β-Elimination Reactions Overview
dehydrogenation of alkanes: (enzymatic and limited industrial use)
X=Y=H
5.8
Preparation of Alkenes:
Elimination Reactions
dehydration of alcohols:
X = H; Y = OH
dehydrohalogenation of alkyl halides:
X = H; Y = Br, etc.
X βC
C
Cα Y
+
C
X
Y
Dehydration of Alcohols
CH3CH2OH
5.9
Dehydration of Alcohols
OH
H2SO4
160°
160°C
H 2C
CH2 +
H 2O
+
H 2O
H2SO4
140°
140°C
(79-87%)
CH3
H 3C
C
OH
CH3
H2SO4
H 3C
C
heat
H 3C
CH2
+
H 2O
(82%)
Question
Answer
The dehydration of 2-methyl-2-propanol cannot
be accomplished by using which of the
following reagents?
The dehydration of 2-methyl-2-propanol cannot
be accomplished by using which of the
following reagents?
A) H2SO4
A) H2SO4
B) H3PO4
B) H3PO4
C) HCl
C) HCl
D) K2SO4
D) K2SO4
R'
Relative
Reactivity
R
C
OH
tertiary:
most reactive
R"
Of the isomeric alcohols having the molecular
formula C5H12O, which one will undergo
acid-catalyzed dehydration most readily?
R'
R
C
OH
A) 2-pentanol
H
B) 2-methyl-1-butanol
H
R
C
Question
C) 2-methyl-2-butanol
OH
H
primary:
least reactive
D) 3-methyl-2-butanol
Answer
Of the isomeric alcohols having the molecular
formula C5H12O, which one will undergo
acid-catalyzed dehydration most readily?
A) 2-pentanol
5.10
Regioselectivity in Alcohol Dehydration:
The Zaitsev Rule
B) 2-methyl-1-butanol
C) 2-methyl-2-butanol
D) 3-methyl-2-butanol
Regioselectivity
Regioselectivity
CH3
HO
H2SO4
+
80°
80°C
10 %
CH3
OH
90 %
A reaction that can proceed in more than one
direction, but in which one direction
predominates, is said to be regioselective.
H3PO4
CH3
+
heat
84 %
16 %
A reaction that can proceed in more than one
way to produce different products involving
different carbon atoms, where one
predominates. It is said to be regioselective.
regioselective.
The Zaitsev Rule
The Zaitsev Rule
When elimination can occur in more than one
direction, the principal alkene is the one formed
by loss of H from the β carbon having the
fewest hydrogens.
R
R
OH
C
C
H
CH3
CH2R
When elimination can occur in more than one
direction, the principal alkene is the one formed
by loss of H from the β carbon having the
fewest hydrogens.
R
R
OH
C
C
H
CH3
CH2R
two protons on this β carbon
three protons on this β carbon
The Zaitsev Rule
Question
When elimination can occur in more than one
direction, the principal alkene is the one formed
by loss of H from the β carbon having the
fewest hydrogens.
R
R
OH
C
C
H
CH3
R
C
CH2R
R
What is the major product in the reaction of 2methyl-2-butanol with H2SO4 at 80°
80°C?
CH2R
A)
B)
CH3
C)
D)
C
only one proton on this β carbon
Answer
Question
What is the major product in the reaction of 2methyl-2-butanol with H2SO4 at 80°
80°C?
What is the major product of the dehydration of
2-methylcyclohexanol?
A) 1-methylcyclohexene
A)
B)
B) 3-methylcyclohexene
C) 4-methylcyclohexene
C)
D)
D) cyclohexene
Answer
What is the major product of the dehydration of
2-methylcyclohexanol?
5.11
Stereoselectivity in Alcohol Dehydration
A) 1-methylcyclohexene
B) 3-methylcyclohexene
C) 4-methylcyclohexene
D) cyclohexene
Stereoselectivity
H2SO4
The major product of the dehydration of 1phenyl-2-propanol is
+
heat
OH
Question
(25%)
(75%)
A)
B)
C)
D)
A stereoselective reaction is one in which
a single starting material can yield two or more
stereoisomeric products, but one is produced
in greater amounts than any other.
Stereospecificity
Answer
dehydrogenase
The major product of the dehydration of 1phenyl-2-propanol is
OH
A)
+
(0%)
(100%)
B)
A stereospecific reaction is one in which a single
starting material yields only a single stereoisomer
C)
D)
although other steroeisomers are possible. Enzymes
do this commonly, but there are not many processes
invented by man which do.
A Connecting Point...
Dehydration of alcohols and the reaction
of alcohols with hydrogen halides share the
following common features:
5.12
The E1 and E2 Mechanisms
of Alcohol Dehydration
1) Both reactions are promoted by acids
2) The relative rates/ reactivity decreases in the
order tertiary > secondary > primary
These similarities promote the idea that carbocations are
intermediates in the acid-catalyzed dehydration of
alcohols, just as they are in the reaction of alcohols
with hydrogen halides.
