5.14 dehydrohalogenation of alkyl halides. x y dehydrogenation of alkanes: x = y = h dehydration of...
TRANSCRIPT
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5.145.14
Dehydrohalogenation of Dehydrohalogenation of
Alkyl Halides Alkyl Halides
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XX YY
dehydrogenation of alkanes:dehydrogenation of alkanes:X = Y = HX = Y = H
dehydration of alcohols:dehydration of alcohols:X = H; Y = OHX = H; Y = OH
dehydrohalogenation of alkyl halides:dehydrohalogenation of alkyl halides:X = H; Y = Br, etc.X = H; Y = Br, etc.
CC CCCC CC ++ XX YY
-Elimination Reactions Overview-Elimination Reactions Overview
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XX YY
dehydrogenation of alkanes:dehydrogenation of alkanes:industrial process; not regioselectiveindustrial process; not regioselective
dehydration of alcohols:dehydration of alcohols:acid-catalyzedacid-catalyzed
dehydrohalogenation of alkyl halides:dehydrohalogenation of alkyl halides:consumes baseconsumes base
CC CCCC CC ++ XX YY
-Elimination Reactions Overview-Elimination Reactions Overview
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is a useful method for the preparation of alkenesis a useful method for the preparation of alkenes
(100 %)(100 %)
likewise, NaOCHlikewise, NaOCH33 in methanol, or KOH in ethanol in methanol, or KOH in ethanol
NaOCHNaOCH22CHCH33
ethanol, 55°Cethanol, 55°C
DehydrohalogenationDehydrohalogenation
ClCl
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CHCH33(CH(CH22))1515CHCH22CHCH22ClCl
When the alkyl halide is When the alkyl halide is primaryprimary, potassium, potassiumterttert-butoxide in dimethyl sulfoxide is the -butoxide in dimethyl sulfoxide is the base/solvent system that is normally used. base/solvent system that is normally used.
KOC(CHKOC(CH33))33
dimethyl sulfoxidedimethyl sulfoxide
(86%)(86%)
CHCH22CHCH33(CH(CH22))1515CHCH
DehydrohalogenationDehydrohalogenation
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BrBr
29 %29 % 71 %71 %
++
RegioselectivityRegioselectivity
follows Zaitsev's rulefollows Zaitsev's rule
more highly substituted double bond predominatesmore highly substituted double bond predominates
KOCHKOCH22CHCH33
ethanol, 70°Cethanol, 70°C
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more stable configurationmore stable configurationof double bond predominatesof double bond predominates
StereoselectivityStereoselectivity
KOCHKOCH22CHCH33
ethanolethanol
BrBr
++
(23%)(23%) (77%)(77%)
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more stable configurationmore stable configurationof double bond predominatesof double bond predominates
StereoselectivityStereoselectivity
KOCHKOCH22CHCH33
ethanolethanol
++
(85%)(85%) (15%)(15%)
BrBr
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5.155.15Mechanism of theMechanism of the
Dehydrohalogenation of Alkyl Dehydrohalogenation of Alkyl Halides:Halides:
The E2 MechanismThe E2 Mechanism
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FactsFacts
(1)(1) Dehydrohalogenation of alkyl halides Dehydrohalogenation of alkyl halides exhibits second-order kineticsexhibits second-order kinetics
first order in alkyl halidefirst order in alkyl halidefirst order in basefirst order in baserate = rate = kk[alkyl halide][base][alkyl halide][base]
implies that rate-determining step implies that rate-determining step involves both base and alkyl halide; involves both base and alkyl halide; i.e., it is bimoleculari.e., it is bimolecular
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FactsFacts
(2)(2) Rate of elimination depends on halogenRate of elimination depends on halogen
weaker C—X bond; faster rateweaker C—X bond; faster raterate: RI > RBr > RCl > RFrate: RI > RBr > RCl > RF
implies that carbon-halogen bond breaks in implies that carbon-halogen bond breaks in the rate-determining stepthe rate-determining step
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concerted (one-step) bimolecular processconcerted (one-step) bimolecular process
single transition statesingle transition state
C—H bond breaksC—H bond breaks
component of double bond formscomponent of double bond forms
C—X bond breaksC—X bond breaks
The E2 MechanismThe E2 Mechanism
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––
OORR..
