ALKENES. Alkene Nomenclature AlkenesAlkenes Alkenes are hydrocarbons that contain a carbon-carbon double bond also called "olefins" characterized by

ALKENES

Alkene NomenclatureAlkene Nomenclature

AlkenesAlkenesAlkenesAlkenes

Alkenes are hydrocarbons that contain a Alkenes are hydrocarbons that contain a carbon-carbon double bondcarbon-carbon double bond

also called "olefins"also called "olefins"

characterized by molecular formula Ccharacterized by molecular formula CnnHH2n2n

said to be "unsaturated"said to be "unsaturated"

Alkene NomenclatureAlkene NomenclatureAlkene NomenclatureAlkene Nomenclature

HH22CC CHCH22 HH22CC CHCHCHCH33

EtheneEtheneoror

EthyleneEthylene(both are acceptable(both are acceptable

IUPAC names)IUPAC names)

PropenePropene

(Propylene is(Propylene issometimes used sometimes used

but is not an acceptablebut is not an acceptableIUPAC name)IUPAC name)


1) Find the longest continuous chain that 1) Find the longest continuous chain that includes the double bond.includes the double bond.

2) Replace the -2) Replace the -aneane ending of the unbranched ending of the unbranched alkane having the same number of carbons alkane having the same number of carbons by -by -eneene..

3) Number the chain in the direction that gives 3) Number the chain in the direction that gives the lowest number to the doubly bonded the lowest number to the doubly bonded carbon.carbon.

HH22CC CHCHCHCH22CHCH33 1-Butene1-Butene


4) If a substituent is present, identify its position 4) If a substituent is present, identify its position by number. The double bond takes by number. The double bond takes precedence over alkyl groups and halogens precedence over alkyl groups and halogens when the chain is numbered.when the chain is numbered.

The compound shown above isThe compound shown above is4-bromo-3-methyl-1-butene.4-bromo-3-methyl-1-butene.

HH22CC CHCHCHCHCHCH22BrBr

CHCH33


4) If a substituent is present, identify its position 4) If a substituent is present, identify its position by number. by number. HydroxylHydroxyl groups take groups take precedence over the double bond when the precedence over the double bond when the chain is numbered.chain is numbered.

The compound shown above isThe compound shown above is2-methyl-3-buten-1-ol.2-methyl-3-buten-1-ol.

HH22CC CHCHCHCHCHCH22OHOH

CHCH33

Alkenyl groupsAlkenyl groupsAlkenyl groupsAlkenyl groups

methylenemethylene

vinylvinyl

allylallyl

isopropenylisopropenyl

CHCHHH22CC

HH22CC CHCHCHCH22

HH22CC CCHCCH33

HH22CC

Cycloalkene NomenclatureCycloalkene NomenclatureCycloalkene NomenclatureCycloalkene Nomenclature

1) Replace the -1) Replace the -aneane ending of the cycloalkane ending of the cycloalkane having the same number of carbons by -having the same number of carbons by -eneene..

CyclohexeneCyclohexene



2) Number 2) Number throughthrough the double bond in the the double bond in thedirection that gives the lower number to the direction that gives the lower number to the first-appearing substituent.first-appearing substituent.

CHCH33

CHCH22CHCH33



2) Number 2) Number throughthrough the double bond in the the double bond in thedirection that gives the lower number to the direction that gives the lower number to the first-appearing substituent.first-appearing substituent.

6-Ethyl-1-methylcyclohexene6-Ethyl-1-methylcyclohexeneCHCH33

CHCH22CHCH33

Structure and Bonding in Structure and Bonding in AlkenesAlkenes

Structure of EthyleneStructure of EthyleneStructure of EthyleneStructure of Ethylene

bond angles: bond angles: H-C-H = 117°H-C-H = 117°

H-C-C = 121°H-C-C = 121°

bond distances: bond distances: C—H = 110 pmC—H = 110 pm

C=C = 134 pmC=C = 134 pm

planarplanar

• Framework of bonds• Each carbon is sp2 hybridized

Bonding in Ethylene

• Each carbon has a half-filled p orbital

Bonding in Ethylene

• Side-by-side overlap of half-filled p orbitals gives a bond

Bonding in Ethylene

Isomerism in AlkenesIsomerism in Alkenes

IsomersIsomersIsomersIsomers

Isomers are different compounds thatIsomers are different compounds thathave the same molecular formula.have the same molecular formula.


