Dehydrohalogenation of Alkyl Halides E2 and E1 Reactions ... · dehydrohalogenation of alkyl...
Transcript of Dehydrohalogenation of Alkyl Halides E2 and E1 Reactions ... · dehydrohalogenation of alkyl...
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Dehydrohalogenation of
Alkyl Halides
E2 and E1 Reactions in Detail
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X Y
dehydration of alcohols:
X = H; Y = OH
dehydrohalogenation of alkyl halides:
X = H; Y = Br, etc.
abC CC C + X Y
b-Elimination Reactions Overview
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X Y
dehydration of alcohols:
acid-catalyzed
dehydrohalogenation of alkyl halides:
consumes base
abC CC C + X Y
b-Elimination Reactions Overview
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is a useful method for the preparation of alkenes
(100 %)
likewise, NaOCH3 in methanol, or KOH in ethanol
NaOCH2CH3
ethanol, 55°C
Dehydrohalogenation
Cl
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CH3(CH2)15CH2CH2Cl
When the alkyl halide is primary, potassium
tert-butoxide in dimethyl sulfoxide is the
base/solvent system that is normally used.
KOC(CH3)3
dimethyl sulfoxide
(86%)
CH2CH3(CH2)15CH
Dehydrohalogenation
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Br
29 % 71 %
+
Regioselectivity
follows Zaitsev's rule
More highly substituted double bond
predominates = More Stable
KOCH2CH3
ethanol, 70°C
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Zaitsev’s Rule
The more substituted alkene is
obtained when a proton is removed
from the b-carbon that is bonded to
the fewest hydrogens
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Conjugated alkenes are preferred !
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Steric hindrance effects the product distribution
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more stable configurationof double bond predominates
Stereoselectivity
KOCH2CH3
ethanol
Br
+
(23%) (77%)
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more stable configurationof double bond predominates
Stereoselectivity
KOCH2CH3
ethanol
+
(85%) (15%)
Br
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Mechanism of the
Dehydrohalogenation of Alkyl Halides:
The E2 Mechanism
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Facts
Dehydrohalogenation of alkyl halides
exhibits second-order kinetics
first order in alkyl halide
first order in base
rate = k[alkyl halide][base]
implies that rate-determining step
involves both base and alkyl halide;
i.e., it is bimolecular
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Facts
Rate of elimination depends on halogen
weaker C—X bond; faster rate
rate: RI > RBr > RCl > RF
implies that carbon-halogen bond breaks in
the rate-determining step
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concerted (one-step) bimolecular process
single transition state
C—H bond breaks
p component of double bond forms
C—X bond breaks
The E2 Mechanism
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The E2 Mechanism
QuickTime™ and aGraphics decompressor
are needed to see this picture.
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–
OR..
.. :
C C
H
X..::
Reactants
The E2 Mechanism
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–
OR..
.. :
C C
H
X..::
Reactants
The E2 Mechanism
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C C
d–
OR..
..H
X..::d–
Transition state
The E2 Mechanism
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OR..
..H
C C
–X..
::..
Products
The E2 Mechanism
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Stereoelectronic Effects
Anti Elimination in E2 Reactions
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Stereochemistry of the E2 Reaction
Remember: The bonds to the eliminated groups (H
and X) must be in the same plane and anti to each
other
H
XMore stable conformation than syn-eclipsed
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The best orbital overlap of the interacting orbitals is
achieved through back side attack of the leaving
group X as in an SN2 displacement.
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Regioselectivity
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Configuration of the Reactant
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Elimination from Cyclic Compounds
Configuration must be trans, which is (anti).