.... ::
CC CC
HH
XX....::::
ReactantsReactants
The E2 MechanismThe E2 Mechanism
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––
OORR..
.... ::
CC CC
HH
XX....::::
ReactantsReactants
The E2 MechanismThe E2 Mechanism
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CC CC
––
OORR..
.... HH
XX....::::––
Transition stateTransition state
The E2 MechanismThe E2 Mechanism
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OORR....
.... HH
CC CC
––XX....::::....
ProductsProducts
The E2 MechanismThe E2 Mechanism
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Stereoelectronic EffectsStereoelectronic Effects
5.165.16
Anti Elimination in E2 ReactionsAnti Elimination in E2 Reactions
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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 effectStereoelectronic 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 effectStereoelectronic effectStereoelectronic effect
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(CH(CH33))33CC
(CH(CH33))33CC
BrBr
(CH(CH33))33CCBrBr
KOC(CHKOC(CH33))33
(CH(CH33))33COHCOH
KOC(CHKOC(CH33))33
(CH(CH33))33COHCOH
ciscis
transtrans
Rate constant for Rate constant for dehydrohalogenation dehydrohalogenation of cis is 500 times of cis is 500 times greater than that of greater than that of transtrans
Stereoelectronic effectStereoelectronic effectStereoelectronic 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 H that is removed by base must be anti periplanar to Brperiplanar to Br
Two anti periplanar H atoms in cis Two anti periplanar H atoms in cis stereoisomerstereoisomer
HHHH
Stereoelectronic effectStereoelectronic effectStereoelectronic 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 H that is removed by base must be anti periplanar to Brperiplanar to Br
No anti periplanar H atoms in trans No anti periplanar H atoms in trans stereoisomer; all vicinal H atoms are stereoisomer; all vicinal H atoms are gauche to Brgauche to Br
HHHH
(CH(CH33))33CCBrBr
HH
HH
Stereoelectronic effectStereoelectronic effectStereoelectronic effectStereoelectronic effect
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ciscis
more reactivemore reactive
transtrans
less reactiveless reactive
Stereoelectronic effectStereoelectronic effectStereoelectronic effectStereoelectronic effect
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Stereoelectronic effectStereoelectronic effectStereoelectronic effectStereoelectronic effect
An effect on reactivity that has its origin An effect on reactivity that has its origin in the spatial arrangement of orbitals or bonds in the spatial arrangement of orbitals or bonds is called a stereoelectronic effect.is called a stereoelectronic effect.
The preference for an anti periplanar The preference for an anti periplanar arrangement of H and Br in the transition arrangement of H and Br in the transition state for E2 dehydrohalogenation is an state for E2 dehydrohalogenation is an example of a stereoelectronic effectexample of a stereoelectronic effect..
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5.175.17
A Different Mechanism for Alkyl A Different Mechanism for Alkyl
Halide Elimination:Halide Elimination:
The E1 MechanismThe E1 Mechanism
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CHCH33 CHCH22CHCH33
BrBr
CHCH33
Ethanol, heatEthanol, heat
++
(25%)(25%) (75%)(75%)
CC
HH33CC
CHCH33
CC CC
HH33CC
HH
CHCH22CHCH33
CHCH33
CCHH22CC
ExampleExample
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1. Alkyl halides can undergo elimination in 1. Alkyl halides can undergo elimination in absence of base.absence of base.
2. Carbocation is intermediate2. Carbocation is intermediate
3. Rate-determining step is unimolecular 3. Rate-determining step is unimolecular ionization of alkyl halide.ionization of alkyl halide.
The E1 MechanismThe E1 Mechanism
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slow, unimolecularslow, unimolecular
CCCHCH22CHCH33CHCH33
CHCH33
++
CHCH33 CHCH22CHCH33
BrBr
CHCH33
CC
::....:
:
::....:
: BrBr.... ––
Step 1Step 1
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CCCHCH22CHCH33CHCH33
CHCH33
++
CCCHCH22CHCH33CHCH33
CHCH22
++ CCCHCHCHCH33CHCH33
CHCH33
– – HH++
Step 2Step 2