StereoisomersStereoisomersStereoisomersStereoisomersConstitutional isomersConstitutional isomersConstitutional isomersConstitutional isomers



different connectivitydifferent connectivitysame connectivity;same connectivity;

different arrangementdifferent arrangementof atoms in spaceof atoms in space



consider the isomeric alkenes of consider the isomeric alkenes of

molecular formula Cmolecular formula C44HH88

2-Methylpropene2-Methylpropene1-Butene1-Butene

cis-cis-2-Butene2-Butene trans-trans-2-Butene2-Butene

CC CC

HH

HH HH

CHCH22CHCH33

HH33CC

CC CC

CHCH33

HH

HHHH

CHCH33

CC CC

HH33CC

HH

CC CC

HH

HHHH33CC

HH33CC


cis-cis-2-Butene2-Butene

CC CC

HH

HH HH

CHCH22CHCH33

HH

CHCH33

CC CC

HH33CC

HH

CC CC

HH

HHHH33CC

HH33CC

Constitutional isomersConstitutional isomersConstitutional isomersConstitutional isomers


trans-trans-2-Butene2-Butene

CC CC

HH

HH HH

CHCH22CHCH33

HH33CC

CC CC

CHCH33

HH

HH

CC CC

HH

HHHH33CC

HH33CC

Constitutional isomersConstitutional isomersConstitutional isomersConstitutional isomers

cis-cis-2-Butene2-Butene trans-trans-2-Butene2-Butene

HH33CC

CC CC

CHCH33

HH

HHHH

CHCH33

CC CC

HH33CC

HH

StereoisomersStereoisomersStereoisomersStereoisomers

Stereochemical NotationStereochemical NotationStereochemical NotationStereochemical Notation

cis (identical or cis (identical or analogous substituents analogous substituents on same side)on same side)

trans (identical or trans (identical or analogous substitutents analogous substitutents on opposite sides)on opposite sides)

Figure 5.2Figure 5.2Figure 5.2Figure 5.2

transtransciscis

Interconversion of stereoisomericInterconversion of stereoisomericalkenes does not normally occur.alkenes does not normally occur.

Requires that Requires that component of doublecomponent of doublebond be broken.bond be broken.

Figure 5.2Figure 5.2Figure 5.2Figure 5.2

transtransciscis

Naming Stereoisomeric Naming Stereoisomeric Alkenes by the Alkenes by the E-ZE-Z Notational Notational

SystemSystem

Stereochemical NotationStereochemical NotationStereochemical NotationStereochemical Notation

cis and trans are useful when substituents are cis and trans are useful when substituents are identical or analogous (oleic acid has a cis identical or analogous (oleic acid has a cis double bond)double bond)

cis and trans are ambiguous when analogies cis and trans are ambiguous when analogies are not obviousare not obvious

CC CC

CHCH33(CH(CH22))66CHCH22 CHCH22(CH(CH22))66COCO22HH

HH HH

Oleic acidOleic acid

ExampleExampleExampleExample

What is needed:What is needed:

1) 1) systematic body of rules for ranking systematic body of rules for ranking substituentssubstituents

2)2) new set of stereochemical symbols othernew set of stereochemical symbols otherthan cis and transthan cis and trans

CC CC

HH FF

ClCl BrBr

CC CC

The E-Z Notational SystemThe E-Z Notational SystemThe E-Z Notational SystemThe E-Z Notational System

EE : : higher ranked substituents on higher ranked substituents on oppositeopposite sides sides

ZZ : : higher ranked substituents on higher ranked substituents on samesame side side

higherhigher

lowerlower

CC CC




higherhigher

lowerlower

CC CC




EntgegenEntgegen

higherhigher

higherhigherlowerlower

lowerlower

CC CCCC CC




EntgegenEntgegen ZusammenZusammen

higherhigher


lowerlower

lowerlower

higherhigher

lowerlower

higherhigher

CC CCCC CC


Answer: Answer: They are ranked in order of They are ranked in order of decreasing atomic number. decreasing atomic number.

EntgegenEntgegen ZusammenZusammen

higherhigher


lowerlower

lowerlower

higherhigher

lowerlower

higherhigher

Question: How are substituents ranked?Question: How are substituents ranked?

The Cahn-Ingold-Prelog (CIP) SystemThe Cahn-Ingold-Prelog (CIP) SystemThe Cahn-Ingold-Prelog (CIP) SystemThe Cahn-Ingold-Prelog (CIP) System

The system that we use was devised byThe system that we use was devised byR. S. CahnR. S. CahnSir Christopher IngoldSir Christopher IngoldVladimir PrelogVladimir Prelog

Their rules for ranking groups were devised in Their rules for ranking groups were devised in connection with a different kind of connection with a different kind of stereochemistry—one that we will discuss in stereochemistry—one that we will discuss in Chapter 7—but have been adapted to alkene Chapter 7—but have been adapted to alkene stereochemistry.stereochemistry.

