β-Elimination Reactions
dehydrohalogenation of alkyl halides:
X = H; Y = Br, etc.
5.14
Dehydrohalogenation of
Alkyl Halides
X βC
C
Cα Y
β-Elimination Reactions
C
+
X
Y
Dehydrohalogenation
dehydrohalogenation of alkyl halides:
X = H; Y = Br, etc.
X C
β
C
Cα Y
C
+
X
Cl
Y
NaOCH2CH3
ethanol, 55°
55°C
(100 %)
requires base
likewise, NaOCH3 in methanol, or KOH in ethanol
Regioselectivity
Dehydrohalogenation
When the alkyl halide is primary,
primary, potassium
terttert-butoxide in dimethyl sulfoxide (DMSO), a
strong non-protic
non-protic polar solvent is the
base/solvent system that is normally used.
CH3(CH2)15CH2CH2Cl
KOC(CH3)3
dimethyl sulfoxide
CH3(CH2)15CH
(86%)
CH2
Br
KOCH2CH3
+
ethanol, 70°
70°C
29 %
71 %
follows Zaitsev's rule
more highly substituted double bond predominates
Stereoselectivity
Question
How many alkenes would you expect to be
formed from the E2 elimination of
3-bromo-2-methylpentane?
KOCH2CH3
ethanol
Br
A) 2
B) 3
+
C) 4
(23%)
(77%)
D) 5
more stable configuration
of double bond predominates
Stereoselectivity
Br
KOCH2CH3
ethanol
5.15
The E2 Mechanism of
Dehydrohalogenation of Alkyl Halides
+
(85%)
(15%)
more stable configuration
of double bond predominates
Facts
(1) 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
Question
The reaction of 2-bromobutane with
KOCH2CH3 in ethanol produces trans-2butene. If the concentration of both
reactants is doubled, what would be the
effect on the rate of the reaction?
A) halve the rate
B) double the rate
C) quadruple the rate
D) no effect on the rate
Facts
The E2 Mechanism
(2) Rate of elimination depends on halogen
weaker C—
C—X bond; faster rate
rate: RI > RBr > RCl > RF
implies that carbon-halogen bond breaks in
the rate-determining step
concerted (one-step) bimolecular process
single transition state
C—H bond breaks
π component of double bond forms
C—X bond breaks
The E2 Mechanism
R
.. –
:
O
..
The E2 Mechanism
δ–
..
O
..
Transition state
H
R
H
C
C
C
C
: X:
..
δ–
: X:
..
Reactants
The E2 Mechanism
R
..
O
..
H
C
C
.. –
: X:
..
Products
Question
Which one of the following best describes a mechanistic
feature of the reaction of 3-bromopentane with sodium
ethoxide?
ethoxide?
A) The reaction occurs in a single step which is
bimolecular.
B) The reaction occurs in two steps, both of which are
unimolecular.
unimolecular.
C) The rate-determining step involves the formation
of the carbocation (CH 3CH 2)2CH +.
D) The carbon-halogen bond breaks in a rapid
step
that follows the rate-determining step.
5.16
Stereochemistry:
Anti Elimination in E2 Reactions
Stereoelectronic Effects
Stereoelectronic Effect
Stereoelectronic Effect
Br
KOC(CH3)3
(CH3)3COH
(CH3)3C
cis-1-Bromo-4tertcis-1-Bromo-4-tertbutylcyclohexane
(CH3)3C
(CH3)3C
trans-1-Bromo-4terttrans-1-Bromo-4-tertbutylcyclohexane
Br
(CH3)3C
Stereoelectronic Effect
cis
Br
(CH3)3C
Rate constant for
dehydrohalogenation
of 1,4- cis is >500
times than that of 1,4trans
Br
(CH3)3C
trans
Stereoelectronic Effect
cis
KOC(CH3)3
(CH3)3COH
KOC(CH3)3
(CH3)3COH
Br
KOC(CH3)3
(CH3)3COH
(CH3)3C
H H
(CH3)3C
KOC(CH3)3
(CH3)3COH
(CH3)3C
H that is removed by base must be anti
coplanar to Br
Two anti coplanar H atoms in cis
stereoisomer
Stereoelectronic Effect
Stereoelectronic Effect
trans
H
Br
(CH3)3C
H
1,4- cis
more reactive
KOC(CH3)3
(CH3)3COH
H H
(CH3)3C
H that is removed by base must be anti
coplanar to Br
No anti coplanar H atoms in trans stereoisomer;
all vicinal H atoms are gauche to Br; therefore
infinitesimal or no product is formed
Question
Which would react with KOC(CH3)3/(CH3)3COH
faster?
1,4- trans
much less reactive
Question
Which would react with KOCH2CH3 in ethanol
faster?
A) cis-3tert-butylcyclohexyl
-butylcyclohexyl bromide
cis-3-tert
A) cis-2tert-butylcyclohexyl
-butylcyclohexyl bromide
cis-2-tert
B) trans-3tert-butylcyclohexyl
-butylcyclohexyl bromide
trans-3-tert
B) trans-2tert-butylcyclohexyl
-butylcyclohexyl bromide
trans-2-tert
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 coplanar
arrangement of H and Br in the transition
state for E2 dehydrohalogenation is an
example of a stereoelectronic effect.
effect.
5.17
Isotopes Effects And The E2
Mechanism
The Isotope Effect
A C-D bond is ≈12 kJ/mol stronger than a C-H bond.
The activation energy for breaking a C-D bond is
greater than for breaking a C-H bond.
The rate constant k for an elementary step where
C-D breaks is smaller than for a C-H bond.
The difference in rate is expressed as a ratio kH/kD,
and is a kinetic isotope effect.
effect.
Because it compares 2H to 1H, it is called a deuterium
isotope effect.
effect.
The Isotope Effect
The rate is slower in the reaction below for deuterium, 2H, vs. 1H.
Since in the rate determining step of the E2 mechanism,
a base removes a proton from a β carbon.
The mechanism accounts for the observed deuterium isotope
effect.
Example
CH3
CH3
5.18
The E1 Mechanism of
Dehydrohalogenation of Alkyl
Halides
C
CH2CH3
Br
Ethanol, heat
H 3C
CH3
H 2C
+
C
CH2CH3
(25%)
H
C
H 3C
C
CH3
(75%)
CH3
Step 1
The E1 Mechanism
CH3
slow, unimolecular
CH3
CH3
CH3
CH3
C
+
CH2CH3
CH3
CH2
CH3
+
CH2CH3
C
+ CH2CH3
.. –
: Br :
..
Question
– H+
C
CH2CH3
:
: Br
..
1. Alkyl halides can undergo elimination in protic
solvents in the absence of base.
2. Carbocation is intermediate.
3. Rate-determining step is unimolecular
ionization of alkyl halide.
Step 2
C
CH3
C
CHCH 3
Which reaction would be most likely to proceed
by an E1 mechanism?
A) 2-chloro-2-methylbutane + NaOCH2CH3 in
ethanol (heat)
B) 1-bromo-2-methylbutane + KOC(CH3)3 in
DMSO
C) 2-bromo-2-methylbutane in ethanol
(heat)
D) 2-methyl-2-butanol + KOH