AlquenosReacciones

1

Reacciones de Alquenos:

2

Introduction: Additions to Alkenes

Generally the reaction is exothermic because one and one bond

are converted to two bonds

The electrons of the double bond are loosely held and are a source

of electron density, i.e. they are nucleophilic

Alkenes react with electrophiles such as H+ from a hydrogen

halide to form a carbocation

3

The carbocation produced is an electrophile

It can react with a nucleophile such as a halide

Insert top scheme pg 331

In addition reactions the alkene changes from a

nucleophile in the first step to an electrophile in the

second

Electron-rich atoms or molecules (nucleophiles) are

attracted to electron-deficient atoms or molecules

(electrophiles)

5

Addition Reactions

The pi ( ) bond is easily broken, which

makes double and

triple bonds very

reactive.

In the addition reaction, reactants

are added to the

carbon atoms in the

double or triple bond.

6

Reacciones de Alquenos

Alkenes react with many electrophiles to give useful

products by addition (often through special reagents)

Haluros (adicion H-X)

alcoholes (adicion H-OH)

dihaluros (adicion X-X)

alkanes (adicion H-H)

halohidrinas (adicion HO-X)

dioles (adicion HO-OH)

7

REACCIONES DE ADICION

Son caractersticas de los compuestos insaturados: alquenos y alquinos

Reaccin de adicin a un alqueno se rompe el enlace y el par de e- se usa en la formacin de dos nuevos

enlaces

C C

sp2 sp3

YZ Y Z

Se combinan dos molculas para dar una sola

Inversa de la eliminacin

Nube de electrones disponibles para electrfilos

ADICION ELECTROFILICA C C

Electrophilic Addition of HX to Alkenes

General reaction mechanism: electrophilic addition

Attack of electrophile (such as HBr) on bond of alkene produces carbocation

and bromide ion

Carbocation is itself an electrophile, reacting with nucleophilic bromide ion

Writing Organic Reactions

No established convention shorthand

Can be formal kinetic expression

Not necessarily balanced

Reactants can be before or on arrow

Solvent, temperature, details, on arrow

Curved Arrows in Reaction Mechanisms

Movement of a pair of electrons

Movement of one electron

Curved Arrows Show the Flow of

Electrons

electron-rich center

electron-deficient center

Rules on Using Curved Arrows

1. Arrows are drawn in the direction of the electron flow

2. Curved arrows indicate the movement of electrons

3. The head of a curved arrow always points at an atom

CH3COCH3

O

HO -

+ CH3COCH3

O -

OH

CH3COCH3

O

HO -

+ CH3COCH3

O -

OH

correct incorrect

4. The arrow starts at the electron source

16

REACCIONES DE ADICION ELECTROFILICA

C C

Hidratacin C C

H OH

Halgenos

C C

Cl Cl

Hidroxilacin

C C

OH OH

Hidrogenacion

C C

H H

Halohidrinas

C C

Cl OH

Halogenuros de hidrgeno

C C

H Cl

18

Electrophilic Addition of HX to

Alkenes

General reaction mechanism: electrophilic

addition

Attack of electrophile (such as HBr) on

bond of alkene produces carbocation and

bromide ion

Carbocation is itself an electrophile,

reacting with nucleophilic bromide ion

19

In hydrohalogenation, the atoms of a hydrogen

halide add to the carbon atoms of a double

bond or triple bond.

Hydrohalogenation

CH3 CH CH CH3

H Cl

+ HClCH3 CH CH CH3

H

Br

+ HBr

20

When an unsymmetrical alkene undergoes hydrohalogenation, the H in HX adds to the carbon in the double bond that has the greater number of H.