(1)(1) Higher atomic number outranks lower Higher atomic number outranks lower atomic numberatomic number

Br > FBr > F Cl > HCl > H

higherhigher

lowerlower

BrBr

FF

ClCl

HH

higherhigher

lowerlower

CC CC

Table 5.1 CIP RulesTable 5.1 CIP RulesTable 5.1 CIP RulesTable 5.1 CIP Rules

(1)(1) Higher atomic number outranks lower Higher atomic number outranks lower atomic numberatomic number

Br > FBr > F Cl > HCl > H

((Z Z )-1-Bromo-2-chloro-1-fluoroethene)-1-Bromo-2-chloro-1-fluoroethene

higherhigher

lowerlower

BrBr

FF

ClCl

HH

higherhigher

lowerlower

CC CC


(2) When two atoms are identical, compare the (2) When two atoms are identical, compare the atoms attached to them on the basis of their atoms attached to them on the basis of their atomic numbers. Precedence is established atomic numbers. Precedence is established at the first point of difference. at the first point of difference.

——CCHH22CCHH33 outranks — outranks —CCHH33

——CC((CC,H,H),H,H)


——CC(H,H,H)(H,H,H)

(3) Work outward from the point of attachment, (3) Work outward from the point of attachment, comparing all the atoms attached to a comparing all the atoms attached to a particular atom before proceeding furtherparticular atom before proceeding furtheralong the chain. along the chain.

——CCH(H(CCHH33))22 outranks outranks —C—CHH22CCHH22OHOH

——CC((CC,,CC,H),H) ——CC((CC,H,H),H,H)


(4) (4) Evaluate substituents one by one. Evaluate substituents one by one. Don't add atomic numbers within groups.Don't add atomic numbers within groups.

——CCHH22OOH outranks H outranks —C—C(CH(CH33))33

——CC((OO,H,H),H,H) ——CC(C,C,C)(C,C,C)


(5)(5) An atom that is multiply bonded to another An atom that is multiply bonded to another atom is considered to be replicated as a atom is considered to be replicated as a

substituent on that atom.substituent on that atom.

——CCH=H=OO outranks outranks —C—CHH22OOHH

——CC((OO,,OO,H),H) ——CC((OO,H,H),H,H)


(A table of commonly encountered substituents ranked according to (A table of commonly encountered substituents ranked according to

precedence is given on the inside back cover of the text.)precedence is given on the inside back cover of the text.)

Physical Properties of AlkenesPhysical Properties of Alkenes

= 0 D= 0 D

CC CC

HH HH

HHHH

= 0.3 D= 0.3 D

HH

HH HH

CC CC

HH33CC

Dipole momentsDipole momentsDipole momentsDipole moments

What is direction of What is direction of dipole moment?dipole moment?

Does a methyl group Does a methyl group donate electrons to the donate electrons to the double bond, or does it double bond, or does it withdraw them?withdraw them?

= 0 D= 0 D

CC CC

HH HH

HHHH

= 1.4 D= 1.4 D

CC CC

HH HH

ClClHH

= 0.3 D= 0.3 D

HH

HH HH

CC CC

HH33CC


Chlorine is Chlorine is electronegative and electronegative and attracts electrons.attracts electrons.

= 1.4 D= 1.4 D

CC CC

HH HH

ClClHH

= 0.3 D= 0.3 D

HH

HH HH

CC CC

HH33CC = 1.7 D= 1.7 D

HH

HH ClCl

CC CC

HH33CC


Dipole moment Dipole moment of 1-of 1-chloropropene chloropropene is equal to the is equal to the sum of the sum of the dipole dipole moments of moments of vinyl chloride vinyl chloride and propene.and propene.

= 1.7 D= 1.7 D

= 1.4 D= 1.4 D

CC CC

HH HH

ClClHH

= 0.3 D= 0.3 D

HH

HH HH

CC CC

HH33CC

HH

HH ClCl

CC CC

HH33CC


Therefore, a Therefore, a methyl group methyl group donates donates electrons to electrons to the double the double bond.bond.

Alkyl groups stabilize Alkyl groups stabilize spsp22 hybridized hybridized carbon by releasing electrons carbon by releasing electrons

Alkyl groups stabilize Alkyl groups stabilize spsp22 hybridized hybridized carbon by releasing electrons carbon by releasing electrons

....

