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Chapter 14 ALKENES AND ALKYNES II.

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Chapter 14 ALKENES AND ALKYNES II. Oxidation and Reduction Oxymercuration and Hydroboration Simple Ring Formation Assigned Sections Sections 14.1 through 14.4 (c. 352 ... – PowerPoint PPT presentation

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Title: Chapter 14 ALKENES AND ALKYNES II.


1
Chapter 14 ALKENES AND ALKYNES II.
Oxidation and Reduction Oxymercuration and
Hydroboration Simple Ring Formation
2
Assigned Sections
  • Sections 14.1 through 14.4 (c. 352)
  • Summary (Table 14-1)
  • Skip section 14.3
  • Sections 14.5-14.13 will be in c. 353

3
Problem Assignment
  • Chemistry 352 and 353
  • In-Text Problems 1 3 4b 6 - 8 10-18
  • End-of-Chapter Problems 25 - 26 28 31
    33 35
  • 38 42 (all but a) 45- 47
  • 19, 20 22 - 24 39 41 44 48

4
What is Oxidation?
  • Gain of oxygen
  • Loss of hydrogen
  • Skim section between p. 1 and 2

Oxidation
5
OXIDATIONS OF ALKENES
glycol formation
epoxidation
complete oxidation
cleavage
n
H2O
6
What is Reduction?
  • Gain of hydrogen
  • Loss of oxygen

reduced
7
Sect. 14.1 EPOXIDES anti-glycol
formation
8
EPOXIDATION AND ANTI GLYCOL FORMATION
First we will look at
1) the formation of epoxides, and
2) the opening of epoxides to form glycols
generally trans
anti
glycol
epoxide
9
Carboxylic Acid
Hydrogen peroxide
Organic Peroxide
A PERACID
10
TWO COMMONLY-USED PERACIDS
peracetic acid
m-chloroperbenzoic acid (MCPBA)
11
EPOXIDATION WITH A PERACID
A CONCERTED REACTION
both bonds formed at the same time
epoxide
STEREOSPECIFIC
12
anti-Glycol Formation
EPOXIDATION
anti ring opening
RING OPENING
13
OPENING EPOXIDES IN BASE
See Chapter 12, Section 12.8

..
..
..









..


SN2 hydroxyl attacks at least substituted
carbon,
STEREOSPECIFIC
and on the opposite side
14
OPENING EPOXIDES IN ACID
  • See Chapter 12, Section 12.8 for a review.
  • My Chapter 12 webpage give some examples

15
Sect. 14.2 syn-GLYCOL FORMATION

16
syn -Glycol Formation
syn glycols are made with OsO4 or KMnO4
OsO4
osmium tetroxide
KMnO4
potassium permanganate
17
SYN ADDITION GIVES CIS GLYCOLS
C
H
3
C
H
H
3
2
O
H
H
O
H
cis
C
H
3
C
H
H
3
C
H
C
syn conformation
C
H
C
H
3
3
C
H
O
H
C
H
O
H
meso
cis -2-butene
remember addition is syn
result is cis
18
syn-Glycol Formation (I)
osmium tetroxide
syn hydroxylation
19
Mechanism
Notice the transfer of 2e- onto Os
REDUCTION
OXIDIZED
Both of the hydroxyl oxygens in the glycol come
from OsO4
REDUCED
20
syn-Glycol Formation (II)
potassium permanganate
syn hydroxylation
21
Making epoxides via bromohydrins
Bromohydrin (Chapter 8)
( peracid )
22
Skip Sect. 14.3
23
Sect. 14.4 OZONOLYSIS
24
Ozone
electric discharge
or
cosmic rays
..
..
..
..