Markovnikovs Rule

CH3 CH CH2

Cl H

CH3 CH CH2 + HCl

CH3 CH CH2

H Cl

Does not form

C with the most H

Product that forms

21

ADICION DE HALUROS DE HIDROGENO

CH2=CH2 + HX CH3CH2X

CH CH CH2=CHX CH3CHX2 HX HX

MECANISMO

CH3CH=CH2

H Cl

Paso 1 (lento)

Paso 2 (rpido)

CH3CH CH2

H

intermediario carbocatin

+ Cl-

CH3CH CH2

HCl-

CH3CHClCH3

- +

Reactivo: HBr (g) , HCl (g)

22

CH3CH=CH2 CH3CHClCH3

HCl

CH3CH2CH2Cl

REGIOSELECTIVIDAD DE LA ADICIN DE HX A ALQUENOS

Regla de Markovnikov

En la adicin de HX a alquenos no

simtricos, el H+ de HX se dirige al

carbono con > nmero de hidrgenos

CH3CH=CH2

H Cl

CH3CH CH2

H Se va a formar el carbocatin ms estable que es el ms

sustituido

ORDEN DE ESTABILIDAD DE CARBOCATIONES

< < < <

ESTABILIDAD

CH3+

metilo

CH2CH3+

primario

(CH3)2CH+

secundario

(CH3)3C+

terciario

CH2=CH-CH2+

allico

benclico

CH2+

23

Two step

process

First transition

state is high

energy point

Electrophilic Addition Energy

Diagram:

25

Stability of Carbocations and

Markovnikovs Rule More stable carbocation forms faster

Tertiary cations and associated transition states are more stable

than primary cations

26

Example of Electrophilic

Addition

Addition of hydrogen

bromide to 2-Methyl-

propene

H-Br transfers proton to

C=C

Forms carbocation

intermediate

More stable cation

forms

Bromide adds to

carbocation

27

Electrophilic Addition for Syntheses

The reaction is successful with HCl and with HI as well

as HBr. Note that HI is generated from KI and

phosphoric acid

28

Orientacion de la adicion electrofilica:

Regla de Markovnikov En un alqueno asimetrico, el reactivo HX puede

adicionarse de dos maneras distintas, pero una es preferida sobre la otra.

Si una orientacion predomina, se dice que la reaccion es regiospecifica

Markovnikov observo en el siglo XIX que en la adicion de HX a un alqueno, el H se une al carbono con mas hidrogenos y el X se une al otro carbono (aquel con mayor numero de sustituyentes alquilicos)

Esta es la Regla de Markovnikov

29

Addition of HCl to 2-methylpropene is regiospecific one product forms where two are possible

If both ends have similar substitution, then the reaction is not regiospecific

Example of Markovnikovs Rule

30

Same example:

32

Practice Problem:

33

Solution:

34

Practice Problem:

35

Solution:

36

Problem : Major products?

37

Problem : Which alkene?

38

Mecanismo adicion electrofilica de alquenos

39

Transition State resembles cation

40

a more stable

carbocation

Carbocation Formation Is the Rate-

Limiting Step

41

Carbocation Stabilities

Alkyl groups decrease the concentration of positive

charge in the carbocation

42

Carbocation Structure and Stability

Carbocations are planar

The positively charged carbon is

surrounded by only 6 electrons in

three sp2 orbitals

The fourth orbital on carbon is a

vacant p-orbital

44

Carbocation Structure and Stability

The stability of the carbocation

(measured by energy needed to form it

from R-X) is increased by the presence

of alkyl substituents

Therefore stability of carbocations:

3 > 2 > 1 > +CH3

46

Heterolytic bond dissociations

energies:

47

Stabilizing Carbocations:

48

Electrophilic Addition Reactions Are

Regioselective

49

Mechanism of Electrophilic Addition:

Rearrangements of Carbocations

Carbocations undergo structural

rearrangements following set patterns

1,2-H and 1,2-alkyl shifts occur

Goes to give more stable carbocation

Can go through less stable ions as

intermediates

50

Rearrangement of Carbocation

1,2-hydride shift a more stable carbocation

51

a more stable

carbocation

Rearrangement of Carbocation

1,2-methyl shift

52

Carbocation does not always rearrange

53

Carbocation Rearrangement

a more stable

carbocation

Ring Expansion

54

Carbocation rearrangements:

55

Hydride Shifts

56

Alkyl (methyl) shifts

57

TRANSPOSICIONES

CH3 C CH CH2

CH3

H

CH3 C CH CH3

CH3

H

+ CH3 C CH CH3

CH3

H OH

CH3 C CH CH3

CH3

H+

transposicin de hidruro

CH3 C CH CH3

CH3

HOH

El desplazamiento 1,2 de hidruro convierte al carbocatin secundario en uno

terciario que es ms estable produciendo el alcohol terciario como producto

mayoritario

58

Problem : mechanism?

