alkenes. alkene nomenclature alkenesalkenes alkenes are hydrocarbons that contain a carbon-carbon...
TRANSCRIPT
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ALKENES
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Alkene NomenclatureAlkene Nomenclature
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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"
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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)
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Alkene NomenclatureAlkene NomenclatureAlkene NomenclatureAlkene Nomenclature
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
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Alkene NomenclatureAlkene NomenclatureAlkene NomenclatureAlkene Nomenclature
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
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Alkene NomenclatureAlkene NomenclatureAlkene NomenclatureAlkene Nomenclature
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
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Alkenyl groupsAlkenyl groupsAlkenyl groupsAlkenyl groups
methylenemethylene
vinylvinyl
allylallyl
isopropenylisopropenyl
CHCHHH22CC
HH22CC CHCHCHCH22
HH22CC CCHCCH33
HH22CC
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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
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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..
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
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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..
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
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Structure and Bonding in Structure and Bonding in AlkenesAlkenes
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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
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• Framework of bonds• Each carbon is sp2 hybridized
Bonding in Ethylene
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• Each carbon has a half-filled p orbital
Bonding in Ethylene
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• Side-by-side overlap of half-filled p orbitals gives a bond
Bonding in Ethylene
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Isomerism in AlkenesIsomerism in Alkenes
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IsomersIsomersIsomersIsomers
Isomers are different compounds thatIsomers are different compounds thathave the same molecular formula.have the same molecular formula.
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IsomersIsomersIsomersIsomers
StereoisomersStereoisomersStereoisomersStereoisomersConstitutional isomersConstitutional isomersConstitutional isomersConstitutional isomers
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IsomersIsomersIsomersIsomers
StereoisomersStereoisomersStereoisomersStereoisomersConstitutional isomersConstitutional isomersConstitutional isomersConstitutional isomers
different connectivitydifferent connectivitysame connectivity;same connectivity;
different arrangementdifferent arrangementof atoms in spaceof atoms in space
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IsomersIsomersIsomersIsomers
StereoisomersStereoisomersStereoisomersStereoisomersConstitutional isomersConstitutional isomersConstitutional isomersConstitutional isomers
consider the isomeric alkenes of consider the isomeric alkenes of
molecular formula Cmolecular formula C44HH88
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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
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2-Methylpropene2-Methylpropene1-Butene1-Butene
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
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2-Methylpropene2-Methylpropene1-Butene1-Butene
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
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cis-cis-2-Butene2-Butene trans-trans-2-Butene2-Butene
HH33CC
CC CC
CHCH33
HH
HHHH
CHCH33
CC CC
HH33CC
HH
StereoisomersStereoisomersStereoisomersStereoisomers
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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)
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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.
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Figure 5.2Figure 5.2Figure 5.2Figure 5.2
transtransciscis
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Naming Stereoisomeric Naming Stereoisomeric Alkenes by the Alkenes by the E-ZE-Z Notational Notational
SystemSystem
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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
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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
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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
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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
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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
EntgegenEntgegen
higherhigher
higherhigherlowerlower
lowerlower
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CC CCCC 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
EntgegenEntgegen ZusammenZusammen
higherhigher
higherhigherlowerlower
lowerlower
lowerlower
higherhigher
lowerlower
higherhigher
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CC CCCC CC
The E-Z Notational SystemThe E-Z Notational SystemThe E-Z Notational SystemThe E-Z Notational System
Answer: Answer: They are ranked in order of They are ranked in order of decreasing atomic number. decreasing atomic number.
EntgegenEntgegen ZusammenZusammen
higherhigher
higherhigherlowerlower
lowerlower
lowerlower
higherhigher
lowerlower
higherhigher
Question: How are substituents ranked?Question: How are substituents ranked?
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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.
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(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
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(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
Table 5.1 CIP RulesTable 5.1 CIP RulesTable 5.1 CIP RulesTable 5.1 CIP Rules
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(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)
Table 5.1 CIP RulesTable 5.1 CIP RulesTable 5.1 CIP RulesTable 5.1 CIP Rules
——CC(H,H,H)(H,H,H)
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(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)
Table 5.1 CIP RulesTable 5.1 CIP RulesTable 5.1 CIP RulesTable 5.1 CIP Rules
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(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)
Table 5.1 CIP RulesTable 5.1 CIP RulesTable 5.1 CIP RulesTable 5.1 CIP Rules
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(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)
Table 5.1 CIP RulesTable 5.1 CIP RulesTable 5.1 CIP RulesTable 5.1 CIP Rules
(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.)
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Physical Properties of AlkenesPhysical Properties of Alkenes
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= 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?
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= 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
Dipole momentsDipole momentsDipole momentsDipole moments
Chlorine is Chlorine is electronegative and electronegative and attracts electrons.attracts electrons.
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= 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 momentsDipole momentsDipole momentsDipole moments
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.
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= 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
Dipole momentsDipole momentsDipole momentsDipole moments
Therefore, a Therefore, a methyl group methyl group donates donates electrons to electrons to the double the double bond.bond.
