Download - 6.14 Addition of Halogens to Alkenes
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6.146.14
Addition of HalogensAddition of Halogens
to Alkenesto Alkenes
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++ XX22 XX XX
electrophilic addition to double bondelectrophilic addition to double bondforms a vicinal dihalideforms a vicinal dihalide
CC CCCC CC
General featuresGeneral features
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CHCH33CHCHCHCHCH(CHCH(CH33))22
(100%)(100%)
CHClCHCl33
0°C0°C
CHCHCH(CHCH(CH33))22CHCH33CHCHBrBr22
ExampleExample
BrBr BrBr
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FF22 addition proceeds with explosive violence addition proceeds with explosive violence
II22 addition is endothermic: vicinal diiodides addition is endothermic: vicinal diiodides
dissociate to an alkene and Idissociate to an alkene and I22
limited to Cllimited to Cl22 and Br and Br22
ScopeScope
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anti additionanti additionanti additionanti addition
6.156.15Stereochemistry of Halogen AdditionStereochemistry of Halogen Addition
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BrBr22
transtrans-1,2-Dibromocyclopentane-1,2-Dibromocyclopentane80% yield; only product80% yield; only product
ExampleExample
HH
HH
BrBr
BrBr
HH
HH
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ClCl22
transtrans-1,2-Dichlorocyclooctane-1,2-Dichlorocyclooctane73% yield; only product73% yield; only product
ExampleExample
HH
HH
HH
HH
ClCl
ClCl
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6.166.16Mechanism of Halogen Addition to Mechanism of Halogen Addition to
Alkenes: Halonium IonsAlkenes: Halonium Ions
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BrBr22 is not polar, but it is polarizable is not polar, but it is polarizable
two stepstwo steps(1) (1) formation of bromonium ionformation of bromonium ion(2) (2) nucleophilic attack on bromonium nucleophilic attack on bromonium
ion by bromideion by bromide
Mechanism is electrophilic additionMechanism is electrophilic addition
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ethyleneethylene HH22C=CHC=CH22 11
propenepropene CHCH33CH=CHCH=CH22 6161
2-methylpropene 2-methylpropene (CH(CH33))22C=CHC=CH22 54005400
2,3-dimethyl-2-butene 2,3-dimethyl-2-butene (CH(CH33))22C=C(CHC=C(CH33))22 920,000920,000
More highly substituted double bonds react faster.More highly substituted double bonds react faster.Alkyl groups on the double bond make itAlkyl groups on the double bond make itmore “electron rich.”more “electron rich.”
Relative Rates of BrominationRelative Rates of Bromination
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HH22CC CHCH22 BrBrCHCH22CHCH22BrBr++ BrBr22
??
++CC CC
BrBr
++
:: ::....
....:: ::....BrBr
––
No obvious explanation for anti addition No obvious explanation for anti addition provided by this mechanism.provided by this mechanism.
Mechanism?Mechanism?
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HH22CC CHCH22 BrBrCHCH22CHCH22BrBr++ BrBr22
++CC CC
BrBr:: ::++
....:: ::....BrBr
––
Cyclic bromonium ionCyclic bromonium ion
MechanismMechanism
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BrBr
BrBr
Mutual polarizationMutual polarizationof electron distributionsof electron distributionsof Brof Br22 and alkene and alkene
Formation of Bromonium IonFormation of Bromonium Ion
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BrBr
BrBr
––Electrons flow Electrons flow from alkene toward Brfrom alkene toward Br22
++++
Formation of Bromonium IonFormation of Bromonium Ion
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BrBr––
BrBr++
electrons of alkeneelectrons of alkenedisplace Brdisplace Br–– from Br from Br
Formation of Bromonium IonFormation of Bromonium Ion
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attack of Brattack of Br–– from side opposite from side opposite
C—Br bond of bromonium ion givesC—Br bond of bromonium ion gives
anti additionanti addition
++
––BrBr:: ::....
....
BrBr....
....
BrBr
BrBr....::
....
....::::
....
StereochemistryStereochemistry
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BrBr22
transtrans-1,2-Dibromocyclopentane-1,2-Dibromocyclopentane80% yield; only product80% yield; only product
ExampleExample
HH
HH
BrBr
BrBr
HH
HH
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Cyclopentene +BrCyclopentene +Br22
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––
Bromonium ionBromonium ion
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––
––
Bromide ion attacks Bromide ion attacks the bromonium ion the bromonium ion from side opposite from side opposite carbon-bromine bondcarbon-bromine bond
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trans-Stereochemistry in trans-Stereochemistry in vicinal dibromidevicinal dibromide
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6.176.17Conversion of Alkenes to Vicinal Conversion of Alkenes to Vicinal
HalohydrinsHalohydrins
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++ X X22 XX XXCC CCCC CC
alkenes react with Xalkenes react with X22 to form vicinal dihalides to form vicinal dihalides
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++ X X22 XX XXCC CCCC CC
++ H H22OO OHOH
+ + HH—X—X
++ X X22 XX CC CCCC CC
alkenes react with Xalkenes react with X22 to form vicinal dihalides to form vicinal dihalides
alkenes react with Xalkenes react with X22 in water to give vicinal in water to give vicinal
halohydrinshalohydrins
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ClCl22
anti anti addition: only productaddition: only product
HH22OO
HH22CC CHCH22 BrBrCHCH22CHCH22OOHH++ BrBr22
HH22OO
(70%)(70%)
ExamplesExamples
HH
HH
OOHH
ClCl
HH
HH
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++BrBr....
....
BrBr....::
....
::....OO
....OO
++
bromonium ion is bromonium ion is intermediateintermediate
water is nucleophile that water is nucleophile that attacks bromonium ionattacks bromonium ion
MechanismMechanism
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ClCl22
anti anti addition: only productaddition: only product
HH22OO
HH22CC CHCH22 BrBrCHCH22CHCH22OOHH++ BrBr22
HH22OO
(70%)(70%)
ExamplesExamples
HH
HH
OOHH
ClCl
HH
HH
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Cyclopentene + ClCyclopentene + Cl22
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Chloronium ionChloronium ion
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Water attacks Water attacks chloronium ion chloronium ion from side from side opposite opposite carbon-chlorine carbon-chlorine bondbond
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trans-Stereochemistry trans-Stereochemistry in oxonium ionin oxonium ion
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trans-2-Chlorocyclopentanoltrans-2-Chlorocyclopentanol
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(77%)(77%)
HH33CC
CC CHCH22
HH33CC
CHCH33
OHOH
CC CHCH22BrBrCHCH33
Markovnikov's rule applied to halohydrin Markovnikov's rule applied to halohydrin formation: formation: the halogen adds to the carbon the halogen adds to the carbon having the greater number of hydrogens.having the greater number of hydrogens.
BrBr22
HH22OO
RegioselectivityRegioselectivity
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CHCH22
HH33CC
CCHH33CC
transition state has for attack of water on bromonium ion transition state has for attack of water on bromonium ion has carbocation character; more stable transition state (left) has carbocation character; more stable transition state (left)
has positive charge on more highly substituted carbonhas positive charge on more highly substituted carbon
HH33CC CHCH22
HH33CC
BrBr:: ::
HH
OOHH ....
BrBr:: ::
HH
OO HH....
CC
ExplanationExplanation