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Addition of Hydrogen HalidesAddition of Hydrogen Halides
toto
Conjugated DienesConjugated Dienes
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ProtonProton adds to end of diene system adds to end of diene system
Carbocation formed is allylicCarbocation formed is allylic
Electrophilic Addition to Conjugated DienesElectrophilic Addition to Conjugated Dienes HH XX
HH
++
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HHClCl
Example:Example: HH
HH
HH
HH
HH
HH
ClCl
HHHH
HH
HH
HH
HHHH
HH
HH
HHHH
HH
ClCl
HHHH
?? ??
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HHClCl
Example:Example: HH
HH
HH
HH
HH
HH
ClCl
HHHH
HH
HH
HH
HHHH
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via:via: HHHH
HH
HH
HH
HHHH
++
HH
HH
HH
HH
HH
HH
HH XX HH
HH
HH
HH
HH
HHHH
++
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and:and: HHHH
HH
HH
HH
HHHH
++ HHHH
HH
HH
HH
HHHH
++
ClCl––
ClCl
HHHH
HH
HH
HH
HHHH
HH
HHHH
HH
HH
HHHH
ClCl
3-Chlorocyclopentene3-Chlorocyclopentene
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1,2-Addition versus 1,4-Addition1,2-Addition versus 1,4-Addition 1,2-addition of XY1,2-addition of XY
XX
YY
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1,2-addition of XY1,2-addition of XY 1,4-addition of XY1,4-addition of XY
XX
YY XX
YY 1,2-Addition versus 1,4-Addition1,2-Addition versus 1,4-Addition
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viavia
1,2-addition of XY1,2-addition of XY 1,4-addition of XY1,4-addition of XY
XX
YY XX
YY XX
++
1,2-Addition versus 1,4-Addition1,2-Addition versus 1,4-Addition
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electrophilic additionelectrophilic addition1,2 and 1,4-addition both observed1,2 and 1,4-addition both observedproduct ratio depends on temperatureproduct ratio depends on temperature
HBr Addition to 1,3-ButadieneHBr Addition to 1,3-Butadiene
HH22CC CHCHCHCH CHCH22
HBrHBr
BrBr
CHCH22CHCH33CHCHCHCH CHCHCHCH22BrBrCHCH33CHCH++
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3-Bromo-1-butene is formed faster than3-Bromo-1-butene is formed faster than1-bromo-2-butene because allylic carbocations 1-bromo-2-butene because allylic carbocations react with nucleophiles preferentially at the carbon react with nucleophiles preferentially at the carbon that bears the greater share of positive charge. that bears the greater share of positive charge.
RationaleRationale
BrBr
CHCH22CHCH33CHCHCHCH CHCHCHCH22BrBrCHCH33CHCH++
CHCH22CHCH33CHCHCHCH CHCHCHCH22CHCH33CHCH
via:via:++ ++
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3-Bromo-1-butene is formed faster than3-Bromo-1-butene is formed faster than1-bromo-2-butene because allylic carbocations 1-bromo-2-butene because allylic carbocations react with nucleophiles preferentially at the carbon react with nucleophiles preferentially at the carbon that bears the greater share of positive charge. that bears the greater share of positive charge.
RationaleRationale
BrBr
CHCH22CHCH33CHCHCHCH CHCHCHCH22BrBrCHCH33CHCH++
formed fasterformed faster
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more stablemore stable
RationaleRationale
BrBr
CHCH22CHCH33CHCHCHCH CHCHCHCH22BrBrCHCH33CHCH++
1-Bromo-2-butene is more stable than1-Bromo-2-butene is more stable than3-bromo-1-butene because it has a3-bromo-1-butene because it has amore highly substituted double bond.more highly substituted double bond.
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major product at -80°Cmajor product at -80°C
RationaleRationale
major product at 25°Cmajor product at 25°C
The two products equilibrate at 25°C.The two products equilibrate at 25°C.Once equilibrium is established, the moreOnce equilibrium is established, the morestable isomer predominates.stable isomer predominates.
BrBr
CHCH22CHCH33CHCHCHCH CHCHCHCH22BrBrCHCH33CHCH
(formed faster)(formed faster) (more stable)(more stable)
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Kinetic ControlKinetic Controlversusversus
Thermodynamic ControlThermodynamic Control
Kinetic control: major product is the one formed Kinetic control: major product is the one formed at the fastest rateat the fastest rate
Thermodynamic control: major product is the Thermodynamic control: major product is the one that is the most stableone that is the most stable
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H2C CHCH CH2
HBr
CH2CH3CHCH CHCH2CH3CH+ +
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CH2CH3CHCH+
CHCH2CH3CH+
Br
CH2CH3CHCH
CHCH2BrCH3CH
higher activation energy
formed more slowly
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Addition of hydrogen chloride to Addition of hydrogen chloride to 2-methyl-1,3-butadiene is a kinetically controlled 2-methyl-1,3-butadiene is a kinetically controlled reaction and gives one product in much greater reaction and gives one product in much greater amounts than any isomers. What is this product?amounts than any isomers. What is this product?
++ HClHCl ??
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Think mechanistically.Think mechanistically.
Protonation occurs:Protonation occurs:
at end of diene systemat end of diene system
in direction that gives most stable carbocationin direction that gives most stable carbocation
Kinetically controlled product corresponds to attack by Kinetically controlled product corresponds to attack by
chloride ion at carbon that has the greatest share of chloride ion at carbon that has the greatest share of
positive charge in the carbocationpositive charge in the carbocation
++ HClHCl
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Think mechanistically Think mechanistically HH ClCl
++ ++
one resonance form is one resonance form is
tertiary carbocation; tertiary carbocation;
other is primaryother is primary
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HH ClCl
++ ++
one resonance form is one resonance form is
secondary carbocation; secondary carbocation;
other is primaryother is primary
one resonance form is one resonance form is
tertiary carbocation; tertiary carbocation;
other is primaryother is primary
ClCl HH
++
++
Think mechanistically Think mechanistically
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HH ClCl
++ ++
one resonance form is one resonance form is
tertiary carbocation; tertiary carbocation;
other is primaryother is primary
More stable carbocationMore stable carbocation
Is attacked by chloride ion Is attacked by chloride ion
at carbon that bears at carbon that bears
greater share of positive greater share of positive
chargecharge
Think mechanistically Think mechanistically
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HH ClCl
++ ++
one resonance form is one resonance form is
tertiary carbocation; tertiary carbocation;
other is primaryother is primary
ClClClCl––
majormajorproductproduct
Think mechanistically Think mechanistically
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gives mixtures of 1,2 and gives mixtures of 1,2 and 1,4-addition products1,4-addition products
Halogen Addition to DienesHalogen Addition to Dienes
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ExampleExample
HH22CC CHCHCHCH CHCH22
BrBr22
BrBr
CHCH22BrBrCHCH22CHCHCHCH CHCHCHCH22BrBrBrBrCHCH22CHCH++
(37%)(37%) (63%)(63%)