RR—C+—C+ HH—C+—C+is more stable thanis more stable than

RR—C—C HH—C—Cis more stable thanis more stable than

RR—C—C is more stable thanis more stable than HH—C—C

Relative Stabilities of AlkenesRelative Stabilities of Alkenes

Double bonds are classified according toDouble bonds are classified according tothe number of carbons attached to them.the number of carbons attached to them.


HH

CC CC

RR

HH

HH

monosubstitutedmonosubstituted

R'R'

CC CC

RR

HH

HH

disubstituteddisubstitutedHH

CC CC

RR

HH

R'R'

disubstituteddisubstituted

HH

CC CC

RR HH

R'R'

disubstituteddisubstituted



R'R'

CC CC

RR

HH

R"R"

trisubstitutedtrisubstituted

R'R'

CC CC

RR

R"'R"'

R"R"

tetrasubstitutedtetrasubstituted

Substituent effects on alkene stabilitySubstituent effects on alkene stabilitySubstituent effects on alkene stabilitySubstituent effects on alkene stability

ElectronicElectronic

disubstituted alkenes are more stable disubstituted alkenes are more stable than monosubstituted alkenesthan monosubstituted alkenes

StericSteric

transtrans alkenes are more stable than alkenes are more stable than ciscis alkenes alkenes

+ 6O+ 6O22

4CO4CO22 + 8H + 8H22OO

2700 kJ/mol2700 kJ/mol2700 kJ/mol2700 kJ/mol




Fig. 5.4 Heats of Fig. 5.4 Heats of combustion of Ccombustion of C44HH88

isomers.isomers.

Fig. 5.4 Heats of Fig. 5.4 Heats of combustion of Ccombustion of C44HH88

isomers.isomers.


ElectronicElectronic

alkyl groups stabilize double bonds more than Halkyl groups stabilize double bonds more than H

more highly substituted double bonds are moremore highly substituted double bonds are morestable than less highly substituted ones.stable than less highly substituted ones.

Problem 5.8Problem 5.8Problem 5.8Problem 5.8

Give the structure or make a molecular model of Give the structure or make a molecular model of the most stable Cthe most stable C66HH1212 alkene. alkene.

CC CC

Problem 5.8Problem 5.8Problem 5.8Problem 5.8

Give the structure or make a molecular model of Give the structure or make a molecular model of the most stable Cthe most stable C66HH1212 alkene. alkene.

CC CC

HH33CC

HH33CC CHCH33

CHCH33


Steric effectsSteric effects

transtrans alkenes are more stable than alkenes are more stable than ciscis alkenes alkenes

ciscis alkenes are destabilized by van der Waals alkenes are destabilized by van der Waalsstrain strain

ciscis-2-butene-2-butene transtrans-2-butene-2-butene

van der Waals strainvan der Waals straindue to crowding ofdue to crowding ofcis-methyl groupscis-methyl groups

Figure 5.5 Figure 5.5 cis and trans-2-Butenecis and trans-2-Butene

Fig. 5.5Fig. 5.5cis and trans-2-butenecis and trans-2-butene

Fig. 5.5Fig. 5.5cis and trans-2-butenecis and trans-2-butene

ciscis-2-butene-2-butene transtrans-2-butene-2-butene

van der Waals strainvan der Waals straindue to crowding ofdue to crowding ofcis-methyl groupscis-methyl groups

Van der Waals StrainVan der Waals StrainVan der Waals StrainVan der Waals Strain

Steric effect causes a large difference in stabilitySteric effect causes a large difference in stabilitybetween between ciscis and and transtrans-(CH-(CH33))33CCH=CHC(CHCCH=CHC(CH33))33

ciscis is 44 kJ/mol less stable than is 44 kJ/mol less stable than transtrans

CC CC

HH HH

CCCC CHCH33

CHCH33HH33CC

HH33CC

HH33CC CHCH33

CycloalkenesCycloalkenes

Cyclopropene and cyclobutene have angle Cyclopropene and cyclobutene have angle strain.strain.

Larger cycloalkenes, such as cyclopenteneLarger cycloalkenes, such as cyclopenteneand cyclohexene, can incorporate a double and cyclohexene, can incorporate a double bond into the ring with little or no angle strain. bond into the ring with little or no angle strain.