..
..
..
-




..
..
-
EQUIVALENT RESONANCE STRUCTURES
25
Ozonolysis
FORMATION OF AN OZONIDE
unstable
HYDROLYSIS OF THE OZONIDE (WORKUP)
aldehydes or ketones
26
WORKUP PROCEDURES FOR OZONOLYSIS
Two types of workup (decomposition of the
ozonide) are possible
1. OXIDATIVE
Hydrogen peroxide is present
Aldehydes are oxidized to carboxylic
acids. Formaldehyde is oxidized to carbon
dioxide, which
is lost as a gas.
2. REDUCTIVE
Add Zn and H2O or H3O
METHOD A
Reduce the ozonide with Pd / H2 , and
then add acid ( H3O ).
METHOD B
Aldehydes survive intact and are not oxidized
with reductive conditions.
27
EXAMPLES
O3
Zn / H2O
OR
O3
1) Pd/H2
  1. O3

2) H3O
2) H2O2 H3O
H2O
28
AT ONE TIME OZONOLYSIS WAS WIDELY USED FOR
STRUCTURE PROOF BY DEGRADATION
Broken apart ( or degraded ) to simpler pieces
that are easier to identify.
Unknown compound
The original structure can be deduced by
reassembling the pieces.
29
PROBLEM TO SOLVE
1) O3 / CH2Cl2
C7H12
2) H3O
Pd / H2
C7H14
answer
30
WHAT WAS THE ORIGINAL STRUCTURE ?
H2O2
31
ACETYLENES
1) O3, CH2Cl2 2) H3O
KMnO4 or
Oxidation of acetylenes, whether by KMnO4 or
ozone, normally yields carboxylic acids.
32
NATURAL SOURCES OF SMOG
temperature inversion traps bioemissions
reacts with terpenes
Terpenes
Spruce, Cedar, Fir or Pine Forest
33
Sect. 14.5 Hydrogenation of Alkenes
catalyst

C
C
H
H
C
C
H
H
fine powder, very porous, suspended in solution
a syn addition reaction
The catalyst is Pt, PtO2, Pd, or Ni
(in special cases Ru, Rh, Re)
34
REVIEW OF HYDROGENATION
CATALYTIC HYDROGENATION
IS COVERED IN SEVERAL PLACES IN THE TEXT,
PRINCIPALLY
Ch3 Section 3.18 Hydrogenation of Alkenes
Ch4 Section 4.13 Relative Stabilities of
Alkenes
Ch6 Section 6.12 Heat of Hydrogenation
Section 6.13 Resonance Energy of
Benzene
Ch8 Section 8.11 Hydrogenation of the
Double Bond
Pages 723-725 (Addition to Triple
Bonds)
Ch 14 Section 14.4 Hydrogenation of Alkynes
35
Hydrogenation of Alkynes
36
Lindlar Catalyst syn addition
syn addition
Lindlar is a special catalyst that allows the
hydrogenation of an alkyne to stop after one mole
of hydrogen is added.
Most amines, and compounds containing sulfur,
reduce the activity of catalysts or poison them.
quinoline
37
Sect 14.6 Dissolving Metal Reduction
anti
This reaction proceeds with anti addition (trans
compound).
Catalytic reduction proceeds with syn addition,
hence we have a choice of methods.
38
MECHANISM OF Na-LIQUID-NH3 REDUCTIONS
electron transfer 1
.
-
All intermediates prefer the trans geometry.
( from (NH3)n )
2H
2e
radical- anion
anion
anti addition
radical
NH2-
electron transfer 2
39
TWO DIFFERENT REAGENTS !
SODIUM IN LIQUID AMMONIA
Reducing Agent

Na / NH3(l)
Strong Base

SODIUM AMIDE IN LIQUID AMMONIA
NaNH2 / NH3(l)
40
Sect. 14.7 Oxymercuration of an Alkene
41
Mechanism of Oxymercuration
Formation of Bridged Ion step 1
42
Step 2 Attack by water
43
Step 3 Reduction
44
Another example
Continued next slide
45
(No Transcript)
46
Sect. 14.8 Hydroboration of an Alkene
47
Herbert Brown
Nobel Prize, 1979
Source Michigan State University, Department of
Chemistry http//www.chemistry.msu.edu/Portraits/P
ortraitsHH_collection.shtml
48
Preparation of Diborane
Electron deficient
49
DIBORANE ADDS TO ALKENES ANTI-MARKOVNIKOFF
substituent stabilizes transient carbocation
center
R
R
hydride transfer
electrophile
sp2
empty 2p orbital
R
Since there are 3 B-H bonds, addition can occur
three times (3 moles of alkene)
syn addition
anti-Markovnikoff
Boron adds to the carbon with the most hydrogens
(!) which is the reverse of the Markovnikoff Rule.
50
Concerted versus Carbocation Intermediate
R
R