59

H

Br

HBr

H-shift

Br-1

60


61

CH3

H3C

CH2

CH3

H3C

CH3

CH3

CH3

CH3

CH3

CH3

CH3

Cl

62

Problem : What will be the

rearranged cations?

63


64

Problem : carbocation structure?

65

ADICION DE HBr ANTI-MARKOVNIKOV

CH3CH=CH2

HBr

CH3CH2CH2Br perxidos

MECANISMO RADICALES

1. Formacin del radical Br

ROOR 2 RO

RO + HBr ROH + Br

2. Adicin de Br al alqueno

Br + CH3CH=CH2 CH3CHBrCH2 CH3CHCH2Br

3. Formacin del producto

CH3CHCH2Br

+ H-Br CH3CH2CH2Br + Br

C-O

O

O-C

O

C-O

O

35 kcal/mol

66

In hydration, H and OH from water add to the

carbon atoms of a double bond or triple bond

to form alcohols (OH).

The reaction is catalyzed by acid H+.

Hidratacion de alquenos: Alcoholes

+ HOHH

OH

CH3 CH CH2 + HOH

CH3 CH CH2

OH H

H+

H+

67

Addition of Water to Alkenes:

Hydration

Hydration of an alkene is the addition of H-OH

to to give an alcohol

Acid catalysts are used in high temperature

industrial processes: ethylene is converted to

ethanol

68

Addition of HOH to Alkene

What is the electrophile?

What nucleophile is present in the greatest concentration?

69

HIDRATACION

CH3CH=CH2 + H2O CH3CH CH3

OHH+

MECANISMO 2 pasos como adicin de HX

1.

CH3CH CH2

H

CH3CH=CH2 + H+

CH3CH CH2

H

+ H2O CH3CH CH3

OH2

+

- H+ CH3CH CH3

OH

Sigue la regla de

Markovnikov produciendo

el alcohol ms sustituido

Es la inversa de la

deshidratacin de

alcoholes E1

Catlisis cida

ALQUINOS

CH3CH2C CH + H2O HgSO4

H2SO4 CH3CH2C CH2

OH

CH3CH2C CH3

O

70

Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright 2000 The McGraw-Hill Companies, Inc. All rights reserved.

Mechanism of acid-catalyzed hydration

of 2-methylpropeneFigure 6.9

73

Addition of Water to Alkenes:

Hydroboration

Herbert Brown (HB) invented hydroboration (HB)

Borane (BH3) is electron deficient: a Lewis acid

Borane adds to an alkene to give an organoborane

74

BH3 is a Lewis Acid

Six electrons in outer shell

Coordinates to oxygen electron pairs in ethers

75

Addition of H-BH2 (from BH3-THF complex) to three alkenes gives a trialkylborane

Oxidation with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene

Hydroboration-Oxidation Forms an

Alcohol from an Alkene

76

Orientation in Hydration via

Hydroboration Regiochemistry is opposite to Markovnikov

orientation (anti Markovnikov )

OH is added to carbon with most Hs

H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition)

77

Mechanism of

Hydroboration Borane is a Lewis acid

Alkene is Lewis base

Transition state involves

anionic development on B

The components of BH3

are across C=C

78

Hydroboration: Orientation in

Addition Step

79

Hydroboration, Electronic Effects Give

anti-Markovnikov

More stable carbocation is also consistent with steric

preferences

80

Steric Preference

81

Hydroboration - Oxygen

Insertion Step

H2O2, OH- inserts OH in place of B

Retains syn orientation

82

HIDRATACION ANTI-MARKOVNIKOV HIDROBORACION-OXIDACION

CH3CH=CH2 1.BH3

2. H2O2, OH-

CH3CH2CH2OH

B2H6

(CH3CH2O)2O

2 BH3

gas txico

MECANISMO

CH3 CH CH2

H BH2- +

CH3 CH CH2

H BH2

hidrgeno en el carbono

ms sustituido

CH2=CH2 BH2CH2CH3

CH2=CH2 BH(CH2CH3)2

CH2=CH2

B(CH2CH3)3 H2O2, OH

- CH3CH2OH

CH3 CH CH2

H OH

alcohol menos sustituido

83

ESTEREOQUMICA DE LA HIDROBORACIN

CH3H

H2O2/ HO-

CH3

OHH

H

CH3H

OH H

HBH2

H

CH3H

BH2

ADICION SYN

El tomo de boro y el hidruro se adicionan a los dos

tomos del doble enlace simultneamente

Ambos se adicionan del mismo lado del doble enlace

Cuando el organoborano se oxida al alcohol el grupo OH- ocupa la posicin del boro

84

Hidrogenacion alquenos

85

In hydrogenation, hydrogen atoms add to the carbon atoms of a double bond or triple bond.