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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
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Relative Stabilities of AlkenesRelative Stabilities of Alkenes
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Double bonds are classified according toDouble bonds are classified according tothe number of carbons attached to them.the number of carbons attached to them.
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
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Double bonds are classified according toDouble bonds are classified according tothe number of carbons attached to them.the number of carbons attached to them.
Double bonds are classified according toDouble bonds are classified according tothe number of carbons attached to them.the number of carbons attached to them.
R'R'
CC CC
RR
HH
R"R"
trisubstitutedtrisubstituted
R'R'
CC CC
RR
R"'R"'
R"R"
tetrasubstitutedtetrasubstituted
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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
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+ 6O+ 6O22
4CO4CO22 + 8H + 8H22OO
2700 kJ/mol2700 kJ/mol2700 kJ/mol2700 kJ/mol
2707 kJ/mol2707 kJ/mol2707 kJ/mol2707 kJ/mol
2717 kJ/mol2717 kJ/mol2717 kJ/mol2717 kJ/mol
2710 kJ/mol2710 kJ/mol2710 kJ/mol2710 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.
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Substituent effects on alkene stabilitySubstituent effects on alkene stabilitySubstituent effects on alkene stabilitySubstituent effects on alkene stability
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.
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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
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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
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Substituent effects on alkene stabilitySubstituent effects on alkene stabilitySubstituent effects on alkene stabilitySubstituent effects on alkene stability
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
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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
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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
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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
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CycloalkenesCycloalkenes
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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
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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
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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
Stereoisomeric cycloalkenesStereoisomeric cycloalkenesStereoisomeric cycloalkenesStereoisomeric cycloalkenes
transtrans-Cyclooctene-Cyclooctene
HHHH
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Stereoisomeric cycloalkenesStereoisomeric cycloalkenesStereoisomeric cycloalkenesStereoisomeric cycloalkenes
transtrans-Cyclododecene-Cyclododeceneciscis-Cyclododecene-Cyclododecene
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
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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
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.
Stereoisomeric cycloalkenesStereoisomeric cycloalkenesStereoisomeric cycloalkenesStereoisomeric cycloalkenes
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Preparation of Alkenes:Elimination Reactions
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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
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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
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Dehydration of AlcoholsDehydration of Alcohols
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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
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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
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Regioselectivity in Alcohol Dehydration:Regioselectivity in Alcohol Dehydration:The Zaitsev RuleThe Zaitsev Rule
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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.
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RegioselectivityRegioselectivity
• 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
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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
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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
two protons on this two protons on this carbon carbon
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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
only one proton on this only one proton on this carbon carbon
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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
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
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The Zaitsev RuleThe Zaitsev RuleThe Zaitsev RuleThe Zaitsev Rule
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.
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StereoselectivityStereoselectivityinin
Alcohol Dehydration Alcohol Dehydration
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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
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The Mechanism of the The Mechanism of the Acid-Catalyzed Dehydration of Acid-Catalyzed Dehydration of
AlcoholsAlcohols
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• 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...
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•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
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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
::
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MechanismMechanismMechanismMechanism
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
++
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MechanismMechanismMechanismMechanism
Step 3: Step 3: Deprotonation of Deprotonation of terttert-butyl cation.-butyl cation.
fast, bimolecularfast, bimolecular
++
OO
HH
::
HH
::
CHCH22++
HH33CC
CC
HH33CC
HH
CHCH22
HH33CC
CC
HH33CC
++ OO
HH
::
HH
HH++
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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)
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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
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Step 1: Step 1: Proton transfer from acid to ethanolProton transfer from acid to ethanol
HH
....:: HH OO++OOCHCH33CHCH22
....
HH
HH
HH
OO :: ++
HH
++
fast, bimolecularfast, bimolecular
Ethyloxonium ionEthyloxonium ion
CHCH33CHCH22 OO
HH
::
HH
::
Mechanism
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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
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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.
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Rearrangements in Alcohol Rearrangements in Alcohol DehydrationDehydration
Sometimes the alkene product does not have the same carbon skeleton as the starting alcohol.
