5.8 Preparation of Alkenes: Elimination Reactions - PowerPoint PPT Presentation

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5.8 Preparation of Alkenes: Elimination Reactions

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limited to industrial syntheses of ethylene, propene, 1,3 ... fewest hydrogens. R. OH. CH3. C. C. H. R. CH2R. three protons on this b carbon. The Zaitsev Rule ... – PowerPoint PPT presentation

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Title: 5.8 Preparation of Alkenes: Elimination Reactions


1
5.8 Preparation of AlkenesElimination Reactions
2
b-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
3
Dehydrogenation
  • 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
4
5.9Dehydration of Alcohols
5
Dehydration of Alcohols
H2SO4

H2O
CH3CH2OH
160C
H2SO4

H2O
140C
(79-87)

H2O
(82)
6
Relative Reactivity
tertiarymost reactive
primaryleast reactive
7
5.10Regioselectivity in Alcohol DehydrationThe
Zaitsev Rule
8
Regioselectivity

90
10
  • A reaction that can proceed in more than one
    direction, but in which one direction
    predominates, is said to be regioselective.

9
Regioselectivity

84
16
  • A reaction that can proceed in more than one
    direction, but in which one direction
    predominates, is said to be regioselective.

10
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 b carbon having
    thefewest hydrogens.

three protons on this b carbon
11
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 b carbon having
    thefewest hydrogens.

two protons on this b carbon
12
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 b carbon having
    thefewest hydrogens.

only one proton on this b carbon
13
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 b carbon having
    thefewest hydrogens.

only one proton on this b carbon
14
5.11Stereoselectivity in Alcohol Dehydration
15
Stereoselectivity
  • 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.

16
5.12The Mechanism of Acid-CatalyzedDehydration
of Alcohols
17
A 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.

18
Dehydration of tert-Butyl Alcohol

H2O
  • first two steps of mechanism are identical
    tothose for the reaction of tert-butyl alcohol
    withhydrogen halides

19
Mechanism
Step 1 Proton transfer to tert-butyl alcohol
..



O
  • (CH3)3C

H
fast, bimolecular



O
H
tert-Butyloxonium ion
20
Mechanism
Step 2 Dissociation of tert-butyloxonium
ion to carbocation
slow, unimolecular


(CH3)3C
tert-Butyl cation
21
Mechanism
Step 3 Deprotonation of tert-butyl cation.

fast, bimolecular


H
22
Carbocations
  • are intermediates in the acid-catalyzed
    dehydration of tertiary and secondary alcohols
  • carbocations can
  • react with nucleophileslose a b-proton to form
    an alkene

23
Dehydration 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

24
Mechanism
Step 1 Proton transfer from acid to ethanol
..


CH3CH2
O
H
fast, bimolecular
H



O
CH3CH2
H
Ethyloxonium ion
25
Mechanism
Step 2 Oxonium ion loses both a proton and a
water molecule in the same step.

slow, bimolecular


26
5.13Rearrangements in Alcohol Dehydration
  • Sometimes the alkene product does not have the
    same carbon skeleton as the starting alcohol.

27
Example
OH
H3PO4, heat


3
33
64
28
Rearrangement 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
29
Rearrangement involves alkyl group migration

CH3
CH3
  • tertiary carbocation more stable

3
30
Rearrangement involves alkyl group migration

CH3
CH3
3
31
Another rearrangement
  • CH3CH2CH2CH2OH

H3PO4, heat

mixture of cis (32)and trans-2-butene (56)
12
32
Rearrangement 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

33
Rearrangement 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

34
Rearrangement involves hydride shift
CH3CH2CHCH3

mixture of cisand trans-2-butene
35
Hydride Shift
H
36
Carbocations can...
  • react with nucleophiles
  • lose a proton from the b-carbon to form an alkene
  • rearrange (less stable to more stable)
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