dr. wolf's chm 201 & 202 5-1 5.14 dehydrohalogenation of alkyl halides
TRANSCRIPT
Dr. Wolf's CHM 201 & 202 5-1
5.14 Dehydrohalogenation of
Alkyl Halides
Dr. Wolf's CHM 201 & 202 5-2
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
Dr. Wolf's CHM 201 & 202 5-3
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
Dr. Wolf's CHM 201 & 202 5-4
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
Dr. Wolf's CHM 201 & 202 5-5
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
Dr. Wolf's CHM 201 & 202 5-6
BrBr
29 %29 % 71 %71 %
++
RegioselectivityRegioselectivity
follows Zaitsev's rule:
more highly substituted double bond predominates
KOCHKOCH22CHCH33
ethanol, 70°Cethanol, 70°C
Dr. Wolf's CHM 201 & 202 5-7
•more stable configurationof double bond predominates
StereoselectivityStereoselectivity KOCHKOCH22CHCH33
ethanolethanol
BrBr
++
(23%)(23%)(77%)(77%)
Dr. Wolf's CHM 201 & 202 5-8
•more stable configurationof double bond predominates
StereoselectivityStereoselectivity
KOCHKOCH22CHCH33
ethanolethanol
++
(85%)(85%) (15%)(15%)
BrBr
Dr. Wolf's CHM 201 & 202 5-9
5.155.15Mechanism of the Mechanism of the
Dehydrohalogenation of Alkyl Halides: Dehydrohalogenation of Alkyl Halides: The E2 MechanismThe E2 Mechanism
Dr. Wolf's CHM 201 & 202 5-10
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)
Dr. Wolf's CHM 201 & 202 5-11
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
Dr. Wolf's CHM 201 & 202 5-12
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
Dr. Wolf's CHM 201 & 202 5-13
The E2 MechanismThe E2 MechanismThe E2 MechanismThe E2 Mechanism
––
OORR..
....::
CC CC
HH
XX....::::
ReactantsReactants
Dr. Wolf's CHM 201 & 202 5-14
The E2 MechanismThe E2 MechanismThe E2 MechanismThe E2 Mechanism
––
OORR..
....::
CC CC
HH
XX....::::
ReactantsReactants
Dr. Wolf's CHM 201 & 202 5-15
CC CC
The E2 MechanismThe E2 MechanismThe E2 MechanismThe E2 Mechanism
––
OORR..
.... HH
XX....::::––
Transition stateTransition state
Dr. Wolf's CHM 201 & 202 5-16
The E2 MechanismThe E2 MechanismThe E2 MechanismThe E2 Mechanism
OORR....
.... HH
CC CC
––XX....
::::....
ProductsProducts
Dr. Wolf's CHM 201 & 202 5-17
5.16 & 5.175.16 & 5.17Anti Elimination in E2 Anti Elimination in E2
ReactionsReactions
Stereoelectronic Effects
Isotope Effects
Dr. Wolf's CHM 201 & 202 5-18
(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
Dr. Wolf's CHM 201 & 202 5-19
(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
Dr. Wolf's CHM 201 & 202 5-20
(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
Dr. Wolf's CHM 201 & 202 5-21
(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
Dr. Wolf's CHM 201 & 202 5-22
(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
Dr. Wolf's CHM 201 & 202 5-23
ciscis
more reactivemore reactive
transtrans
less reactiveless reactive
Stereoelectronic effectStereoelectronic effect
Dr. Wolf's CHM 201 & 202 5-24
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.
Dr. Wolf's CHM 201 & 202 5-25
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.
Dr. Wolf's CHM 201 & 202 5-26
5.185.18A Different Mechanism for A Different Mechanism for
Alkyl Halide Elimination:Alkyl Halide Elimination:The E1 MechanismThe E1 Mechanism
Dr. Wolf's CHM 201 & 202 5-27
ExampleExampleExampleExample
CHCH33 CHCH22CHCH33
BrBr
CHCH33
Ethanol, heatEthanol, heat
++
(25%)(25%) (75%)(75%)
CC
HH33CC
CHCH33
CC CC
HH33CC
HH
CHCH22CHCH33
CHCH33
CCHH22CC
Dr. Wolf's CHM 201 & 202 5-28
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
Dr. Wolf's CHM 201 & 202 5-29
Step 1Step 1Step 1Step 1
slow, unimolecularslow, unimolecular
CCCHCH22CHCH33CHCH33
CHCH33
++
CHCH33 CHCH22CHCH33
BrBr
CHCH33
CC
::....::
::....:: BrBr.... ––
Dr. Wolf's CHM 201 & 202 5-30
Step 2Step 2Step 2Step 2
CCCHCH22CHCH33CHCH33
CHCH33
++
CCCHCH22CHCH33CHCH33
CHCH22
++ CCCHCHCHCH33CHCH33
CHCH33
– – HH++
Dr. Wolf's CHM 201 & 202 5-31
End of Chapter 5