A catalyst such as Pt or Ni is used to speed up the reaction.

Hydrogenation

HC CH + 2H2Ni

HC CH

H H

H H

H2C CH2

H HPt

H2H2C CH2 +

86

HIDROGENACION CATALITICA

CH3CH=CH2 H2 / Pt

CH3CH2CH3

CH3C CCH3 H2 / Pt CH3CH2CH2CH3

H2 / Pd Na2CO3

Pb(AcO)4 quinolina

C C

H3C CH3

H H

Na / NH3 (lq)

cis

CH3

H

C C

H3C

H

trans

CATALIZADOR

Metal finamente dividido

o adsorbido sobre un

soporte insoluble e

inerte como carbono

o carbonato de bario

Para reducir alquenos

Pt Pd Ni Cu

Catalizador envenenado

es aquel que est

parcialmente desactivado

por tratamiento del metal

(Lindlar)

87

progreso de la reaccin

E

CH3CH=CH2

CH3CHCH3 +

CH3CH2CH2+

DIAGRAMA DE ENERGA PARA LA PROTONACION DEL PROPENO

Ea2

Ea1

Ea2 < Ea1

carbocatin secundario

Efecto inductivo

Desplazamiento de electrones a travs de

enlaces debido a la diferencia de electro-

negatividad entre los tomos de un enlace

H C+ C

R

R

R

120

C+

Hiperconjugacin

Superposicin del orbital p vaco del

sp2 con el orbital de un enlace C-H vecino

Los e- del enlace estabilizan la carga positiva del carbocacin dispersndola

88

MECANISMO

H H R2C=CR2 C C

R

R R

RH H

C C

R

R R

R

H H H

C C

R

R R

R

H

Primero el H2 se adsorbe sobre la superficie del metal, luego se rompen los

enlaces y se forman enlaces H-metal. Se adsorbe el alqueno en la superficie

del metal y su orbital interactua con los orbitales vacos del metal

La molcula de alqueno se desplaza sobre la superficie hasta que colisiona con

un tomo de hidrgeno unido al metal, se produce la reaccin y se regenera

el catalizador

89

Mechanism of Catalytic

Hydrogenation

Heterogeneous reaction between phases

Catalytic Hydrogenation of an Alkene

Hydrogenation

Alkene + H2 Alkane

Catalyst required, usually Pt, Pd, or Ni.

Finely divided metal, heterogeneous

Syn addition

=>

92

ESTEREOQUIMICA DE LA HIDROGENACION CATALITICA

ADICION SYN Los dos tomos de hidrgeno se adicionan a la misma cara

del doble enlace

H

H

H

H

CH3

CH3

CH3

CH3

CH3H3C

H3C

H

CH3H3C

H

CH3H2

Pd

H2

PtO2

- pineno reduccin exclusivamente en la

cara inferior por bloqueo del metilo

cis-1,2-dimetilciclohexano

93

Reduction of Alkenes:

Hydrogenation

Requires Pt or Pd as powders on carbon and H2 Hydrogen is first adsorbed on catalyst

Reaction is heterogeneous (process is not in solution)

94

Syn Addition

95

Steric effect:

96

Hydrogen Addition- Selectivity

With Pt or Pd catalysts, selective for C=C.

No reaction with C=O, C=N

97

Hydrogen Addition- Selectivity

98

Problem : Major Products?

99

Solid Fats from Liquid Oils

100

When hydrogen adds to the

double bonds in

vegetable oils, the

products are

solids at room

temperature.

Hydrogenation of Oils

101

In halogenation, halogen atoms add to the

carbon atoms of a double bond or triple

bond.