CycloalkenesCycloalkenesCycloalkenesCycloalkenes

ciscis-cyclooctene and -cyclooctene and transtrans-cyclooctene-cycloocteneare stereoisomersare stereoisomers

ciscis-cyclooctene is 39 kJ/ mol more stable-cyclooctene is 39 kJ/ mol more stablethan than transtrans-cyclooctene-cyclooctene

Stereoisomeric cycloalkenesStereoisomeric cycloalkenesStereoisomeric cycloalkenesStereoisomeric cycloalkenes

ciscis-Cyclooctene-Cyclooctene transtrans-Cyclooctene-Cyclooctene

HH

HH HHHH

transtrans-cyclooctene is smallest -cyclooctene is smallest transtrans-cycloalkene -cycloalkene that is stable at room temperature that is stable at room temperature

ciscis stereoisomer is more stable than stereoisomer is more stable than transtrans through C through C11 11 cycloalkenescycloalkenes

ciscis and and transtrans-cyclododecene are approximately -cyclododecene are approximately equal in stability equal in stability


transtrans-Cyclooctene-Cyclooctene

HHHH


transtrans-Cyclododecene-Cyclododeceneciscis-Cyclododecene-Cyclododecene







When there are more than 12 carbons in theWhen there are more than 12 carbons in thering, ring, transtrans-cycloalkenes are more stable than -cycloalkenes are more stable than ciscis..The ring is large enough so the cycloalkene behavesThe ring is large enough so the cycloalkene behavesmuch like a noncyclic one.much like a noncyclic one.


Preparation of Alkenes:Elimination Reactions

HH YY

•dehydrogenation of alkanes: H; Y = H

•dehydration of alcohols: H; Y = OH

•dehydrohalogenation of alkyl halides: H; Y = Br, etc.

-Elimination Reactions Overview-Elimination Reactions Overview-Elimination Reactions Overview-Elimination Reactions Overview

CC CCCC CC ++ HH YY

DehydrogenationDehydrogenationDehydrogenationDehydrogenation

• limited to industrial syntheses of ethylene, propene, 1,3-butadiene, and styrene

• important economically, but rarely used in laboratory-scale syntheses

750°C750°CCHCH33CHCH33

750°C750°CCHCH33CHCH22CHCH33

HH22CC CHCH22 ++ HH22

HH22CC CHCHCHCH33 ++ HH22

Dehydration of AlcoholsDehydration of Alcohols

Dehydration of AlcoholsDehydration of AlcoholsDehydration of AlcoholsDehydration of Alcohols

(79-87%)(79-87%)

(82%)(82%)

HH22SOSO44

160°C160°CCHCH33CHCH22OHOH HH22CC CHCH22 ++ HH22OO

OHOHHH22SOSO44

140°C140°C

++ HH22OO

CC OHOH

CHCH33

CHCH33

HH33CC HH22SOSO44

heatheatCHCH22

HH33CC

CC

HH33CC

++ HH22OO

RR

R'R'

R"R"

OHOHCC

RR

R'R'

HH

OHOHCC

RR

HH

HH

OHOHCC

Relative Reactivity

tertiary:tertiary:most reactivemost reactive

primary:primary:least reactiveleast reactive

Regioselectivity in Alcohol Dehydration:Regioselectivity in Alcohol Dehydration:The Zaitsev RuleThe Zaitsev Rule

10 %10 % 90 %90 %

HOHO

HH22SOSO44

80°C80°C++

RegioselectivityRegioselectivity

• A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective.


• A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective.

84 %84 % 16 %16 %

HH33POPO44

heatheat

CHCH33

OHOH

CHCH33

++

CHCH33

The Zaitsev RuleThe Zaitsev RuleThe Zaitsev RuleThe Zaitsev Rule

• When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the carbon having thefewest hydrogens.

RR OHOH

CHCH33

CC CC

HH

RR CHCH22RR

three protons on this three protons on this carbon carbon



RR OHOH

CHCH33

CC CC

HH

RR CHCH22RR

two protons on this two protons on this carbon carbon



RR OHOH

CHCH33

CC CC

HH

RR CHCH22RR

only one proton on this only one proton on this carbon carbon



RR

RR

CHCH22RR

CHCH33

CC CC

RR OHOH

CHCH33

CC CC

HH

RR CHCH22RR

only one proton on this only one proton on this carbon carbon


Zaitsev Rule states that the elimination Zaitsev Rule states that the elimination reaction yields the more highly substitutedreaction yields the more highly substitutedalkene as the major product.alkene as the major product.

The more stable alkene product The more stable alkene product predominates.predominates.

StereoselectivityStereoselectivityinin

Alcohol Dehydration Alcohol Dehydration

StereoselectivityStereoselectivityStereoselectivityStereoselectivity

• A stereoselective reaction is one in which a single starting material can yield two or more stereoisomeric products, but gives one of them in greater amounts than any other.