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Example Example Example Example
OHOH
HH33POPO44, heat, heat
3%3% 64%64% 33%33%
++ ++
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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
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Rearrangement involves alkyl group migration Rearrangement involves alkyl group migration Rearrangement involves alkyl group migration Rearrangement involves alkyl group migration
3%3%
CHCH33
CHCHCHCH33
CHCH33
++CHCH33
97%97% CHCH33
CHCHCHCH33CHCH33
CHCH33
++CC CC
• tertiary carbocation; more stable
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Rearrangement involves alkyl group migration Rearrangement involves alkyl group migration Rearrangement involves alkyl group migration Rearrangement involves alkyl group migration
3%3%
CHCH33
CHCHCHCH33
CHCH33
++CHCH33
97%97% CHCH33
CHCHCHCH33CHCH33
CHCH33
++CC CC
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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
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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
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Rearrangement involves hydride shiftRearrangement involves hydride shiftRearrangement involves hydride shiftRearrangement involves hydride shift
hydrogen migrates with its pair of electrons from C-2 to C-1 as water is lost
carbocation formed by hydride shift is secondary
CHCH33CHCH22CHCH22CHCH22 OO
HH
HH
++::
CHCH22CHCH33CHCH22CHCH
CHCH33CHCH22CHCHCHCH33++
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Hydride shiftHydride shiftHydride shiftHydride shift
HH
HH
CHCH33CHCH22CCHHCCHH22 OO
HH
++::
++CHCH33CHCH22CCHHCCHH22 ++
HH
OO
HH
HH
::::
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Rearrangement involves hydride shiftRearrangement involves hydride shiftRearrangement involves hydride shiftRearrangement involves hydride shift
CHCH33CHCH22CHCH22CHCH22 OO
HH
HH
++::
CHCH22CHCH33CHCH22CHCH
CHCH33CHCH22CHCHCHCH33++
mixture of mixture of ciscisand and transtrans-2-butene-2-butene
CHCHCHCH33CHCH33CHCH
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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)
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Dehydrohalogenation of Alkyl Halides
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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
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HH YY
•dehydrogenation of alkanes:industrial process; not regioselective
•dehydration of alcohols:acid-catalyzed
•dehydrohalogenation of alkyl halides:consumes base
-Elimination Reactions Overview-Elimination Reactions Overview-Elimination Reactions Overview-Elimination Reactions Overview
CC CCCC CC ++ HH YY
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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
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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
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BrBr
29 %29 % 71 %71 %
++
RegioselectivityRegioselectivity
follows Zaitsev's rule:
more highly substituted double bond predominates
KOCHKOCH22CHCH33
ethanol, 70°Cethanol, 70°C
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•more stable configurationof double bond predominates
StereoselectivityStereoselectivity KOCHKOCH22CHCH33
ethanolethanol
BrBr
++
(23%)(23%)(77%)(77%)
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•more stable configurationof double bond predominates
StereoselectivityStereoselectivity
KOCHKOCH22CHCH33
ethanolethanol
++
(85%)(85%) (15%)(15%)
BrBr
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Mechanism of the Mechanism of the Dehydrohalogenation of Alkyl Halides: Dehydrohalogenation of Alkyl Halides:
The E2 MechanismThe E2 Mechanism
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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)
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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
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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
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The E2 MechanismThe E2 MechanismThe E2 MechanismThe E2 Mechanism
––
OORR..
....::
CC CC
HH
XX....::::
ReactantsReactants
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The E2 MechanismThe E2 MechanismThe E2 MechanismThe E2 Mechanism
––
OORR..
....::
CC CC
HH
XX....::::
ReactantsReactants
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CC CC
The E2 MechanismThe E2 MechanismThe E2 MechanismThe E2 Mechanism
––
OORR..
.... HH
XX....::::––
Transition stateTransition state
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The E2 MechanismThe E2 MechanismThe E2 MechanismThe E2 Mechanism
OORR....
.... HH
CC CC
––XX....
::::....
ProductsProducts
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Anti Elimination in E2 Anti Elimination in E2 ReactionsReactions
Stereoelectronic Effects
Isotope Effects
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(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
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(CH(CH33))33CC
(CH(CH33))33CCBrBr KOC(CHKOC(CH33))33
(CH(CH33))33COHCOH
transtrans-1-Bromo-4--1-Bromo-4-tert-tert- butylcyclohexanebutylcyclohexane
Stereoelectronic effectStereoelectronic effect
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(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
Stereoelectronic effectStereoelectronic effect
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(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
Stereoelectronic effectStereoelectronic effect
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(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
Stereoelectronic effectStereoelectronic effect
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ciscis
more reactivemore reactive
transtrans
less reactiveless reactive
Stereoelectronic effectStereoelectronic effect
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Stereoelectronic effectStereoelectronic effect
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.
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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.
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A Different Mechanism for A Different Mechanism for Alkyl Halide Elimination:Alkyl Halide Elimination:
The E1 MechanismThe E1 Mechanism
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ExampleExampleExampleExample
CHCH33 CHCH22CHCH33
BrBr
CHCH33
Ethanol, heatEthanol, heat
++
(25%)(25%) (75%)(75%)
CC
HH33CC
CHCH33
CC CC
HH33CC
HH
CHCH22CHCH33
CHCH33
CCHH22CC
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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.
The E1 MechanismThe E1 Mechanism
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Step 1Step 1Step 1Step 1
slow, unimolecularslow, unimolecular
CCCHCH22CHCH33CHCH33
CHCH33
++
CHCH33 CHCH22CHCH33
BrBr
CHCH33
CC
::....::
::....:: BrBr.... ––
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Step 2Step 2Step 2Step 2
CCCHCH22CHCH33CHCH33
CHCH33
++
CCCHCH22CHCH33CHCH33
CHCH22
++ CCCHCHCHCH33CHCH33
CHCH33
– – HH++