Mechanism
Dehydration of tert-Butyl Alcohol
Step 1:
CH3
H 3C
C
OH
H2SO4
H 3C
CH2
C
heat
+
H 2O
Proton transfer to tert-butyl
tert-butyl alcohol
H
..
(CH3)3C O : + H O +
..
H
H
fast, bimolecular
H 3C
CH3
H
+
(CH3)3C O :
first two steps of mechanism are identical to
those for the reaction of tert-butyl
tert-butyl alcohol with
hydrogen halides
H
+
: O:
H
H
terttert-Butyloxonium ion
Mechanism
Step 2:
Dissociation of terttert-butyloxonium ion
to carbocation
H
+
(CH3)3C O :
Because rate-determining
step is unimolecular,
unimolecular, this
is called the E1 mechanism.
(CH3)3C+
Mechanism
Step 3:
Deprotonation of tert-butyl
tert-butyl cation
H
H 3C
+C
H
slow, unimolecular
H
+
: O:
tert-Butyl
tert-Butyl cation
H
H
+
:O :
H
CH2
H 3C
fast, bimolecular
H 3C
C
H 3C
CH2
+
H
H
+
O:
H
Carbocations
Dehydration of Primary Alcohols
CH3CH2OH
H2SO4
160°
160°C
Carbocation intermediates can:
1) react with nucleophiles
H 2C
CH2 +
avoids carbocation because primary carbocations
are not thermodynamically stable
and/or
2) lose a β-proton to form an alkene
oxonium ion loses water and a proton in a
bimolecular step
Mechanism
Step 1:
Proton transfer from acid to ethanol
H
..
CH3CH2 O : + H O
..
H
H
Mechanism
Step 2:
fast, bimolecular
H
Oxonium ion loses both a proton and
a water molecule in the same step.
H
H
+
: O : + H CH2 CH2 O :
H
H
H
+
CH3CH2 O :
+
slow, bimolecular
H
:O :
H
H 2O
+
:O
H
H
Ethyloxonium ion
H
H
+
H 2C
CH2
+
:O:
H
Mechanism
Step 2:
H
+
:O
H
Oxonium ion loses both a proton and
a water molecule in the same step.
H
H
+
: O : + H CH2 CH2 O :
H
H
Because rate-determining
step is bimolecular,
thisbimolecular
slow,
is called the E2 mechanism. H
H
+
H 2C
CH2
+
:O:
H
5.13
Rearrangements in Alcohol Dehydration
The alkene product may not have
the same carbon skeleton as the starting alcohol.
Rearrangement Involves Alkyl Group Migration
Example
OH
Carbocation can lose
a proton as shown;
CH3
CH3 C
H3PO4, heat
CHCH 3
+
or it can undergo a
1,2- methyl migration.
CH3
3%
+
3%
+
33%
64%
Rearrangement Involves Alkyl Group Migration
CH3
CH3 C
CHCH 3
+
CH3 group migrates
with its pair of
electrons to adjacent
positively charged
carbon.
97%
CH3
CH3
+
C
CHCH 3
CH3
CH3
Rearrangement Involves Alkyl Group Migration
CH3
CH3 C
CHCH 3
+
CH3
+
C
CH3
CHCH 3
CH3
CH3
tertiary carbocation;
more stable
3%
97%
3%
Rearrangement Involves Hydride Shift
Another Rearrangement
CH3CH2CH2CH2
CH3CH2CH2CH2OH
H
+
O:
H
H3PO4, heat
CH3CH2CH
12%
CH2
+
CH3CH
CHCH 3
mixture of cis (32%)
and trans-2-butene
trans-2-butene (56%)
CH3CH2CH
CH2
Oxonium ion can lose
water and a proton
(from C-2) to give
1-butene;
doesn't give a
carbocation directly
because primary
carbocations are too
unstable.
Rearrangement Involves Hydride Shift
CH3CH2CH2CH2
H
+
:
O
CH3CH2CHCH 3
+
H
Rearrangement Involves Hydride Shift
CH3CH2CH2CH2
H
+
:
O
CH3CH2CHCH 3
+
H
Hydrogen migrates
with its pair of
electrons from C-2 to
C-1 as water is lost;
CH3CH2CH
carbocation formed by
hydride shift is
secondary.
CH2
CH3CH2CH
Carbocations Can...
H
+
O:
• react with nucleophiles
H
H
CHCH 3
mixture of cis
and trans-2-butene
trans-2-butene
Hydride Shift
CH3CH2CHCH2
CH3CH
CH2
• lose a proton from the β-carbon to form an alkene
• rearrange (less stable to more stable)
+
CH3CH2CHCH2 +
H
: O:
H
H
Question
Answer
What is the major product in the reaction of 2methyl-1-butanol with H2SO4 at 80°
80°C?
What is the major product in the reaction of 2methyl-2-butanol with H2SO4 at 80°
80°C?
A)
B)
A)
B)
C)
D)
C)
D)