ar  Ar  NN NN++

 Ar  Ar  FF Ar  Ar  CNCN

 Ar  Ar  HH

 Ar  Ar  OHOH

 Ar  Ar  II

Transformations of Aryl Diazonium SaltsTransformations of Aryl Diazonium Salts

hypophosphorous acid (Hhypophosphorous acid (H33POPO22) reduces diazonium) reduces diazonium

 Ar  Ar  NN NN++

 

this is calledthis is called reductive deaminationreductive deamination

 Ar  Ar  HH

ExampleExample

aa 22,, 22 44,,

HH33POPO22

NNHH22

CHCH33 CHCH33

(70(70--75%)75%)

Value of Diazonium SaltsValue of Diazonium Salts

1)1) allows introduction of substituents such asallows introduction of substituents such as

, , ,, , ,

2)2) allows preparation of otherwise difficultlyallows preparation of otherwise difficultly

accessible substitution patternsaccessible substitution patterns

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  NNHH22

ExampleExample

NaNONaNO22, H, H22SOSO44,,

NNHH22

Br Br 

HH22O, CHO, CH33CHCH22OHOHBr Br 22

HH22OO

Br Br Br Br 

(74(74--77%)77%)

(100%)(100%)

Br Br 

 Azo Azo CouplingCoupling

 Azo Coupling Azo Coupling

Diazonium salts are weak electrophiles.Diazonium salts are weak electrophiles.

eac w s rong y ac va e aroma ceac w s rong y ac va e aroma c

compounds by electrophilic aromaticcompounds by electrophilic aromatic

substitution.substitution.

 Azo Coupling Azo Coupling

Diazonium salts are weakDiazonium salts are weak electrophileselectrophiles..

eac w s rong y ac va e aroma ceac w s rong y ac va e aroma c

compounds bycompounds by electrophilicelectrophilic aromaticaromatic

substitution.substitution.

 Ar  Ar  NN NN++

 Ar' Ar' HH++  Ar  Ar  NN NN  Ar' Ar'

an azo compoundan azo compound

 Ar' Ar' must bear a strongly electronmust bear a strongly electron--releasing groupreleasing group

such as OH, OR, or NRsuch as OH, OR, or NR22..

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ExampleExample

  OHOH

++++ CC66HH55NN NN ClCl –  – 

 

OHOH

NN NCNC66HH55

HalobenzenesHalobenzenes and Nucleophilicand Nucleophilic

 Aromatic Substitution Aromatic Substitution

ClCl OHOHNaOH, 350CNaOH, 350C

300 atm.300 atm.

Reactivity increases as we attach more electronReactivity increases as we attach more electron

withdrawing groups:withdrawing groups:

NaOH, 160CNaOH, 160C

300 atm.300 atm.ClClOO22NN OHOHOO22NN

NONO22NONO22

HH22O, warmO, warmClCl

 

OO22NN

NONO22

OHOHOO22NN

NONO22

Mechanism:

If the ring is substituted by an electron withdrawing group the

reaction proceeds through a two-step carbanion intermediate.

O2N Cl O2N

Cl

O2N OH

Slow Fast

OH-

OH

If the ring is unsubstituted, the reaction proceeds through a

benzyne intermediate.

Cl NH2Slow

NH2-, NH3-HCl