(25%)(25%) (75%)(75%)

++

OHOH

HH22SOSO44

heatheat

The Mechanism of the The Mechanism of the Acid-Catalyzed Dehydration of Acid-Catalyzed Dehydration of

AlcoholsAlcohols

• The dehydration of alcohols and the reaction of alcohols with hydrogen halides share thefollowing common features:

• 1) Both reactions are promoted by acids

• 2) The relative reactivity decreases in theorder tertiary > secondary > primary

These similarities suggest that carbocationsare intermediates in the acid-catalyzeddehydration of alcohols, just as they are inthe reaction of alcohols with hydrogen halides.

A connecting point...A connecting point...

•first two steps of mechanism are identical tothose for the reaction of tert-butyl alcohol withhydrogen halides

Dehydration of tert-Butyl AlcoholDehydration of tert-Butyl Alcohol

CC OHOH

CHCH33

CHCH33

HH33CCHH22SOSO44

heatheatCHCH22

HH33CC

CC

HH33CC

++ HH22OO

MechanismMechanismMechanismMechanism

Step 1: Step 1: Proton transfer to Proton transfer to terttert-butyl alcohol -butyl alcohol

(CH3)3C OO

HH

....:: HH OO++

HH

OO :: ++(CH(CH33))33CC

HH

++

fast, bimolecularfast, bimolecular

terttert-Butyloxonium ion-Butyloxonium ion

....

HH

HH

++

OO

HH

::

HH

::


Step 2: Step 2: Dissociation of Dissociation of terttert-butyloxonium ion-butyloxonium ionto carbocation to carbocation

++

(CH(CH33))33CC OO

HH

::

HH++

slow, unimolecularslow, unimolecular

(CH(CH33))33CC OO

HH

::

HH

::

terttert-Butyl cation-Butyl cation

++


Step 3: Step 3: Deprotonation of Deprotonation of terttert-butyl cation.-butyl cation.


++

OO

HH

::

HH

::

CHCH22++

HH33CC

CC

HH33CC

HH

CHCH22

HH33CC

CC

HH33CC

++ OO

HH

::

HH

HH++

CarbocationsCarbocationsCarbocationsCarbocations

are intermediates in the acid-catalyzeddehydration of tertiary and secondary alcohols

Carbocations can:

•react with nucleophiles

•lose a -proton to form an alkene (Called an E1 mechanism)

Dehydration of primary alcoholsDehydration of primary alcoholsDehydration of primary alcoholsDehydration of primary alcohols

•A different mechanism from 3 o or 2 o alcohols

•avoids carbocation because primarycarbocations are too unstable

•oxonium ion loses water and a proton in abimolecular step

HH22SOSO44

160°C160°CCHCH33CHCH22OHOH HH22CC CHCH22 ++ HH22OO

Step 1: Step 1: Proton transfer from acid to ethanolProton transfer from acid to ethanol

HH

....:: HH OO++OOCHCH33CHCH22

....

HH

HH

HH

OO :: ++

HH

++


Ethyloxonium ionEthyloxonium ion

CHCH33CHCH22 OO

HH

::

HH

::

Mechanism

Step 2: Step 2: Oxonium ion loses both a proton and Oxonium ion loses both a proton and a water molecule in the same step.a water molecule in the same step.

++

HH

OO ::

HH

++CHCH22CHCH22HHOO

HH

::

HH

::

slow, bimolecularslow, bimolecular

++ OO

HH

::

HH

::OO

HH

HH

:: HH++

HH22CC CHCH22++

Mechanism

Step 2: Step 2: Oxonium ion loses both a proton and Oxonium ion loses both a proton and a water molecule in the same step.a water molecule in the same step.

++

HH

OO ::

HH

++CHCH22CHCH22HHOO

HH

::

HH

::

slow, bimolecularslow, bimolecular

++ OO

HH

::

HH

::OO

HH

HH

:: HH++

HH22CC CHCH22++

Mechanism

Because rate-determiningstep is bimolecular, thisis called the E2 mechanism.

Rearrangements in Alcohol Rearrangements in Alcohol DehydrationDehydration

Sometimes the alkene product does not have the same carbon skeleton as the starting alcohol.

