Preparation of Alkenes:
Crude Oil & Elimination Reactions
Alkenes: Crude Oil Sources & Products
-Elimination Reactions / Alkenes
-Elimination Reactions Overview
dehydrogenation of alkanes: (enzymatic and limited industrial use)
X=Y=H
dehydration of alcohols:
X = H; Y = OH
dehydrohalogenation of alkyl halides:
X = H; Y = Br, etc.
X
C

C Y
C
C
+
X
Y
Alkene Stability
Because of steric strain, cis isomers are generally
less stable than trans.
The difference in stability can be quantified by
comparing the heats of combustion.
Alkene Stability
Alkene Stability
Consider the following stability trend:
What pattern do you see?
See SKILLBUILDER 8.3.
Question
What is the correct order of stability (most stable to
least stable) for alkenes?
A. Tetrasubstituted > cis-disubstituted > transdisubstituted > monosubstituted
B. Trisubstituted > terminal disubstituted > transdisubstituted > monosubstituted
C. Tetrasubstituted > trans-disubstituted > cis-
disubstituted > trisubstituted
D. Monosubstituted > cis-disubstituted > transdisubstituted > trisubstituted
Alkene Nomenclature
Alkene Nomenclature
H2C
CHCH2CH3
1-Butene or
But-1-ene
1) Find the longest continuous chain that
includes the double bond.
2) Replace the -ane ending of the unbranched
alkane having the same number of carbons
with -ene.
3) Number the chain in the direction that gives
the lowest number to the doubly bonded
carbon.
Alkene Nomenclature
H2C
CHCHCH2Br
CH3
4) If a substituent is present, identify its position
by number. The double bond takes
precedence over alkyl groups and halogens
when the chain is numbered.
The compound shown above is
4-bromo-3-methyl-1-butene.
(or 4-bromo-3-methylbut-1-ene)
Alkene Nomenclature
H2C
CHCHCH2OH
CH3
4) If a substituent is present, identify its position
by number. Alcohol groups take precedence
over the double bond when the chain is
numbered.
The compound shown above is
2-methyl-3-buten-1-ol.
(or 2-methyl-but-3-en-1-ol)
Common Alkenyl Groups
methylene
H2C
vinyl
H2C
CH
allyl
H2C
CHCH2
Cycloalkene Nomenclature
3
4
2
1
CH3
6-Ethyl-1-methylcyclohexene
5 6
CH2CH3
1) Replace the -ane ending of the cycloalkane
having the same number of carbons with -ene.
2) Number through the double bond in the
direction that gives the lower number to the
first-appearing substituent.
Question
Name the alkene below according to the
IUPAC system.
A) 2-bromo-5-heptane
B) 6-bromo-2-trans-heptene
C) 2-bromo-5-cis-heptene
D) 5-bromo-2-heptene
Question
What is the correct IUPAC name for the following structure?
A. (E, 2S)-2-methyl-3-propylhept-5-ene
B. (E, 5S)-6-methyl-5-propylhept-2-ene
C. (E, 5S)-5-isopropyloct-2-ene
D. (E, 2R)- 2-methyl-3-propylhept-5-ene
E. (E, 5R)- 5-isopropyloct-2-ene
Dehydration of Alcohols
Dehydration of Alcohols
CH3CH2OH
OH
H2SO4
160°C
H2C
CH2 +
H2O
+
H2O
H2SO4
140°C
(79-87%)
CH3
H3C
C
CH3
OH
H2SO4
H3C
C
heat
H3C
CH2
(82%)
+
H2O
Question
The dehydration of 2-methyl-2-propanol cannot
be accomplished by using which of the
following reagents?
A) H2SO4
B) H3PO4
C) HCl
D) K2SO4
R'
Relative
Reactivity
R
C
OH
tertiary:
most reactive
R"
R'
R
C
OH
H
H
R
C
H
OH
primary:
least reactive
Question
Of the isomeric alcohols having the molecular
formula C5H12O, which one will undergo
acid-catalyzed dehydration most readily?
A) 2-pentanol
B) 2-methyl-1-butanol
C) 2-methyl-2-butanol
D) 3-methyl-2-butanol
Regioselectivity in Alcohol Dehydration:
The Zaitsev Rule
OH
Regioselectivity
H2SO4
HO
+
80°C
10 %
90 %
A reaction that can proceed in more than one
direction, but in which one direction
predominates, is said to be regioselective.
Regioselectivity
CH3
CH3
OH
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.
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  carbon having the
fewest hydrogens.
R
R
OH
C
C
H
CH3
CH2R
three protons on this  carbon
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  carbon having the
fewest hydrogens.
R
R
OH
C
C
H
CH3
CH2R
two protons on this  carbon
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  carbon having the
fewest hydrogens.
R
R
OH
C
C
H
CH3
R
CH2R
C
CH2R
R
C
CH3
only one proton on this  carbon
Question
What is the major product in the reaction of 2methyl-2-butanol with H2SO4 at 80°C?
A)
B)
C)
D)
Question
What is the major product of the dehydration of
2-methylcyclohexanol?