H
Br
H
Br
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(CH3)3C
(CH3)3C
Br
KOC(CH3)3
(CH3)3COH
cis-1-Bromo-4-tert-
butylcyclohexane
Stereoelectronic effect
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(CH3)3C
(CH3)3C
Br KOC(CH3)3
(CH3)3COH
trans-1-Bromo-4-tert-
butylcyclohexane
Stereoelectronic effect
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(CH3)3C
(CH3)3C
Br
(CH3)3C
Br
KOC(CH3)3
(CH3)3COH
KOC(CH3)3
(CH3)3COH
cis
trans
Rate constant for
dehydrohalogenation
of cis is 500 times
greater than that of
trans
Stereoelectronic effect
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(CH3)3C
(CH3)3C
Br
KOC(CH3)3
(CH3)3COH
cis
H that is removed by base must be anti
periplanar to Br
Two anti periplanar H atoms in cis
stereoisomer
HH
Stereoelectronic effect
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(CH3)3C
KOC(CH3)3
(CH3)3COH
trans
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
HH
(CH3)3C
BrH
H
Stereoelectronic effect
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cis
more reactive
trans
less reactive
Stereoelectronic effect
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Stereoelectronic 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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E2 in a cyclohexane ring
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E2 in a cyclohexane ring
Cl
C
H
3
C
H
3
C
H
3
C
H
3
C
H
3
C
H
3
Cl
C
H
3
C
H
2
O
-
C
H
3
C
H
2
O
-
+
+
menthyl
neomenthyl
Can you predict the products?
Cis or trans?
Axial or equatorial?
a,e e,a
e,e a,a
C
H
3
C
H
3
C
H
3
C
H
3
C
H
3
C
H
3
+
80% 20%
Can you explain the products?
C
H
3
C
H
3
C
H
3
100%
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Cyclohexane Stereochemistry Revisited
http://www.csir.co.za/biochemtek/newsletter/aug/menthol.html
l-menthol
http://www.library.ucsf.edu/tobacco/batco/html/9000/9036/
How many stereoisomers are possible for menthol?
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A Different Mechanism for Alkyl
Halide Elimination:
The E1 Mechanism
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CH3 CH2CH3
Br
CH3
Ethanol, heat
+
(25%) (75%)
C
H3C
CH3
C C
H3C
H
CH2CH3
CH3
CH2C
Example
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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.
The E1 Mechanism
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slow, unimolecular
CCH2CH3CH3
CH3
+
CH3 CH2CH3
Br
CH3
C
:..
:
:..
: Br.. –
Step 1
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CCH2CH3CH3
CH3
+
CCH2CH3CH3
CH2
+ CCHCH3CH3
CH3
– H+
Step 2
Which alkene is more stable and why?
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Reaction coordinate diagram for the E1 reaction of
2-chloro-2-methylbutane
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Must consider possible carbocation rearrangement
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Stereochemistry of the E1 Reaction
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E1 Elimination from Cyclic Compounds
E1 mechanism involves both syn and anti elimination
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Summary & Applications (Synthesis)
SN1 / E1 vs. SN2 / E2
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E2 and E1 Reactions
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Substitution vs. Elimination
Alkyl halides can undergo SN2, SN1, E2 and E1 Reactions
1) Which reaction conditions favor SN2/E2 or SN1/E1?
•SN2/E2 reactions are favored by a high
concentration of nucleophile/strong base
•SN1/E1 reactions are favored by a poor
nucleophile/weak base
2) What will be the relative distribution of substitution product
vs. elimination product?
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Consider the Substrate
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NOTE: a bulky base encourages elimination over substitution
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Returning to Sn2 and E2:
Considering the differences
Can you predict the products?
Br
O
C
H
3
B
r
C
H
3
O
-
+
+
O
C
H
3
Can you explain the products?
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Substitution and Elimination Reactions
in Synthesis
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A hindered alkyl halide should be used if you
want to synthesize an alkene
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Which reaction produces an ether?
CH3CH2Br CH3CO-
CH3
CH3
+
CH3CH2O- CH3CBr
CH3
CH3
+
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Consecutive E2 Elimination Reactions:
Alkynes
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Intermolecular vs. Intramolecular Reactions
• A low concentration of reactant favors an intramolecular
reaction
• The intramolecular reaction is also favored when a five-
or six-membered ring is formed
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Three- and four-membered rings are less easily formed
Three-membered ring compounds are formed more
easily than four-membered ring compounds
The likelihood of the reacting groups finding each other
decreases sharply when the groups are in compounds
that would form seven-membered and larger rings.
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Designing a synthesis …
?
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?CH3 CH3
Br
Br
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