Example Example Example Example

OHOH

HH33POPO44, heat, heat

3%3% 64%64% 33%33%

++ ++

Rearrangement involves alkyl group migration Rearrangement involves alkyl group migration Rearrangement involves alkyl group migration Rearrangement involves alkyl group migration

3%3% CHCH33

CHCHCHCH33CHCH33

CHCH33

++CC

• carbocation can lose a proton as shown

• or it can undergo a methyl migration

• CH3 group migrates with its pair of electrons to adjacent positively charged carbon


3%3%

CHCH33

CHCHCHCH33

CHCH33

++CHCH33

97%97% CHCH33

CHCHCHCH33CHCH33

CHCH33

++CC CC

• tertiary carbocation; more stable


3%3%

CHCH33

CHCHCHCH33

CHCH33

++CHCH33

97%97% CHCH33

CHCHCHCH33CHCH33

CHCH33

++CC CC

Another rearrangementAnother rearrangementAnother rearrangementAnother rearrangement

CH3CH2CH2CH2OH

HH33POPO44, heat, heat

12%12%

++

mixture of mixture of ciscis (32%) (32%)and and transtrans-2-butene (56%)-2-butene (56%)

CHCH22CHCH33CHCH22CHCH CHCHCHCH33CHCH33CHCH

Rearrangement involves hydride shiftRearrangement involves hydride shiftRearrangement involves hydride shiftRearrangement involves hydride shift

oxonium ion can lose water and a proton (from C-2) to give1-butene

doesn't give a carbocation directlybecause primarycarbocations are too unstable

CHCH33CHCH22CHCH22CHCH22 OO

HH

HH

++::

CHCH22CHCH33CHCH22CHCH


hydrogen migrates with its pair of electrons from C-2 to C-1 as water is lost

carbocation formed by hydride shift is secondary


HH

HH

++::


CHCH33CHCH22CHCHCHCH33++

Hydride shiftHydride shiftHydride shiftHydride shift

HH

HH

CHCH33CHCH22CCHHCCHH22 OO

HH

++::

++CHCH33CHCH22CCHHCCHH22 ++

HH

OO

HH

HH

::::



HH

HH

++::


CHCH33CHCH22CHCHCHCH33++

mixture of mixture of ciscisand and transtrans-2-butene-2-butene

CHCHCHCH33CHCH33CHCH

Carbocations can...Carbocations can...Carbocations can...Carbocations can...

•react with nucleophiles

•lose a proton from the -carbon to form an alkene

•rearrange (less stable to more stable) (alkyl shift or hydride shift)

Dehydrohalogenation of Alkyl Halides

HH YY

•dehydrogenation of alkanes: H; Y = H

•dehydration of alcohols: H; Y = OH

•dehydrohalogenation of alkyl halides: H; Y = Br, etc.


CC CCCC CC ++ HH YY

HH YY

•dehydrogenation of alkanes:industrial process; not regioselective

•dehydration of alcohols:acid-catalyzed

•dehydrohalogenation of alkyl halides:consumes base


CC CCCC CC ++ HH YY

DehydrohalogenationDehydrohalogenationDehydrohalogenationDehydrohalogenation

A useful method for the preparation of alkenes

ClCl

(100 %)(100 %)

likewise, NaOCHlikewise, NaOCH33 in methanol, or KOH in ethanol in methanol, or KOH in ethanol

NaOCHNaOCH22CHCH33

ethanol, 55°Cethanol, 55°C

CHCH33(CH(CH22))1515CHCH22CHCH22ClCl

When the alkyl halide is primary, potassiumtert-butoxide in dimethyl sulfoxide is the base/solvent system that is normally used.

KOC(CHKOC(CH33))33

dimethyl sulfoxidedimethyl sulfoxide

(86%)(86%)

DehydrohalogenationDehydrohalogenationDehydrohalogenationDehydrohalogenation

CHCH22CHCH33(CH(CH22))1515CHCH

BrBr

29 %29 % 71 %71 %

++


follows Zaitsev's rule:

more highly substituted double bond predominates

KOCHKOCH22CHCH33

ethanol, 70°Cethanol, 70°C

•more stable configurationof double bond predominates

StereoselectivityStereoselectivity KOCHKOCH22CHCH33

ethanolethanol

BrBr

++

(23%)(23%)(77%)(77%)

•more stable configurationof double bond predominates

StereoselectivityStereoselectivity

KOCHKOCH22CHCH33

ethanolethanol

++

(85%)(85%) (15%)(15%)

BrBr

Mechanism of the Mechanism of the Dehydrohalogenation of Alkyl Halides: Dehydrohalogenation of Alkyl Halides:

The E2 MechanismThe E2 Mechanism

FactsFacts

• (1) Dehydrohalogenation of alkyl halides exhibits second-order kinetics

first order in alkyl halidefirst order in baserate = k[alkyl halide][base]

implies that rate-determining step involves both base and alkyl halide; i.e., it is bimolecular (second-order)

FactsFacts

• (2) Rate of elimination depends on halogen

weaker C—X bond; faster raterate: RI > RBr > RCl > RF

implies that carbon-halogen bond breaks in the rate-determining step

The E2 MechanismThe E2 MechanismThe E2 MechanismThe E2 Mechanism

•concerted (one-step) bimolecular process

•single transition state

C—H bond breaks

component of double bond forms

C—X bond breaks


––

OORR..

