Title: 5.8 Preparation of Alkenes: Elimination Reactions
15.8 Preparation of AlkenesElimination Reactions
2b-Elimination Reactions Overview
- dehydrogenation of alkanes X Y H
- dehydration of alcohols X H Y OH
- dehydrohalogenation of alkyl halides X H Y
Br, etc.
Y
X
a
b
3Dehydrogenation
- limited to industrial syntheses of ethylene,
propene, 1,3-butadiene, and styrene - important economically, but rarely used in
laboratory-scale syntheses
750C
CH3CH3
750C
CH3CH2CH3
45.9Dehydration of Alcohols
5Dehydration of Alcohols
H2SO4
H2O
CH3CH2OH
160C
H2SO4
H2O
140C
(79-87)
H2O
(82)
6Relative Reactivity
tertiarymost reactive
primaryleast reactive
75.10Regioselectivity in Alcohol DehydrationThe
Zaitsev Rule
8Regioselectivity
90
10
- A reaction that can proceed in more than one
direction, but in which one direction
predominates, is said to be regioselective.
9Regioselectivity
84
16
- A reaction that can proceed in more than one
direction, but in which one direction
predominates, is said to be regioselective.
10The Zaitsev Rule
- When elimination can occur in more than one
direction, the principal alkene is the one
formed by loss of H from the b carbon having
thefewest hydrogens.
three protons on this b carbon
11The Zaitsev Rule
- When elimination can occur in more than one
direction, the principal alkene is the one
formed by loss of H from the b carbon having
thefewest hydrogens.
two protons on this b carbon
12The Zaitsev Rule
- When elimination can occur in more than one
direction, the principal alkene is the one
formed by loss of H from the b carbon having
thefewest hydrogens.
only one proton on this b carbon
13The Zaitsev Rule
- When elimination can occur in more than one
direction, the principal alkene is the one
formed by loss of H from the b carbon having
thefewest hydrogens.
only one proton on this b carbon
145.11Stereoselectivity in Alcohol Dehydration
15Stereoselectivity
- A stereoselective reaction is one in which a
single starting material can yield two or more
stereoisomeric products, but gives one of them
in greater amounts than any other.
165.12The Mechanism of Acid-CatalyzedDehydration
of Alcohols
17A connecting point...
- The dehydration of alcohols and the reaction of
alcohols with hydrogen halides share
thefollowing common features - 1) Both reactions are promoted by acids
- 2) The relative reactivity decreases in
the order tertiary gt secondary gt primary - These similarities suggest that carbocations
areintermediates in the acid-catalyzed
dehydration ofalcohols, just as they are in the
reaction of alcoholswith hydrogen halides.
18Dehydration of tert-Butyl Alcohol
H2O
- first two steps of mechanism are identical
tothose for the reaction of tert-butyl alcohol
withhydrogen halides
19Mechanism
Step 1 Proton transfer to tert-butyl alcohol
..
O
H
fast, bimolecular
O
H
tert-Butyloxonium ion
20Mechanism
Step 2 Dissociation of tert-butyloxonium
ion to carbocation
slow, unimolecular
(CH3)3C
tert-Butyl cation
21Mechanism
Step 3 Deprotonation of tert-butyl cation.
fast, bimolecular
H
22Carbocations
- are intermediates in the acid-catalyzed
dehydration of tertiary and secondary alcohols - carbocations can
- react with nucleophileslose a b-proton to form
an alkene
23Dehydration of Primary Alcohols
H2SO4
H2O
CH3CH2OH
160C
- avoids carbocation because primary carbocations
are too unstable - oxonium ion loses water and a proton in
abimolecular step
24Mechanism
Step 1 Proton transfer from acid to ethanol
..
CH3CH2
O
H
fast, bimolecular
H
O
CH3CH2
H
Ethyloxonium ion
25Mechanism
Step 2 Oxonium ion loses both a proton and a
water molecule in the same step.
slow, bimolecular
265.13Rearrangements in Alcohol Dehydration
- Sometimes the alkene product does not have the
same carbon skeleton as the starting alcohol.
27Example
OH
H3PO4, heat
3
33
64
28Rearrangement involves alkyl group migration
- carbocation can lose a proton as shown
- or it can undergo a methyl migration
- CH3 group migrates with its pair of electrons to
adjacent positively charged carbon
3
29Rearrangement involves alkyl group migration
CH3
CH3
- tertiary carbocation more stable
3
30Rearrangement involves alkyl group migration
CH3
CH3
3
31Another rearrangement
H3PO4, heat
mixture of cis (32)and trans-2-butene (56)
12
32Rearrangement involves hydride shift
- oxonium ion can losewater and a proton(from
C-2) to give1-butene - doesn't give a carbocation directlybecause
primarycarbocations are toounstable
33Rearrangement involves hydride shift
CH3CH2CHCH3
- hydrogen migrates with its pair of electrons
from C-2 to C-1 as water is lost - carbocation formed by hydride shift is
secondary
34Rearrangement involves hydride shift
CH3CH2CHCH3
mixture of cisand trans-2-butene
35Hydride Shift
H
36Carbocations can...
- react with nucleophiles
- lose a proton from the b-carbon to form an alkene
- rearrange (less stable to more stable)