Halogenation

+ HC C CH3 2Cl2C C

Cl Cl

Cl Cl

CH3H

H2C CH2

Br Br

Br2H2C CH2 +

102

Testing for Double and Triple

Bonds

When bromine (Br2) is

added to an alkane, the

red color of bromine

persists.

When bromine (Br2) is

added to an alkene or

alkyne, the red color of

bromine disappears

immediately.

103

Fin hidrogenacion alkenes

104

Addition of Halogens to Alkenes

Bromine and chlorine add to alkenes to give 1,2-

dihaldes, an industrially important process

F2 is too reactive and I2 too unreactive

Cl2 reacts as Cl+ Cl-; Br2 is similar

105

Addition of Br2 to Cyclopentene

Addition is exclusively trans (stereospecific)

106

Mechanism of Bromine Addition

Br+ adds to an alkene producing a cyclic cation: a bromonium ion, in which bromine shares charge with carbon

107

Mechanism of Bromine Addition

Since the Br blocks one face, one must get anti (trans) addition

108

Addition of Br2 to Cyclopentene

110

The Reality of Bromonium Ions Bromonium were postulated more than 60 years ago to explain

the stereochemical course of the addition (to give the trans-dibromide from a cyclic alkene

George Olah (Nobel Prize 1994) showed that bromonium ions are stable in liquid SO2 with SbF5 and can be studied directly by nuclear magnetic resonance spectroscopy

111

Halohydrin Formation

This is formally the addition of HO-X to an

alkene (with +OH as the electrophile) to give a

1,2-halo alcohol, called a halohydrin

The actual reagent is the dihalogen (Br2 or Cl2

with water in an organic solvent)

112

Mechanism of Formation of a

Bromohydrin

113


Bromohydrin

114


Bromohydrin

116

An Alternative to Bromine

Bromine is a difficult reagent to use for this reaction

N-Bromosuccinimide (NBS) produces bromine in organic solvents and is a safer source

117

MECANISMO

CH3CH CH2

BrBr

- Br -

bromonio asimtrico

CH3CH CH2

Br

+

Br - bromuro ataca del lado opuesto

al que est el bromo ADICION ANTI

Br H

H

CH3C CH

Br

EVIDENCIA DE ADICION ANTI

H

H

Br

Br

H

H

Br

HH +

Br-

Br- Br

Br

H

H + trans

cis

Br2

118

FORMACION DE HALOHIDRINAS

R2C=CH2

X2, H2O

OH

R2C CH2

Br

1,2-halohidrina

X: Cl o Br

MECANISMO

CH3CH CH2

BrBr

- Br - CH3CH CH2

Br

+

H2O

OH

CH3C CH2

BrH

OH2

CH3C CH2

BrH

+

-H+

119

ADICION DE HALOGENOS

Br2 CH3CH CHCH3

CH3C CCH3

Br

CH3CH CHCH3

Br

Br Br

Br

CH3C CCH3

BrBr2

Bromo y cloro se

adicionan al doble

o triple enlace

Fluor da reacciones

explosivas

Iodo se adiciona pero

el producto es inestable

CH2=CH2 < RCH=CH2 < R2C=CH2 < R2C=CHR < R2C=CR2

ORDEN DE REACTIVIDAD DE LOS ALQUENOS

REACTIVIDAD

OxidationofAlkenes:Hydroxylation

HydroxylationaddsOHtoeachendofC=C Oxidizingagentisosmiumtetroxide Stereochemistryofadditionissyn Productisa1,2dialcoholordiol (alsocalleda

glycol)

OXIDACIONDEALQUENOS

Variosproductosdependiendodelalquenoydeloxidante

OXIDACIONDELENLACE SINRUPTURADELENLACEOXIDACIONDELENLACE CONRUPTURADELENLACE

OXIDACIONSINRUPTURA FORMACIONDEDIOLES

OHOH

HHO

O

HH

O-

O

Mn

OO

HH

O

O

Os

H

H

KMnO4,aq

25C

OsO4

Na2SO3

H2O

HO

cis1,2ciclohexanodiol

ADICIONSYN

FormationofDiolsviaOsmiumTetroxide

Hydroxylation convertstosyndiol Osmiumtetroxide,thensodiumbisulfate Viacyclicosmatediester

Problem:WhichAlkene?