....::

CC CC

HH

XX....::::

ReactantsReactants


––

OORR..

....::

CC CC

HH

XX....::::

ReactantsReactants

CC CC


––

OORR..

.... HH

XX....::::––

Transition stateTransition state


OORR....

.... HH

CC CC

––XX....

::::....

ProductsProducts

Anti Elimination in E2 Anti Elimination in E2 ReactionsReactions

Stereoelectronic Effects

Isotope Effects

(CH(CH33))33CC

(CH(CH33))33CC

BrBr

KOC(CHKOC(CH33))33

(CH(CH33))33COHCOH

ciscis-1-Bromo-4--1-Bromo-4-tert-tert- butylcyclohexanebutylcyclohexane

Stereoelectronic effectStereoelectronic effect

(CH(CH33))33CC

(CH(CH33))33CCBrBr KOC(CHKOC(CH33))33

(CH(CH33))33COHCOH

transtrans-1-Bromo-4--1-Bromo-4-tert-tert- butylcyclohexanebutylcyclohexane


(CH(CH33))33CC

(CH(CH33))33CC

BrBr

(CH(CH33))33CC

BrBr

KOC(CHKOC(CH33))33

(CH(CH33))33COHCOH

KOC(CHKOC(CH33))33

(CH(CH33))33COHCOH

ciscis

transtrans

Rate constant for dehydrohalogenation of cis is 500 times greater than that of trans


(CH(CH33))33CC

(CH(CH33))33CC

BrBr

KOC(CHKOC(CH33))33

(CH(CH33))33COHCOH

ciscis

H that is removed by base must be anti periplanar to Br

Two anti periplanar H atoms in cis stereoisomer

HHHH


(CH(CH33))33CC

KOC(CHKOC(CH33))33

(CH(CH33))33COHCOH

transtrans

H that is removed by base must be anti periplanar to Br

No anti periplanar H atoms in trans stereoisomer; all vicinal H atoms are gauche to Br

HHHH

(CH(CH33))33CC

BrBrHH

HH


ciscis

more reactivemore reactive

transtrans

less reactiveless reactive



An effect on reactivity that has its origin in the spatial arrangement of orbitals or bonds is called a stereoelectronic effect.

The preference for an anti periplanar arrangement of H and Br in the transition state for E2 dehydrohalogenation is an example of a stereoelectronic effect.

Isotope effectIsotope effectDeuterium,D, is a heavy isotope of

hydrogen but will undergo the same reactions. But the C-D bond is stronger so a reaction where the rate involves breaking a C-H (C-D) bond, the deuterated sample will have a reaction rate 3-8 times slower.

In other words comparing the two rates, i.e. kH/kD = 3-8 WHEN the rate determining step involves breaking the C-H bond.

A Different Mechanism for A Different Mechanism for Alkyl Halide Elimination:Alkyl Halide Elimination:


ExampleExampleExampleExample

CHCH33 CHCH22CHCH33

BrBr

CHCH33

Ethanol, heatEthanol, heat

++

(25%)(25%) (75%)(75%)

CC

HH33CC

CHCH33

CC CC

HH33CC

HH

CHCH22CHCH33

CHCH33

CCHH22CC

1. Alkyl halides can undergo elimination in absence of base.

2. Carbocation is intermediate

3. Rate-determining step is unimolecular ionization of alkyl halide.

4. Generally with tertiary halide, base is weak and at low concentration.


Step 1Step 1Step 1Step 1

slow, unimolecularslow, unimolecular

CCCHCH22CHCH33CHCH33

CHCH33

++

CHCH33 CHCH22CHCH33

BrBr

CHCH33

CC

::....::

::....:: BrBr.... ––

Step 2Step 2Step 2Step 2


CHCH33

++


CHCH22

++ CCCHCHCHCH33CHCH33

CHCH33

– – HH++

Documents

ALKENES. Alkene Nomenclature AlkenesAlkenes Alkenes are hydrocarbons that contain a carbon-carbon double bond also called "olefins" characterized by