A) 1-methylcyclohexene
B) 3-methylcyclohexene
C) 4-methylcyclohexene
D) cyclohexene
Stereoselectivity in Alcohol Dehydration
Stereoselectivity
H2SO4
+
heat
OH
(25%)
(75%)
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.
Question
The major product of the dehydration of 1phenyl-2-propanol is
A)
B)
C)
D)
Stereospecificity
dehydrogenase
+
OH
(0%)
(100%)
A stereospecific reaction is one in which a single
starting material yields only a single stereoisomer
although other steroeisomers are possible. Enzymes
do this commonly, but there are not many processes
invented by man which do.
The E1 & E2 Mechanisms
of Alcohol Dehydration
A Connecting Point...
Dehydration of alcohols and the reaction
of alcohols with hydrogen halides share the
following common features:
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.
Dehydration of tert-Butyl Alcohol
CH3
H3C
C
CH3
OH
H2SO4
H3C
C
heat
CH2
+
H3C
first two steps of mechanism are identical to
those for the reaction of tert-butyl alcohol with
hydrogen halides
H2O
Dehydration of tert-Butyl Alcohol
CH3
H3C
C
CH3
OH
H2SO4
H3C
C
heat
CH2
+
H3C
first two steps of mechanism are identical to
those for the reaction of tert-butyl alcohol with
hydrogen halides
H2O
Mechanism
Step 1:
Proton transfer to tert-butyl alcohol
H
..
+
(CH3)3C O : + H O
..
H
H
fast, bimolecular
H
+
(CH3)3C O :
H
+
H
tert-Butyloxonium ion
:O:
H
Mechanism
Step 2:
Dissociation of tert-butyloxonium ion
to carbocation
H
+
(CH3)3C O :
H
Because rate-determining
step is unimolecular, this
is called the E1 mechanism.
slow, unimolecular
H
+
+
:O:
tert-Butyl cation
H
(CH3)3C
Mechanism
Step 3:
Deprotonation of tert-butyl cation
H
H3C
+C
H
+
:O:
H
CH2
H3C
fast, bimolecular
H
H3C
C
H3C
CH2
+
H
+
O:
H
Carbocations
Carbocation intermediates can:
1) react with nucleophiles
and/or
2) lose a -proton to form an alkene
Dehydration of Primary Alcohols
CH3CH2OH
H2SO4
160°C
H2C
CH2 +
H2O
avoids carbocation because primary carbocations
are not thermodynamically stable
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
fast, bimolecular
H
+
CH3CH2 O :
H
Ethyloxonium ion
H
+
:O:
H
Mechanism
Step 2:
Oxonium ion loses both a proton and
a water molecule in the same step.
H
H
+
: O : + H CH2 CH2 O :
H
H
slow, bimolecular
H
+
:O
H
H
H
+
H2C
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
+
H2C
CH2
+
:O:
H
Rearrangements in Alcohol Dehydration
The alkene product may not have
the same carbon skeleton as the starting alcohol.
Example
OH
H3PO4, heat
+
3%
+
64%
33%
Example
Rearrangement Involves Alkyl Group Migration
CH3
CH3 C
Carbocation can lose
a proton as shown;
CHCH3
+
or it can undergo a
1,2- methyl migration.
CH3
3%
CH3 group migrates
with its pair of
electrons to adjacent
positively charged
carbon.
Rearrangement Involves Alkyl Group Migration
CH3
CH3
CH3 C
CHCH3
+
97%
CH3
+
C
CHCH3
CH3
CH3
3%
tertiary carbocation;
more stable
Rearrangement Involves Alkyl Group Migration
CH3
CH3
CH3 C
CHCH3
+
97%
CH3
+
C
CH3
CH3
3%
CHCH3
Example
Example
Another Rearrangement
CH3CH2CH2CH2OH
H3PO4, heat
CH3CH2CH
12%
CH2
+
CH3CH
CHCH3
mixture of cis (32%)
and trans-2-butene (56%)
Rearrangement Involves Hydride Shift
CH3CH2CH2CH2
H
+
O:
H
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:
CH3CH2CHCH3
+
H
Hydrogen migrates
with its pair of
electrons from C-2 to
C-1 as water is lost;
CH3CH2CH
CH2
carbocation formed by
hydride shift is
secondary.
Rearrangement Involves Hydride Shift
CH3CH2CH2CH2
H
+
O:
CH3CH2CHCH3
+
H
CH3CH2CH
CH2
CH3CH
CHCH3
mixture of cis
and trans-2-butene
Hydride Shift
H
CH3CH2CHCH2
+
O:
H
H
+
CH3CH2CHCH2 +
H
H
: O:
H
Carbocations Can...
• react with nucleophiles
• lose a proton from the -carbon to form an alkene
• rearrange (less stable to more stable)
Question
What is the major product in the reaction of 2methyl-1-butanol with H2SO4 at 80°C?
A)
B)
C)
D)
Question
What is the major product of the following reaction?
H2SO4/Heat
OH
A.
B.
C.
D.