OXIDACIONCONRUPTURA OZONOLISIS

OO O- +

OO O-

+O

O O-+ O

O O-+

Ozono:molculaformadaportrestomosdeoxgeno

Oznolisis1.oxidacindelalquenoporozonoparadarunoznido

2.oxidacinoreduccindeloznido

Resultadodelaoznolisiseslarupturadeldobleenlacedelalquenoparaformardoscompuestoscarbonlicos,unoacadaladodledobleenlaceoriginal

C C C C+

Esunareaccindedegradacinyseusaparalocalizarlaposicindeundobleenlaceydeducirlaestructuradeunalquenodesconocido

Reactivo

OzonolysisofAlkenes CleavageofalkeneswithozoneandworkupwithzincinaceticacidleadstolesshighlyoxidizedcarbonsthanproductsfromcleavagewithhotKMnO4

Unsubstitutedcarbonsareoxidizedtoformaldehyde,monosubstitutedcarbonsareoxidizedtoaldehydesanddisubstitutedcarbonsareoxidizedtoketones

Ozoneaddsacrossthedoublebondtoformtheinitialozonidewhichrearrangestoahighlyunstableozonide

Theozonidesreactwithzincandaceticacidtoeffectthecleavage

C CH3C CH3

H CH3

O O

OREDUCTIVA

aldehdocetona

+Zn/AcOH

OXIDATIVA

cido

2.Oxidacinoreduccindeloznido

H2O2 /H+ +

CH3CH

O

acetaldehdo

CH3CCH3

O

acetona

CH3CCH3

O

acetona

CH3COH

O

cidoactico

1.Oxidacindelalqueno

H

O

CH3-C C-CH3CH3

O OC C

H3C CH3

H CH3

O O

O

C CH3C CH3

H CH3

1,2,3trioxolano1,2,4trioxolano

OZONIDO

O3

CCl4

OzonolysisofAlkenes CleavageofalkeneswithozoneandworkupwithzincinaceticacidleadstolesshighlyoxidizedcarbonsthanproductsfromcleavagewithhotKMnO4

Unsubstitutedcarbonsareoxidizedtoformaldehyde,monosubstitutedcarbonsareoxidizedtoaldehydesanddisubstitutedcarbonsareoxidizedtoketones

SolvedProblem AnunknownalkenewithformulaC7H12 yieldsonlythefollowingproductonoxidationwithhotKMnO4

Answer:Sincenocarbonsaremissingintheproduct,thealkenemustbepartofaringintheoriginalmolecule

AdditionofRadicalstoAlkenes:Polymers

Apolymer isaverylargemoleculeconsistingofrepeatingunitsofsimplermolecules(monomers),formedbypolymerization

Alkenesreactwithradicalcatalyststoundergoradicalpolymerization

Ethyleneispolymerizedtopolyethylene

FreeRadicalPolymerizationofAlkenes

Alkenescombinemanytimestogivepolymer Reactivityinducedbyformationoffreeradicals

FreeRadicalPolymerization:Initiation

Initiation afewradicalsaregeneratedbythehomolysisoftheOObondinbenzoylperoxide

Thebenzoyloxyradicaladdstoethylenetoformacarbonradical

Polymerization:Propagation

Radicalfrominitiationaddstoalkenetogeneratealkenederivedradical

Thisradicaladdstoanotheralkene,andsoonmanytimes

Polymerization:Termination

Chainpropagationendswhentworadicalchainscombine,orreactwithsomeradicalscavenger

Notcontrolledspecificallybutaffectedbyreactivityandconcentration

OtherPolymers

UnsymmetricalMonomers

Ifthedoublebondisunsymmetrical,thereactionisviamorehighlysubstituted(morestable)radical

ChainBranchingDuringPolymerization

Duringradicalpropagationchaincandevelopforksleadingtobranching

Onemechanismofbranchingisshortchainbranchinginwhichaninternalhydrogenisabstracted

LongChainBranching

Inlongchains,ahydrogenfromanotherchainisabstracted

CationicPolymerization

VinylmonomersreactwithBrnstedorLewisacidtoproduceareactivecarbocationthataddstoalkenesandpropagatesvialengtheningcarbocations

Monomers

NaturalRubber:

Madefromthesapofanumberofplants,principallyHeveabrasiliensis

Cruderubber(latex)isZpolyisoprene

Alkenesobtention:EliminationReactions:

Alkenes are commonly made by elimination of HX from alkyl halide: (dehydrohalogenation); using heat and KOH and dehydration from alcohols.