-
Carbocation intermediate would allow rotation
the reaction would not be stereospecific, giving
both syn anti addition.
Concerted reaction, no intermediate syn addition
51
Hydroboration of an Alkene
bubble B2H6 gas through the solution
3 moles


note abbreviation above
a trialkylboron compound
( or a trialkylborane )
52
Stereochemistry of Hydroboration
Syn-additiion
53
Sect. 14.9 Hydroboration - Protonolysis
54
Sect. 14.10 Hydroboration - Oxidation
55
trialkylborane
3
3
basic hydrogen peroxide
OH-
H2O2
acid
H3O
3
alcohol ( 3 moles)

trialkoxyborane
B(OH)3
This work up step hydrolyzes the trialkoxyboron
compound.
56
..
-
-


H-O-O-H OH
O-O-H H-O-H
..
cast-off
-
..
-


internal SN2
adds into empty orbital
hydroperoxide ion
2 more times
-
trialkoxyborane

57
Stereochemistry of Hydroboration-Oxidation
syn addition
cis
(work up)
H3O
retains cis stereochemistry
syn
58
Summary of Hydroboration Reactions
59
Hydroboration-Protonolysis of Alkynes
A second reaction sequence is required to react
the second pi bond.
60
Hydroboration - Oxidation of Alkynes
3

_
H3O
3
immediate tautomerism
Ketone
Enol
unfavored tautomer !
61
OXYMERCURATION VS. HYDROBORATION-OXIDATION
result similar to 3M H2SO4/H2O
OXYMERCURATION
HYDROBORATION-OXIDATION
1) Hg(OAc)2
1) B2H6
2) NaBH4
2) H2O2, NaOH
3) H3O
anti
syn
Markovnikoff Addition
anti-Markovnikoff Addition
62
SYNTHESIS PROBLEM
Show how to synthesize all five compounds from
acetylene and methanol.
and cis isomer
Requires both Na / NH3(liq) and NaNH2 / NH3(liq).
63
Sect. 14.11 The Diels-Alder Reaction
64
Otto Diels and Kurt Alder
Source Michigan State University, Department of
Chemistry http//www.chemistry.msu.edu/Portraits/P
ortraitsHH_collection.shtml
65
Diels-Alder Reaction
HOMO
The reaction is concerted - all of the
orbitals are aligned in a 6-ring.
Push
The HOMO of the diene donates elec- trons into
the LUMO of the dienophile.
Pull
LUMO
66
BUTADIENE
ETHYLENE
p4
LUMO
p
p3
HOMO
p2
p
p1
67
EXAMPLE - WITH ELECTRONIC FACTORS
pull
push
D
Diene
Dienophile
A Cyclohexene
Works best if the dienophile has
electron-withdrawing groups and the diene
has electron-donating groups.
The HOMO of the diene donates electrons into the
LUMO of the dienophile.
68
FORMATION OF A BICYCLIC COMPOUND
D
69
FORMATION OF A BICYCLIC COMPOUND
D
D
70
Sect. 14.12 Carbene formation and reactions
  • Carbenes are divalent carbon atoms
  • Carbenes are electrophiles and react readily with
    double bonds
  • Carbenes add syn to alkene
  • Carbene addition is usually stereospecific

71
Formation of dichlorocarbene
72
Reactions of dichlorocarbene with alkenes
73
Diazomethane reactions
74
Another diazomethane reaction
Light
75
Sect. 14.13 Carbenoid reactions
  • These reactions are like carbene reactions, but
    do not give formal carbenes as intermediates.
    An example is the Simmons-Smith reaction that
    uses Zn-Cu couple and CH2I2
  • Products formed from carbenoid reactions are
    identical to those from pure carbene reactions.
  • syn-addition to alkene and stereospecific.

76
Simmons-Smith Reaction
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