Elimination by the E1 mechanismFigure 5.12

ZaitsevsRule:FormationoftheMostSubstitutedAlkeneisFavoredwithaSmallBase

Somehydrogenhalidescaneliminatetogivetwodifferentalkeneproducts

ZaitzevsRule:whentwodifferentalkeneproductsarepossibleinanelimination,themosthighlysubstituted(moststable)alkenewillbethemajorproduct

Thisistrueonlyifasmallbasesuchasethoxideisused

Heatsofhydrogenationofthreebuteneisomers:

OverallRelativeStabilitiesofAlkenes Thegreaterthenumberofattachedalkylgroups(i.e.themorehighlysubstitutedthecarbonatomsofthedoublebond),thegreaterthealkenesstability

SynthesisofAlkenesviaEliminationReactions Dehydrohalogenation

ReactionsbyanE2mechanismaremostuseful E1reactionscanbeproblematic

E2reactionarefavoredby: Secondaryortertiaryalkylhalides Alkoxidebasessuchassodiumethoxideorpotassiumtertbutoxide

BulkybasessuchaspotassiumtertbutoxideshouldbeusedforE2reactionsofprimaryalkylhalides

FormationoftheLeastSubstitutedAlkeneUsingaBulkyBase

Bulkybasessuchaspotassiumtertbutoxidehavedifficultyremovingstericallyhinderedhydrogensandgenerallyonlyreactwithmoreaccessiblehydrogens(e.g.primaryhydrogens)

AcidCatalyzedDehydrationofAlcohols

Recallthateliminationisfavoredoversubstitutionathighertemperatures

Typicalacidsusedindehydrationaresulfuricacidandphosphoricacid Thetemperatureandconcentrationofacidrequiredtodehydrate

dependsonthestructureofthealcohol

Primaryalcoholsaremostdifficulttodehydrate,tertiaryaretheeasiest

Rearrangementsofthecarbonskeletoncanoccur

MechanismforDehydrationofSecondaryandTertiaryAlcohols:AnE1Reaction

Onlyacatalyticamountofacidisrequiredsinceitisregeneratedinthefinalstepofthereaction


Mechanism of acid-catalyzed dehydration of tert-butyl alcoholFigure 5.6

CarbocationStabilityandtheTransitionState Recallthestabilityofcarbocationsis:

ThesecondstepoftheE1mechanisminwhichthecarbocationformsisratedetermining

Thetransitionstateforthisreactionhascarbocationcharacter Tertiaryalcoholsreactthefastestbecausetheyhavethemoststabletertiarycarbocationliketransitionstateinthesecondstep

TherelativeheightsofG forthesecondstepofE1dehydrationindicatethatprimaryalcoholshaveaprohibitivelylargeenergybarrier

CarbocationStabilityandtheOccurrenceofMolecularRearrangements RearrangementsDuringDehydrationofSecondaryAlcohols

Rearrangementsofcarbocationsoccurifamorestablecarbocationcanbeobtained

Example

ThefirsttwostepsaretosameasforanyE1dehydration

Prob.:Howmanyalkeneproductsfromdehydration?

Solution:

AlquenosReaccionesIAlkenescontOxidation of Alkenes: HydroxylationSlide Number 2Formation of Diols via Osmium TetroxideProblem : Which Alkene?Slide Number 5Slide Number 6Slide Number 7Slide Number 8Slide Number 9Slide Number 10Addition of Radicals to Alkenes: PolymersFree Radical Polymerization of AlkenesFree Radical Polymerization: InitiationPolymerization: PropagationPolymerization: TerminationOther PolymersUnsymmetrical MonomersChain Branching During PolymerizationLong Chain BranchingCationic PolymerizationSlide Number 21MonomersNatural Rubber:Alkenes obtention: Elimination Reactions:Slide Number 25Slide Number 26Slide Number 27Slide Number 28Slide Number 29Slide Number 30Slide Number 31Slide Number 32Slide Number 33Slide Number 34Slide Number 35Slide Number 36Prob. : How many alkene products from dehydration?Solution:

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